60 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?

Four independent methods, averaged, give about 2.54 million light-years.

The Andromeda Galaxy is described as which type of galaxy?

Visible-light images suggested an ordinary spiral, but infrared surveys revealed a bar, just like the Milky Way's.

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

The group is dumbbell-shaped, with the Milky Way and its satellites in one lobe and Andromeda and its satellites in the other.

With an apparent magnitude of about 3.4, how can the Andromeda Galaxy be seen from Earth?

It can be spotted even from areas with moderate light pollution and is one of the most distant naked-eye objects.

The galaxy's virial mass is estimated at roughly how many solar masses?

It was long thought a quarter to a half heavier than the Milky Way, but recent studies suggest the two are closer to equal, or the Milky Way heavier.

Which Persian astronomer described the Andromeda Galaxy as a "small cloud" in his Book of Fixed Stars around 964 CE?

It is the earliest known reference to any galaxy beyond the Milky Way; star charts of the era called it the Little Cloud.

Which German astronomer gave an early telescopic description of Andromeda in 1612?

Messier later mistakenly credited him as its discoverer.

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

His book was Universal Natural History and Theory of the Heavens; Maupertuis had made a similar conjecture in 1745.

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

That put it at roughly 18,000 light-years, wildly short; he had noted a faint reddish hue in the core.

Who made a drawing of Andromeda's spiral structure in 1850?

His giant reflector, the Leviathan of Parsonstown, had revealed spiral structure in nebulae five years earlier.

In 1864 William Huggins found that Andromeda's spectrum resembled that of what, rather than a gaseous nebula?

The continuum with dark absorption lines showed the object was made of stars.

The only supernova ever observed in the Andromeda Galaxy, S Andromedae, appeared in which year?

It was called "Nova 1885" at the time, since the difference between novae and supernovae was not yet understood.

Which Welsh amateur astronomer took one of the first photographs of Andromeda in 1888?

He mistook Andromeda and similar spiral nebulae for star systems in the process of forming.

In 1912 Vesto Slipher measured Andromeda's radial velocity at about 300 km/s, the largest yet recorded. Which technique did he use?

He went on to discover that most spiral nebulae are redshifted, laying the groundwork for the expanding universe.

In 1917 Heber Curtis used novae in Andromeda to estimate its distance, in light-years, at about how far?

About five times too small, but the first estimate correct to within an order of magnitude.

The 1920 Great Debate over whether spiral nebulae were separate galaxies pitted Heber Curtis against which astronomer?

Curtis argued for island universes and was ultimately proven right on that main point.

Edwin Hubble settled the debate in 1925 by identifying which kind of star in photographs of Andromeda?

Their period-luminosity relation, discovered by Henrietta Leavitt, made them standard candles for measuring distance.

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

It sat on Mount Wilson above Los Angeles.

Which Estonian astronomer estimated Andromeda's distance in 1922 from the velocities of its stars, placing it far outside the Milky Way?

His figure of about 450 kiloparsecs was closer to the truth than Hubble's first result.

In 1943 Walter Baade resolved stars in Andromeda's core and divided stars into two populations. What did he name them?

His later discovery of two kinds of Cepheids doubled the estimated distance to Andromeda, and to the rest of the universe.

The estimated distance to Andromeda was doubled in 1953 following what discovery?

Baade found the two Cepheid types while working at Mount Wilson.

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.

Andromeda's most massive globular cluster, nicknamed Globular One, is catalogued under what name?

It is about twice as luminous as Omega Centauri and may be the stripped core of a swallowed dwarf galaxy.

Approximately how many globular clusters are associated with the Andromeda Galaxy?

Unlike the Milky Way's, they span a wide range of ages, from as old as the galaxy to a few hundred million years.

What is the designation of the supermassive black hole at Andromeda's centre?

It is a low-luminosity active nucleus, detected only in radio and X-rays, with a 2005 mass estimate of 110-230 million suns.

Hubble Space Telescope images in 1991 revealed Andromeda's nucleus to consist of how many concentrations of stars?

Two concentrations, P1 and P2, sit 1.5 parsecs apart; the brighter one is offset from the true centre.

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?

M32 seems to have punched through Andromeda's disk more than 200 million years ago, creating its inner dust ring.

Andromeda's satellite M32 was discovered in 1749 by which French astronomer?

It is a compact dwarf about 2.49 million light-years away.

Andromeda's satellite NGC 205 was the last object added to the Messier list. Who proposed its number, in 1967?

Messier had drawn it in 1773 beside M32 but never listed it.

Roughly how many light-years separate the Triangulum Galaxy, M33, from Andromeda?

Whether it is actually a satellite of Andromeda remains unknown.

