Commonly Observed Properties of Galaxies
Some notable observed features of galaxy structure (including our own Milky Way) that astronomers wish to explain with galactic formation theories include (but are certainly not limited to) the following:
- Spiral galaxies and the galactic disk are quite thin, dense, and rotate very fast. The Milky Way disk is 100 times longer than it is thick.
- The majority of mass in galaxies is made up of dark matter, a substance which is not directly observable, and does not interact through any means except gravity.
- Halo stars are typically much older and have much lower metallicities (that is to say, they are almost exclusively composed of hydrogen and helium) than disk stars.
- Many disk galaxies have a puffed up outer disk (often called the "thick disk") that is composed of old stars.
- Globular clusters are typically old and metal-poor as well, but there are a few that are not nearly as metal-poor as most, and/or have some younger stars. Some stars in globular clusters appear to be as old as the universe itself (by entirely different measurement and analysis methods).
- High velocity clouds, clouds of neutral hydrogen are "raining" down on the galaxy, and presumably have been from the beginning (this would be the necessary source of a gas disk from which the disk stars formed).
- Galaxies come in a great variety of shapes and sizes (see the Hubble Sequence), from giant, featureless blobs of old stars (called elliptical galaxies) to thin disks with gas and stars arranged in highly-ordered spirals.
- The majority of giant galaxies contain a supermassive black hole in their centers, ranging in mass from millions to billions of times the mass of our Sun. The black hole mass is tied to properties of its host galaxy.
- Many of the properties of galaxies (including the galaxy color-magnitude diagram) indicate that there are fundamentally two types of galaxies. These groups divide into blue star-forming galaxies that are more like spiral types, and red nonstar forming galaxies that are more like elliptical galaxies.
Read more about this topic: Galaxy Formation And Evolution
Famous quotes containing the words commonly, observed, properties and/or galaxies:
“The opening of large tracts by the ice-cutters commonly causes a pond to break up earlier; for the water, agitated by the wind, even in cold weather, wears away the surrounding ice.”
—Henry David Thoreau (1817–1862)
“... the only way in which Mr. Brooke could be coerced into thinking of the right arguments at the right time was to be well plied with them till they took up all the room in his brain. But here there was the difficulty of finding room, so many things having been taken in beforehand. Mr. Brooke himself observed that his ideas stood rather in his way when he was speaking.”
—George Eliot [Mary Ann (or Marian)
“The reason why men enter into society, is the preservation of their property; and the end why they choose and authorize a legislative, is, that there may be laws made, and rules set, as guards and fences to the properties of all the members of the society: to limit the power, and moderate the dominion, of every part and member of the society.”
—John Locke (1632–1704)
“Don’t you see what’s at stake here? The ultimate aim of all science—to penetrate the unknown. Do you realize we know less about the earth we live on than about the stars and the galaxies of outer space? The greatest mystery is right here, right under our feet.”
—Walter Reisch (1903–1963)