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Posts Tagged ‘Orion nebula’

In this blog I discussed the Messier catalogue, including its historical background. As I mentioned in that blog, many of the 110 objects in the Messier catalogue are amongst the most beautiful and interesting objects in the sky. In a series of blogs I am going to discuss some of my favourite objects in the Messier catalogue in more detail, including how to find them.

First off, Messier 31.

Messier 31 – The Andromeda galaxy

I have already talked about this galaxy a few times before. In this blog I explained how Edwin Hubble showed in 1923 that M31 lies outside of our Milky Way galaxy. It was the first object ever shown to lie outside the Milky Way, and of course led to the realisation that many similar objects also lay beyond our Galaxy. This discovery by Hubble in 1923 totally changed our perception of our Universe and our place in it. It provided conclusive proof that our Milky Way galaxy was not the entire Universe, but just one galaxy in many. In my blog introducing the Messier catalogue it was one of the four objects I illustrated.

Messier 31 is an example of a spiral galaxy. However, in visible light its spiral arms are not very prominent. Along with our Milky Way, Messier 31 and our home Galaxy form the two largest galaxies in our Local Group. There are other galaxies in the local group, but they are all so-called dwarf galaxies and are either satellite galaxies of our Milky Way or of M31.


Messier 31 - The Andromeda galaxy. This is the most distant object that can be seen with the naked eye.

Messier 31 – The Andromeda galaxy.


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Our current best measurements suggest that M31 is about 2 million light years away. That is, the light has taken 2 million years from when it left M31 to reach us. You are seeing the object as it was 2 million years ago! This makes it the most distant object visible to the naked eye. However, to see it with the naked eye you have to be (a) in a dark place away from light pollution and (b) know where to look. Here is a finding chart to find M31.

How to find Messier 31


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Messier 31 is best seen in the late Summer and Autumn sky. The easiest way to find M31 is to first find the Square of Pegasus, an easily located asterism which forms part of the constellation Pegasus. The bright bluish-white star in the North-Eastern corner (top left as seen from the Northern Hemisphere) is called Alpheratz. If one moves to the left (East) from this star the Andromeda constellation forms a thin v-shape. The lower branch of the “v” has a bright red star called Mirach (the second star along from Alpheratz). From Mirach go up to pi-Andromodae (quite a faint star), and in a straight line an equal distance from Mirach to pi-Andromodae is where you should find the Andromeda galaxy.

However, even if you are in a dark place it is extremely difficult to see, and with your naked eye you will probably only be able to see it using your peripheral vision, the part of the eye which is more sensitive to low light levels. If you are having difficulty finding it with your eye, it is quite a good idea to find it with a low powered magnification in a telescope, and then use this to help you know what you are looking for with just your un-aided eye.

What you will see with your eye, whether un-aided or through a telescope, will probably not look much like the photograph above. Rather, you will see a fuzzy blob. Remember, the word “nebula” comes from the Latin word for cloud, and Messier 31 looks like a fuzzy cloud. Until astronomers gained a better understanding of objects in the Messier catalogue, a galaxy like Messier 31 was referred to as a “nebula” just as Messier 42, the Orion nebula, had the same term attached to it, even though we now know they are very different kinds of objects.

Messier 31 in ultra-violet light and infrared light

Although the spiral arms are not very prominent when we take an image of M31 in visible light, its appearance is quite different when we observe it in e.g. ultra-violet light and infrared light. The uv image below was taken by a mission called Galex, and shows a prominent ring of stars emitting in the uv part of the spectrum. The nucleus of the galaxy is also seen to be emitting quite strongly in uv light. It is only the hotter, younger stars which emit in the uv, cooler and older stars like our Sun are just not hot enough to emit much in ultra-violet light. The hot young stars are found in the spiral arms of spiral galaxies, in fact it is the presence of these hot young stars which give such galaxies their spiral appearance.


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A prominent ring is also visible in this infrared image of M31 shown below. This image was taken with the Herschel Space Observatory, and was taken at a wavelength of 250 microns. What we are seeing in this image is not radiation from stars, but rather the emission from interstellar dust. At 250 microns we can see both warm dust heated by hot young stars and cooler dust that is heated by the more general stellar population. That is why you see you reasonable but not exact correspondence between the uv image and the infrared image.


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Studying objects at different wavelengths has allowed to learn a lot more about them than if we just observe them with visible light. Nowadays, astronomers have the facilities to observe astrophysical objects from the X-ray to the radio part of the spectrum, and through doing this gain a much deeper understanding of the object than if it were just observed in visible light alone.

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