Sunday, March 13, 2016

Messier 31: The Andromeda Galaxy

Next to our own Milky Way galaxy, Andromeda is the most well-known galaxy in our universe. At the distance of 2.5 million light-years away, it is the most distant thing we can see with our naked eye. A spiral galaxy, approximately 260,000 light-years across, is the largest of our local group of galaxies, which includes our own Milky Way spanning 100,000 light-years across. On a clear and dark winter night, M31 can be seen as a fuzzy patch of light. It’s an inviting target for binoculars or a telescope. We are best able to see it starting in the fall when it’s high enough in the sky to be seen from nightfall until daybreak. In late September and early October Andromeda shines in the eastern sky at nightfall and stands high in the west at the onset of dawn. Winter evenings are also good for viewing.

How do we find the Andromeda galaxy? The easier way I have found is to use the constellation Cassiopeia, the Queen. It’s easy to recognize because it is in the shape of a “W.” I generally look northward on the sky’s dome to find this constellation. By finding Polaris (the North Star) and by finding the Big Dipper nearby, I can easily see that the Big Dipper and Cassiopeia move around Polaris like hands on a clock. In Cassiopeia, the star “Schedar” points right to Andromeda. Schedar is the second bottom star in the “W.”

When Andromeda was first photographed in 1900 it was thought to be a cloud of gas within our Milky Way and called the Andromeda Nebula. It wasn’t until the 1920’s that Edwin Hubble determined that Andromeda was outside the Milky Way, that it was an individual galaxy, and that these points of light were indeed stars. It wasn’t until 2006 that the Spitzer Space Telescope revealed that Andromeda contained one trillion stars. That’s over twice the amount of stars in the Milky Way, which is estimated to be 200-400 billion.

So what have we learned since 964 when a Persian astronomer found Andromeda and described it as a “small cloud?”  Well, new stars, as well as old stars, could be found there with a dense concentration toward the center. It has a double nucleus, which I interpret as a double “massive black hole,” at the center. As a matter of fact, 26 black holes have been found in the galaxy to date, but not all are massive. There are 450 globular clusters orbiting in and around Andromeda. More recently the KELT North Telescope has detected two large planets in this galaxy: KELT-1b and KELT-1ab. An ancient companion galaxy was ripped up and consumed by Andromeda; the clouds are the remains of the stars of this prior galaxy. There are 14 dwarf galaxies nearby that it regularly bullies. Andromeda is blue-shifted, which means that it and the Milky Way galaxies are on a collision course. But we needn’t worry; this won’t happen for another 4 billion years.  

References:,,, HubbleSite,, CalTech,, and