# How can the Andromeda Galaxy alone have more mass than the Local Group?

I was reading about the Local Group, and it is stated that the total mass of the group is $1.29±0.14 × 10^{12}$ Solar masses. The number actually felt a bit low to me because I know the Milky Way galaxy alone is close to a trillion solar masses, so I checked the Andromeda Galaxy's mass to find it $1.5 × 10^{12}$ Solar masses.

The Local Group also contains more than 54 galaxies, with the Milky Way and Andromeda being the most massive, so by only summing the masses of the Milky Way and Andromeda, we already exceed the mass estimated for the group. Not to mention the other 52 less massive galaxies.

So how could this be correct ? Does it have something to do with the calculation of the dark matter mass ?

• Note the "dubious" claim next to that cited valued for the Local Group. This paper claims $2.4\pm2\times10^{12}\,M_\odot$ for the LG (huge error bars!). There are others that cite closer to $2\times10^{12}\,M_\odot$, if you do an ADS/arXiv search for them. Nov 26, 2014 at 2:43

## 1 Answer

It is difficult to estimate the masses of either galaxies or clusters of galaxies, and it depends on what source you consult. You can get different values from different sources, for instance in this reference: the local group is estimated to have $5.27\times 10^{12} M_\odot$ (which does include dark matter). I didn't have access to the wikipedia source.

• @ Jonny, please be kind, what is this "Local group"? I never herd of. With thanks in advance. Dec 6, 2014 at 0:36
• @Sofia I am not a native speaker, so I am not sure what you meant in the comment, did I something wrong or is it a joke? :)
– user65081
Dec 6, 2014 at 0:47
• @ Jonny, I apologize, I am looking at your answer from 26 Nov. You say there, I quote, "the local group is estimated to have 5.27×10^12 $M_⊙$." This is what I ask, what is the local group. (Neither my mother-tongue is English.) Dec 6, 2014 at 2:37
• @Sofia The Local group is the name of the galactic cluster to which the Milky Way belongs, the two larger galaxies in the cluster are Andromeda and the milky way, the rest are smaler ones but there are quite a few, I do not remember the exact number.
– user65081
Dec 6, 2014 at 2:40
• @ Jonny, the star closest to us is Alpha-Centauri. So, the constellation Centaurus belongs to our Galaxy, doesn't it? I apologize for flooding you with questions. Dec 6, 2014 at 3:25