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M31, Andromeda Galaxy in Andromeda

TargetTypeR.A.Dec.ConstellationMagnitudeSize
M31 (NGC 224)Spiral Galaxy00h 42m 44s+41° 16′ 08″Andromeda3.3178×70 arcmin

Overview

There are two members of the Messier catalog vying for title of “most famous deep sky object”. One of them is M42, the Orion Nebula, the other is this, the closest large galaxy to Earth and the most distant object that can be seen with the naked eye: the Andromeda Galaxy.

This mass of maybe one trillion stars (that’s 1000 x one billion) is about 150,000 light-years across but over 2.5 million light-years away. Our Milky Way galaxy and Andromeda are on a collision course in around five billion years’ time. Today, the gap between us is closing at almost 200 miles per second.

When Messier first pointed his telescope towards Andromeda, galaxies had neither been discovered nor conceived of, which is why he famously recorded it as “…a beautiful nebula…shaped like a spindle.”

It is easily the largest galaxy in the Local Group and is one of the largest galaxies ever discovered. Having said that, due to its brightness, there is no ‘discovery’ as such for Andromeda because it can be seen with the naked eye. However, like all large DSOs, the magnitude of 3.3 feels like a lie. This is how bright it would be if all that light were concentrated to a point, like a star. Instead, the light is spread over a large area (much larger than the full moon) and so it has a much dimmer surface brightness.

The dwarf galaxy M31 is very close by in our eyepieces and is, indeed, a satellite of M31. Another satellite galaxy, M110, was the last object added to Messier’s catalog after his death but based on notes he made while alive.

M31, captured by 2MASS at the University of Massachusetts. See also M32, the bright light above and just to the left of the core of M31, and M110 the hazy light slightly below M31 (source)

In the next section, you’ll learn how to find this wonderful galaxy in your own eyepiece.

Finder Charts

The first finder chart, which uses SkySafari 6, shows Andromeda Galaxy high in the west at 9 p.m. in the middle of December. Stars are shown to magnitude 5.0, but the moon and planets are not shown.

Two of the most obvious markers we can use to orientate ourselves are the Great Square of Pegasus, and the obvious ‘W’ of Cassiopeia, both of which are highlighted orange in the first chart. M31 itself is shown as a green oval between these two features.

[M31-1] Use the Great Square of Pegasus and Cassiopeia to locate Andromeda. Click for full-screen.

The most obvious feature of the constellation of Andromeda is the two ‘spider legs’ originating from Alpheratz, at the corner of the Great Square, and both extending through three, similarly staged, bright stars.

Follow both of these legs along from Alpheratz to the second stars, Mirach and Mu Andromedae. Both of these are bright enough to be seen in moderate light pollution.

If you imagine a line joining Mirach to Mu Andromedae that continues onwards for a similar distance, this will bring you to M31.

[M31-2] Run a line from Mirach to Mu Andromedae and then onwards for a similar distance to hit Andromeda Galaxy. Click for full-screen.

This third chart shows the path in more detail and stars are now shown to magnitude 7.

[M31-3] Mirach and Mu Andromedae with stars to magnitude 7. Click for full-screen, or download the pdf below.

With the galaxy now in your eyepiece, let’s explore what you should be seeing.

Individual Telescope Views

The following views will help you find M31 in different telescope types by presenting the images as your telescope will show them. Stars are shown to magnitude 10 including Mu Andromedae from the charts above.

The first image is with a black sky and white stars, the second picture is the same image but presented in inverse monochrome. Black stars on a white background is often easier to use at the telescope. Each image can be clicked on for a full-screen version.

Upright View – This is what your eyes see unaided and through a reflex or red-dot finderscope

[M31-4]
[M31-5]

Upside-down view – This is what reflectors and magnifying finders show, and refractors / Cassegrains without a star diagonal

[M31-6]
[M31-7]

Mirrored View – Refractors and Cassegrain models with a star diagonal show this view

[M31-8]
[M31-9]

With this lovely galaxy now in your eyepiece, what should you expect to see?

Observation

As mentioned earlier, this galaxy is bright enough to be seen as a faint, hazy light with the naked eye, but you won’t see any detail without magnification.

The most immediately apparent elements of the galaxy are its long oval shape and the concentrated bright core that readily dims towards the edges. You’ll also be able to pick out the brighter dots of M32 and M110 when you’re using low magnification and/or have a larger field of view (M32 and M8 will just about squeeze inside the same 1° field of view).

Larger scopes reveal the dust lane that runs along the galaxy’s northwest rim. They also show just how large this galaxy is because it won’t all fit in the eyepiece now. What we can see is better graduation between the bright core, the not-quite-as-bright fringes around it, and then the softer, fainter fading at the farthest reaches of the galaxy’s edge.

One of the best things about having such a large galaxy so ‘close’ is that we can tease out more and more detail with each viewing. Familiarity does not, in this case, breed contempt. Rather, the more we visit this distant island world, the more we want to explore it.


PDFs for Printing

Each of the star maps above is reproduced as a pdf below. Each star map has a number in [square brackets] beneath it which corresponds to the file number below. If you want image [M31-1], for example, click the ‘download’ button next to it below and you’ll be able to open it as a printable pdf.

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