Observing at Skyhound



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The Andromeda Galaxy (M31)
Nearby Galaxy
M31, NGC 224, Uppsala 454, MCG 7-2-16, PGC 2557
Integrated Visual Magnitude: 4.3
Apparent Diameter: 1.0o x 3.2o
Distance: 2.3 Mly

Minimum requirements to view: naked eye and very dark skies

The Andromeda Galaxy is the brightest galaxy visible from the northern hemisphere. Most of you have probably observed it many times. Some may even ignore it now because it's often considered a beginner's object. In any case, whether you have never looked upon it or haven't for a long while, I urge you to take a good long look at this magnificent galaxy. Like the Great nebula in Orion, Andromeda at the very least deserves a yearly visit.

You should be able to see M31 with your unaided eyes--quite a feat for something that lies over 2 million light years away. From a relatively light-polluted location all you will see is the feeble glow of the nucleus. It may look like nothing more than a slightly fuzzy star. From a dark site you can see much of the full extent of the galaxy -- up to 6 times the diameter of the full moon! You may need to use averted vision and take into account that the galaxy generally becomes fainter as you move away from the nucleus.

M31 as seen at about 8PM local time.

In the telescope, use your lowest-power, widest-field eyepiece.

The first thing to look for is the very bright, nearly stellar nuclear core. The Hubble Space Telescope has observed the spectral signature of fast moving hot gas in this core. The gas is moving rapidly around a massive, unseen object -- probably a "supermassive" black hole.

The bright, elongated glow surrounding the core is the nucleus. Those observing from light-polluted locations may mistake this nuclear region for the entire galaxy. Look carefully at the western edge of the nucleus. Compare what you see to the eastern edge. The western edge is much more abrupt. In fact, what we are seeing is actually a lane of dark matter. Look for another line of haze that parallels the abrupt edge; this is the other edge of the dark lane. Follow the abrupt edge along the long axis of the galaxy. Can you see it become a thin dark line that arcs about the nucleus?

Now look for the fainter haze that surrounds the nucleus. Try to follow it as far as you can, particularly along the long axis. At the ends it can be helpful to move the scope away and then bring it back again. As you move it back look for the sky background to become brighter. This is the galaxy.

Look for a bright, hazy spot less than 1/2o to the south of the nucleus. This is the galaxy M32, a small galaxy that is orbiting the much larger M31. Look for the bright, star-like core.

A bit more than 1/2o to the northwest lies another small galaxy, M110. This galaxy is larger and more diffuse than M32. These two galaxies are analogous to the Large and Small Magellanic clouds which orbit our own Milky Way. In fact, our Milky Way is similar in many ways to the Andromeda galaxy. An observer looking back at us from Andromeda would see a similar view to yours.

Above is a 3o x 3o region of sky centered on the nucleus of M31. The circle represents approximately a 1o field of view, which is similar to that of most wide field eyepieces. North is up and East is left. An image of the galaxies derived from the Digital Sky Survey has been overlaid to provide a realistic representation of what you may see in the eyepiece. You may need to adjust your monitor for best results.

Millennium Star Atlas Vol I Chart 105
Sky Atlas 2000 Chart 4
Uranometria 2000 Vol I Chart 60