Big questions about space
315 questions people ask about the Sun, black holes, the universe and how it ends. Each has a short answer here and a full one, with its sources, on the guide it comes from.
By The Orbital Record editorial teamLast reviewed
Free, no sign-up. Teachers: see classroom resources.
The biggest ones
- How old is the universe?The universe is 13.8 billion years old (13.787 billion, Planck 2018). How it is measured two independent ways, and why the 26.7 billion claim fails.
- What is the fourth dimension?In physics the fourth dimension is time, joined to space in relativity's spacetime. Extra spatial dimensions are a hypothesis no test has confirmed.
- Is the universe infinite?Nobody knows. The observable universe is 93 billion light-years across and space is flat to within 0.2%, which allows, but does not prove, infinity.
- What came before the Big Bang?Nobody knows what came before the Big Bang, or if 'before' means anything. The five serious scientific ideas, what each predicts, how to test them.
- Is dark energy weakening?Maybe. DESI's 2025 data prefer a weakening dark energy at 2.8 to 4.2 sigma, depending on the supernovae used. Not a discovery yet: what would settle it.
- Is the universe young?The universe is 13.8 billion years old, yet the age of stars has 100 trillion years to run. Scaled to a human life, it is about five days old. The math.
- Do gravitational waves travel at the speed of light?Yes. In 2017 the gravitational waves and gamma rays of a neutron star merger arrived 1.7 seconds apart after a 130-million-year trip. What that proved.
- How does LIGO work?LIGO splits a laser down two 4 km arms and recombines it. A passing gravitational wave changes the arm lengths by far less than a proton's width.
- When will the Sun die?The Sun will leave its stable phase in about 5 billion years, swell into a red giant and end as a white dwarf. The timeline, and what it means for Earth.
- What has JWST discovered?JWST has found the most distant galaxies known, surprising early black holes and the chemistry of exoplanet air. Its biggest results to October 2026.
Cosmology
Read the cosmology guideWhat is cosmology?
Cosmology is the branch of astronomy and physics that studies the universe as a single object: how it began, what it is made of, how it has changed over time and how it will end.
Full answer in CosmologyIs the universe still expanding?
Yes. Distant galaxies are still moving apart, and the expansion has been speeding up for roughly the last five billion years.
Full answer in CosmologyWho proposed the Big Bang theory?
Georges Lemaître, a Belgian priest and physicist, proposed in 1927 that the universe is expanding and later traced it back to a dense 'primeval atom'.
Full answer in CosmologyIs the Big Bang theory proven?
Science does not prove theories, but the Big Bang is as well supported as anything in astronomy.
Full answer in CosmologyWhat is the cosmic microwave background?
The cosmic microwave background is the oldest light in the universe, released about 380,000 years after the Big Bang when the cooling gas first became transparent.
Full answer in CosmologyWhy is the universe expanding?
The universe expands because it started out expanding: the Big Bang set space itself stretching, and gravity has been slowing or shaping that motion ever since.
Full answer in CosmologyWhat is the Lambda-CDM model?
Lambda-CDM is the standard model of cosmology. It describes a universe that began hot and dense, contains about 5% ordinary matter, 27% cold dark matter and 68% dark energy in the form of a cosmological constant (Lambda), and is fully described by just six measured numbers.
Full answer in CosmologyHow old is the universe?
The universe is about 13.8 billion years old. The most precise figure, 13.787 billion years with an uncertainty of about 20 million years, comes from the European Space Agency's Planck satellite, which measured the cosmic microwave background.
Full answer in How old is the universe?How do we know the age of the universe?
Astronomers use two independent methods. One fits the standard cosmological model to the cosmic microwave background and runs the expansion backward to the start.
Full answer in How old is the universe?How old is the universe, 13.7 or 13.8 billion years?
Both numbers come from real measurements. NASA's WMAP satellite gave 13.7 billion years in 2003, and many textbooks still use it.
Full answer in How old is the universe?Is the universe 26.7 billion years old?
No. A 2023 paper by Rajendra Gupta proposed 26.7 billion years by mixing expansion with 'tired light' and changing physical constants.
Full answer in How old is the universe?Can stars be older than the universe?
No real star can be, and none is. In the 1990s some star ages briefly exceeded estimates of the universe's age, but better distances and a better expansion model fixed the mismatch.
Full answer in How old is the universe?Does the Hubble tension change the age of the universe?
It could. The age depends on the expansion rate. If the true rate is near the distance-ladder value of 73 rather than 67, and nothing else in the model changes, the universe would come out roughly a billion years younger, close to the ages of the oldest stars.
Full answer in How old is the universe?What is the oldest thing in the universe?
The oldest light we can see is the cosmic microwave background, released about 380,000 years after the Big Bang.
Full answer in How old is the universe?What is the fourth dimension?
In physics, the fourth dimension is time. Since Hermann Minkowski's 1908 lecture, relativity treats the three dimensions of space and one of time as a single four-dimensional spacetime.
Full answer in What is the fourth dimension?Is time the fourth dimension?
Yes, in relativity. Locating any event takes four numbers, three for place and one for time, and observers moving relative to each other mix space and time differently.
Full answer in What is the fourth dimension?Are there extra dimensions?
Nobody knows. String theory and several models of gravity predict them, but no experiment has found one.
Full answer in What is the fourth dimension?How many dimensions are there?
We observe four: three of space and one of time. Gravitational waves from the 2017 neutron star merger GW170817 confirmed that gravity spreads through about four spacetime dimensions over cosmic distances.
Full answer in What is the fourth dimension?Can we see the fourth dimension?
Not a fourth dimension of space: our senses and instruments work in three. We experience time, the fourth dimension of spacetime, as change.
Full answer in What is the fourth dimension?What is the fifth dimension in physics?
It is a proposed extra dimension beyond the four of spacetime. Theodor Kaluza introduced it in 1921 to unite gravity and electromagnetism, and Oskar Klein suggested in 1926 that it is curled into a tiny circle.
Full answer in What is the fourth dimension?What is the Hubble tension?
The Hubble tension is the disagreement between two ways of measuring how fast the universe expands today.
Full answer in The Hubble tensionWhat is the Hubble constant?
The Hubble constant is the expansion rate of the universe today. It says how much faster a galaxy recedes for each extra unit of distance, in kilometers per second per megaparsec.
Full answer in The Hubble tensionIs the Hubble tension real?
Most cosmologists now accept that the two measurement families genuinely disagree, because each side has been checked with independent data and telescopes.
Full answer in The Hubble tensionWhat is the current value of the Hubble constant?
It depends on the method. Planck's cosmic microwave background analysis gives 67.4 plus or minus 0.5.
Full answer in The Hubble tensionCould the Hubble tension be solved by new physics?
Possibly. The leading proposals change the early universe, for example a brief burst of 'early dark energy' before the cosmic microwave background was released, which shrinks the cosmic sound-wave ruler.
Full answer in The Hubble tensionHow did JWST affect the Hubble tension?
The James Webb Space Telescope re-observed Cepheid stars with sharper infrared images to test whether crowding by neighboring stars had biased Hubble's measurements.
Full answer in The Hubble tensionWhy does the Hubble tension matter?
The expansion rate sets the age, size and future of the universe. If the standard cosmological model predicts it wrongly, the model is incomplete, which would be the first crack in a theory that has passed nearly every other test for 25 years.
Full answer in The Hubble tensionIs the universe infinite?
Nobody knows. The part we can observe is finite, about 93 billion light-years across. Measurements show space is flat to within about 0.2%, and a flat universe can be infinite, but it could also be finite yet enormous, or curved slightly beyond what we can detect.
Full answer in Is the universe infinite?How big is the observable universe?
The observable universe is about 93 billion light-years in diameter. Light from its edge has traveled for nearly 13.8 billion years, but space kept expanding during that time, so the regions that emitted it are now roughly 46 billion light-years away in every direction.
Full answer in Is the universe infinite?Is the universe flat?
As far as we can measure, yes. Combining Planck's cosmic microwave background maps with baryon acoustic oscillation data gives a curvature of 0.0007 plus or minus 0.0019, consistent with zero.
Full answer in Is the universe infinite?What is beyond the observable universe?
Almost certainly more universe much like ours: more galaxies, spread just as evenly. The observable universe is a limit set by how far light has traveled since the Big Bang, not a physical boundary.
Full answer in Is the universe infinite?Does the universe have an edge?
No known model of the universe has an edge. An infinite universe goes on forever, and a finite one could close back on itself like the surface of a sphere or a video game screen that wraps around.
Full answer in Is the universe infinite?How can the observable universe be 93 billion light-years wide if it is 13.8 billion years old?
