190 Fun Facts About Astronomy

What is the name of the Sun's visible surface layer?

The photosphere is about 5,500°C, yet the corona above it reaches millions of degrees, and why the corona is so much hotter is still unsolved.

Our Sun is classified as which type of star?

Yellow Dwarf is a misnomer: the Sun is actually white, and only looks yellow through Earth's atmosphere.

What type of galaxy is the Milky Way?

The Milky Way is a barred spiral, and the Sun takes more than 200 million years to complete one orbit around its centre.

Which American author said he came in with Halley's Comet and expected to go out with it?

Mark Twain was born two weeks after the comet's 1835 approach, and he died in 1910, one day after it next swung closest to the Sun.

In astronomy, what kind of quantity does a 'light-year' measure?

Despite the name, a light-year is a distance: how far light travels in one Julian year of 365.25 days.

Which Bell Labs radio astronomers accidentally found the cosmic microwave background in 1964?

Penzias and Wilson even scraped pigeon droppings out of their horn antenna to kill the hiss, which turned out to be the afterglow of the Big Bang and won them a Nobel Prize.

What is a nebula in astronomy?

A nebula is a giant cloud of dust and gas in space, and many are 'star nurseries' where new stars are beginning to form.

Which of these best describes 'dark matter'?

Dark matter is invisible mass detected only through its gravity; it seems to outweigh ordinary matter more than five to one, yet no experiment has detected it directly.

The Sun is in which long, stable phase where most stars spend most of their lives?

The Sun has spent about 4.5 billion years on the main sequence, a little under halfway through, before it will swell into a red giant.

What is a quasar?

A quasar is a galaxy's core lit by a feeding black hole, and the most powerful shine thousands of times brighter than the entire Milky Way.

What is a solar flare?

A solar flare is an intense burst of radiation from the Sun's atmosphere; the first ever recorded was seen in 1859 by two English astronomers working independently.

Which volcano is the tallest planetary mountain in the Solar System?

Olympus Mons rises over 21 km, well over twice Mount Everest's height above sea level.

Whose 1543 book placed the Sun, not the Earth, at the centre of the solar system?

Nicolaus Copernicus's De revolutionibus revived the Sun-centred model — Aristarchus of Samos had proposed one some 1,800 years earlier — and challenged the prevailing geocentric view.

Which spacecraft became the first to enter interstellar space, in August 2012?

Voyager 1 crossed into interstellar space 35 years after launch, and its radio signals now take more than 22 hours to reach Earth.

What is the final evolutionary stage for a low-mass star like our Sun?

A white dwarf packs about half the Sun's mass or more into a ball only about the size of Earth.

Which constellation, home to the bright supergiant Rigel, has a famous belt of stars?

Orion is named after a hunter in Greek myth, and the fuzzy middle 'star' of his sword is really the Orion Nebula, where new stars are forming.

Which famous space telescope, launched in 1990, has provided iconic images of the universe?

The Hubble Space Telescope launched with a mirror ground too flat at its edge, so its images were blurry until astronauts fitted corrective optics in 1993.

What is the name of the boundary around a black hole beyond which nothing, not even light, can escape?

An event horizon isn't a solid surface: someone falling through the event horizon of a giant black hole would notice nothing special as they crossed.

Which region of icy bodies lies beyond the outermost planet, 30 to 50 times as far from the Sun as Earth?

The Kuiper belt is named after Gerard Kuiper, yet he believed such icy bodies would have been swept clear long ago, leaving few or none there today.

What name is given to the average distance between Earth and the Sun?

Sunlight takes a little over 8 minutes to cross one Astronomical Unit, so we always see the Sun as it was about 8 minutes earlier.

What type of nebula is formed from the expanding shell of gas ejected by an aging, dying star?

A planetary nebula has nothing to do with planets: early astronomers named these glowing shells for their round, planet-like look through small telescopes.

What is the leading explanation for how Earth's Moon formed?

A Mars-sized body smashing into Earth most likely made the Moon, and Moon rocks brought back by astronauts share Earth's isotopic fingerprint.

What phenomenon provides the primary observational evidence that the universe is expanding?

The redshift of distant galaxies grows the farther away they are: a galaxy twice as far is receding about twice as fast, as an expanding universe predicts.

Who was the first human to walk on the Moon?

Neil Armstrong's first steps were broadcast live to hundreds of millions of viewers, among the most-watched television events in history.

Which color of star indicates the highest surface temperature?

A star's color is directly related to its surface temperature; blue stars are the hottest, while red stars are the coolest.

Which NASA rover landed in Mars's Gale Crater in August 2012?

Curiosity was lowered onto Mars on cables from a hovering rocket-powered 'sky crane', which then flew off and crash-landed hundreds of metres away.

What are comets mainly made of?

