October - The Andromeda Galaxy

Finding the Andromeda Galaxy in the Sky

On a clear, moonless night in October, you can see a marvelous world over 2 million light years away: the Andromeda Galaxy. The Andromeda Galaxy, also called Messier 31 or M31, is a large spiral galaxy containing about a trillion stars as well as gas, dust and dark matter. This galaxy can be seen – even from locations with mild light pollution – in the northern hemisphere toward the Andromeda constellation (near the Great Square of Pegasus) in the northeast. You can find observing tips and resources in the October 2009 IYA Discovery Guide and in the Exploring Further section.

  • Galileo's MoonsThe Andromeda Galaxy as seen through binoculars.


Getting to Know A Neighbor

When Galileo pioneered the use of telescopes in astronomy, he resolved the band of our Milky Way Galaxy into millions of individual stars. For three centuries after that, telescopic views of the “Andromeda Nebula” (as M31 was then called) only showed a faint smudge bigger than the full moon. It was only in the 20th century that astronomers, using much more sensitive and discerning telescopes, resolved individual stars in Andromeda.

Certain kinds of these individually resolved stars helped astronomers determine how far away Andromeda is from the Sun. At about the same time, a reliable estimate of the size of our Milky Way Galaxy was made. Together, these observations conclusively proved Andromeda to be a separate galaxy, located well outside the Milky Way. In a modern version of the Copernican revolution, we discovered that, like the Sun being a “suburban” star about half way from our Galaxy’s center, our big and beautiful galaxy is not in any particularly special place. We now know it is just one among a hundred billion galaxies, including Andromeda, that make up the visible universe.

Two and a half million light years away, Andromeda is still one of our Galaxy’s closest neighbors. This closeness makes Andromeda a valuable prototype to study a wide range of astrophysical phenomena in galaxies, which are subtle, complex, and dynamic entities. Furthermore, since we only get to study the Milky Way from within, Andromeda’s similarity to it gives us an “exterior view” of our own Galaxy’s architecture.

In visible light that our eyes are sensitive to, Andromeda appears to be a normal spiral galaxy, seen almost edge-on. Its central bulge is bigger and its spiral arms are wound tighter than our Galaxy’s. Astronomers using both ground-based and space-based telescopes such as NASA’s Spitzer Space Telescope, Galaxy Evolution Explorer and Chandra X-ray Observatory, have acquired data at infrared, ultraviolet and X-ray wavelengths that complement deep, visible light and radio observations. These observations shatter the outward appearance of Andromeda as a serene spiral, and allude to Andromeda being quite a disturbed galaxy.

The multi-wavelength observations track the locations of cold gas and warm dust in which new stars are born, and of ionized gas regions that shine by the light of hot, young stars. The data show that Andromeda has a central supermassive black hole, some distance from which begin a starry bar and broken, twisting spiral arms veiled by cosmic dust. Framing these are glowing concentric rings of star-formation. Intriguingly, Andromeda may be forming stars at a more leisurely pace than the Milky Way. Astronomers are busy trying to solve the mystery of what makes similar galaxies form stars at different rates and in different regions.

The newly discovered breakages in the spiral arms and presence of the star-forming rings make astronomers suspect that Andromeda may not quite share a family resemblance to the Milky Way. One possibility is that Andromeda has been distorted by gravitational interactions with the bevy of its smaller, satellite galaxies. Indeed, very deep optical observations that expose the outer halo of the galaxy show ghostly star streams that may be the debris of smaller galaxies devoured in the on-going construction of Andromeda.

Closing In

Furthermore, our Milky Way and Andromeda, which share a home in the Local Group of about 40 galaxies, are so close that only about 20 comparable galaxies would fit between them. And they are getting closer every second. Approaching at a speed of nearly 0.5 million kilometers per hour, the pair is on course to a majestic collision in about 3 billion years.

Details of this predicted collision are still uncertain because we don’t quite know the precise speed and direction of Andromeda’s motion relative to the Milky Way (it is not head-on), nor the precise mass and size of either galaxy. Nevertheless, from observations of dozens of other such galactic collisions both in the nearby and distant universe, astronomers predict that the Milky Way and Andromeda will merge into one gigantic galaxy. Over the next few billion years, they would morph from two pretty spiral galaxies into a diffuse, elliptical one like Messier 87 in the nearby Virgo cluster of galaxies.

What would happen to the Solar System in this cosmic crash? One likely scenario is that we would lose our spot at the suburbs of our Galaxy, and be shuffled to a very different location in the budding elliptical galaxy, where our sky would no longer have the familiar disk of starlight that we call the Milky Way. By then, the Sun would be running out of its core nuclear fuel and evolving into a red giant. Life on Earth could be quite unlike anything we know now.


The Andromeda Galaxy.

The Andromeda Galaxy - the nearest major galaxy to our Milky Way - is one of the most distant objects visible to the unaided eye. Credit: Adam Block/NOAO/AURA/NSF

False color composite.

Infrared light detected by the Spitzer Space Telescope (represented in red) and ultraviolet light detected by the Galaxy Evolution Explorer (represented in blue and green) highlight Andromeda’s spiral nature.

Central region of Andromeda.

The central region of Andromeda glows in X-rays in this image taken by the Chandra X-ray Observatory. Millions of degree hot, diffuse gas and point-like interacting pairs of stars are prominent.

Venus transit

Double Nucleus

Hubble Space Telescope finds a double nucleus at the center of the Andromeda Galaxy.