Nothing traveled faster than light through space. Space itself expanded while the light was in transit.
Full answer in Is the universe infinite?What was before the Big Bang?
Nobody knows. General relativity, the theory that describes the expanding universe, breaks down when run back to the very beginning, so it cannot say what came before.
Full answer in What came before the Big Bang?Did time begin at the Big Bang?
Possibly. In classical general relativity, time and space both start at the initial singularity, so there is no 'before'.
Full answer in What came before the Big Bang?What caused the Big Bang?
There is no accepted cause. In the most popular picture, cosmic inflation, a brief burst of exponential expansion set up the hot Big Bang, but inflation itself needs a starting point.
Full answer in What came before the Big Bang?Was there a universe before ours?
Some serious models say yes. Bouncing cosmologies propose that a previous universe contracted and rebounded, and Roger Penrose's conformal cyclic cosmology proposes an endless chain of universes.
Full answer in What came before the Big Bang?Can we ever see before the Big Bang?
Not with light. The cosmic microwave background, released 380,000 years after the Big Bang, is a wall we cannot see behind with telescopes.
Full answer in What came before the Big Bang?What is the no-boundary proposal?
The no-boundary proposal, published by James Hartle and Stephen Hawking in 1983, describes the universe with a quantum wave function in which time behaves like a direction of space near the beginning.
Full answer in What came before the Big Bang?
Future of the universe
Read the future of the universe guideWill the universe ever end?
Not with a single bang, on current evidence. The leading forecast is a gradual heat death: stars stop forming within about 100 trillion years, black holes evaporate by roughly 10100 years, and space keeps expanding into a cold, near-empty state.
Full answer in The future of the universeWhen will the universe end?
There is no date. In the standard model the universe never stops existing, it just runs down.
Full answer in The future of the universeHow long until the heat death of the universe?
Heat death is a process rather than a moment. Normal star formation ends around 1014 years, stellar remnants dominate until roughly 1039 years, and the biggest black holes finish evaporating near 10100 years.
Full answer in The future of the universeWill the universe expand forever?
Probably. Since 1998, supernova data have shown the expansion is speeding up, driven by dark energy.
Full answer in The future of the universeIs the universe dying?
Slowly, in one sense. The universe forms stars at a far lower rate than at its peak about 10 billion years ago.
Full answer in The future of the universeWhat will be the last stars in the universe?
Red dwarfs, the smallest true stars. A red dwarf of 0.08 solar masses burns hydrogen for about 12 trillion years, heats up into a blue dwarf and fades as a white dwarf.
Full answer in The future of the universeCan the universe end at any moment?
In principle, yes, through vacuum decay: if the Higgs field sits in a metastable state, a bubble of lower energy could form and spread at nearly light speed.
Full answer in The future of the universeWill the heat death of the universe happen in 2036?
No. The 2036 date is an internet meme with no scientific basis. No published physics predicts any end of the universe in this century.
Full answer in The future of the universeWhat is the difference between the Big Rip, Big Crunch and Big Freeze?
All three are possible endings set by dark energy. In the Big Freeze, expansion continues and the universe slowly goes cold and dark.
Full answer in Big Rip vs Big Crunch vs Big FreezeWhich is more likely, the Big Rip or the Big Crunch?
Neither is the favorite. The Big Freeze remains the leading scenario. Since 2024, DESI data have leaned toward dark energy that is weakening today, which makes a Big Rip less likely.
Full answer in Big Rip vs Big Crunch vs Big FreezeDid DESI prove that dark energy is changing?
No. DESI found a preference for evolving dark energy at 2.8 to 4.2 sigma in 2025, depending on which supernova sample is added.
Full answer in Big Rip vs Big Crunch vs Big FreezeWhen would a Big Rip happen?
It depends on how strongly dark energy grows. In the 2003 paper that named it, an example with equation of state w = -1.5 ends the universe about 22 billion years from now.
Full answer in Big Rip vs Big Crunch vs Big FreezeCould the universe collapse in a Big Crunch?
Only if dark energy becomes negative. A 2025 model by Luu, Qiu and Tye, fitted to DES and DESI data, predicts expansion stopping in about 11 billion years and a crunch about 20 billion years from now.
Full answer in Big Rip vs Big Crunch vs Big FreezeWhat is the heat death of the universe?
It is the state an ever-expanding universe approaches when no usable energy is left: stars have burned out, black holes have evaporated, and everything sits at nearly the same extremely low temperature.
Full answer in Heat death of the universeWhen will the heat death of the universe happen?
Gradually, not on a date. Star formation should end around 1014 years after the Big Bang, and the largest black holes should finish evaporating around 10100 years.
Full answer in Heat death of the universeIs the heat death of the universe inevitable?
It is the leading forecast, not a certainty. It assumes dark energy stays constant or fades gently.
Full answer in Heat death of the universeDoes heat death mean the universe gets hot?
No. The name comes from 19th-century thermodynamics, where heat death meant all heat evenly spread so none could flow.
Full answer in Heat death of the universeIs the heat death the same as the Big Freeze?
Mostly, yes. Big Freeze describes the cooling of an ever-expanding universe, and heat death describes the end state where no useful energy remains.
Full answer in Heat death of the universeIs the universe young or old?
Young, measured in years against its own future. The universe is 13.8 billion years old, but stars will keep forming for about 100 trillion years and black holes will last around 10100 years.
Full answer in Is the universe young?If the universe were a human, how old would it be?
If the age of stars, about 1014 years, were a 100-year human life, the universe today would be about five days old, roughly 121 hours.
Full answer in Is the universe young?What era of the universe are we in?
The Stelliferous Era, the age of stars. In the scheme of Fred Adams and Gregory Laughlin it runs from about 106 to 1014 years after the Big Bang.
Full answer in Is the universe young?Is the universe past its prime?
In one respect, yes. The rate at which galaxies form new stars peaked about 3.5 billion years after the Big Bang and has fallen steadily since.
Full answer in Is the universe young?What is vacuum decay?
Vacuum decay is a hypothetical transition of empty space from a higher-energy state, a false vacuum, to a lower-energy true vacuum.
Full answer in Vacuum decayCan vacuum decay happen at any moment?
In principle, yes: tunneling is random, so a bubble could form at any time. In practice, Standard Model calculations put the expected wait far beyond the age of the universe, with central estimates from about 10139 to 10790 years.
Full answer in Vacuum decayWould we see vacuum decay coming?
No. The bubble wall would move at nearly the speed of light, so no light signal could outrun it to warn us.
Full answer in Vacuum decayIs our universe in a false vacuum?
Possibly. If the Standard Model of particle physics holds up to very high energies, the measured masses of the Higgs boson and top quark place our vacuum in a metastable state, long-lived but not the lowest possible.
Full answer in Vacuum decayCould a particle collider trigger vacuum decay?
No. In 1983 Piet Hut and Martin Rees pointed out that cosmic rays have been colliding with matter at energies far above any accelerator's for billions of years.
Full answer in Vacuum decayWhat is the Primordial Era?
The first of the five eras of the universe in the scheme of Fred Adams and Gregory Laughlin: the hot, dense phase from about the Planck time to roughly 100,000 years after the Big Bang.
Full answer in The Primordial EraHow long did the Primordial Era last?
About 100,000 years in ordinary time, cosmological decades minus 50 to 5. On a logarithmic clock, though, it covers 55 powers of ten, more than any other era except the Black Hole Era and the endless Dark Era, because so many physical changes happened in its first fractions of a second.
Full answer in The Primordial EraWhat came after the Primordial Era?
The Stelliferous Era, the age of stars, which we live in. Around 380,000 years after the Big Bang, atoms formed and light could travel freely.
Full answer in The Primordial EraDo we have evidence for the Primordial Era?
Yes. The cosmic microwave background, discovered in 1965, is light from the end of this era, and its blackbody spectrum was confirmed by COBE in 1990.
Full answer in The Primordial EraWhat is the Stelliferous Era?
The age of stars: the second of five eras in the timeline of Fred Adams and Gregory Laughlin, when most of the energy in the universe comes from nuclear fusion in stars.
Full answer in The Stelliferous EraHow long will the Stelliferous Era last?
About 100 trillion (1014) years in total. Galaxies should exhaust their star-forming gas within 1 to 10 trillion years, and the smallest red dwarfs burn for about 12 trillion years.
Full answer in The Stelliferous EraWhere are we in the Stelliferous Era?
Near the start in ordinary years: 13.8 billion years is about 0.014% of 1014 years. On a logarithmic clock, which counts powers of ten, we are close to the middle.
