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: EarthSky.org, NASA.org, Space.com,
HubbleSite, AstronomyNow.com, CalTech, WISE.ssi.berkley.edu, and Wikipedia.org.
