Astronomy — The Universe
The Universe
- Our galaxy is only one of billions of galaxies in the Universe, with each galaxy containing billions of stars.
- The average distance between galaxies is 1 million light-years.
- Scientists predict the ‘Big Bang’ occurred 20 billion years ago.
- Galactic morphology:
- Elliptical galaxies — almost no visible dust, no hot bright stars, probably a single generation of stars. E0 denotes the least- elliptical of the elliptical galaxies, with E7 denoting the most-elliptical of the elliptical galaxies.
- Spiral galaxies — disk component, gas, dust, and hot new stars.
- Irregular galaxies — large clouds of dust and gas mixed with old and new stars.
- To measure distances to galaxies, we use megaparsecs (Mpc).
- One megaparsec = 1 million parsecs = 3,260,000 light-years = 2 x 1019 miles.
- M101 is 7 Mpc away.
- The Large Magellanic Cloud is 50 Kpc away.
- The nearest large galaxy (Andromeda Galaxy) is 0.66 Mpc away (2,150,000 light years)
- Light that we see coming from the Andromeda galaxy has traveled for 2.15 million years to reach us.
- If the entire Andromeda galaxy was visible in the night sky, it would span a distance 10-times the diameter of the full moon.
- The largest galaxies are 5-times larger than our Milky Way galaxy.
- The Hubble Law: A galaxy’s velocity of recession is equal to a constant (α) times its distance from us. Thus, the more distant a galaxy is, the faster it is receding from us. The universe is expanding. In other words, the larger the distance between 2 galaxies, the faster they are receding from each other.
- Our galaxy is a member of a galaxy-cluster containing over 20 galaxies, and over 2700 other clusters have been catalogued within 4 million light-years of our galaxy.
- Rich galaxy clusters contain greater than 1000 galaxies, mostly elliptical. Poor galaxy clusters contain less than 1000 galaxies, mostly irregular.
- Quasars (quasi-stellar objects) are probably very distant galaxies. They are incredibly far away (109 light years). They have 10 to 1000 times the luminosity of a large galaxy. Some quasars are receding at about 90% of the speed of light. The light now arriving from the most distant quasars left when the universe was only one-fourth its current age.
- A black hole with the mass of a billion suns has been found in Galaxy NGC3115 in the constellation Sextans, 30 million light-years away.
- Big Bang Theory: the universe began a finite time ago in a high temperature, high density state. The expansion of the universe since that time has allowed matter to cool and from galaxies, and the universe continues to expand.
- Cosmology: the study of the universe as a whole.
- Basic assumptions of the universe:
- Homogeneity: matter is uniformly spread throughout space (on the large scale)
- Isotropy: the universe looks the same in every direction
- Universality: the physical laws on Earth apply everywhere in the universe
- The cosmological principle: any observer in the universe sees the same general features of the universe. If we accept this premise, then the universe has no center or edge.
- Basic assumptions of the universe:
- Up to 95% of the universe consists of a matter that doesn’t emit radiation that can be detected.
- Galaxies occur in clusters. Our own galaxy is a member of the Local Group, a small cluster. A galaxy in a rich cluster may collide with other galaxies more often that a galaxy in a poor cluster, and such collisions can force a galaxy to form new stars and use up its gas and dust. Collisions can also strip gas out of a galaxy. This may explain why elliptical galaxies are more common in rich clusters than in poor clusters. Spiral galaxies may be star systems that haven’t experienced many collisions. Clusters seem to be organized into super-clusters and super-clusters into filaments.
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