Full answer in The Stelliferous EraWhat will be the last stars to shine?
Red dwarfs, the smallest true stars. They stay fully mixed by convection, burn nearly all their hydrogen, grow hotter and bluer with age rather than becoming red giants, and finally fade as helium white dwarfs.
Full answer in The Stelliferous EraWhat is the Degenerate Era?
The third of the five eras of the universe, after the age of stars. From about 1015 to 1039 years after the Big Bang, most ordinary matter is locked in stellar remnants: white dwarfs, brown dwarfs and neutron stars, held up by degeneracy pressure.
Full answer in The Degenerate EraWhy is it called the Degenerate Era?
Because its main objects are degenerate stars. In white dwarfs and neutron stars, matter is so compressed that a quantum effect, degeneracy pressure, holds them up instead of heat from fusion.
Full answer in The Degenerate EraDoes proton decay really happen?
Nobody knows. Grand unified theories predict it, and Adams and Laughlin assumed a lifetime of 1037 years to end the Degenerate Era.
Full answer in The Degenerate EraWhat happens to galaxies in the Degenerate Era?
They evaporate. Over countless close encounters, dead stars gain energy and are flung into intergalactic space, while a minority sink into the central black hole.
Full answer in The Degenerate EraWhat is the Black Hole Era?
The fourth of the five eras of the universe, from about 1040 to 10100 years after the Big Bang.
Full answer in The Black Hole EraHow long does it take a black hole to evaporate?
The lifetime grows as the cube of the mass. A black hole of one solar mass needs roughly 1065 to 1067 years, depending on which particles it can emit.
Full answer in The Black Hole EraDo black holes evaporate today?
Not yet, for astrophysical black holes. They are colder than the 2.7-kelvin cosmic microwave background, so they absorb more energy than they emit and grow slightly.
Full answer in The Black Hole EraWhat is the Dark Era?
The fifth and last era of the universe in the scheme of Fred Adams and Gregory Laughlin. It begins after about 10100 years, once black holes have evaporated.
Full answer in The Dark EraDoes the Dark Era ever end?
Not in the standard picture. If dark energy is a cosmological constant, space expands forever and the Dark Era has no end.
Full answer in The Dark EraIs the Dark Era the heat death of the universe?
It is the closest thing to it. Adams and Laughlin found that the universe keeps producing energy and entropy until about 10100 years, so cosmological heat death is postponed until the Dark Era.
Full answer in The Dark EraWhat is the temperature of the universe in the Dark Era?
Close to, but not exactly, absolute zero. In a universe dominated by a cosmological constant, the cosmic horizon has a temperature of its own, about 2 x 10−30 kelvin with today's measured expansion rate.
Full answer in The Dark Era
Black holes
Read the black holes guideWhat is inside a black hole?
Nobody knows for certain. General relativity predicts that everything that crosses the event horizon is crushed into a singularity of infinite density at the center, but that prediction marks where the theory breaks down.
Full answer in Black holesCan black holes die?
Yes, in principle. Stephen Hawking showed in 1974 that quantum effects make black holes emit faint radiation and slowly lose mass.
Full answer in Black holesCan a black hole swallow the Earth?
Not in any realistic scenario. Black holes do not suck things in from afar: at a safe distance their gravity is the same as any object of equal mass.
Full answer in Black holesWho discovered black holes?
No single person. John Michell imagined 'dark stars' in 1784, Karl Schwarzschild found the mathematical solution in 1916, and Oppenheimer and Snyder showed in 1939 how a star could collapse into one.
Full answer in Black holesHas Hawking radiation been observed?
No. Hawking radiation from a stellar or supermassive black hole would be far colder than the cosmic microwave background, so it is undetectable with any telescope.
Full answer in Black holesHow far away is the nearest black hole?
About 1,560 light-years, or 480 parsecs. That is the distance to Gaia BH1, the nearest black hole confirmed as of October 2026, a dormant black hole of about 9.6 solar masses with a Sun-like companion.
Full answer in The black hole closest to EarthIs there a black hole in our solar system?
No. A black hole in the solar system would betray itself through its gravity, tugging on the planets, asteroids and comets whose orbits are tracked with great precision, and nothing like that is seen.
Full answer in The black hole closest to EarthCan the nearest black hole affect Earth?
No. At 1,560 light-years, the gravity of Gaia BH1 on Earth is far weaker than the pull of ordinary nearby stars, and it is not heading our way.
Full answer in The black hole closest to EarthHow many black holes are in the Milky Way?
NASA says scientists think there may be as many as 100 million stellar-mass black holes in our galaxy, but only about 50 suspected or confirmed ones have been found through binary systems.
Full answer in The black hole closest to EarthWas HR 6819 the closest black hole?
It was announced as one in 2020, about 1,000 light-years away, but the claim did not survive.
Full answer in The black hole closest to EarthWhy are nearby black holes so hard to find?
Because a black hole emits no light of its own. The famous ones, like Cygnus X-1, glow in X-rays only because they pull gas from a close companion star.
Full answer in The black hole closest to EarthWhat is the closest supermassive black hole?
Sagittarius A*, at the center of the Milky Way, about 26,000 light-years away. It has about 4.3 million times the mass of the Sun, according to the GRAVITY collaboration in 2022, and the Event Horizon Telescope published the first image of it in May 2022.
Full answer in The black hole closest to EarthWhat are little red dots?
Little red dots are small, compact, unusually red objects that the James Webb Space Telescope finds in large numbers in the early universe, mostly within the first 1.5 billion years after the Big Bang.
Full answer in Little red dotsAre little red dots black holes?
Most astronomers now think so. A Nature paper published in January 2026 argued that they are young supermassive black holes, about 100,000 to 10 million times the Sun's mass, buried in dense cocoons of ionized gas.
Full answer in Little red dotsWhy are little red dots red?
In the cocoon picture, light from the growing black hole passes through a thick envelope of dense gas that absorbs and reprocesses it, giving the objects their red optical color and a sharp jump in brightness at the Balmer limit.
Full answer in Little red dotsHow far away are little red dots?
Most little red dots are seen at redshifts between about 4 and 8, roughly 600 million to 1.5 billion years after the Big Bang, and the largest samples span redshifts of about 2 to 11.
Full answer in Little red dotsWhy did little red dots disappear?
Little red dots are common in the early universe but rare later on. A 2024 survey of 341 of them found their numbers drop rapidly after a redshift of about 4.5.
Full answer in Little red dotsWhat is a black hole star?
A 'black hole star' is a proposed object in which a growing black hole is surrounded by gas so dense that it behaves like the atmosphere of an enormous star.
Full answer in Little red dotsDo little red dots break cosmology?
No. When they were first found, some looked like impossibly massive galaxies very early in cosmic history, which seemed to challenge the standard model.
Full answer in Little red dotsWhat is the difference between a supermassive black hole and a black hole?
A supermassive black hole is a black hole, just a far heavier one. Stellar-mass black holes form when single massive stars collapse and weigh a few to a few tens of Suns.
Full answer in Supermassive black holesWhat is the largest black hole?
Among those with a direct measurement, the heaviest in the nearby universe is the black hole in the galaxy Holm 15A, about 40 billion solar masses (Mehrgan et al.
Full answer in Supermassive black holesIs there a supermassive black hole in the Milky Way?
Yes. Sagittarius A*, at the center of our galaxy about 26,000 light-years away, has about 4.3 million times the Sun's mass, measured from the orbits of nearby stars by the GRAVITY collaboration.
Full answer in Supermassive black holesHow do supermassive black holes form?
Nobody knows for certain. They may have started as light seeds, the 10 to 100 solar-mass remains of the first stars, or as heavy seeds of 10,000 to 100,000 solar masses formed by the direct collapse of gas clouds.
Full answer in Supermassive black holesCan a supermassive black hole swallow a galaxy?
No. Even a very massive black hole is tiny compared with its galaxy, typically about half a percent of the mass of the galaxy's central bulge.
Full answer in Supermassive black holesWhat is a quasar?
A quasar is a supermassive black hole feeding so fast that the gas spiraling into it outshines its whole galaxy.
Full answer in Supermassive black holes
Gravitational waves
Read the gravitational waves guideWho discovered gravitational waves?
Albert Einstein predicted gravitational waves in 1916. Russell Hulse and Joseph Taylor found the first indirect evidence with a binary pulsar discovered in 1974, earning the 1993 Nobel Prize.
Full answer in Gravitational wavesWhen were gravitational waves first detected?
Gravitational waves were first detected directly on September 14, 2015, at 09:50:45 UTC, by both LIGO detectors in the United States.
