Andromeda: Our Sister Galaxy

In Greek mythology, Andromeda, the daughter of Cepheus and Cassiopeia, was stripped and chained to a rock, only to be saved from certain death in the claws of a sea monster by Perseus (Figure 1 shows a wonderful depiction of the myth by Lord Frederic Leighton).

Figure 1. 'Perseus and Andromeda' by Lord Frederic Leighton. Walker Art Gallery, Liverpool. At Google Cultural Institute.

In the northern sky, a constellation is named Andromeda, and it contains the galaxy M31 (so cataloged by astronomer Charles Messier on August 3rd, 1764), commonly known as the Andromeda galaxy. At a distance of 2.5 million light years, the Andromeda galaxy is next door in astronomical terms. Its mass is only about twice that of the Milky Way, making the two galaxies if not quite twins, then close sisters.

By measuring very precisely the motion of Andromeda relative to the Milky Way, astronomers using the Hubble Space Telescope were able to determine in 2012 that the Milky Way and Andromeda are destined for a head-on collision in about 4 billion years. About two billion years later, the two sister spiral galaxies will completely merge, most probably producing an elliptical galaxy. While solar system will not be destroyed during the collision, it is very likely that it will be flung into a new region of the merged galaxy.

Figure 2 is a photo illustration of what the night sky may look like as the two galaxies will be on their way to that fateful encounter. This view was inspired by detailed computer modeling of the future collision.

Figure 2. Illustration of the expected collision between the Milky Way and the Andromeda galaxy.

Recently, astronomers combined data from two large surveys to discover that, like its mythological namesake, Andromeda has experienced a rather violent history. One of the surveys (SPLASH) used the Keck telescope to measure the radial (in our direction) velocities of more than 10,000 stars. The other survey (the Panchromatic Hubble Andromeda Treasury; PHAT) used the sharp vision of the Hubble Space Telescope to produce an unprecedented, high-definition image of a part of Andromeda (see Figure 3 and the Zoom into M31 video on the web page of STScI News Release Number: STScI-2015-02).

Figure 3. A high-definition panoramic view of a part of the Andromeda galaxy. A product of the Panchromatic Hubble Andromeda Treasury (PHAT) program. (Credit: NASA, ESA, J. Dalcanton, B.F. Williams, and L.C. Johnson [University of Washington], the PHAT team, and R. Gendler.)

The two sets of observations revealed an intriguing distinction between young and old stars in Andromeda. While we normally associate youth with rebellion, and old age with discipline, the youngest stars were found to rotate in an orderly fashion around Andromeda's center, while the older stars displayed a less ordered, more chaotic motion.

Possible explanations for these observations include a series of past bombardments of Andromeda by a number of smaller satellite galaxies, and an evolution of Andromeda's disk from a more puffed-up configuration to a thinner one. Either way, it appears that our sister galaxy may have had a rougher past than the Milky Way. This fascinating object is just about the most distant thing we can see in the night's sky with the naked eye.