Teach Astronomy - Galaxy Evolution скачать в хорошем качестве

Teach Astronomy - Galaxy Evolution

http://www.teachastronomy.com/ Galaxies appear unchanging from year to year, and on a human timescale they do not change their properties. But on cosmic timescales of billions of years galaxies must evolve. Any stellar population as it ages will become dimmer and redder. However, this does not mean that one type of galaxy changes into another. For example, spirals, although they're mostly composed of young blue stars, do not change into elliptical galaxies which are primarily composed of older, redder stars. This is because the angular momentum and stellar densities of spirals and ellipticals are completely different, so an isolated elliptical cannot be formed from an isolated spiral galaxy. In general galaxies do not behave as if they are closed boxes. For example, much of the gas that is lost from stars in the late phases of evolution of elliptical galaxies must be removed from the stellar systems, usually because ellipticals live in clusters of galaxies, whereas in spiral galaxies the gas is recycled into the interstellar medium where it can fuel subsequent generations of stars. There is however insufficient gas to explain the star formation rate of spirals, so some gas must come in from outside the galaxy.