Full answer in Gravitational wavesDo gravitational waves travel at the speed of light?
Yes. In August 2017 the neutron star merger GW170817 was seen in gravitational waves and, 1.7 seconds later, in gamma rays, after both had traveled about 130 million light-years.
Full answer in Gravitational wavesAre gravitational waves dangerous?
No. By the time gravitational waves from cosmic collisions reach Earth they stretch and squeeze space by about one part in 1021, changing the length of a 4-kilometer detector arm by far less than the width of a proton.
Full answer in Gravitational wavesCan gravitational waves escape a black hole?
No. Nothing that starts inside the event horizon can get out, including gravitational waves.
Full answer in Gravitational wavesHow many gravitational waves has LIGO detected?
As of the GWTC-5.0 catalog released in May 2026, the LIGO-Virgo-KAGRA network has confirmed 390 gravitational-wave events since 2015.
Full answer in Gravitational wavesWhy are gravitational waves so hard to detect?
Because spacetime is extremely stiff. Even the merger of two black holes, one of the most violent events in the universe, produces a stretch of only about 10−21 by the time it reaches Earth.
Full answer in Gravitational wavesWhat is the speed of gravity?
As far as anyone can measure, the speed of gravity equals the speed of light, about 299,792 kilometers per second.
Full answer in Do gravitational waves travel at the speed of light?Is gravity faster than light?
No. Changes in gravity, carried by gravitational waves, travel at the speed of light, not faster.
Full answer in Do gravitational waves travel at the speed of light?If the Sun disappeared, when would Earth feel it?
About eight minutes and 20 seconds later, the same time sunlight takes to reach us. Because changes in gravity travel at the speed of light, Earth would keep orbiting the empty point where the Sun had been until the news arrived, and the sky would go dark at the same moment.
Full answer in Do gravitational waves travel at the speed of light?Why did the gamma rays arrive 1.7 seconds after the gravitational waves?
The gap does not mean light is slower. The gamma rays need not have left at the exact instant the neutron stars merged, and nobody knows the precise emission time.
Full answer in Do gravitational waves travel at the speed of light?How did GW170817 affect theories of dark energy?
It ruled out many of them. A large family of modified-gravity theories, proposed to explain cosmic acceleration without a cosmological constant, predicted that gravitational waves would travel at a different speed from light today.
Full answer in Do gravitational waves travel at the speed of light?Can gravitational waves be slowed down?
In general relativity, no medium slows gravitational waves the way glass slows light, because they interact so weakly with matter.
Full answer in Do gravitational waves travel at the speed of light?How does LIGO detect gravitational waves?
LIGO splits a laser beam in two and sends the halves down two 4-kilometer arms at right angles.
Full answer in How does LIGO work?How sensitive is LIGO?
Extremely. The first signal LIGO detected, in 2015, changed distances by about one part in a billion trillion at its peak, so each 4-kilometer arm changed by a few thousandths of a proton's width.
Full answer in How does LIGO work?Why does LIGO have two detectors?
To tell real signals from local noise. A truck, an earthquake or a power glitch affects one site only, while a gravitational wave reaches both.
Full answer in How does LIGO work?Why are LIGO's arms 4 kilometers long?
Because a gravitational wave changes lengths by a fixed fraction, so a longer arm means a larger, more measurable change.
Full answer in How does LIGO work?What is squeezed light in LIGO?
Squeezed light is a quantum trick that reduces the random flicker of photons, which limits LIGO's sensitivity.
Full answer in How does LIGO work?Where are the LIGO detectors?
LIGO has two observatories in the United States: Hanford, in Washington State, and Livingston, in Louisiana.
Full answer in How does LIGO work?What happens when two neutron stars collide?
They spiral together for millions of years, then merge in a fraction of a second. The collision sends out gravitational waves, can launch a short gamma-ray burst, and throws out hot, neutron-rich debris in which heavy elements such as gold and platinum form.
Full answer in Neutron star mergersWhat is a kilonova?
A kilonova is the glow of radioactive debris thrown out by a neutron star merger. The name, proposed by Brian Metzger and colleagues in 2010, reflects a peak brightness about 1,000 times that of a typical nova.
Full answer in Neutron star mergersDo neutron star mergers make gold?
Yes. Spectra of the 2017 kilonova showed freshly made heavy elements from the rapid neutron-capture process, or r-process.
Full answer in Neutron star mergersHow often do neutron stars merge?
Rarely enough that LIGO and Virgo caught only two confirmed mergers of two neutron stars in their first three observing runs, GW170817 and GW190425.
Full answer in Neutron star mergersCan a neutron star merger affect Earth?
Not from any known system. GW170817 was about 130 million light-years away, and its gamma rays and gravitational waves passed harmlessly through Earth.
Full answer in Neutron star mergersWhat is left after two neutron stars merge?
Either a heavier neutron star or a black hole, depending on the final mass. If the combined remnant exceeds the maximum mass a neutron star can support, a limit thought to lie a little above about 2.2 solar masses, it collapses into a black hole.
Full answer in Neutron star mergers
Dark matter
Read the dark matter guideIs dark matter real?
Almost all astronomers think so. Independent evidence from galaxy rotation, galaxy clusters, gravitational lensing, the cosmic microwave background and the growth of cosmic structure all points to far more mass than we can see.
Full answer in Dark matterWhat is dark matter made of?
Nobody knows yet. The leading candidates are new particles that barely interact with ordinary matter, such as weakly interacting massive particles (WIMPs) or extremely light axions.
Full answer in Dark matterWho discovered dark matter?
Fritz Zwicky first inferred dark matter in 1933, when galaxies in the Coma Cluster moved too fast to be held together by visible mass alone; he called it 'dunkle Materie'.
Full answer in Dark matterHow did Vera Rubin discover dark matter?
Vera Rubin and Kent Ford measured how fast gas clouds orbit at different distances from the center of the Andromeda galaxy, published in 1970.
Full answer in Dark matterCan dark matter be detected?
Not directly, so far. Underground detectors such as LZ, XENONnT and PandaX look for rare collisions between dark matter particles and atoms, and none has found a confirmed signal.
Full answer in Dark matterIs dark matter the same as antimatter?
No. Antimatter is ordinary matter with opposite charges: it emits and absorbs light, and when it meets matter both annihilate in a flash of gamma rays.
Full answer in Dark matterDoes dark matter exist on Earth?
Probably, but only in tiny amounts. If dark matter is a particle, the Milky Way's dark halo means some is streaming through Earth, and through you, right now, almost without interacting.
Full answer in Dark matterWhat is an axion?
An axion is a hypothetical elementary particle, predicted in 1978 by Steven Weinberg and Frank Wilczek, that would solve a long-standing puzzle about the strong nuclear force called the strong CP problem.
Full answer in Axion dark matterAre axions dark matter?
They could be, but nobody has detected one. Axions produced in the early universe would form a cold, invisible sea of particles with exactly the properties dark matter needs.
Full answer in Axion dark matterWhy are axions called axions?
Frank Wilczek named them after a brand of laundry detergent. In his Nobel lecture he explained that the name fit because the particles 'clean up a problem with an axial current', a technical feature of the strong force theory.
Full answer in Axion dark matterWhat is the difference between axions and WIMPs?
Both are proposed dark matter particles, but their masses differ enormously. WIMPs would weigh roughly as much as a proton or more and are hunted through collisions in underground detectors.
Full answer in Axion dark matterHow do scientists search for axions?
Mostly with haloscopes, such as ADMX, which place a tuned microwave cavity inside a powerful magnet.
Full answer in Axion dark matterHas an axion been detected?
No. Several experiments have reported candidate signals that disappeared under scrutiny. In July 2026, for example, a haloscope team published that an excess near 1.036 gigahertz did not persist in follow-up tests and was not an axion.
Full answer in Axion dark matterWhat is fuzzy dark matter?
Fuzzy dark matter is the idea that dark matter consists of extraordinarily light particles, around 10−22 electronvolts, often axion-like.
Full answer in Axion dark matterWhat is the difference between dark matter and dark energy?
Dark matter is invisible mass that pulls through gravity and holds galaxies and clusters together.
Full answer in Dark matter vs dark energyAre dark matter and dark energy the same thing?
No. They act in opposite ways: dark matter attracts and builds structure, dark energy drives the universe apart.
Full answer in Dark matter vs dark energyWhat percentage of the universe is dark matter and dark energy?
According to the Planck satellite's 2018 results, the universe's energy budget is about 68% dark energy, 27% dark matter and 5% ordinary matter.
Full answer in Dark matter vs dark energyAre dark matter and dark energy related?