Comets are frozen gases, dust, and rock, and when one nears the Sun, sunlight and the solar wind push its glowing tail to point away from the Sun.

Which of Kepler's laws says the line from a planet to the Sun sweeps out equal areas in equal times?

Second Law, really a speed rule: a planet hurries near the Sun and dawdles far from it; the first is ellipses, the third links period to distance.

What exoplanet detection method watches a star dim as a planet passes in front of it?

The transit method can also sniff out a planet's air: some starlight passes through its atmosphere during the crossing, revealing what that atmosphere contains.

What gives Mars its reddish colour?

Rusty minerals in its soil make Mars red, and strong winds can whip up storms that cover the entire planet.

What is the dense core of a collapsing interstellar cloud that will eventually form a star called?

A protostar is still gathering mass from its cloud, and it glows from the heat of that collapse long before nuclear fusion begins.

Which planet, spotted in 1781, was the first ever discovered with a telescope?

Uranus is faintly visible to the naked eye, yet even its discoverer first reported it as a comet before others recognised it as a planet.

What is the largest and most massive moon in the Solar System?

Ganymede hides a saltwater ocean beneath its icy crust that may hold more water than all of Earth's oceans.

Which galaxy is the closest large spiral galaxy to the Milky Way?

The Andromeda Galaxy is approaching us at more than 100 km per second, yet newer estimates give only about even odds that the two galaxies merge within 10 billion years.

Which supermassive black hole sits at the centre of the Milky Way?

Astronomers weighed Sagittarius A* at about 4 million Suns by tracking stars whipping around it, work that earned a share of the 2020 Nobel Prize in Physics.

Which astronomer lost part of his nose in a 1566 sword duel and wore a metal prosthetic?

Tycho Brahe lost the duel over who was the better mathematician, and his precise records of Mars later let Kepler work out the laws of planetary motion.

What is the brightest star in Earth's night sky?

Sirius outshines every other star in our night sky partly because it is one of our nearest neighbours, only about 8.6 light-years away.

What name is given to an object being stretched long and thin as it falls into a black hole?

Spaghettification happens because a black hole pulls much harder on the nearer end of an object than on its far end, stretching it while squeezing its sides.

Long-period comets are thought to come from which distant reservoir?

The Oort cloud is inferred only from the orbits of comets nudged inward, sometimes by passing stars; it may reach a light-year or more from the Sun.

What is the point in the sky directly above an observer called?

Zenith comes from medieval scribes misreading an Arabic phrase meaning 'path over the head', and the point straight below you is the nadir.

The Sun's sunspot activity rises and falls in a cycle averaging roughly how many years?

Solar flares and solar storms wax and wane with the same 11-year cycle, peaking when sunspots are most numerous.

Which planet was found by telescope in 1846 within a degree of where calculations had placed it?

Neptune's position was worked out by Urbain Le Verrier from unexplained tugs on a neighbouring planet, yet Galileo had observed it in 1612 and mistaken it for a star.

Henrietta Swan Leavitt, paid 30 cents an hour as a Harvard 'computer', found the period-luminosity law for which pulsating stars?

Cepheid variables became cosmic 'standard candles', but Leavitt died in 1921, before a planned Nobel Prize nomination for her discovery could go ahead.

What astronomical phenomenon is observed when Earth passes through a stream of debris left by a comet?

Many meteor shower particles are no bigger than a grain of sand, yet they flare brightly as they burn up in the atmosphere.

Which moon is so large compared with Pluto that the two orbit a point in empty space between them?

Charon is just over half Pluto's width, and the pair are tidally locked, each always showing the other the same face.

Which moon is the most volcanically active world in the Solar System?

Io became the first world beyond Earth seen erupting, with nine volcanic plumes spotted during a single spacecraft flyby in 1979.

Jocelyn Bell's 1967 repeating radio signal, nicknamed LGM-1, was the first known what?

That first pulsar was nicknamed LGM-1 for 'little green men', yet the 1974 Nobel Prize for discovering pulsars went to her supervisor, Antony Hewish, not to Bell.

What mainly causes the rise and fall of Earth's ocean tides?

The Moon's gravity raises a double bulge of water, one on each side of Earth, which is why most coasts get two high tides a day.

Excluding the Sun, what is the closest known star to Earth?

Proxima Centauri is the nearest star to the Sun, yet it is a dim red dwarf far too faint to see with the naked eye.

Which ancient Greek astronomer compiled the West's first known comprehensive star catalog, around 150 BC?

Hipparchus also compiled the first known trigonometric table, which is why he is called the father of trigonometry.

Which planet in our solar system is primarily known for its extensive and prominent ring system?

Saturn's main rings are typically only about 10 metres thick, so when Earth lined up with their edge in 1612, Galileo found they had vanished.

What celestial event occurs when the Moon passes directly between the Sun and Earth?

