Top Document: [sci.astro] Galaxies (Astronomy Frequently Asked Questions) (8/9) Previous Document: H.01 How many stars, galaxies, clusters, QSO's etc. in the Universe? Next Document: H.01.3 How many globular clusters in the Milky Way? See reader questions & answers on this topic! - Help others by sharing your knowledge A widely-distributed press release about the Hubble Deep Field observations, <URL:http://oposite.stsci.edu/pubinfo/PR/96/01.html>, reported the discovery of a vast number of new galaxies. The existence of many galaxies too faint to be hitherto detected was no surprise, and calculations of the number of galaxies in the observable Universe and searches for how they change with cosmic time must always allow for the ones we can't detect, through some combination of intrinsic faintness and great distance. What was of great interest in the Hubble Deep field (and similar) data was just how any faint galaxies were detected and what their colors and forms are. Depending on just what level of statistical error can be tolerated, catalogs of galaxies in the Hubble Deep Field list about 3000. This field covers an area of sky of only about 0.04 degrees on a side, meaning that we would need 27,000,000 such patches to cover the whole sky. Ignoring such factors as absorption by dust in our own Galaxy, which make it harder to see outside in some directions, the Hubble telescope is capable of detecting about 80 billion galaxies (although not all of these within the foreseeable future!). In fact, there must be many more than this, even within the observable Universe, since the most common kind of galaxy in our own neighborhood is the faint dwarfs which are difficult enough to see nearby, much less at large cosmological distances. For example, in our own local group, there are 3 or 4 giant galaxies which would be detectable at a billion light-years or more (Andromeda, the Milky Way, the Pinwheel in Triangulum, and maybe the Large Magellanic Cloud). However, there are at least another 20 faint members, which would be difficult to find at 100 million light-years, much less the billions of light years to which the brightest galaxies can be seen. User Contributions:Top Document: [sci.astro] Galaxies (Astronomy Frequently Asked Questions) (8/9) Previous Document: H.01 How many stars, galaxies, clusters, QSO's etc. in the Universe? Next Document: H.01.3 How many globular clusters in the Milky Way? 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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