56 Fun Facts About Andromeda Galaxy

What is the Andromeda Galaxy's number in Charles Messier's catalogue?

Messier logged it in 1764 and wrongly credited a German astronomer with discovering an object visible to the naked eye.

Roughly how far from Earth is the Andromeda Galaxy?

2.5 million light-years. That means the glow you can see tonight left Andromeda about 2.5 million years ago, long before our own species existed.

The Andromeda Galaxy is described as which type of galaxy?

Barred spiral. Dust hid the bar in ordinary photographs, so it took near-infrared surveys in the 2000s, which see through the dust, to pick out the boxy central bulge that gives a bar away.

Andromeda is the largest member, by extent, of which galaxy grouping that also includes the Milky Way?

The Local Group. Mutual gravity holds its galaxies together, which is why Andromeda is falling toward the Milky Way while far-off galaxies rush away from us.

At a minimum, how can the Andromeda Galaxy be seen from Earth?

With the naked eye on moonless nights, no instrument is needed, which is why skywatchers had put it on star charts centuries before the telescope was invented.

With more than twice the Milky Way's star count, Andromeda holds roughly how many stars?

1 trillion. More stars need not mean more weight, though: several estimates of its total mass put Andromeda level with the Milky Way or lighter.

In the 10th century, the Persian astronomer al-Sufi made the first known record of Andromeda, describing it as what?

A little cloud. Eight centuries later Charles Messier still credited the discovery to a German astronomer who had merely turned a telescope on it.

Which rival of Galileo named Jupiter's four big moons and first described Andromeda through a telescope?

Simon Marius. He and Galileo each claimed to have found the moons first, and Galileo accused him of plagiarism, yet the names Marius chose, Io, Europa, Ganymede and Callisto, are the ones that stuck.

Which philosopher argued in 1755 that elliptical nebulae like Andromeda were star systems similar to the Milky Way?

Immanuel Kant. He reasoned that the Milky Way itself, seen from far enough away, would look just as small and faint, so the nebulae astronomers kept finding could be other Milky Ways.

William Herschel guessed in 1785 that Andromeda was no more than 2,000 times the distance of which star?

Sirius. Herschel judged the distance from the nebula's colour and brightness, and his guess fell more than a hundred times short of the real figure.

In 1864 William Huggins found that Andromeda's light, split into colours, resembled that of what, rather than a gaseous nebula?

Individual stars. Glowing gas shines in a few bright lines, but Andromeda's light spread into a continuous band like starlight, an early hint that the 'nebula' was really a vast crowd of suns.

What was S Andromedae, which blazed up in Andromeda in 1885?

A supernova. In 1885 nobody knew such explosions existed, so it was logged as an ordinary nova, and no one grasped that they had watched a star blow up in another galaxy.

The reddish patches that dot Andromeda's disc in photographs are star-forming regions glowing with which gas?

Hydrogen. Fierce light from newborn stars sets the gas around them glowing, so the red patches work as a map of where Andromeda is still making stars.

Which technique did Vesto Slipher use in 1912 to clock Andromeda at about 300 km/s, the greatest speed then measured?

Spectroscopy. Slipher read the speed from the way the lines in Andromeda's light were displaced, then found most other such nebulae racing away, the first clue that the universe is expanding.

In 2012 NASA predicted that Andromeda would collide with the Milky Way in about how long?

4 billion years. The first crash would only be the start: the simulations had the pair taking about 2 billion more years to settle into a single galaxy.

In the 1920 Great Debate, Heber Curtis argued that nebulae like Andromeda were separate galaxies against which rival astronomer?

Harlow Shapley. He had just shown that the Sun lies far from the Milky Way's centre, but he made our galaxy so vast that he thought it held every nebula, Andromeda included.

Edwin Hubble settled the debate over Andromeda's distance in 1925 by identifying which kind of star in photographs of it?

Cepheid variables. These stars pulse at a rate set by their true brightness, so timing one in Andromeda gave Hubble its distance, which was far too great to fit inside the Milky Way.

Hubble's Andromeda photographs were taken with which instrument, then the largest in the world?

The 100-inch Hooker reflector. Having used it to prove Andromeda lies outside our galaxy, Hubble turned the same telescope on fainter galaxies and found that the universe is expanding.

Vera Rubin's measurements of how fast stars orbit in Andromeda's outskirts became key evidence for what?

