brian boru wrote:I have seen a number estimates on the size of M31 on this site and others, and there seem to be widely varying figures given. Some claim that M31 has about 200 billion solar masses while other sources claim that it might have as many as a trillion.
Also, distances given to various galaxies seem to differ wildly. For example, on the NASA site the distance given to M104, Sombrero, varies from 28 mly to 60 mly.
Can anyone clarify these figures a bit?
It's quite difficult to answer the questions here in a general sense ...
For starters, all masses and distances are only estimates, and they are all derived from application of one (or more) methods. Each method relies on (a.k.a. is founded on) a combination of (hundreds or thousands) of observations and theoretical constructs.
In articles in the popular press, or even PRs from observatories, the basis for the quoted masses and distances may not be stated, or even hinted at. In published papers such bases should be clearly spelled out, though occasionally they are not unambiguous.
In online databases, there should be a section which explains where the estimates come from, or how they were obtained. Some databases are 'dumb' - they simply pass on whatever estimate is given in the source; some make an attempt to provide consistent estimates (hyperLeda, for example).
While it is very difficult to say anything definitive about mass estimates in general, my guess is that the biggest source of confusion and possible inconsistency is whether the estimate is that of the stars (and gas and dust) in the galaxy, or the total mass (including the cold dark matter halo).
Distance estimates are, in general, easier to understand: if a particular galaxy has been studied in terms of one or more of the standard methods (e.g. Type Ia supernova standard candle, or Cepheids), then the distance estimate will be that derived from those methods. If the galaxy is in a cluster some of whose other members have had distances estimated (by a standard method), then sometimes the cluster distance can be used. Of course, the further away the cluster, the better; conversely, this doesn't work too well for Virgo cluster galaxies! Finally, many galaxies' redshifts have been observed spectroscopically, and so a 'Hubble distance' estimate can be easily given, using the Hubble relationship. This has several, well-known, limitations.
Do you have some particular examples you'd like to look into in more detail?