15 Interesting Andromeda Galaxy Facts

Andromeda Galaxy (M31)
Image Credit: earlb.com

Part of Local Group

If it were possible to travel anywhere in the galaxy, the next step would be to visit our 54 or so neighbouring minor galaxies, most of them dwarfs, that surround the Milky Way within a distance of 1.4 million light-years. The next step on our epic journey would involve travelling to our nearest major galaxy, Andromeda, which at 2.54 million light years from Earth is the most distant naked eye object you can see in the night sky. Finally, we might venture on to the Triangulum Galaxy located around 3 million light-years from Earth, with all three spiral galaxies mentioned being the largest members of the Local Group, or a cluster of galaxies whose gravitational centre lies between the Milky Way and the Andromeda Galaxy.

Quick Facts

Constellation: Andromeda
Type: Spiral
Diameter: 220,000 light years
Distance: 2.54 million light years
Mass: 400 to 700 billion solar masses
Age: 4.5 billion years
Stars: 1 trillion
Group: Local Group
Apparent magnitude: +3.44
Designation: M31, NGC 224

Andromeda is also called Messier 31, or M31 for short, after French astronomer Charles Messier, who catalogued about 100 stellar objects in 1771. M31 is considerably larger than our own galaxy, being approximately 260,000 light years across compared to only 100,000 light years for the Milky Way.

Visible from Late Summer to Winter

From Northern Hemisphere latitudes, the Andromeda Galaxy is best seen from August to February, and in the Southern Hemisphere can be seen low on the northern horizon from October to December.

Find Using Cassiopeia or Pegasus

Cassiopeia and Andromeda Galaxy
Image Credit: www.earlb.com

The constellations of Pegasus and Cassiopeia will be familiar to many people because they are big, obvious shapes, and fortunately for stargazers they can be used to locate Andromeda:

– Pegasus, which includes the Great Square of Pegasus asterism, is huge, and clearly marked by four bright 2nd magnitude stars. Drawing a line from one corner to the other gives a good idea of where to look for the Andromeda Galaxy.

– Cassiopeia, on the other hand, is the large letter W formed by five stars and it rotates around Polaris depending on the season. If you follow the deeper “V” two times its depth, you come straight to the Andromeda Galaxy.

Named After the Andromeda Constellation

The Andromeda Galaxy is named after the area of the sky in which it appears, namely the constellation of Andromeda. This Northern Hemisphere constellation can be seen from between +90° and -40° of latitude, with its brightest star being the hot blue subgiant called Alpheratz, which is located 97 light years away and shines with a magnitude of+2.06.

In Greek mythology, Andromeda was a princess whose parents, king Cepheus and queen Cassiopeia, chained her to a rock as a sacrifice to appease the sea-god Poseidon, who took exception to Cassiopeia’s boast that her daughter was more beautiful than the sea nymphs (Nereids) who often accompanied him. Their sacrifice was intended to stop Poseidon from destroying the city of Aethiopia where they reigned, but Andromeda was saved at the last moment by the hero Perseus riding on the winged-horse Pegasus. All five characters were subsequently commemorated in the night sky as the constellations, Cepheus, Cassiopeia, Andromeda, Perseus and Pegasus. The name Andromeda derives from the Greek for “ruler of men”.

Once Believed to be a Nebula

Until the 1920s, the consensus among astronomers was that the Milky Way comprised the entire Universe, and that the fuzzy patches that could not be resolved into individual stars using early 20th century telescopes were simply clouds of gas, known as nebulae. M31 was therefore known as the Andromeda Nebula.

Appears Six Times Size of Moon

Andromeda Full Moon Comparison
Image Credit: Background image: Stephen Rahn. Andromeda Galaxy: NASA/JPL-Caltech

The Andromeda Galaxy is best observed when it’s reasonably dark outside, because then it is very easy to spot, but it may come as a surprise to most people that the Andromeda galaxy appears six times wider than the full Moon in the night sky. Even the naked-eye portion, that appears as a cloud of faint light, is about the size of the Moon (0.5 degrees of sky), while 10×50 binoculars will reveal a much larger image. Using an 8-inch (200mm) telescope, M31 presents a spectacular sight with its diffuse dust lanes stretching out from its bright centre, and if these vast spiral arms are included, the Andromeda Galaxy would cover around 20 degrees of sky, or 40 full Moons.

One Trillion Suns

There are around one trillion suns in the Andromeda Galaxy, compared to between 100 and 400 billion stars for our own Milky Way galaxy. Despite the physical size of M31 being over two and a half times that of the Milky Way, and containing several times its number of its stars, the mass of the Andromeda Galaxy is only 400 billion times that of our own Sun. The Milky Way is imagined to be considerably heavier because it probably possesses more Dark Matter and Dark Energy than M31.