As far as anyone can tell, no. In the standard model of cosmology they are independent ingredients with separate measured amounts.
Full answer in Dark matter vs dark energyWhich is stronger, dark matter or dark energy?
It depends on the scale and the era. Inside galaxies and clusters, dark matter's gravity wins and holds everything together.
Full answer in Dark matter vs dark energyWhy are they both called dark?
Both names signal ignorance rather than color. Dark matter is 'dark' because it does not emit, absorb or reflect any light.
Full answer in Dark matter vs dark energy
Dark energy
Read the dark energy guideWho discovered dark energy?
Two competing teams discovered the accelerating expansion of the universe in 1998: the High-Z Supernova Search Team, led by Brian Schmidt with Adam Riess as lead author, and the Supernova Cosmology Project, led by Saul Perlmutter.
Full answer in Dark energyHow was dark energy discovered?
Astronomers measured distant type Ia supernovae, exploding stars of nearly known brightness, to see how much the expansion of the universe was slowing down.
Full answer in Dark energyIs dark energy constant?
In the standard model of cosmology it is: a cosmological constant with the same density everywhere and always.
Full answer in Dark energyDoes dark energy cause the universe to expand?
Not exactly. The universe has been expanding since the Big Bang, and that expansion does not need dark energy.
Full answer in Dark energyWill dark energy run out?
If dark energy is a cosmological constant, no: it is a property of space itself, and each new volume of space brings the same amount.
Full answer in Dark energyIs dark energy real?
The accelerating expansion is real: supernovae, the cosmic microwave background, baryon acoustic oscillations and the growth of galaxy clusters all independently require it. 'Dark energy' is simply the name for its unknown cause.
Full answer in Dark energyWhat is dark energy made of?
Nobody knows. The leading idea is that it is the energy of empty space itself, which would make it Einstein's cosmological constant.
Full answer in Dark energyIs dark energy weakening?
Possibly, but it is not established. In March 2025 the Dark Energy Spectroscopic Instrument (DESI) reported that its data, combined with the cosmic microwave background and supernovae, prefer a dark energy that weakens over time at 2.8 to 4.2 sigma.
Full answer in Is dark energy weakening?Has DESI proven that dark energy is changing?
No. DESI's results are a preference, not proof. The strength of the evidence depends on which supernova sample is combined with DESI, ranging from 2.8 to 4.2 sigma, and none reaches 5 sigma.
Full answer in Is dark energy weakening?What do w0 and wa mean?
They describe dark energy's equation of state, w, the ratio of its pressure to its energy density.
Full answer in Is dark energy weakening?What does 4.2 sigma mean?
Sigma measures how unlikely a result would be if the simplest explanation, here a constant dark energy, were true.
Full answer in Is dark energy weakening?What would confirm that dark energy is weakening?
Confirmation would need the signal to reach 5 sigma and to appear in independent data. The key tests are DESI's full five-year results expected in 2027, Euclid's cosmology results expected from mid-2027, and new supernova samples from the Rubin Observatory and the Roman Space Telescope, all pointing the same way.
Full answer in Is dark energy weakening?If dark energy is weakening, will the universe end in a Big Crunch?
Not necessarily. A weakening dark energy rules out the Big Rip and makes the future less certain.
Full answer in Is dark energy weakening?Why do different supernova samples give different answers?
Supernova samples differ in which telescopes found them, how their brightness was calibrated and how nearby and distant events were tied together.
Full answer in Is dark energy weakening?
Exoplanets
Read the exoplanets guideHow many exoplanets have been discovered?
As of October 8, 2026, NASA's Exoplanet Archive lists 6,445 confirmed exoplanets, with thousands more candidates awaiting confirmation.
Full answer in ExoplanetsWhat was the first exoplanet discovered?
The first confirmed exoplanets were two rocky worlds orbiting the pulsar PSR B1257+12, announced by Aleksander Wolszczan and Dale Frail in 1992.
Full answer in ExoplanetsHow are exoplanets discovered?
Mostly indirectly. The transit method watches for a star dimming slightly as a planet crosses it, and the radial velocity method detects the star wobbling as the planet tugs on it.
Full answer in ExoplanetsCan exoplanets support life?
Some might. Dozens of rocky planets orbit in their stars' habitable zones, where liquid water could exist on the surface.
Full answer in ExoplanetsCan exoplanets be seen from Earth?
Not with the naked eye or a backyard telescope. Planets are billions of times fainter than their stars and sit very close to them on the sky.
Full answer in ExoplanetsWhy are exoplanets so hard to detect?
Because they are small, dark and drowned in starlight. An Earth-sized planet crossing a Sun-like star dims it by less than 0.01%, and an Earth-mass planet makes the star wobble by only about 9 centimeters per second.
Full answer in ExoplanetsHow are exoplanets named?
An exoplanet's scientific name is its star's name followed by a lowercase letter, starting with b in order of discovery, so the first planet found around 51 Pegasi is 51 Pegasi b.
Full answer in ExoplanetsWhat is a rogue planet?
A rogue planet drifts through space without orbiting any star, probably after being thrown out of its home system or forming alone from a collapsing cloud.
Full answer in ExoplanetsWhat exoplanet is most like Earth?
There is no single answer, because we cannot yet measure the surface or air of any rocky exoplanet in the habitable zone.
Full answer in The most Earth-like exoplanetsWhat is the Earth Similarity Index?
The Earth Similarity Index, proposed by Dirk Schulze-Makuch and colleagues in 2011, scores planets from 0 to 1 by how close their radius, density, escape velocity and temperature are to Earth's.
Full answer in The most Earth-like exoplanetsWhat is the closest Earth-like exoplanet?
Proxima b, about 4.2 light-years away around the nearest star, Proxima Centauri. It has at least about one Earth mass and orbits in the habitable zone every 11.2 days.
Full answer in The most Earth-like exoplanetsHas JWST found an atmosphere on an Earth-like planet?
Not convincingly. JWST has shown that several rocky planets around red dwarfs, including TRAPPIST-1 b, c and d, lack thick atmospheres.
Full answer in The most Earth-like exoplanetsWhy do most Earth-like candidates orbit red dwarfs?
Because they are easiest to find. Red dwarfs are small and dim, so their habitable zones are close in, planets transit often, and a small planet blocks a larger share of the light.
Full answer in The most Earth-like exoplanetsIs Kepler-452b Earth's twin?
It was called Earth's cousin when announced in 2015, because it orbits a Sun-like star in 385 days.
Full answer in The most Earth-like exoplanetsHow far away is Proxima b?
About 4.24 light-years, the distance to its star, Proxima Centauri, the closest star to the Sun.
Full answer in Proxima bCan humans live on Proxima b?
Nobody knows, and it is far beyond reach anyway. Proxima b gets a similar amount of energy from its star as Earth gets from the Sun, so liquid water is possible in principle.
Full answer in Proxima bDoes Proxima b have an atmosphere?
Unknown as of October 2026. Proxima b does not pass in front of its star as seen from Earth, so astronomers cannot use the transit method that JWST relies on to read atmospheres.
Full answer in Proxima bHow was Proxima b discovered?
Through the radial velocity method. In 2016 the Pale Red Dot campaign, led by Guillem Anglada-Escudé, used the HARPS spectrograph on ESO's 3.6-meter telescope at La Silla to detect Proxima Centauri wobbling toward and away from Earth every 11.2 days, the pull of an orbiting planet of at least about one Earth mass.
Full answer in Proxima bIs Proxima b bigger than Earth?
Probably about the same size, but its size has never been measured directly. Its minimum mass is about 1.06 Earth masses, according to 2025 measurements with the NIRPS spectrograph.
Full answer in Proxima bHow many planets orbit Proxima Centauri?
Two are confirmed as of October 2026: Proxima b, in the habitable zone, and Proxima d, a small planet of about a quarter of Earth's mass on a 5.1-day orbit, confirmed in 2025.
Full answer in Proxima bHow far away is TRAPPIST-1?
About 40 light-years, or 12.4 parsecs. NASA puts it at about 235 trillion miles, roughly 380 trillion kilometers.
Full answer in TRAPPIST-1How many planets does TRAPPIST-1 have?
Seven known planets, named b through h from the inside out. All are roughly Earth-sized, with radii between about 0.76 and 1.13 times Earth's, and all orbit closer to their star than Mercury does to the Sun, taking between 1.5 and 18.8 days per orbit.
Full answer in TRAPPIST-1Which TRAPPIST-1 planets are in the habitable zone?
NASA counts three: TRAPPIST-1 e, f and g, which receive amounts of starlight that could allow liquid water on their surfaces if they have suitable atmospheres.
