Top Document: [sci.astro] Galaxies (Astronomy Frequently Asked Questions) (8/9) Previous Document: H.00 Galaxies, Clusters, and Quasars (QSOs) Next Document: H.01.2 How many galaxies in the Universe? See reader questions & answers on this topic! - Help others by sharing your knowledge The various parts of this question will be considered separately. Also, rather consider how many stars there are in the Universe, we'll consider how many stars there are in the Milky Way. The number of stars in the Universe can be estimated by multiplying the number of stars in the Milky Way by the number of galaxies in the Universe. ------------------------------ Subject H.01.1 How many stars are there in the Milky Way? Author: William Keel <keel@bildad.astr.ua.edu> My standard answer in introductory astronomy classes is "about as many as the number of hamburgers sold by McDonald's." Being more precise requires an extrapolation, because we can't see all the individual stars in the Milky Way for two reasons---distance and dust absorption. Both factors make stars appear dimmer. Observations at visible wavelengths are limited to a region of (more or less) 5000 light-years radius about the Sun, with a few windows in the intervening dust giving us glimpses of more distant areas (especially near the Galactic center). Our map of the Galaxy gets correspondingly more sketchy with distance. Guided somewhat by observations of other spiral galaxies, we think that the overall run of star density with radius is fairly well known. Getting a total stellar head count is more of a problem, because the stars that we can see to the greatest distances are also the rarest. Measurements of the relative numbers of stars with different absolute brightness (known in the trade as the luminosity function) shows that, for example, for every Sun-like star there are about 200 faint red M dwarfs. These are so faint that the closest, Proxima Centauri, despite being closer to the Sun than any other (known) star, takes very large binoculars or a telescope to find. So, to get the total stellar population in the Milky Way, we must take the number of luminous stars that we can see at large distances and assume that we know how many fainter stars go along with them. Recent numbers give about 400,000,000,000 (400 billion) stars, but a 50% error either way is quite plausible. Much of the interest in "brown dwarfs" stems from a similar issue---a huge number of brown dwarfs would not change how bright the Galaxy appears (at visible wavelengths), but would change its total mass quite substantially. Oddly enough, within a particular region, we probably know the total mass and luminosity rather more accurately than we do just how many stars are producing that light (since the most common stars are by far the dimmest). User Contributions:Top Document: [sci.astro] Galaxies (Astronomy Frequently Asked Questions) (8/9) Previous Document: H.00 Galaxies, Clusters, and Quasars (QSOs) Next Document: H.01.2 How many galaxies in the Universe? Part0 - Part1 - Part2 - Part3 - Part4 - Part5 - Part6 - Part7 - Part8 - Single Page [ Usenet FAQs | Web FAQs | Documents | RFC Index ] Send corrections/additions to the FAQ Maintainer: jlazio@patriot.net
Last Update March 27 2014 @ 02:11 PM
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