A solar eclipse occurs when the Moon passes between the Sun and Earth, blocking the Sun's light and casting a shadow on our planet. This event can only happen during a new moon.

Which pair announced 51 Pegasi b, the first planet found orbiting a Sun-like star, in 1995?

Mayor and Queloz's planet circles its star in about four days, a shock that made astronomers rethink how planets form, and it won them a share of the 2019 Nobel Prize.

What name is given to a planet that drifts through space without orbiting any star?

A rogue planet can be flung out of the system where it formed, and the Milky Way may hold billions or even trillions of them.

The cosmic microwave background has a black-body temperature of roughly how many kelvin?

At about 2.7 kelvin, the cosmic microwave background sits just above absolute zero, because the expanding universe has stretched and cooled the Big Bang's leftover light.

Which Catholic priest and physicist proposed in 1927 that the universe is expanding?

Georges Lemaître later traced the expansion back to a single 'primeval atom', the idea that grew into the Big Bang theory.

A typical neutron star packs about 1.4 solar masses into a sphere with a radius of roughly what?

With a radius of about 10 kilometres, a neutron star is roughly the size of a city like Manhattan, yet it packs in more mass than the Sun.

Which physicist predicted in 1974 that black holes emit faint radiation and slowly evaporate?

Stephen Hawking's predicted glow, now called Hawking radiation, is so faint that it has never been detected from a real black hole.

If the Sun were compressed into a black hole, its Schwarzschild radius would be about how big?

Squeezed to about 3 kilometres, the Sun would become a black hole; Karl Schwarzschild worked out the solution behind this radius in 1916.

The first direct detection of gravitational waves, on 14 September 2015, came from what event?

The merger of two black holes converted roughly 3 solar masses into gravitational waves in a fraction of a second.

The first image of a black hole, released in April 2019, showed the one at the centre of which galaxy?

Messier 87's black hole weighs about 6.5 billion Suns, and the image came from linking radio dishes around the world into one virtual telescope nearly as wide as Earth.

Type Ia supernovae are prized as 'standard candles' because they all occur in what kind of system?

A binary with a white dwarf lets the dwarf pull matter from its partner until it nears a set mass limit and explodes, which is why these blasts peak at similar brightness.

In her 1925 doctoral thesis, Cecilia Payne showed that stars are made mostly of what?

Hydrogen and helium dominated her results, but Henry Norris Russell talked Payne out of that conclusion, then reached it himself in 1929.

What name is given to whatever is speeding up the universe's expansion?

Dark energy makes up roughly 68% of the universe's mass-energy, yet nobody knows what it actually is.

Which physicist developed the general theory of relativity?

Albert Einstein became an international celebrity in 1919, when astronomers measured starlight bending around the Sun by the amount his theory predicted.

The Hertzsprung-Russell diagram plots a star's luminosity against what?

Its temperature or spectral class runs along the bottom, but backwards: the hottest stars sit on the left and the coolest on the right.

Which physicist showed in 1860 that the Sun's dark spectral lines come from absorption in its atmosphere?

Gustav Kirchhoff and chemist Robert Bunsen matched the Sun's dark lines to elements heated in the lab, showing sunlight reveals what the Sun is made of.

Which planet rotates clockwise on its axis, opposite to its path around the Sun?

Venus has retrograde rotation; a Venusian day (243 Earth days) is longer than its year.

Which planet is the smallest in the Solar System?

Mercury has been shrinking as its interior cools, wrinkling its surface into cliffs that can stretch around 1,000 km.

Which two lumpy little moons orbit Mars?

Phobos and Deimos are named after the Greek brothers of fear and dread, sons of Ares, the war god the Romans called Mars.

Which American astronomer discovered the two moons of Mars in 1877?

Asaph Hall nearly abandoned the search until his wife, Angeline Stickney, urged him on; the largest crater on the inner moon is named Stickney after her.

The asteroid that wiped out the dinosaurs left which buried impact crater?

Chicxulub lies buried under Mexico's Yucatán Peninsula; geophysicists surveying for the state oil company Pemex first spotted its outline in 1978.

Which spacecraft remains the only one to have visited both of the Solar System's ice giants?

Voyager 2 rode a rare late-1970s alignment of the outer planets, swinging from one giant to the next, and is the only probe to study all four up close.

Which Greek scholar estimated Earth's circumference by comparing noon shadows at Syene and Alexandria?

Eratosthenes was also chief librarian at the Library of Alexandria, and he may have coined the word 'geography'.

What name was given to the huge storm spotted in 1989 on the Solar System's outermost planet?

The Great Dark Spot had vanished when a space telescope looked again in 1994, though other dark storms have formed on the planet since.

Which is the only large moon in the Solar System that orbits backwards, against its planet's spin?

Triton's backward orbit suggests it didn't form where it is: it is thought to be a dwarf planet captured from the outer Solar System.

