# Ob Jective In This Lab You Will Learn That We Live In The Milky Way Galaxy Our Solar 2264657

Ob jective In this lab, you will learn that we live in the Milky Way Galaxy. Our solar system and all the stars you can see with your own eyes are in this galaxy. You will learn where we are in the galaxy, what it looks like, and how old it is. II. Exercises Our Milky Way Galaxy is made up of a disk (which you see in the sky as the band of the Milky Way), a bulge (a large clump of stars surrounding the galactic center), and a halo (a larger, spherical cloud of stars that surrounds the entire galaxy). The halo is much larger than the bulge. Our Milky Way Galaxy is made up of mostly stars, gas, and dust. The dust blocks out light from distant stars, and makes it hard to see a lot of the galaxy, especially the bulge and parts of the disk. Now, we know that we live in a galaxy that has a disk-like shape. But, where in the disk do we live? Are we at the center, or o on one side? In the early 1920’s, an American astronomer named Harlow Shapley studied this question. He found that there were clusters of stars, called globular clusters, that were spread around the center of the galaxy evenly in a spherical distribution in the halo. Harlow Shapley realized that depending on where you are in the galaxy, the pattern of globular clusters will look di erent. Let’s try to perform Harlow Shapley’s experiment and nd the center of the galaxy from the globular clusters. We must start, however, with a discussion of the coordinates involved. Skip the next text section if you are already familiar with right ascension and declination. SKY COORDINATES The celestial sphere is divided up in a system like the longitude-latitude” system on the Earth’s surface. The latitude of an ob ject in the sky is called the declination; an ob ject (like Polaris) whose position is over the Earth’s north pole has a declination of +90 degrees; an ob ject over the south pole is at 90 degrees. Ob jects on the celestial equator have a declination of zero. Longitude on the sky is called right ascension. On Earth, the line de ning zero degrees longitude is fairly arbitrary | it’s the circle that goes from the north pole to the south pole which passes through Greenwich, England. Right ascension (RA) on the sky also has an arbitrary zero point | it’s a circle from the north celestial pole (Polaris) to the south celestial pole that passes through one of the points where the ecliptic crosses the celestial equator (in the constellation of Pisces). The only di erence between longitude on earth and right ascension on the sky is that longitude is usually measured in degrees (from 0 to 360 degrees), while right ascension is measured in HOURS (from 0 to 24 hours). The point to the above discussion is that the coordinate system astronomers have for ob jects in the sky is similar to the coordinate system that map makers have on earth. Just as every location on earth has a longitude and latitude, every ob ject in the sky has a right ascension and declination

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