REM%IMSSS@SU-AI.ARPA (Robert Elton Maas) (03/10/86)
CW> Date: 8 Mar 86 03:52:35 GMT CW> From: hplabs!hao!seismo!umcp-cs!mangoe@ucbvax.berkeley.edu CW> Subject: Re: Size of the Galaxy CW> If memory serves me, current numbers list Milky Way as one of the biggest CW> single galaxies we know of. Is there an astronomer in the house? I have a completely different impression, after reading Sky&Telescope and other journals for many years. The giant elliptical galaxies such as the one in the center of the Virgo cluster (is that M85?) are much larger than the Milky Way. Even our twin the Andromeda galaxy (M31) is believed to be slightly larger than ours, although scales of distance aren't accurate enough to be sure. But considering spiral galaxies only, excluding elliptical galaxies, ours and M31 are typical of full-sized galaxies, not exceeded greatly by others. (But I'm not sure of that part; perhaps Diana Hadley or Lynn.es@Xerox will help me out?) (-: perhaps you were confusing apparent and actual diameter? Milky Way is viewed from very near, thus appears larger than any others viewed from afar :-)
Lynn.ES@XEROX.COM (03/15/86)
Message from ...!mangoe of 8 Mar 86: >If memory serves me, current numbers list Milky Way as one of the biggest >single galaxies we know of. Is there an astronomer in the house? I am not a real astronomer, but I own lots of books on the subject. Besides, REM thinks I can answer this, so why not? The first counter-example I thought of is the Andromeda galaxy (M31). It is a spiral and is usually given as about 180,000 light years in diameter to the dimmest ends of the arms that we can detect (other measurements to the brighter parts of the arms or to the surrounding globulars give smaller or larger numbers. Also very old figures give vastly different sizes, but astronomers think they know its distance, and therefore its size, fairly accurately now). The Milky Way cannot be measured the same way, since we are on the inside, but the best number seems to be 100,000 ly across the arms, so the Andromeda is much larger. Burnham's book says that the Andromeda is one of the largest and one of the most luminous galaxies known, and that most spirals are in the 50,000 ly area. So the Milky Way is big as spirals go, but not among the biggest. The recent discoveries that there is some matter way outside the arms of the Milky Way doesn't really make it suddenly larger than all the other galaxies. It just means there is likely a halo of junk too difficult to detect around many galaxies; so for comparing galaxy sizes we will have to stick with the visible arms. M87 always seems to be referred to as a "giant elliptical" galaxy. Burnham's says it is one of the largest ellipticals of the Virgo cluster, a group of thousands of galaxies that are near enough, in cosmic terms, to be well surveyed. However, diameters of ellipticals are smaller than spirals. So a giant elliptical is lucky to be the diameter of an average spiral. M87 is probably 35,000 ly across, though certain reputable astronomers have put the whole Virgo cluster much farther than the most accepted distance (of 40 million ly), so M87 could possibly be up to 60,000 ly across. M87 is, however, about twice as massive as Andromeda (which is probably twice as massive as the Milky Way), and is yet more luminous than the bright Andromeda. I couldn't find any figures for luminosity of the Milky Way, probably because that is hard to figure when you are inside the dust lanes of it. We can guess that it is proportionately dimmer than the Andromeda. So the Milky Way does not qualify as one of the largest by mass, and probably not by luminosity. Summary: the Milky Way is a good galaxy, but not a great galaxy. :-) /Don Lynn