Full answer in TRAPPIST-1Does TRAPPIST-1e have an atmosphere?
Not yet known. JWST observations published in September 2025 showed that TRAPPIST-1 e no longer has its original hydrogen-rich atmosphere and that a carbon dioxide-dominated one is unlikely.
Full answer in TRAPPIST-1Could there be life on TRAPPIST-1 planets?
There is no evidence either way. The outer planets could hold liquid water if they kept atmospheres, but their star is an active red dwarf whose flares may strip air away, and JWST has found the two innermost planets airless or nearly so.
Full answer in TRAPPIST-1How old is TRAPPIST-1?
About 7.6 billion years, plus or minus 2.2 billion, according to a 2017 study by Burgasser and Mamajek in The Astrophysical Journal.
Full answer in TRAPPIST-1
Astrobiology
Read the astrobiology guideIs there life in the universe besides Earth?
Nobody knows yet. As of October 2026, no life beyond Earth has been confirmed. Scientists have found promising places, such as ancient lake beds on Mars and the hidden oceans of Europa and Enceladus, and some intriguing chemical hints, but no observation so far has ruled out non-biological explanations.
Full answer in Astrobiology: the search for life in the universeAre we alone in the universe?
We do not know. The Milky Way alone holds hundreds of billions of stars, most with planets, and the chemistry of life appears to be common.
Full answer in Astrobiology: the search for life in the universeIs there life on Europa?
Unknown. Europa almost certainly has a salty liquid water ocean beneath its ice, with more water than all of Earth's oceans, which makes it one of the most promising places to look.
Full answer in Astrobiology: the search for life in the universeHas NASA found life on Mars?
No. In September 2025 NASA announced that a rock sampled by the Perseverance rover holds a potential biosignature: mineral spots that on Earth often form with help from microbes.
Full answer in Astrobiology: the search for life in the universeWhen will we find alien life?
There is no date, but the next two decades bring real tests: Europa Clipper at Jupiter from 2030, the Dragonfly lander at Titan in 2034, ESO's Extremely Large Telescope from the early 2030s, and NASA's proposed Habitable Worlds Observatory in the 2040s, designed to search Earth-like exoplanets for signs of life.
Full answer in Astrobiology: the search for life in the universeWhat is a biosignature?
A biosignature is any substance, pattern or structure that could be evidence of past or present life, such as certain gases in an atmosphere, minerals shaped by microbes or complex organic molecules.
Full answer in Astrobiology: the search for life in the universeWhat is astrobiology?
Astrobiology is the science of the origin, evolution, distribution and future of life in the universe.
Full answer in Astrobiology: the search for life in the universeWhat is the Drake equation?
The Drake equation, written by astronomer Frank Drake in 1961, estimates the number of detectable civilizations in the Milky Way by multiplying seven factors, from the rate of star formation to how long civilizations remain detectable.
Full answer in Astrobiology: the search for life in the universeWho came up with the Fermi paradox?
It is named after the physicist Enrico Fermi, who asked colleagues at a lunch at Los Alamos in 1950 a question usually remembered as where is everybody.
Full answer in The Fermi paradoxWhat is the Great Filter?
The Great Filter is the idea, proposed by economist Robin Hanson in the late 1990s, that some step between dead chemistry and a galaxy-spanning civilization is extremely unlikely.
Full answer in The Fermi paradoxIs the Fermi paradox a real paradox?
Not in the logical sense. It is a tension between two reasonable expectations: that intelligent life should be common given the number of planets, and that it should have spread or become visible by now.
Full answer in The Fermi paradoxWhat is the zoo hypothesis?
The zoo hypothesis, published by astronomer John Ball in 1973, suggests that advanced civilizations exist but deliberately avoid contact, leaving Earth alone the way a nature reserve is left undisturbed.
Full answer in The Fermi paradoxHow long would it take to colonize the galaxy?
Estimates are usually a few million to a few tens of millions of years, even with ships much slower than light, because settlements could spread outward in waves.
Full answer in The Fermi paradoxHas SETI found anything?
No confirmed signal from an alien civilization has ever been found. The best-known candidate, the Wow! signal detected by Ohio State's Big Ear telescope in 1977, was never seen again.
Full answer in The Fermi paradoxDoes K2-18b have life?
There is no confirmed evidence of life on K2-18b. A 2025 study reported a possible signal of dimethyl sulfide, a gas made by marine life on Earth, at about 3 sigma.
Full answer in K2-18bWhat is dimethyl sulfide and why does it matter?
Dimethyl sulfide, or DMS, is a sulfur gas that on Earth comes mostly from marine plankton.
Full answer in K2-18bWhat is a hycean world?
A hycean world is a hypothetical planet with a global liquid water ocean under a thick hydrogen-rich atmosphere.
Full answer in K2-18bHow far away is K2-18b?
About 124 light-years, in the constellation Leo. K2-18b orbits a cool red dwarf star, K2-18, every 33 days, inside the star's habitable zone, where it receives roughly the same amount of starlight as Earth gets from the Sun.
Full answer in K2-18bIs K2-18b a habitable planet?
It is in the habitable zone, but that only describes how much starlight it receives. With about 8.6 Earth masses and 2.6 Earth radii, K2-18b is a sub-Neptune with a deep hydrogen atmosphere.
Full answer in K2-18bWhat happens next with K2-18b?
More JWST data. The 2025 team estimated that 16 to 24 more hours of observations could settle the dimethyl sulfide question, while a critical reanalysis suggested about 25 more transits with JWST's mid-infrared instrument would be needed.
Full answer in K2-18b
Galaxies
Read the galaxies guideHow many galaxies are in the universe?
Nobody has counted them all. Estimates range from a few hundred billion galaxies that telescopes could in principle detect to about two trillion when faint, unseen dwarf galaxies are extrapolated, the figure from a 2016 study by Christopher Conselice and colleagues.
Full answer in GalaxiesWhat galaxy do we live in?
We live in the Milky Way, a barred spiral galaxy roughly 100,000 light-years across with somewhere between 100 billion and 400 billion stars.
Full answer in GalaxiesWhat are the different types of galaxies?
Astronomers sort galaxies by shape into spirals, barred spirals, ellipticals, lenticulars and irregulars, a scheme Edwin Hubble introduced in 1926.
Full answer in GalaxiesWhat is the oldest galaxy ever found?
As of October 2026 the most distant confirmed galaxy is MoM-z14, at a redshift of 14.44, seen about 280 million years after the Big Bang by the James Webb Space Telescope.
Full answer in GalaxiesIs the Milky Way moving through space?
Yes. The Sun circles the galactic center once every 200 to 250 million years, and the Milky Way and its Local Group move at about 630 kilometers per second relative to the cosmic microwave background, pulled toward mass concentrations such as the Great Attractor in the Laniakea supercluster.
Full answer in GalaxiesWhat galaxy is closest to the Milky Way?
The closest galaxies are small satellites of our own. The Sagittarius Dwarf, about 70,000 light-years from Earth, is being torn apart by the Milky Way, and the Large Magellanic Cloud lies about 160,000 light-years away.
Full answer in GalaxiesWho discovered that other galaxies exist?
Edwin Hubble settled the question in the 1920s. Using Henrietta Leavitt's 1912 law for Cepheid variable stars, he measured the distance to Cepheids in the Andromeda nebula and showed it lies far outside the Milky Way.
Full answer in GalaxiesWhen will Andromeda collide with the Milky Way?
Possibly never, and almost certainly not in the next 5 billion years. A 2025 study in Nature Astronomy found about a 50% chance the two galaxies merge within 10 billion years, with a typical merger time near 7.6 billion years if it happens.
Full answer in The Andromeda-Milky Way collisionWhat would happen to Earth if the Milky Way and Andromeda merged?
Probably nothing dramatic. Stars are so far apart that direct collisions would be extremely rare, and the solar system would most likely survive, perhaps flung into a new orbit.
Full answer in The Andromeda-Milky Way collisionWhat is Milkomeda?
Milkomeda, sometimes written Milkdromeda, is the nickname for the single galaxy that would form if the Milky Way and Andromeda merge.
Full answer in The Andromeda-Milky Way collisionWhy did the collision prediction change?
Because the measurements improved and more galaxies were included. Till Sawala and colleagues combined newer Hubble and Gaia data and added the gravity of the Large Magellanic Cloud and the Triangulum Galaxy, then ran tens of thousands of simulations covering the measurement uncertainties.
Full answer in The Andromeda-Milky Way collisionWill the Milky Way merge with the Large Magellanic Cloud?
