Andromeda, Our Sister Galaxy

The fall observing season is now in full swing.  Last Saturday night was close to perfect — a few early clouds cleared away, there was no Moon, humidity was low.  I had the telescope and camera out for the first time in months, more on that later.

Among the literally dozens of amazing things we can see in the Fall night sky, one of the most fascinating is the great Andromeda Galaxy.  This brightest of major galaxies can be seen from our area on a night like last Saturday night with the unaided eye.  Starting with the smallest of binoculars, this dim fuzzy patch in the sky becomes increasingly amazing as we gather more and more light using larger telescopes.

A galaxy is a stupendously large collection of stars, all bound together with their vast gravitation, and slowly orbiting about their collective center.  Our own galaxy, which has been named the Milky Way since the times of Ancient Greece (in fact the etymology of galaxy has its root in the Greek word for milk), contains approximately 300,000,000,000 stars. 

This immense collection of matter is visible as a faint stream of light grey running from the constellation Cassiopeia in the northeast evening sky, through Cygnus, down to the southwest, and continuing through the southern hemisphere skies. The galaxy has the shape of a flattened whirlpool, with several separated spiraling arms, one of which holds all of the visible stars, including our own sun. As we look in the direction of the Milky Way, we look along the width of this whirlpool at the light of billions of stars far too distant to be seen as separate points of light, but merged into a majestic subtle band.

Because we are within the galaxy (about 2/3rds of the way from the center toward the edge), we cannot easily envision the structure of the Milky Way.  However, we are fortunate enough to have a sister galaxy next door that we can observe in great detail.  The Andromeda Galaxy is similar in size and structure to the Milky Way, and is located a mere 2,500,000 light years away.

2,500,000 light years.  Let's spend a moment on that.  Andromeda is at a distance such that light, traveling at 186,000 miles every second takes two and a half million years to reach Earth.  Put another way, when we observe Andromeda, we do not see it as it currently is, but as it was to and a half million years ago.  Before Man walked the Earth.  Some of the stars producing the light that we see from Andromeda have ceased to exist, and the great spiral arms of that galaxy are now at very different positions circling Andromeda's center than they appear to us today.  Attempting to grasp this will make us dizzy in a hurry.

Finding Andromeda for the first (or even 5th) time can be a bit challenging.  I recommend using binoculars to search for it, and the chart I've attached as a guide.  Find Cassiopeia (which now looks like a big W lying sideways in the northeast in the evening), and work your way south eastward, finding the brighter stars in the chart, until you are close. Then point your binoculars in that direction, and scan the sky slowly.  I doubt you will miss it when you find it — it will appear as a small grey elliptical cloud against the darker surrounding sky.

When we observe or photograph Andromeda through a very large telescope, we can see not only the spiral structures, but much finer details that have enabled astronomers to better understand the contents of our own galaxy.  We find enormous complexes of glowing gas — nurseries in which new stars are being created that will become star clusters after a few hundred million years.  In our own galaxy we have found dozens of these structures, perhaps the most famous being the Orion Nebula that will be visible this winter, as well as the remains of stars destroyed in supernovae, and dark lanes of interstellar dust.

We can also observe Andromeda's satellite galaxies and globular clusters.  Even in a small telescope, near Andromeda we see a second, and possibly a third smaller cloud.  These are M32 and M110, each a galaxy in its own right with billions of stars, orbiting about the much larger galaxy like planets around a sun, but on an inconceivably vaster scale.  Our own Milky Way galaxy has several similar satellites — the Large and Small Magellanic Clouds visible in the southern hemisphere, and at least 10 other smaller galaxies, most sufficiently small and distant to require a modest telescope to observe.

With the largest of telescopes, we can see the huge collection of globular clusters orbiting Andromeda.  These can be thought of as mini-galaxies of 10's-100's of thousands of stars.  Over 300 globular clusters have been mapped in the Andromeda system forming a halo centered on the galaxy's core.  Our own globulars are much easier to observe, including the Great Hercules cluster (discussed here back in July), along with hundreds of others. 

I have somewhat of a fascination with globular clusters, in particular they are only moderately difficult to photograph using my modest equipment.  In the attached photo from this weekend I captured one of the smaller globulars of the Milky Way, M71, seen through a myriad of foreground stars within the Milky Way region of the sky.  As we look at this photograph, we see through the galaxy's spiral arm, out 12,000 light years into a pile of thousands of stars.