Andromeda is approaching the Milky Way at roughly what speed?

Relative to the Sun it closes at about 300 km/s, making it one of about 100 blueshifted galaxies.

A 2025 study using Gaia and Hubble data put the chance of the Milky Way and Andromeda colliding within 10 billion years at about what?

Once the Large Magellanic Cloud and Triangulum are included, a head-on collision in the next 4-5 billion years drops to about 2%.

What name has been given to the hypothetical galaxy that would result from a Milky Way-Andromeda merger?

The product would likely be a giant elliptical or a large disc galaxy.

Why is the chance of any two stars actually colliding during a galaxy merger considered negligible?

Andromeda holds about a trillion stars and the Milky Way about 300 billion, yet they are spread across vast emptiness.

A 2007 estimate gave what probability that the Solar System would be ejected entirely from the merged galaxy?

There is an even chance it would instead be swept to three times its current distance from the core.

The 1999 microlensing event PA-99-N2 hinted at what possible first in Andromeda?

The candidate would be about 6.34 Jupiter masses, but anomalies in the event were later found.

In 2012 astronomers detected what in Andromeda for the first time outside the Milky Way?

The radio burst came from a black hole of about 10 solar masses near the galactic centre.

The 2025 Hubble mosaic of Andromeda was assembled from roughly how many separate fields of view?

It resolves an estimated 200 million stars hotter than the Sun, still a fraction of the total.

How does Andromeda's current star-formation rate compare with the Milky Way's?

The Milky Way makes 3-5 solar masses a year and produces twice as many novae.

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

Green-valley galaxies are running out of star-forming gas; simulations expect Andromeda's star formation to end within about five billion years.

Andromeda is tilted at roughly what angle relative to Earth's line of sight, where 90° would be edge-on?

Its disk also shows a pronounced S-shaped warp, possibly caused by its satellites.

Andromeda's halo of hot gas is estimated to stretch about how far from the galaxy?

That is roughly halfway to the Milky Way, and the two halos may already be touching.

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

It rises in the east in the early evening in September and sets in the west in February.

Which two constellations are usually used to help find Andromeda in the sky?

The galaxy sits off the star Mirach, a short hop from the Great Square.

In Greek myth, Andromeda was chained to a rock to appease Poseidon after her mother boasted about her beauty. Who was her mother?

Perseus rescued her on his way home from beheading Medusa and married her.

Which sea monster did Poseidon send to ravage Aethiopia in the Andromeda myth?

Perseus turned it to stone with Medusa's head; the story is seen as the forerunner of the princess-and-dragon motif.

The constellation Andromeda is known in English by what nickname?

It was one of Ptolemy's original 48 constellations in the 2nd-century Almagest.

Medieval Arab astronomers marked the hazy patch we now know as the galaxy as the nose of what figure overlapping Andromeda?

They knew Ptolemy's constellations but added a second figure across Andromeda's body.

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

It marks one corner of the Great Square.

The supernova in Andromeda was first spotted on 17 August by which French astronomer at a public stargazing event?

Ernst Hartwig at Dorpat made the independent detection that was actually telegraphed to the world.

How long ago did a major merger of two galaxies, mass ratio about 4, occur at Andromeda's location?

Modelling suggests the collision built most of the metal-rich halo, the Giant Stream and the thick disk, and briefly made Andromeda a luminous infrared galaxy.

Which infrared surveys revealed that Andromeda is actually a barred spiral?

In visible light it is classed SA(s)b; the bar's major axis sits 55 degrees anticlockwise from the disc's.

Roughly how many light-years across is Andromeda's disc by the standard D25 isophote (RC3, 1991)?

A 2005 Keck study found a diffuse halo of stars extending to about 220,000 light-years, with far more disorderly orbits than the disc.

Walter Baade described the HII regions outlining Andromeda's spiral arms as resembling what?

Because the galaxy is seen nearly edge-on, its spiral structure is hard to trace; rectified images show two trailing arms.

Which satellite's near head-on plunge is thought to have made Andromeda's inner dust ring?

Simulations show the smaller galaxy plunged through the disc along the polar axis over 200 million years ago, losing more than half its mass.

How many black hole or neutron star X-ray sources did NASA's NuSTAR mission identify in Andromeda?

These compact objects heat infalling gas to millions of kelvins; the hard X-ray glow above 25 keV traces to a single accreting binary.

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

That non-random layout hints the dwarfs may share a common tidal origin rather than having arrived independently.

Roughly how many stars hotter than the Sun does the 2025 Hubble mosaic of Andromeda resolve?

Even that vast count is only a fraction of the galaxy's total stellar population; the mosaic took ten years of observations.