Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Rishya on 11/11/2009 19:30:02
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Rishya asked the Naked Scientists:
What are quasars and galaxies?
What do you think?
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Rishya asked the Naked Scientists:
What are quasars and galaxies?
What do you think?
A quasar (or Quasi-Stellar Radio Source) occurs when gas near a black hole goes into the black hole (at very high speed). When the gas gets close to the black hole, the gas heats up because of friction. Therefore, the gas glows very brightly, and this light is visible on the other side of the Universe. It is often brighter than the whole galaxy that quasar is in. Quasars were discovered in 1960 and are still actively studied by astronomers today.
Astronomers now think that when a galaxy has a quasar, the quasar changes the galaxy. Gas and dust from the galaxy falls onto the quasar, and the bright quasar heats up gas in the galaxy. This stops new stars from forming in the galaxy, so many of the elliptical galaxies we see in the Universe now may have once had a quasar in their centers. When the gas and dust stop falling onto the quasar, it stops being so bright and the black hole becomes very hard to see.
A galaxy is a group of many stars, along with gas, dust, and dark matter. Gravity holds galaxies together. Everything in a galaxy moves around a centre. The name galaxy is taken from the Greek word Galaxia meaning milky, a reference to the our own galaxy, the Milky Way.
There are three main types of galaxies: ellipticals, spirals, and irregulars. All galaxies exist inside the universe. There are an estimated 100 billion galaxies within distance we can see or the Observable Universe. Each galaxy contains roughly 100,000 to 1 trillion stars. This makes the number of stars in the universe more than every grain of sand on every beach on Earth.
We can describe galaxies by the number of stars they have. The galaxy we call the Small Magellanic Cloud has about 1 billion stars in it. This is a small galaxy in comparison to most, but it is not the smallest: Leo I and Leo II have about 1 million stars in them, and the Draco System has "only" a few hundred thousand stars. Astronomers call these galaxies "dwarf galaxies."
In general, smaller groups are called "star clusters," not galaxies (a "cluster" is a group of something, like a cluster of grapes.) The largest star cluster, a globular cluster called Messier 15 has about 6 million stars, so we see that for small galaxies, there is a blurring together of what we mean by a galaxy and a large star cluster.
In addition to their mass and numbers of stars, a galaxy is a collection of stars and gas which move through the universe independently of the Milky Way. Globular clusters are roundish swarms of stars that orbit the Milky Way, while the Leo and Draco Systems seem to be independent collections of stars.
Many galaxies also continue to form new generations of stars. The Milky Way, and all spiral shaped galaxies like it (see above image of NGC 2997), produce new stars at a rate of one or two stars per year. These stars are formed in the vast interstellar clouds that account for about 1% to 10% of the mass of these galaxies. Globular star clusters, on the other hand, are not currently forming stars because this activity happened billions of years ago and then stopped once all of the gas and dust clouds were used up.
It is hard to say if some of the galaxies we see are what we think they are, because many galaxies are very far away and almost too faint to see. Another problem is that we can only see galaxies from one direction, so we cannot see the same galaxy from the top and the side at the same time.
Thank ewe wikipedia.
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This is a picture of a spiral galaxy :
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nice eh ?