Figure 1.3 shows a typical egg-shaped elliptical galaxy. The smallest elliptical galaxies are as small as some globular clusters. Giant elliptical galaxies, on the other hand, can contain over a trillion stars. Elliptical galaxies are reddish to yellowish in color because they contain mostly old stars. Most elliptical galaxies contain very little gas and dust because the gas and dust have already formed into stars. However, some elliptical galaxies, such as the one shown in Figure 1.4, contain lots of dust. Why might some elliptical galaxies contain dust?
Spiral galaxies spin, so they appear as a rotating disk of stars and dust, with a bulge in the middle, like the Sombrero Galaxy shown in Figure 1.2. Several arms spiral outward in the Pinwheel Galaxy (seen in Figure 1.2) and are appropriately called spiral arms. Spiral galaxies have lots of gas and dust and lots of young stars. The Andromeda Galaxy is a large spiral galaxy similar to the Milky Way. (a) The Sombrero Galaxy is a spiral galaxy that we see from the side so the disk and central bulge are visible. (b) The Pinwheel Galaxy is a spiral galaxy that we see face-on so we can see the spiral arms. Because they contain lots of young stars, spiral arms tend to be blue.
Galaxies are the biggest groups of stars and can contain anywhere from a few million stars to many billions of stars. Every star that is visible in the night sky is part of the Milky Way Galaxy. To the naked eye, the closest major galaxy the Andromeda Galaxy, shown in Figure 1.1 looks like only a dim, fuzzy spot. But that fuzzy spot contains one trillion 1,000,000,000,000 stars! Galaxies are divided into three types according to shape: spiral galaxies, elliptical galaxies, and irregular galaxies.
Dwarf galaxies are small galaxies containing only a few million to a few billion stars. Dwarf galaxies are the most common type in the universe. However, because they are relatively small and dim, we dont see as many dwarf galaxies from Earth. Most dwarf galaxies are irregular in shape. However, there are also dwarf elliptical galaxies and dwarf spiral galaxies. Look back at the picture of the elliptical galaxy. In the figure, you can see two dwarf elliptical galaxies that are companions to the Andromeda Galaxy. One is a bright sphere to the left of center, and the other is a long ellipse below and to the right of center. Dwarf galaxies are often found near larger galaxies. They sometimes collide with and merge into their larger neighbors. Click image to the left or use the URL below. URL: Click image to the left or use the URL below. URL:
Is the galaxy in Figure 1.5 a spiral galaxy or an elliptical galaxy? It is neither one! Galaxies that are not clearly elliptical galaxies or spiral galaxies are irregular galaxies. How might an irregular galaxy form? Most irregular galaxies were once spiral or elliptical galaxies that were then deformed either by gravitational attraction to a larger galaxy or by a collision with another galaxy. This galaxy, called NGC 1427A, has nei- ther a spiral nor an elliptical shape.
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galaxies can contain billions or trillions of stars.
--> a) true b) false
earth is a part of the ___.
a) sombrero galaxy --> b) milky way galaxy c) andromeda galaxy d) pinwheel galaxy
the closest galaxy to us is the __.
a) sombrero galaxy b) milky way galaxy --> c) andromeda galaxy d) pinwheel galaxy
which of these is not a type of galaxy?
a) spiral b) elliptical c) irregular --> d) circular
what is true about spiral galaxies?
a) they appear as a rotating disk of stars. b) they appear to have a bulge in the middle. c) they have spiral arms. --> d) all of the above.
the arms in a spiral galaxy tend to be _ because they contain _.
a) red; old stars b) yellow; middle-aged stars --> c) blue; young stars d) all colors; all age stars
irregular galaxies may be deformed by gravitational attraction to another galaxy.
--> a) true b) false
an elliptical galaxy
a) does not contain dust because the dust has come together to form stars. b) may contain dust if it has collided with another galaxy of similar size. c) is likely to be reddish to yellowish because it contains mostly old stars. --> d) all of the above.
what is true about dwarf galaxies?
--> a) they are relatively small and dim. b) they are all irregular in shape. c) they may merge with other galaxies. d) all of the above.
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