Which astronomer discovered Pluto in 1930?

Clyde Tombaugh was a Kansas farm boy who couldn't afford college; drawings of planets he made through homemade telescopes won him a job at an Arizona observatory.

Which spacecraft made the first flyby of Pluto on July 14, 2015?

New Horizons launched in January 2006 to visit what was then the ninth planet; that August, the IAU reclassified Pluto as a dwarf planet.

Which dwarf planet, found in 2005 and first thought to be bigger than Pluto, sparked the debate over Pluto's status?

Eris is named after the Greek goddess of strife and discord, fitting the row it caused; it proved slightly smaller than Pluto but about 27% more massive.

Cassini discovered water-rich plumes venting from the south pole of which Saturn moon?

The plumes point to a subsurface ocean, making Enceladus a top target in the search for life.

Which smooth, icy moon of Jupiter is thought to hide a global ocean, making it a top candidate for life?

Europa's ocean may hold twice the water of all Earth's oceans, kept liquid by tidal flexing; NASA's Europa Clipper launched in 2024 to look.

Galileo named Jupiter's four largest moons after which powerful Italian family, hoping to win its patronage?

Galileo called them the Medicean Stars, after his former pupil Cosimo II de' Medici and Cosimo's three brothers; today they're the Galilean moons.

What was the title of Galileo's 1610 treatise reporting his first telescope observations?

Sidereus Nuncius revealed a Moon rough with mountains and a Milky Way made of countless densely packed stars.

Kepler's first two laws of planetary motion appeared in which 1609 book?

Astronomia Nova came from Kepler's ten-year struggle to fit one planet's orbit with circles, which precise observations kept rejecting until he tried an ellipse.

Which astronomer first showed in 1676 that light travels at a finite speed?

Ole Rømer saw eclipses of one of Jupiter's moons arrive later when Earth was farther from Jupiter, and read the delay as light's travel time.

Which effect changes the light of a star moving toward or away from us, like a passing siren's pitch?

Christian Doppler proposed the Doppler effect in 1842, and it was first tested in 1845 with musicians playing a steady note aboard a moving train.

Halley paid to publish which masterpiece after the Royal Society's funds ran out?

The Royal Society had spent its budget on a flop about fish, so Halley funded Newton's Principia and was later paid his salary in unsold copies of the fish book.

Who is regarded as the first woman ever paid a salary as a scientist?

Caroline Herschel was paid £50 a year by King George III as her brother William's assistant, and she went on to find eight comets.

Which was the first artificial satellite to orbit Earth?

Sputnik 1's surprise launch in 1957 set off a panic in the United States, which created NASA the following year.

Which astronomer hosted the original 1980 TV series Cosmos?

Carl Sagan also wrote the novel Contact, which became the 1997 film starring Jodie Foster.

Which American astronomer fuelled speculation about canals on Mars and hunted 'Planet X'?

Percival Lowell died in 1916, 14 years before Pluto turned up at his observatory, and Pluto's symbol is a P-L monogram of his initials.

Which NASA spacecraft went into orbit around Jupiter in July 2016?

Juno is named for Jupiter's wife in Roman myth, who could see through the veil of clouds he drew around himself to hide his mischief.

Which astronomer coined the term 'Big Bang' during a 1949 BBC radio broadcast?

Fred Hoyle preferred the rival steady-state model, but he denied that 'Big Bang' was meant as an insult, calling it just a striking image for radio.

Which physicist developed the idea of cosmic inflation in 1979?

Alan Guth shared the 2014 Kavli Prize with Alexei Starobinsky and Andrei Linde; Starobinsky's own 1979 model had stayed unknown outside the Soviet Union.

Who proposed in 1933 that unseen mass explained the Coma Cluster's fast-moving galaxies?

Fritz Zwicky found the Coma galaxies moving too fast for their visible matter to hold them together, and called the missing mass 'dunkle Materie': dark matter.

Whose measurements of how fast galaxies spin gave some of the strongest early evidence for dark matter?

Vera Rubin found stars at the edges of galaxies orbiting as fast as those near the centre; a giant survey observatory in Chile now bears her name.

Han Solo's Kessel Run boast used which unit?

A parsec is about 3.26 light-years, so the boast sounded like a blunder until the 2018 film Solo showed Han cutting the route short near a black hole.

On which backwards scale does the Sun rate about −27 and the faintest naked-eye stars about +6?

Apparent magnitude runs backwards because Greek astronomers called the brightest stars 'first magnitude' and the faintest they could see 'sixth magnitude'.

In the OBAFGKM stellar classification, which letter denotes the hottest stars?

O-type stars burn through their fuel so fast that the most massive die within about four million years.

Which Harvard astronomer is credited with creating the Harvard Classification Scheme?

Annie Jump Cannon classified over 350,000 stars by eye, and the scheme she refined is still the one astronomers use.

