Andromeda Galaxy (M31) – Fascinating Facts

Andromeda Galaxy (M31) – Fascinating Facts
The Andromeda Galaxy with two of its satellite galaxies [Flickr user Adam Evans, EarthSky]

Imagine yourself under a dark autumn or winter northern hemisphere sky. You're looking towards the “W” of Cassiopeia, and notice a hazy patch in the sky between Cassiopeia and the constellation Andromeda. That is the Andromeda Galaxy (M31). Here are some fascinating facts about this stunning object.

The Andromeda Galaxy is the most distant object we can see without optical aids.
It's our nearest large neighboring galaxy, and if you can see it with your unaided eyes, you're seeing 2.5 million light years into deep space. You're also looking back 2.5 million years in time, since that's how long it takes Andromeda's light to reach us.

At the start of the 20th century most astronomers thought the Milky Way – the Galaxy – was the whole Universe.
Telescopes could show spiral structure in M31 and other spiral galaxies, but couldn't resolve individual stars. Astronomers usually called these objects “spiral nebulae” and assumed them to be gaseous nebulae in our Galaxy. M31 was known as the Great Andromeda Nebula. But some astronomers, such as American astronomer Heber Curtis (1872-1942), argued that the objects were actually distant galaxies, what German philosopher Immanuel Kant (1724-1804) had called island universes.

Edwin Hubble (1889-1953) resolved the spiral nebula question with the 100-inch Hooker Telescope on Mount Wilson in California.
The Hooker Telescope was then the world's largest telescope. Hubble was able to get more detailed images than were ever before possible. In his observations he found a Cepheid variable star in M31. Here is a copy of the photographic plate containing the Cepheid. You can see that Hubble excitedly labeled it “VAR!” Here was the breakthrough in calculating the distance to M31. Henrietta Leavitt (1868-1921) had discovered that there was a relationship between the period of the light variation in a Cepheid and its actual luminosity. Knowing this, you could calculate the distance to such a star. Although Hubble's distance calculation put it at much less than today's values, the Great Andromeda Nebula was obviously not in the Milky Way.

The Andromeda Galaxy is, like the Milky Way, a large spiral galaxy.
Although the Andromeda Galaxy is tilted so that we can't see it face-on, astronomers think the two galaxies look similar, and that both were formed soon after the beginning of the Universe. However Andromeda is larger and more massive than our Galaxy. The Milky Way has 200-400 billion stars, which is a substantial number, but Andromeda contains over a trillion stars. It's also some 220,000 light years across, which is more than double the size of the Milky Way.

M31 and the Milky Way are the dominant galaxies of the Local Group of galaxies.
Galaxies are rarely alone, because even with the enormous distances between them, gravity still has an effect, pulling them into groups and clusters. In addition to the two large spiral galaxies of the Local Group, the group contains the Triangulum Galaxy (M33), a small spiral galaxy with only 40 billion stars. There are also around fifty known dwarf galaxies.

The Andromeda Galaxy has 14 known satellite galaxies.
All of its known satellite galaxies are dwarf galaxies. They're held by M31's gravity. You can see M32 in an amateur telescope, as well as nearby M110. Charles Messier (1730-1817) drew both of them, but only M32 was included in his catalog of nebulae. Caroline Herschel independently discovered it in 1783. Nearly two hundred years after Messier's discovery, it was given the Messier designation M110.

M31 has at least 450 globular clusters, including some extremely dense ones.
A globular cluster is a dense star cluster with a rounded shape. M31 has three times as many of these clusters as our Galaxy does. They're large and dense and contain up to a million stars.

The Andromeda Galaxy has a supermassive black hole at its center.
It appears that supermassive black holes are a feature of galaxies. Andromeda's black hole is 100 million times the mass of the Sun. Yet the small area around it – its event horizon – is so small that even this Hubble Space Telescope image couldn't capture it. The small cluster of bright blue stars in the middle of the inset is where the black hole is.

The Andromeda Galaxy and the Milky Way are on a collision course.
Distant galaxies are moving away from us as the Universe expands. However nearby Andromeda is moving towards us at 110 km per second (68 miles per second). This is about 396,000 km per hour (247,500 mph). But let's not panic. Remember it takes light 2.5 million years to get here, and the Andromeda Galaxy is moving much more slowly than light. We're looking at something that will take around four billion years to happen. Eventually the two galaxies will merge into one giant elliptical galaxy.

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