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:Comment about this article, ask questions, or add new information about this topic: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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with stars, then every direction you looked would eventually end on
the surface of a star, and the whole sky would be as bright as the
surface of the Sun.
Why would anyone assume this? Certainly, we have directions where we look that are dark because something that does not emit light (is not a star) is between us and the light. A close example is in our own solar system. When we look at the Sun (a star) during a solar eclipse the Moon blocks the light. When we look at the inner planets of our solar system (Mercury and Venus) as they pass between us and the Sun, do we not get the same effect, i.e. in the direction of the planet we see no light from the Sun? Those planets simply look like dark spots on the Sun.
Olbers' paradox seems to assume that only stars exist in the universe, but what about the planets? Aren't there more planets than stars, thus more obstructions to light than sources of light?
What may be more interesting is why can we see certain stars seemingly continuously. Are there no planets or other obstructions between them and us? Or is the twinkle in stars just caused by the movement of obstructions across the path of light between the stars and us? I was always told the twinkle defines a star while the steady light reflected by our planets defines a planet. Is that because the planets of our solar system don't have the obstructions between Earth and them to cause a twinkle effect?
9-14-2024 KP