Physical Science 8th grade (1).pdf

Irregular galaxies exhibit peculiar shapes and do not

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Irregular galaxies exhibit peculiar shapes and do not appear to rotate like those galaxies of other shapes. Figure 17.3 shows examples of some galaxy shapes. Galaxies change shape over time The shapes of galaxies change over time. It is impossible to actually see the changes in a single galaxy, since it takes hundreds of millions of years. However, by looking at many galaxies, astronomers can see similar galaxies at different times in their histories. This observational data has allowed astronomers to develop computer based models which calculate how a galaxy changes over hundreds of millions of years. It is now believed that the barred spiral form is just one phase of a regular spiral. Computer simulations show how the “bar” forms and disappears repeatedly as a spiral galaxy rotates. Figure 17.3: Some representative galaxy shapes.
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358 U NIT 6 A STRONOMY Figure 17.4: The Andromeda galaxy is an elliptical galaxy in our local group. Figure 17.5: Two galaxies that are near to colliding. Distances between galaxies Galaxies are a million times farther away than stars The distances between stars are 10,000 times greater than the distances between planets. The distances between galaxies are a million times greater than the distances between stars . For example, the distance from Earth to the nearest star is 4.3 light years, but from Earth to the Whirlpool galaxy is over 30 million light years. The local group of galaxies The Milky Way belongs to a group of about 30 galaxies called the local group. This group includes the Large Magellanic Cloud (179,000 light years) and the Small Magellanic Cloud (210,000 ly). These Magellanic Clouds are small, irregular galaxies of less than 100,000 stars. The local group also includes Andromeda, an elliptical galaxy 2.9 million light years away (Figure 17.4). Galactic collisions Galaxies move through space singly and in groups. Galaxies even collide with each other in slow dances of stars that take millions of years to complete (Figure 17.5) Determining the distance to nearby galaxies Figuring out the distance between galaxies is one of the more difficult tasks in astronomy. A faint (low brightness) object in the night sky could be a dim object that is relatively nearby or a bright object that is far, far away. The most reliable method for estimating the distance to a galaxy is to find a star whose luminosity is known. If the luminosity is known, the inverse square law can be used to find the distance from the observed brightness. Distant galaxies This method works for the closest galaxies. However, the vast majority of galaxies are too far away to see single stars even with the best telescopes. Beyond 150 million light years, astronomers compare size and type with closer galaxies to estimate the luminosity of the farther ones. This method is not as accurate and, consequently, the distances to far galaxies are known only to within a factor of two.
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359 17.1 G ALAXIES C HAPTER 17: G ALAXIES AND THE U NIVERSE The central black hole The center of the galaxy Since we are located in the outer part of the galaxy, dust between the
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