Almost certainly. The same 2025 study found the Large Magellanic Cloud, the Milky Way's biggest satellite galaxy, is set to merge with it in nearly all simulations, with a typical timescale of about 1.3 to 2 billion years.
Full answer in The Andromeda-Milky Way collisionHow fast is Andromeda moving toward us?
Andromeda is approaching the Milky Way at about 110 kilometers per second, roughly 400,000 kilometers per hour.
Full answer in The Andromeda-Milky Way collisionWhat is the Great Attractor?
The Great Attractor is a region of unusually high mass in the nearby universe, in the direction of the southern Milky Way, toward which the Milky Way and thousands of other galaxies are drifting.
Full answer in The Great AttractorWhy can't we see the Great Attractor?
Because the Milky Way is in the way. The Great Attractor lies close to the plane of our galaxy, where stars, gas and dust block much of the view in visible light.
Full answer in The Great AttractorIs the Great Attractor dangerous?
No. The pull is gentle and acts over hundreds of millions of years, and it is overwhelmed on large scales by the expansion of the universe.
Full answer in The Great AttractorWhat is the Laniakea Supercluster?
Laniakea is the supercluster containing the Milky Way, defined in 2014 by Brent Tully and colleagues from the motions of galaxies.
Full answer in The Great AttractorHow fast is the Milky Way moving?
Relative to the cosmic microwave background, the Local Group of galaxies, which includes the Milky Way, moves at about 627 kilometers per second (COBE, 1993), or about 631 km/s in a 2017 analysis.
Full answer in The Great AttractorWhat is the dipole repeller?
The dipole repeller is a large, nearly empty region of space, identified in 2017 by Yehuda Hoffman, Brent Tully and colleagues, that effectively pushes the Milky Way away.
Full answer in The Great Attractor
Stars
Read the stars guideHow long do stars live?
It depends almost entirely on mass. A star like the Sun lives about 10 billion years. A star ten times heavier burns through its fuel in roughly 20 million years, while the smallest red dwarfs, about a tenth of the Sun's mass, should shine for trillions of years, far longer than the universe has existed.
Full answer in StarsWhat happens when a star dies?
A star like the Sun swells into a red giant, sheds its outer layers as a glowing planetary nebula and leaves a white dwarf.
Full answer in StarsHow are stars formed?
Stars form inside cold clouds of gas and dust. When a dense clump becomes too heavy for its internal pressure to support, gravity pulls it inward.
Full answer in StarsWho discovered pulsars?
Jocelyn Bell, then a graduate student at Cambridge, spotted the first pulsar in 1967 in data from a radio array built by her supervisor Antony Hewish.
Full answer in StarsWhat is the difference between a neutron star and a white dwarf?
A white dwarf is the Earth-sized core left by a Sun-like star, held up by electron pressure and limited to about 1.4 solar masses.
Full answer in StarsHow long does it take for a white dwarf to become a black dwarf?
Far longer than the universe has existed: a white dwarf must radiate away all its stored heat before it stops glowing.
Full answer in StarsWhat is the Chandrasekhar limit?
It is the maximum mass, about 1.4 times the Sun's, that a white dwarf can have before electron pressure fails and it collapses or explodes.
Full answer in StarsWhen will Betelgeuse explode?
Probably not for a long time. Most models place Betelgeuse in its core helium-burning phase, which implies roughly 100,000 years before it collapses and explodes.
Full answer in BetelgeuseDoes Betelgeuse have a companion star?
Very probably. A small companion was predicted in 2024 from a six-year brightness cycle, glimpsed at low significance by Gemini North in 2025, and imaged more clearly by ESO's Very Large Telescope in a study published in 2026.
Full answer in BetelgeuseWould a Betelgeuse supernova be dangerous to Earth?
No. A supernova would need to be within roughly 50 light-years to damage Earth's ozone layer, and Betelgeuse is about ten times farther away.
Full answer in BetelgeuseWhat was the Great Dimming of Betelgeuse?
Between late 2019 and early 2020 Betelgeuse faded to roughly a third of its usual brightness, its faintest in more than a century of measurements.
Full answer in BetelgeuseHow far away is Betelgeuse?
About 550 light-years, according to a 2020 study that combined brightness variations with stellar models, with an uncertainty of roughly 50 to 90 light-years.
Full answer in BetelgeuseWhat is Betelgeuse's companion called?
The companion, also called Betelgeuse B, has been given the name Siwarha, announced alongside the 2025 Gemini North detection.
Full answer in BetelgeuseWhat is a neutron star?
A neutron star is the collapsed core left behind when a massive star explodes as a supernova.
Full answer in Neutron starsHow dense is a neutron star?
So dense that, according to NASA, a teaspoon of neutron star matter would weigh about a billion tons on Earth.
Full answer in Neutron starsWhat is the difference between a neutron star and a black hole?
Both are the remains of massive stars, but a neutron star has a surface and is held up by the pressure of packed neutrons, while a black hole has collapsed completely behind an event horizon.
Full answer in Neutron starsWhat is a magnetar?
A magnetar is a neutron star with an extreme magnetic field, from about 100 trillion to a quadrillion gauss, according to NASA, up to a thousand times stronger than a typical neutron star's.
Full answer in Neutron starsWhat is the heaviest neutron star?
The heaviest well-measured neutron star is PSR J0952-0607, at 2.35 solar masses (plus or minus 0.17), measured in 2022.
Full answer in Neutron starsHow many neutron stars are known?
The catalog maintained by Australia's CSIRO lists 4,503 pulsars, the most commonly detected neutron stars, as of its October 2026 version.
Full answer in Neutron starsCan a neutron star turn into a black hole?
Yes, if it gains enough mass. Theory predicts that a neutron star pushed above its maximum mass, by swallowing gas from a companion or by merging with another neutron star, can no longer be supported by neutron pressure and collapses into a black hole.
Full answer in Neutron starsWhat is a white dwarf?
A white dwarf is the dense, hot core left behind when a low- or medium-mass star, like the Sun, runs out of fuel and sheds its outer layers.
Full answer in White dwarfsWill the Sun become a white dwarf?
Yes. NASA says that in about 10 billion years, after its time as a red giant, the Sun will become a white dwarf.
Full answer in White dwarfsHow dense is a white dwarf?
Extremely. NASA gives a density of about a billion kilograms per cubic meter, roughly 200,000 times the density of Earth, and says a teaspoon of white dwarf material would weigh more than a pickup truck.
Full answer in White dwarfsWhat is a black dwarf?
A black dwarf is what a white dwarf would become after cooling so much that it no longer gives off visible light or significant heat.
Full answer in White dwarfsWhat is Sirius B?
Sirius B is the white dwarf companion of Sirius, the brightest star in the night sky, about 8.6 light-years away.
Full answer in White dwarfsCan a white dwarf explode?
Yes, if it gains too much mass. A white dwarf that pulls matter from a companion star, or merges with another white dwarf, can be pushed past the Chandrasekhar limit of about 1.4 solar masses.
Full answer in White dwarfs
Solar System
Read the solar system guideHow old is the solar system?
About 4.567 billion years. The age comes from radioactive dating of the oldest solids known, millimeter-sized calcium- and aluminum-rich grains inside primitive meteorites, which a 2012 study dated to 4,567.3 million years with an uncertainty of about 160,000 years.
Full answer in The solar systemHow did the solar system form?
A dense pocket of an interstellar gas cloud collapsed under its own gravity. Most of the material fell to the center and became the Sun; the rest flattened into a spinning disk.
Full answer in The solar systemDoes Planet Nine exist?
Nobody knows. Konstantin Batygin and Mike Brown proposed it in 2016 to explain why the orbits of several distant icy bodies beyond Neptune are clustered.
Full answer in The solar systemWhere does the solar system end?
It depends on the definition. The Sun's wind of charged particles ends at the heliopause, which Voyager 1 crossed in 2012 at about 122 times Earth's distance from the Sun.
Full answer in The solar systemIs the solar system moving through space?
Yes. The Sun and its planets orbit the center of the Milky Way at more than 200 kilometers per second, completing one lap every 200 to 250 million years.
Full answer in The solar systemWhat is the Kuiper belt?
The Kuiper belt is a broad ring of icy bodies beyond Neptune, roughly 30 to 50 times Earth's distance from the Sun.
Full answer in The solar systemWhat is the Oort cloud?
The Oort cloud is a vast, spherical swarm of icy bodies thought to surround the solar system far beyond the planets, proposed by Dutch astronomer Jan Oort in 1950 to explain long-period comets.
Full answer in The solar systemHow old is the solar system compared to the universe?