Teeming With Black Holes

As with all large galaxies, including our own Milky Way, the Andromeda Galaxy has a supermassive black hole at its centre, in this case with a mass of 100 million Suns. M31 also has a large central bulge much bigger than the one at the core of our own galaxy, which allows for more black holes to form there, and so far astronomers have detected at least 34 additional black holes of a more conventional size of around 5 to 10 times the Sun’s mass.

Blue and Red Stars Near Centre

Andromeda Galaxy CentreSurrounding the supermassive black hole located at the heart of the Andromeda Galaxy’s central bulge is a light-year wide disk of around 400 hot blue stars orbiting it at 2.2 million miles per hour. The disk is situated five light-years away from the black hole, and in turn is encircled by an elliptical ring of older, red stars. The unusual juxtaposition of these blue and red stars suggests that the blue stars are actually older stars that on their way to becoming red giants shed their outer layers to reveal their blue-hot cores. The number of blue stars then falls with the distance away from the centre, and the galaxy’s outer rim delimits the older, more mature red stars.

Double Nucleus

What sets Andromeda apart from our own galaxy is the fact that it has a double-nucleus, likely because somewhere between five and nine billion years ago two smaller galaxies merged together and the nuclei are now in orbit around each other. One of the compact star clusters is designated P1, while P2 is dimmer, but contains a black hole. They will eventually blend into one body in a few billion years.

Orbitted by 14+ Dwarf Galaxies

Andromeda Dwarf Galaxies M32 and M110The Andromeda Galaxy possesses around 450 large globular clusters, the most massive being Mayall II (G1), which is situated 130,000 light-years from the galaxy’s nucleus, consists of more than 300,000 old stars, and is twice as bright as our Milky Way’s most lumious globular cluster, Omega Centauri.

In addition, there are at least 14 dwarf galaxies orbiting the Andromeda Galaxy, the brightest of which are M32 and M110, both located a little over 2.65 million light-years from Earth, and visible using even a basic telescope. Interestingly, the majority of Andromeda’s satellites have formed a one million light years wide and 30,000 light years thick ring around its center, and rotate around it core in an immense plane. Though Andromeda’s globular clusters are about the same size as its dwarf spheroidal galaxies, the former have virtually no Dark Matter and so are considered different objects.

Andromeda and Milky Way to Merge

Andromeda is going to combine with our galaxy, the Milky Way, in the distant future. At this very moment our two galaxies are approaching each other between 100 and 140 kms/s (60-87 miles/sec). In about 4 billion years we’re going to merge, probably having a triple nucleus for a while, while the stars we are familiar with will also form new constellations, but eventually the galactic collision will settle down resulting in a brand new massive elliptical galaxy.

Of course when two galaxies come together you might think there would be a lot of collisions. It seems logical that a whole bunch of little red dwarves, the most common sort of star in any galaxy, would combine to make many more large white suns, like our own. However, the truth is that the space between the stars is so vast that there might be no collisions at all! What will happen is that lots of stars will be flung off into deep space and become wandering solar systems, but the large majority will remain associated and form a large ball of stars. It will look very much like this video.

Sparked Theory of Island Universes

It wasn’t until astronomer Heber Doust Curtis (1872-1942) was studying the Andromeda Nebula and studied novae (exploding stars) that this opinion began to change about whether M31 was in fact an “island universe” beyond our own Milky Way. Curtis observed a nova in Andromeda, and after comparing its data to photographic records of 11 more novae, he noticed that on average they were 10 magnitudes dimmer than the ones which took place within the Milky Way. Doing some calculations, he then came up with a distance of 150,000 parsecs (489,234ly) for the Andromeda Nebula, a distance placing it outside of our own galaxy, and so proposed his “island universes” theory that asserted that spiral nebulae were in fact separate galaxies. In fact, it led to astronomy’s Great Debate that took place at the Smithsonian Museum of Natural History in 1920 between himself and Harlow Shapley, another preeminent astronomer, who maintained that “spiral nebulae”, including Andromeda, were within the Milky Way.

Proved Existence of Galaxies

Parallax MeasurementThe Great Debate remained unresolved until 1925 when Edwin Hubble first calculated the distance to the Andromeda Nebula showing that it was clearly outside of the Milky Way Galaxy. Using his 2.5m (100″) telescope, Hubble produced photos that showed parts of spiral nebulae to be collections of individual stars, while he also used stars called Cepheid Variables to calculate the distance to the Andromeda Galaxy, using parallax measurements.

Every six months, the Earth is on the opposite side of its orbit, creating a baseline for a giant triangle that is 186 million miles long. With the relationship of the Cepheid Variables’s changes in brightness and pulsation period known, the size and distance to Andromeda could then be mathematically calculated using simple trigonometry. Hubble placed the distance to Andromeda at 950,000 light years, but as our ability to measure improved, the number was increased to the modern day figure, nearly three times that which Hubble calculated, of 2.54 million light years. Nevertheless, Hubble still helped prove that our Milky Way galaxy is not the whole Universe, but simply one of many galaxies.

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