Dark matter. Rubin and Kent Ford expected the outer stars to dawdle, as distant planets do around the Sun, but they circled as fast as the inner ones, as if held by unseen mass.

Which wartime measure helped Walter Baade photograph Andromeda's core in new detail in 1943?

Blackouts in Los Angeles. With the city dark below his mountaintop observatory, he could at last make out the faint red stars in Andromeda's heart, and saw that galaxies hold two distinct populations of stars.

Walter Baade's 1950s discovery that Hubble's yardstick stars were not all alike did what to Andromeda's estimated distance?

Doubled it. Every other galaxy's distance grew along with Andromeda's, so the whole universe turned out twice as big, and older, than astronomers had believed.

Radio emissions from Andromeda were first detected in 1950 by Hanbury Brown and Hazard at which observatory?

The galaxy's core is listed as 2C 56 in the second Cambridge radio catalogue.

What did the Hubble Space Telescope find at Andromeda's centre in the early 1990s?

A double nucleus. It is now thought to be an illusion: the two clumps look like parts of one lopsided disc of old stars circling the central black hole, not two separate clusters.

A prominent band of gas and dust about 32,000 light-years from Andromeda's core, where most of its star formation happens, is nicknamed what?

Infrared images suggest the whole galaxy may be turning into a ring galaxy.

Andromeda's two best-known satellite galaxies are M32 and which other?

M110. Charles Messier first saw it beside Andromeda in 1773 and drew it there, yet never listed it, and only in the 1960s did astronomers make it the 110th and last entry in his catalogue.

Isaac Roberts took one of the first photographs of Andromeda in the 1880s and suggested it showed what being born?

A planetary system. Andromeda was then taken for a nearby nebula, so Roberts read his picture as a sun and its planets condensing, when he had in fact photographed a whole galaxy of stars far beyond our own.

The 'Andromeda paradox', in which two passers-by disagree on whether an alien fleet has already launched, illustrates which theory?

Special relativity. Roger Penrose's point was that merely strolling toward or away from Andromeda shifts your 'now' there by days, so the two walkers disagree about what has already happened.

After Andromeda and the Milky Way, which is our neighbourhood's third-largest galaxy?

Triangulum Galaxy. It lies close enough to Andromeda that it may be its companion, and some forecasts have it joining any future pile-up between the two giants.

Andromeda is one of the few galaxies moving toward us, so its light arrives shifted toward which colour?

Blue. Almost every other galaxy's light is stretched toward red as the universe expands, but Andromeda is falling toward the Milky Way at roughly 110 kilometres a second.

A 2025 study found the long-forecast collision of Andromeda and the Milky Way to be what?

Only about a coin toss. In 100,000 simulated futures the pair merged in only about half, partly because a smaller neighbour tugs the Milky Way out of Andromeda's path.

What name has been given to the galaxy that would result if Andromeda merged with our own?

Milkomeda. Simulations suggest it would most likely look like neither parent: the two flat discs would be scrambled into one huge, rounded swarm of stars.

If Andromeda and the Milky Way merge, why would almost none of their stars actually collide?

The gaps between them are enormous. Shrink the Sun to a ping-pong ball and the next nearest star would be a pea more than a thousand kilometres away.

In an Andromeda-Milky Way merger, what most likely happens to the Solar System?

Flung farther out. Simulations put the Sun much farther from the galactic core afterwards, and even leave a remote chance that, partway through, it ends up bound to Andromeda instead.

A 1999 microlensing event in Andromeda hinted at what possible first for astronomy?

An extragalactic planet. As an unseen star drifted in front of a red giant, the magnified light showed an extra blip that a companion about six times the mass of our largest planet would explain.

What powers a microquasar, first found beyond our galaxy in Andromeda in 2012?

A feeding black hole. This one, about ten times the Sun's mass, strips gas from a companion star and fires out jets, a pocket-sized version of the quasars in the hearts of distant galaxies.

Hubble's 2025 mosaic of Andromeda, its largest ever, took roughly how long to photograph?

Ten years. Andromeda sprawls so far across the sky that Hubble, built to study tiny patches, needed about 600 separate pointings spread over more than a decade to tile it.

How does Andromeda's present rate of star birth compare with the Milky Way's?

Several times lower. Astronomers read that as a sign that Andromeda had its great burst of star-making long ago and has since gone quiet, while our own galaxy stays busier.

On the galaxy colour-magnitude diagram, Andromeda lies in which transitional region?

