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Galaxies

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Urooj Khan

on 9 January 2013

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Transcript of Galaxies

9.1 Galaxies The Discovery of
Galaxies The Shapes of
Galaxies Star Clusters The Diameter of the
Milky Way Galaxy Understanding the Milky Way Galaxy Sextans Dwarf Andromeda Galaxy If you ever looked at the night sky, away from the city lights, you may have seen the Milky Way Galaxy. On extremely clear nights, you can see the bright and dim areas in the dark sky. The Milky Way is a galaxy which includes our solar system. A galaxy is a huge collection of stars, planets, gas, and dust that is held together by gravity. The Milky Way comes into view as a hazy white band extending from the southern horizon and across the sky overhead. William Herschel was a talented British astronomer. He and his sister, Caroline Herschel were well-known for constructing and selling fine telescopes. One day, around 1780, he pointed one of his best telescopes at the Milky Way. He expected to see hazy white clouds but was surprised to see a huge number of stars. He discovered that the Milky Way is a gigantic system of stars known as a galaxy. Every star you see in the night sky is part of the Milky Way galaxy. A galaxy is formed when a large, slowly spinning cloud of gas, dust, and stars contract due to gravity. The Sun is estimated to contain 100 billion stars in the Milky Way. All the universal stars belong to a galaxy.
Galaxies come in a variety of different shapes and sizes. They are usually classified as either spiral or elliptical, according to their appearance. Galaxies that do not fit within these common classifications are considered as irregular galaxies. A spiral galaxy looks like a whirlpool if it is viewed from above. It has many long arms spiralling out from the centre core. When a spiral galaxy is viewed from the side, it looks like a plate with a bulge in the middle. Elliptical galaxies come in shapes of perfect spheres to a stretched out ellipse. Elliptical galaxies are composed of some of the oldest stars in the universe. Astronomers suppose that over half of all galaxies are elliptical. The largest galaxies in the universe are elliptical too. Irregular galaxies are galaxies that don’t have a normal shape. They consist of newly formed stars and old stars. Radio waves can travel through clouds; it can also travel through the dust and gas between the stars. Astronomers have been able to determine the Milky Way’s diameter and shape, by mapping the galaxy with radio waves. From the researches, astronomers have determined the diameter of the Milky Way galaxy is about 100 000 light-years and it has a disk-like shape. The Shape of the Milky Way Galaxy The Milky Way galaxy has a disk-like shape, with a central bulge of stars. Astronomers have concluded that the Milky Way is a spiral galaxy. Recent observations show the Milky Way has two main spiral arms and many small arms. The Local Group The Local Group is a group of galaxies which consists of about 40 galaxies. The Milky Way is part of the Local Group. The diameter of the Local Group is approximately 10 million light-years. The Milky Way and the Andromeda galaxies are the largest galaxies in the Local Group. The Milky Way Galaxy Pinwheel Galaxy During the early 20th century, American astronomer Harlow Shapley helped in the discovery of the Milky Way. While examining the Milky Way, he also studied star clusters. Star clusters are a bunch of stars held together by gravity. There are two different types of stars clusters. They are known as open clusters and globular clusters. Open clusters contain 50 to a 1000 stars and are along the main band of the Milky Way. Globular clusters contain 100,000 to 1,000,000 stars. They come in sight in the middle of the Milky Way. Open Cluster Globular Cluster Galaxies Stars occur in clusters within galaxies, and galaxies occur in clusters throughout the universe.
These clusters form superclusters, they contain approximately 4 to 25 clusters of galaxies and may span up to millions of light-years across the universe. Astronomers believe that there may be more than 125 billion galaxies, which are seemed to be organized in clusters. Galaxy Superclusters It took astronomers several years to be educated about the Milky Way galaxy. William Herschel started putting the facts together. By counting stars, Herschel figured out the estimated shape of the Milky Way galaxy. Herschel had in mind that the Milky Way is a huge disk of billions of stars flattened like a dinner plate, in which the Sun is enclosed. He also expected that the Sun might be at the centre of the Milky Way. 1. Which type of galaxy is the largest? Spiral
The Milky Way
Elliptical
Irregular 2. What is a globular cluster? A collection of 50 to 1000 stars.
A collection of 100 000 to a million stars.
A gigantic cluster of 4 to 25 clusters of galaxies. 3. What is a galaxy? A system of stars, planets, gas, and dust, held together by gravitational attraction. 4. What is the Local Group? The cluster of galaxies which consists of 40 galaxies. A collection of 100 000 to a million stars. Elliptical galaxies are the largest. The End!
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