Polaris, the North Star, lies in which constellation?

It is a yellow supergiant; in antiquity the whole of Ursa Minor was used for navigation instead.

A lunar eclipse can happen only during which phase of the Moon?

A total lunar eclipse often turns the full Moon coppery red, since the only sunlight reaching it has passed through the air around Earth.

Antares is the brightest star in which zodiac constellation?

Antares, the brightest star in Scorpius, is so vast that, placed where the Sun is, it would reach out into the asteroid belt.

The Pleiades, or Seven Sisters, is an open star cluster in which constellation?

The Pleiades in Taurus are called Subaru in Japanese, and the carmaker's logo shows six of the cluster's stars.

Apollo 11's lunar module touched down on which dark lunar plain in July 1969?

The Sea of Tranquility is a plain of ancient basalt, and the crew announced the landing with 'Houston, Tranquility Base here. The Eagle has landed.'

The smallest of the 88 official constellations is better known by which name?

The Southern Cross, officially Crux, covers the least sky of any constellation, yet it features on Australia's flag and several others.

Which 'serpent-bearer' constellation crosses the ecliptic yet is left out of the zodiac?

Ophiuchus is often identified with Asclepius, the Greek god of medicine, whose snake-entwined rod is still a symbol of medicine.

Which nebula is the wreckage of the 'guest star' that Chinese astronomers recorded in 1054?

The Crab Nebula's 1054 'guest star' shone so brightly that it stayed visible in daylight for 23 days.

Supernova 1987A exploded in which satellite galaxy of the Milky Way?

Supernova 1987A in the Large Magellanic Cloud was the first supernova whose neutrinos were caught, arriving hours before its light.

The brilliant 'new star' of 1572 appeared in which constellation?

The 1572 star in Cassiopeia showed no parallax, so it lay far beyond the Moon, overturning the belief that the heavens never change.

Which bright star dimmed dramatically between October 2019 and February 2020?

Betelgeuse's 'Great Dimming' set off talk that it was about to explode, but the cause was a veil of dust from gas the star had thrown off.

Which is the largest known globular cluster in the Milky Way?

Omega Centauri may be the leftover core of a dwarf galaxy that the Milky Way captured and stripped long ago.

What name was given in 2014 to the supercluster that contains the Milky Way?

Laniakea, a Hawaiian name, swallowed up the older Virgo Supercluster, which is now counted as just one part of it.

ESA's Huygens probe landed by parachute on which moon in January 2005?

On Titan, Huygens photographed channels apparently carved by methane rain and a landing site strewn with pebbles of water ice.

Cassini ended its mission in September 2017 by doing what?

Burning up in Saturn's atmosphere was deliberate: it removed any risk of Cassini crashing into, and contaminating, moons that might be habitable.

Which space probe carried the Philae lander to the first soft landing on a comet in 2014?

Rosetta was named after the Rosetta Stone, since the mission hoped to help decode comets, and Philae bounced twice before settling.

NASA's DART spacecraft deliberately crashed into which asteroid moonlet in September 2022?

DART's hit shortened Dimorphos's orbit around its parent asteroid by about 32 minutes, showing that an impact can nudge an asteroid's path.

Which 2018 NASA spacecraft was built to fly through the Sun's outer corona?

Parker Solar Probe was the first NASA spacecraft named after a living person: Eugene Parker, who predicted the solar wind in 1958.

The James Webb Space Telescope circles which Lagrange point?

From Sun–Earth L2, about 1.5 million km beyond Earth, the Sun, Earth and Moon all sit on one side, so Webb's sunshield can block them and stay cold.

NASA's Kepler space telescope, launched in 2009, was designed to find what?

Earth-sized exoplanets turned out to be common: Kepler's data suggest a sizeable share of stars host small, possibly rocky planets.

Which comet broke apart and collided with Jupiter in July 1994?

Shoemaker–Levy 9's biggest impact left a dark scar on Jupiter almost as wide as Earth, and the impact scars could be seen for many months.

Which comet, discovered in 1995, stayed visible to the naked eye for about 18 months?

Hale–Bopp was found the same July night by Alan Hale in New Mexico and, independently, amateur Thomas Bopp using a friend's telescope in Arizona.

What was the first confirmed interstellar object seen crossing the Solar System, in 2017?

ʻOumuamua's Hawaiian name means roughly 'a messenger from afar arriving first', fitting the first known visitor from another star system.

Astronomer Jerry Ehman circled a strange 1977 radio signal on a printout and wrote which word beside it?

Despite decades of follow-up searches with better telescopes, the Wow! signal has never been detected again, and no explanation has been confirmed.

Frank Drake's equation estimates the number of what?

Drake wrote it in 1961 as an agenda for the first SETI meeting, to organize discussion of detectable civilizations rather than to produce a firm number.

Which dog became the first animal to orbit Earth?