About one third of the universe's age. The universe is about 13.8 billion years old and the solar system about 4.6 billion, so the Sun formed when the cosmos was already over 9 billion years old, from gas enriched with heavy elements forged by earlier generations of stars.
Full answer in The solar systemWhat is 3I/ATLAS?
3I/ATLAS is a comet from another star system, the third interstellar object ever detected after 'Oumuamua in 2017 and Borisov in 2019.
Full answer in 3I/ATLASIs 3I/ATLAS an alien spacecraft?
No evidence supports that. Telescopes and spacecraft have measured a coma and tail of carbon dioxide, water, carbon monoxide, cyanide and metal vapors, all normal for a comet.
Full answer in 3I/ATLASHow old is 3I/ATLAS?
Probably much older than the Sun. Its path suggests it comes from the Milky Way's old thick disk, with an estimated age above 7 billion years, and 2026 measurements of its carbon, nitrogen and hydrogen isotopes point to formation around an old star more than twice the Sun's age of 4.6 billion years.
Full answer in 3I/ATLASHow fast is 3I/ATLAS moving?
Far from the Sun it travels at about 58 kilometers per second relative to the solar system, much faster than either earlier interstellar object.
Full answer in 3I/ATLASDid 3I/ATLAS come close to Earth?
No. Its closest approach to Earth, on December 19, 2025, was about 1.8 astronomical units, roughly 270 million kilometers, almost twice Earth's distance from the Sun.
Full answer in 3I/ATLASCan I still see 3I/ATLAS?
Not with amateur equipment. By mid-October 2026 it was more than 12 astronomical units from the Sun, beyond the orbit of Saturn, and far too faint for anything but the largest telescopes.
Full answer in 3I/ATLASWill the Sun become a black hole?
No. Only stars born with many times the Sun's mass leave cores heavy enough to collapse into black holes.
Full answer in When will the Sun die?Will the Sun explode as a supernova?
No. Core-collapse supernovae need stars of roughly eight solar masses or more, and the Sun has one.
Full answer in When will the Sun die?Will the Sun swallow Earth?
Probably. A widely cited 2008 model by Klaus-Peter Schröder and Robert Connon Smith found that when the red giant Sun reaches its largest size, about 7.6 billion years from now, tidal forces and drag will pull Earth inward and it will be engulfed.
Full answer in When will the Sun die?How long until Earth becomes uninhabitable?
Roughly a billion years, long before the Sun dies. The Sun brightens by about 10% every billion years, and that extra heat is expected to start evaporating the oceans and push Earth out of the habitable zone.
Full answer in When will the Sun die?How long will the Sun last?
The Sun has spent about 4.6 billion years of a roughly 10-billion-year stable life fusing hydrogen, so it has about 5 billion years left in that phase.
Full answer in When will the Sun die?What will the Sun look like after it dies?
First a planetary nebula, a glowing shell of ejected gas lit up by the exposed hot core, visible for a few tens of thousands of years.
Full answer in When will the Sun die?
Telescopes and missions
Read the telescopes and missions guideWhat space telescopes are in orbit right now?
As of October 2026, major working space telescopes include Hubble, the James Webb Space Telescope, the Chandra X-ray Observatory, ESA's Euclid, NASA's TESS and SPHEREx, and the Japanese-led XRISM.
Full answer in Space telescopes and observatoriesIs JWST still working?
Yes. As of October 2026 the James Webb Space Telescope is fully operational and in its fifth observing cycle, which began on July 1, 2026, after a record 2,930 proposals.
Full answer in Space telescopes and observatoriesWhy does JWST use infrared?
Because the light of the first galaxies has been stretched into the infrared by the expansion of the universe, and because infrared passes through dust that hides newborn stars and planets.
Full answer in Space telescopes and observatoriesWhen did the Roman Space Telescope launch?
NASA's Nancy Grace Roman Space Telescope launched on August 30, 2026, at 7:26 a.m. EDT on a SpaceX Falcon Heavy from Kennedy Space Center.
Full answer in Space telescopes and observatoriesWhat is the Rubin Observatory LSST?
The Legacy Survey of Space and Time is a ten-year survey by the Vera C. Rubin Observatory in Chile, which began in mid-2026.
Full answer in Space telescopes and observatoriesWhat is the largest telescope in the world?
The largest single-dish radio telescope is China's 500-meter FAST. The largest optical telescope in operation is the 10.4-meter Gran Telescopio Canarias in La Palma.
Full answer in Space telescopes and observatoriesWhat are the upcoming space telescopes?
After Roman, the next major space observatories include ESA's PLATO planet hunter, planned for March 2027, ESA's Ariel exoplanet mission in 2031, NASA's proposed far-infrared PRIMA probe around 2033, the LISA gravitational-wave observatory in the mid-2030s, and NASA's Habitable Worlds Observatory concept for the 2040s.
Full answer in Space telescopes and observatoriesHow do telescopes see back in time?
Light travels at a finite speed, so we see distant objects as they were when the light left them.
Full answer in Space telescopes and observatoriesWhat is the farthest thing JWST has seen?
As of October 2026, the most distant galaxy JWST has confirmed is MoM-z14, at a redshift of 14.44, seen as it was about 280 million years after the Big Bang.
Full answer in What has JWST discovered?Has JWST disproved the Big Bang?
No. JWST found more bright galaxies in the early universe than many models predicted, which led to headlines claiming the Big Bang was in trouble.
Full answer in What has JWST discovered?What are the little red dots JWST found?
Little red dots are compact, very red objects that JWST found in large numbers in the first one to two billion years of cosmic history.
Full answer in What has JWST discovered?Did JWST find signs of life?
No. JWST has measured gases such as carbon dioxide and methane in exoplanet atmospheres, but none is confirmed evidence of life.
Full answer in What has JWST discovered?How far back in time can JWST see?
To roughly 280 million years after the Big Bang for confirmed galaxies, about 98% of the way back to the beginning.
Full answer in What has JWST discovered?Has JWST resolved the Hubble tension?
Not yet. JWST observations ruled out crowded Cepheid stars as the explanation, supporting the high value of about 73 km/s/Mpc from one team.
Full answer in What has JWST discovered?What is the main difference between JWST and Hubble?
Wavelength. Hubble mainly sees ultraviolet and visible light, plus some near-infrared, while JWST sees infrared from 0.6 to 28.5 micrometers.
Full answer in JWST vs HubbleIs JWST better than Hubble?
For infrared work, yes: deeper, sharper at long wavelengths and able to see the early universe and cool objects.
Full answer in JWST vs HubbleIs Hubble still working?
Yes. As of October 2026 Hubble is still doing science. Since June 2024 it has operated in one-gyroscope mode after a gyro fault, which costs about 12% of its observing efficiency.
Full answer in JWST vs HubbleWhy does JWST orbit so far from Earth?
Because infrared telescopes must be extremely cold. Near the Sun-Earth L2 point, about 1.5 million kilometers from Earth, JWST can keep the Sun, Earth and Moon on one side of its tennis-court-sized sunshield, cooling its mirror and instruments below 50 kelvin, and its coldest instrument below 7 kelvin.
Full answer in JWST vs HubbleCan JWST be repaired like Hubble?
No. Hubble orbits a few hundred kilometers up and was serviced by Space Shuttle astronauts five times between 1993 and 2009.
Full answer in JWST vs HubbleHow long will JWST last?
Its accurate launch in December 2021 left enough propellant for roughly 20 years of operations, according to NASA, well beyond the original 5 to 10 year mission goal.
Full answer in JWST vs HubbleIs the Roman Space Telescope better than James Webb?
Neither is better overall; they do different jobs. Webb has a much larger mirror and sees farther into the infrared, so it studies single targets in great depth.
Full answer in Roman Space Telescope vs James WebbHow big is Roman's mirror compared with Webb's?
Roman's primary mirror is 2.4 meters across, the same size as Hubble's; it was donated to NASA by the National Reconnaissance Office.
Full answer in Roman Space Telescope vs James WebbWhat will the Roman Space Telescope discover?
Roman is built for three main goals: measuring how dark energy has shaped cosmic expansion, mapping dark matter through its gravity, and taking a census of planets around other stars.
Full answer in Roman Space Telescope vs James WebbWhere are Roman and Webb in space?
Both work near the Sun-Earth L2 point, about 1.5 million kilometers from Earth on the side away from the Sun.
Full answer in Roman Space Telescope vs James WebbWhy is it called the Nancy Grace Roman Space Telescope?
It honors Nancy Grace Roman, NASA's first chief astronomer, who campaigned for telescopes in space that could study the wider universe.
Full answer in Roman Space Telescope vs James Webb