The green valley. It is the halfway house between galaxies still busy making stars and those that have stopped, a spot Andromeda shares with the Milky Way, and it suggests star-making there will all but end within about five billion years.

Why does Andromeda look like a long oval, not a round disc?

We see it nearly edge-on. The disc leans about 77 degrees from face-on, so we look across it instead of down on it, which makes its structure hard to trace.

Andromeda's vast envelope of thin gas is likely already doing what?

Touching our galaxy's halo. The gas spreads about halfway to the Milky Way, so although the stars are still far apart, the outskirts of the two galaxies have, to all intents, already met.

In the Northern Hemisphere, Andromeda passes highest overhead around midnight in which month?

October. It climbs the eastern sky as night falls and wheels overhead by midnight, so northern autumn is its season, yet from mid-northern latitudes it can be caught at some hour on any night of the year.

Which hero of Greek myth rescued and married Andromeda, the princess who gave the galaxy's constellation its name?

Perseus. On his way home with the head of Medusa, he spotted the princess left out for a sea monster, struck a bargain with her parents and killed the beast to win her hand.

In Greek myth, whose boast about beauty led to her daughter Andromeda being offered to a sea monster?

Cassiopeia. The queen claimed that she, or in some tellings her daughter, outshone the sea nymphs, and it was the blameless daughter who paid for it while her mother watched from the shore.

Which sea monster of the Andromeda myth shares its name with the constellation also called the Whale?

Cetus. The name was simply the Latin word for any large sea creature, which is why whales and their kin are still known as cetaceans.

The constellation Andromeda is known in English by what nickname?

The Chained Lady. Star maps have long drawn her with her wrists in irons, as the princess of Greek myth left on the shore for a sea monster, and her old Latin and Arabic names say the same thing.

Medieval Arab star charts placed the Andromeda Galaxy at the mouth of what figure drawn across the constellation?

A fish. Arab stargazers kept their own older star figures, so al-Sufi's atlas lays a Big Fish across the Greek princess, and the galaxy's earliest portrait is a cluster of dots by its mouth.

The brightest star of the constellation Andromeda, Alpheratz, has also been counted as part of which neighbouring constellation?

Pegasus. Ptolemy catalogued the star under the winged horse while noting that it also marked the princess's head, and star maps showed it in both figures until modern borders gave it to Andromeda alone.

Andromeda's approach is fast enough to cover which distance in about an hour?

Earth to the Moon. That is about 110 kilometres every second, yet the gulf between the two galaxies is so wide that the gap closes far too slowly for anyone to notice.

Evidence suggests that 2 to 3 billion years ago Andromeda went through what?

A major galaxy merger. The collision, with a galaxy about a quarter of Andromeda's mass, is thought to have built much of its outer halo and flung out the giant stream of stars that still loops around it.

Which measurement of Andromeda, first made by Hubble in 2012, was needed to judge whether it will hit our galaxy?

Its sideways motion. The speed of Andromeda's approach was long known, but a galaxy drifting fast enough to one side would sail past, and Hubble found the drift so small that it pointed to a direct hit.

Roughly how wide is Andromeda's disc of stars?

About 200,000 light-years. That makes it the widest galaxy in our neighbourhood, although several estimates suggest that it is not the heaviest.

Which sci-fi video game series sent players to the Andromeda Galaxy in its 2017 instalment?

Mass Effect. In Mass Effect: Andromeda the player is a 'Pathfinder' who wakes aboard a sleeper ship and must find humanity a new home far beyond the Milky Way.

Simulations suggest the small galaxy M32 once did what to Andromeda?

Plunged through it. The smaller galaxy is thought to have dropped almost head-on through the disc more than 200 million years ago, sending out ripples of dust and losing over half its own mass.

NASA's NuSTAR telescope picked out dozens of collapsed stars in Andromeda using which kind of light?

X-rays. Gas dragged toward one of these collapsed stars grows so hot on the way in that it blazes in X-rays, which lets astronomers pick them out one by one in another galaxy.

In 2006, nine of Andromeda's satellite galaxies were found to share what arrangement?

They lie in a plane through its core. Galaxies that drifted in one by one should be scattered at random, so the neat sheet hints that these dwarfs share a common origin.

Which claim to fame does Andromeda hold among the Milky Way's neighbours?

The nearest large galaxy. Dozens of small galaxies lie closer, most of them dwarfs in orbit around our own, but nothing of the Milky Way's own class does.