Laika's spacecraft was never designed to come back, and she died of overheating within hours; the true cause stayed secret until 2002.

Atoms of which gas give auroras their most common green glow?

Oxygen glows green roughly 100 to 200 km up and red higher still, while nitrogen adds blues and pinks.

What flattened trees near Siberia's Tunguska River in 1908?

A space rock most likely caused it, exploding several kilometres above the ground, which is why searchers never found an impact crater.

A superbolide injured nearly 1,500 people over which Russian city in February 2013?

In Chelyabinsk the 1,491 injuries came from indirect effects, mostly glass shattered by the shock wave, not from the falling object itself.

Earth's slow axial wobble will make which bright star the North Star in roughly 12,000 years?

Vega was also the North Star around 12,000 BC, because Earth's axis traces its full circle only about every 26,000 years.

Which named argument asks why the night sky is dark in an infinite, eternal universe?

Olbers' paradox is largely answered by the universe's finite age: light from the farthest stars hasn't reached us yet, as Edgar Allan Poe suggested in 1848.

What is a planet's closest point to the Sun in its orbit called?

Earth reaches perihelion in early January, so the Northern Hemisphere's winter comes when Earth is closest to the Sun.

Which body, spotted by Giuseppe Piazzi on New Year's Day 1801, was the first object found in the asteroid belt?

Ceres was hailed as a planet, recast as an asteroid by the 1850s, and finally reclassified as a dwarf planet in 2006.

Which kind of object's giant flare in 2004 briefly disturbed Earth's upper atmosphere from tens of thousands of light-years away?

A magnetar is a neutron star with an extremely powerful magnetic field, and the 2004 flare from SGR 1806−20 briefly expanded Earth's ionosphere.

Brown dwarfs, 13 to 80 times Jupiter's mass, are too light to fuse ordinary hydrogen but heavy enough to fuse which isotope?

Deuterium, heavy hydrogen, fuses at lower temperatures, and about 13 Jupiter masses is the working line astronomers use between planets and brown dwarfs.

The Arecibo Telescope, which collapsed in December 2020, was built into what natural feature?

A sinkhole's natural bowl cradled Arecibo's 305 m dish, whose dramatic setting starred in the climax of the James Bond film GoldenEye.

Which desert is home to ESO's Very Large Telescope?

The Very Large Telescope stands on Cerro Paranal in the Atacama, 2,635 m up and far from the glow of any city.

Inside a planet's Roche limit, orbiting material tends to form what?

Rings form there because a planet's gravity tugs harder on a moonlet's near side than its far side, pulling loose material apart before it can clump into a moon.

Although it looks like a single point of light, Polaris is actually what kind of star system?

Polaris is triple: a yellow supergiant orbited by two smaller companion stars, one close in and one far out.

Approximately how many kilometres are in one light-year?

A light-year is about 9.46 trillion km, or 5.88 trillion miles, and the nearest star beyond the Sun is about four and a quarter of them away.

Which fairy-tale character lends her name to the zone where a planet could hold liquid water?

Goldilocks lends her name because the zone is neither too hot nor too cold, like the porridge she picked. Astronomers formally call it the habitable zone.

A comet's tail always points in which direction, whichever way the comet itself is travelling?

Away from the Sun is the rule, because sunlight and a stream of charged solar particles push the gas and dust outward. A comet can even grow two tails at once, one of dust and one of ions.

The word 'galaxy' comes from the Greek for which liquid, after a pale band across the night sky?

Milk is the root: the Greek galaxias means 'milky', a nod to the Milky Way, the hazy band of our own galaxy seen edge-on from inside.

Which prehistoric monument on England's Salisbury Plain is aligned with sunrise on the summer solstice?

Stonehenge was laid out to face the midsummer sunrise and, in the opposite direction, the midwinter sunset. On the longest day the Sun still rises close to its outlying Heel Stone.

A shooting star is properly called a meteor; what is a fragment called if it survives to hit the ground?

A meteorite is a fragment that makes it all the way down. Before it meets the atmosphere, the same lump drifting through space is a meteoroid.

Which English scientist built the first working telescope that gathered light with a mirror instead of a lens?

Isaac Newton built it in 1668 to escape the rainbow fringes that lenses put around bright objects. Its mirror was only about 3 cm across.

Which dormant Hawaiian volcano hosts a famous cluster of giant telescopes near its 4,200-metre summit?

Mauna Kea's summit sits above most of the water vapour in the atmosphere, giving telescopes dark, dry, steady skies. Measured from its underwater base, the mountain is taller than Everest.

The 'Pillars of Creation', star-forming columns made famous by a 1995 photograph, lie in which nebula?

The Eagle Nebula holds the pillars, some 6,500 light-years away in the constellation Serpens. The leftmost column alone is about four light-years long.

