Hopping to the Andromeda Nebula

The Nebula

One of the most amazing sights in the sky is the Andromeda Nebula.

Nebula means cloud. Unlike stars, which seem like sharp little points of light, a nebula looks fuzzy. The “Great Nebula in Andromeda” is a fuzzy patch of light that is located in the constellation Andromeda. When seen through binoculars or a telescope it becomes a bunch of stars; actually about one trillion stars... a whole galaxy. That galaxy is rushing toward us and eventually will crash into our galaxy.

You can use star-hopping to find the nebula.

A Star-hopping Plan

Here is one way to find the Andromeda galaxy:

1. Start with the pointer stars of the Big Dipper.
2. Use them to find the constellation Pegasus.
3. From Pegasus, find the constellation Andromeda.
4. Using pointer stars in Andromeda, locate the Andromeda galaxy.

Let's try it.

Starting with the Pegasus Constellation

In Greek myth Pegasus is a flying horse, and in the sky, the Pegasus constellation flies around Polaris, the “North Star”. Because the Earth rotates from west to east, the stars seem to turn around Polaris from east to west; that is, counter-clockwise as one looks northward towards Polaris. Therefore, if you look at the North Star for a long time, you will see Pegasus slowly flying around it counter-clockwise, always keeping his belly towards the North Star.

Ursa Major also flies around counter-clockwise like everyone else in the northern sky, but with his belly away from Polaris. (By the way, Draco the dragon is also circling Polaris, and with his belly toward Polaris like Pegasus. However, although he is a dragon, he is not actually flying like Pegasus is. Instead, he is being dragged by his tail, right behind the great bear.)

Why Pegasus is More Useful in the Winter

Like all the stars, Pegasus makes it completely around Polaris in a little less than 24 hours. Therefore, in one hour (1/24th of a day), Pegasus goes about 1/24th of the way around. The fact that Pegasus and all the stars complete their circle in less than 24 hours means that every day Pegasus is a little further ahead of where he was 24 hours before. This extra speed adds up from day to day, so every month Pegasus starts the day way ahead of where he started the previous month. In the Spring and Summer, Pegasus has pretty much flown below the horizon by night time. Of course during the day in Summer, with the Sun being up, the sky is too bright for any of the stars in Pegasus to be visible from the ground. The best time to look for Pegasus is nighttime in the Fall or the Winter, when he is flying high in the sky.

The Great Square of Pegasus is easily visible in the Fall and Winter. In the Fall it will be in the east moving “up” toward Zenith. (Zenith is the highest point in the sky from wherever you are... straight up.) In Winter the square will be overhead moving toward the west.

In Spring it will already be setting in the west by early evening.

Getting to Pegasus

Here is a way to find Pegasus. Look for the asterism known as the Great Square of Pegasus. The Great Square of Pegasus is almost exactly opposite Ursa Major from Polaris, but about twice as far away. Although Ursa Major the bear can lumber around Polaris in a small circle, Pegasus the horse has to go further to get around and therefore must move faster, so he flies.

Each corner of the Great Square is a fairly bright star. The Square is 60° to 75° from Polaris, directly opposite from Merak and Dubhe, the pointer stars of the Big Dipper. Luckily this makes it easy to find by star-hopping.

Here is how. Imagine a line from Merak, through Dubhe to Polaris. Look at the “distance” (that is, the visual angle) from Merak to Polaris. Continue on the imaginary line past Polaris for twice that angle. You have arrived the middle of one side of the Great Square, between the stars Scheat and Markab.

The star farthest from Polaris on the side you just found of the Great Square is Markab, also named or Alpha Pegasi (α Peg). Markab means saddle in Arabic.

The other star on that side of the Square, the one more to the north (that is, closer to Polaris), is Scheat (β Peg), the shoulder of the horse. The forelegs of the flying horse extend a bit towards Polaris, but mostly forwards, in the counter-clockwise direction that Pegasus is flying around Polaris.

To the east of Markab, (that is, behind the shoulder—remember, Pegasus is flying westward) is another corner of the square, namely, Algenib (γ Peg). In Arabic, Algenib means the flank, the side of the horse between his ribs and his hip. (Al in Arabic means “the”.)

The fourth star in the Great Square of Pegasus is Alpheratz (the bellybutton).

Is Alpheratz part of Pegasus? Surprisingly, no. Astronomers long ago decided to end the constellation with just the front part of the flying horse. Alpheratz is part of the constellation Adromeda and, because it is the brightest star of that constellation, it is labeled α And.

Moving on to Andromeda

The star in the northeast corner of the Great Square of Pegasus is Alpheratz, also called Alpha Andromedae (α And).

From α Andromedae move 7° northeast to find two stars 3° apart: Delta Andromedae and Pi Andromedae (δ and π And).

Keep going for 8° more to reach two more stars 3° apart: Beta and Mu Andromedae (β and μ And). The name of β And is Mirach.

Now make a right angle right turn and head northwest for 3° (that’s the same distance as between the two stars you just found). In other words go across Andromeda’s hips, from β through μ And and continue for the same distance. You have reached the Andromeda galaxy, also known as the Great Nebula in Andromeda, as M31 and as NGC224.

A Quicker Way to the Andromeda Nebula

A faster way to find the Andromeda Nebula, and a good way to check that you are really there, is to use Cassiopia.

Cassiopia is easily spotted as the W asterism opposite from Alioth, the bright star in the handle of the Big Dipper that is nearest to the bowl.

Think of the right side of the W as the point of an arrow. Imagine the shaft of the arrow as starting from Alioth and running through Polaris, then ending with the W. That arrow points straight at the Andromeda Nebula. The length of the arrow from Polaris to the W is 30°. If the arrow is shot it arrives at the nebula after flying 20° ahead.

(Remember, your fist is 10°, so you can easily measure the angles to find the Andromeda nebula.