dipper@utastro.UUCP (Debbie Byrd) (10/12/85)
Our Milky Way galaxy is surrounded by a halo. More on what's in the halo -- when we come back. October 12 The Halo of the Milky Way The Milky Way galaxy contains our sun -- and some hundred billion other stars. The galaxy is generally shaped like a pancake -- round and flat -- with a great bulge of stars in its center. The Milky Way also has a halo -- a thin scattering of stars around the flat disk of the galaxy. The halo of the Milky Way contains the galaxy's oldest stars -- some traveling alone -- some in round, compact globular star clusters. Unlike the disk of the galaxy, the halo lacks the gas and dust that are the raw materials for new stars. It's thought that the halo of the Milky Way is a fossil remnant leftover from the formation of the galaxy. When the Milky Way formed -- more than 10 billion years ago -- it probably started out as a more or less round ball of gas, consisting mostly of hydrogen and helium. The ball began to contract under the influence of its own gravity. As it shrank in volume, it fragmented into individual clouds from which the first stars were born. The stars that now make up the halo of the galaxy still lie in that more or less round volume of space surrounding the flat disk of the galaxy. Meanwhile, the rest of the galaxy continued to contract -- to collapse finally into a rotating disk. Today, most of the galaxy -- including our sun -- lies within this rotating disk. But the halo of stars remains outside the disk -- a skeleton left behind from the early evolution of the galaxy. Script by Deborah Byrd. (c) Copyright 1984, 1985 McDonald Observatory, University of Texas at Austin
wmartin@brl-tgr.ARPA (Will Martin ) (10/15/85)
In article <38@utastro.UUCP> dipper@utastro.UUCP (Debbie Byrd) writes: [Extracts:] > >The Milky Way also has a halo -- a thin scattering of stars around the >flat disk of the galaxy. The halo of the Milky Way contains the >galaxy's oldest stars -- some traveling alone -- some in round, compact >globular star clusters. Unlike the disk of the galaxy, the halo lacks ^^^^^ >the gas and dust that are the raw materials for new stars. > >The stars that now make up the halo of the galaxy still lie in that >more or less round volume of space surrounding the flat disk of the >galaxy. Meanwhile, the rest of the galaxy continued to contract -- to >collapse finally into a rotating disk. Today, most of the galaxy -- ^^^^ >including our sun -- lies within this rotating disk. But the halo of >stars remains outside the disk -- a skeleton left behind from the early >evolution of the galaxy. I added the "^^^" pointers above. I am in the midst of reading Michael Disney's THE HIDDEN UNIVERSE, in which the emphasis is on the "missing mass" problem, and, according to this book, our galaxy, and every other large galaxy, is surrounded by a quite massive halo of invisible material -- I think the figure given is that this invisible matter has to be from 10-20 times the mass of the visible material of the galaxy -- which is necessary to explain the form and stability of the various galaxies and clusters. So this would disagree with the points above, wherin the "halo" is not made up of gas and dust, and also where "most" of the galaxy is defined as the inside (visible) matter. So, which is right? Or is this a currently-debated topic in astronomy, and things are not as clearly agreed-upon as the impression I got from the Disney book leads me to think? Will
ethan@utastro.UUCP (Ethan Vishniac) (10/16/85)
> > Or is this a currently-debated topic in astronomy, > and things are not as clearly agreed-upon as the impression I got from > the Disney book leads me to think? > > Will Bingo. Actually, the massive halo need only weigh about as much as the visible galaxy to stabilize the disk. Virial theorem measurements of galaxy masses have led people to propose much larger halo masses. There is some controversy over exactly how massive the halo needs to be to explain these results. If the halo is very massive then it is not composed of stars. This leaves open the possibility that it is composed of black holes or white dwarfs. It is not likely to be dust or monotomic gas. Could be molecular hydrogen but its formation would be a mystery. Could be Jupiters (i.e. generic gas giant objects). Could be Weakly Interacting Massive Particles (acronym is widely used). -- "Superior firepower is an Ethan Vishniac important asset when {charm,ut-sally,ut-ngp,noao}!utastro!ethan entering into ethan@astro.UTEXAS.EDU negotiations" Department of Astronomy University of Texas
wls@astrovax.UUCP (William L. Sebok) (10/19/85)
In article <2140@brl-tgr.ARPA> wmartin@brl-bmd.UUCP writes: >... So this would disagree with the points above, wherin the "halo" is not >made up of gas and dust, and also where "most" of the galaxy is defined >as the inside (visible) matter. > >So, which is right? Or is this a currently-debated topic in astronomy, >and things are not as clearly agreed-upon as the impression I got from >the Disney book leads me to think? Yes it is a currently debated topic, although I am getting the impression that a consensus may be forming about the factor of 10 to 20 of invisible / invisible matter. A meeting of the International Astronomical Union (IAU) was held last June here in Princeton to discuss this subject. Note that the amount of dark matter around a galaxy or inside a cluster of galaxies may be a different issue from the amount of dark matter in inter-galactic space. There are some theories which have it that the density inside the "voids" never got high enough to condense galaxies. These voids would then be full of ordinary matter that never condensed to galaxies. To muddy the waters some more, my own feeling is that it may be likely that there is more than one type of dark matter out there, each type possibly with its own properties. -- Bill Sebok Princeton University, Astrophysics {allegra,akgua,cbosgd,decvax,ihnp4,noao,philabs,princeton,vax135}!astrovax!wls
grl@charm.UUCP (George Lake) (10/23/85)
Part of the confusion is that the word halo does double duty. Some astronomers use the word to talk about the tenous outer reaches of the "bulge", which is the lump you see in the center of most spiral galaxies. In this sense "halo" means the oldest stars that you see far from the center. "Halo" is also the massive dark halo. Stardate refered to one usage, Mike Disney to the other.