Which organisation's 2006 vote in Prague left the Solar System with eight official planets instead of nine?

The International Astronomical Union ruled that a planet must have cleared its orbital neighbourhood, which Pluto has not. Members voted by holding up yellow cards.

Mars rover teams count mission time in Martian days about 24 hours 39 minutes long, known by which name?

Sols, named from the Latin for Sun, each run about 39 minutes longer than a day on Earth. Rover crews have worked on 'Mars time', with shifts synchronised to the landing site rather than home.

Which feature of Earth, rather than its changing distance from the Sun, causes the seasons?

The tilt of its axis leans each hemisphere toward the Sun for half the year, lifting the midday Sun higher and stretching the days. That is why the north's summer is the south's winter.

In which month is Earth closest to the Sun, even though the Northern Hemisphere is then in winter?

January brings the closest approach, around the 3rd, when Earth receives about 7% more solar energy than at its farthest point six months later.

In which direction does Earth rotate, making the Sun, Moon and stars appear to cross the sky the opposite way?

West to east is the way Earth turns, so everything in the sky seems to drift the other way. A point on the equator is carried along at about 1,670 kilometres an hour.

Which effect keeps the same face of the Moon turned toward Earth, so that it spins exactly once per orbit?

Tidal locking slowed the Moon's spin until one rotation matched one trip around Earth. Every large, round moon in the Solar System has ended up locked to its planet in the same way.

Lasers bounced off reflectors left by Apollo astronauts show that the Moon is doing what each year?

Moving away from Earth, by about 3.8 centimetres a year, is what the timing of the returning pulses reveals. Three Apollo crews left the mirrors behind, and they still work.

Roughly how long does light take to travel from the surface of the Sun to Earth?

About 8 minutes, or 8 minutes 20 seconds at the average distance, is the delay, so we always see the Sun as it was a little while ago, never as it is now.

Roughly how many planets the size of Earth would it take to fill the volume of the Sun?

About 1.3 million Earths would fill the Sun, which is some 109 times as wide as our planet. It holds about 99.86% of all the mass in the Solar System.

Which process in the Sun's core converts about four million tonnes of matter into energy every second?

Nuclear fusion squeezes light atomic nuclei together into heavier ones and releases energy as it does. Even at that rate, the Sun has lost only about 0.03% of its mass so far.

On which two days each year, in March and September, does the Sun rise due east and set due west?

The equinoxes take their name from the Latin for 'equal night', because day and night are then almost the same length across the whole planet.

On the June solstice, the noon Sun stands directly overhead along which line of latitude?

The Tropic of Cancer is the most northerly line where the Sun can ever stand directly overhead. It is named for the constellation the Sun sat in at that solstice about two thousand years ago.

Which Renaissance artist explained the faint glow on a crescent Moon's dark side as light reflected from Earth?

Leonardo da Vinci sketched the glow, now called earthshine, in the early 1500s and credited Earth's oceans. Clouds turn out to do most of the reflecting.

Why does the Moon turn coppery red, rather than vanish, during a total lunar eclipse?

Earth's air bends sunlight inward and filters it red, the same effect that colours the sky at dusk. Seen from the Moon, the Sun would appear to be setting behind Earth.

An astronaut standing on the Moon weighs roughly what fraction of their weight on Earth?

One-sixth is the figure, about half the surface pull of Mars. It is still the second strongest of any moon in the Solar System.

Which term describes the Moon when more than half of its face is lit and the bright part is still growing?

Waxing gibbous is the phase in the days before the whole disc is lit. 'Gibbous' comes from a Latin word for hunchbacked, after the bulging shape.

Which resource, vital to any future base, has been confirmed in the permanently dark craters at the Moon's poles?

Water ice survives there because the crater floors never see the Sun. India's Chandrayaan-1 orbiter carried the NASA instrument whose data confirmed it.

The Moon looks dazzling white in the night sky, yet its surface is only about as reflective as what?

Worn asphalt is a fair match: the Moon bounces back only about 12% of the sunlight that strikes it. It dazzles simply because it looms so large against a black sky.

What name is given to the twice-monthly peaks when the gap between high and low water is at its greatest?

Spring tides are named not for the season but for water that seems to spring up. About a week after each one, the range shrinks to its smallest.

Which Latin name is given to the darkest, central part of a shadow, from which an eclipse appears total?

The umbra is where the light source is completely hidden. Stand in the paler region around it, the penumbra, and you see only a partial eclipse.

Which name do astronomers give to the moving line that separates day from night on the Moon or a planet?

The terminator is where the Moon's craters show best, because sunlight strikes at a low angle and stretches every shadow. On Earth it sweeps past you twice a day, at sunrise and sunset.

Which word do astronomers use for three bodies, such as the Sun, Earth and Moon, lying in a straight line?

Syzygy is the line-up behind every eclipse, when one body blocks another's light or casts its shadow on it. Transits and occultations need the same geometry.

Which term describes a model of the Solar System in which Earth and the other planets all circle the Sun?

Heliocentric joins Helios, the Greek word for the Sun, to the word for centre. English picked the term up only in the 1680s, well over a century after the idea was published.

Which astronomer of Roman-era Alexandria set out the Earth-centred system that ruled for over a thousand years?

Ptolemy worked in the 2nd century AD, and his system was still being used to draw up charts of the heavens some 1,500 years later. He wrote on geography and music theory too.

The word 'planet' comes from an ancient Greek term with which meaning?

Wanderer is the meaning, because the planets shift from night to night while the stars hold their patterns. The Greeks and Romans counted seven such lights, the Sun and Moon included.

Which Greek philosopher set a round Earth inside some 55 nested spheres made of a fifth element, aether?

Aristotle gave good evidence that Earth is a sphere, then fixed it motionless in the middle of everything. His authority helped keep that picture standard for many centuries.

Which term describes the backward loop Mars seems to trace against the stars as Earth overtakes it?

Retrograde motion baffled Earth-centred astronomers, who needed extra circles to reproduce it. With the Sun in the middle, it is just the view from a faster planet passing a slower one.

In Earth-centred astronomy, what name was given to the small circle each planet rode around a larger circle?

An epicycle let astronomers match the planets' changing speed and direction remarkably well. 'Adding epicycles' is now a put-down for patching a theory that should be scrapped.

Which ancient Greek proposed a Sun-centred universe some eighteen centuries before the idea finally took hold?

Aristarchus of Samos also had Earth spinning on its axis once a day. His own book is lost, and the idea survives because Archimedes described it in The Sand Reckoner.

Which ancient treatise, known by a title meaning 'the greatest', placed Earth at the centre of the cosmos?

The Almagest owes its name to Arabic translators, and a 12th-century Latin version made from the Arabic carried it into medieval Europe. Its Greek title was simply 'Mathematical Treatise'.

In 1610 Galileo saw which feature of the brightest planet that the ancient Earth-centred system ruled out?

A full set of phases, from thin crescent to nearly round disc, meant the planet had to circle the Sun. In the old system it never went round the far side, so it could not have looked full.

Johannes Kepler made the Sun-centred model fit the sky by replacing perfectly circular orbits with which shape?

Ellipses, with the Sun at one focus, cured a mismatch in the orbit of Mars that no circle could fit. Mars was a lucky test case: its path is more stretched than almost any other planet's.

In 1851 Léon Foucault hung which device beneath the dome of the Paris Panthéon to show that Earth rotates?

A pendulum with a 28 kg bob on a 67-metre wire kept its swing while the floor slowly turned beneath it. An exact copy has been swinging under the same dome since 1995.

Convicted in 1633 for defending a moving Earth, Galileo spent the rest of his life under what sentence?

House arrest kept him at his villa at Arcetri, near Florence, where he went blind in 1638. The original sentence of imprisonment had been softened within a day.

Which defiant phrase is Galileo said to have muttered after the Inquisition forced him to recant?

'And yet it moves' is probably legend: neither the trial records nor his first biographer mention it. The story reached English print only in 1757, over a century after his death.

Which pope acknowledged in 1992 that the Inquisition had erred in condemning Galileo over Earth's motion?

John Paul II had asked for the case to be re-examined back in 1979. His verdict came 359 years after the trial itself.

Which Italian friar, burned at the stake in Rome in 1600, taught that the stars are distant suns with planets?

Giordano Bruno also insisted that the universe is infinite and has no centre at all. Rome raised a statue to him in 1889.

Which tiny yearly shift in nearby stars' positions, proof of Earth's orbit, went unmeasured until the 1830s?

Stellar parallax is minute because even the nearest stars are so remote. For generations its apparent absence was a serious scientific argument against a moving Earth.

In Galileo's 'Dialogue Concerning the Two Chief World Systems', which character defends an Earth-centred cosmos?

Simplicio is said to be named after an ancient philosopher, but in Italian the name also suggests 'simpleton'. Handing him the pope's own argument proved a costly joke.

Which pope, once a friend and admirer of Galileo, had the astronomer brought to Rome to stand trial in 1633?

Urban VIII had encouraged Galileo's work for years. He turned on him after a 1632 book seemed to put the pope's own point of view in the mouth of a fool.

Which English astronomer's 1729 discovery, the aberration of starlight, gave conclusive evidence that Earth moves?

James Bradley stumbled on the effect while hunting for something else, and it let him sharpen the best estimate of the speed of light. He went on to become the third Astronomer Royal.

Which Indian mathematician argued around AD 499 that the stars only seem to move because Earth spins on its axis?

Aryabhata compared it to a boat: someone gliding forward sees fixed things on the bank slide backward. India named its first satellite after him.