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## Our Universe ### Students' Learning Outcomes After studying this chapter, students will be able to: - Explore and understand the terms star, galaxy, Milky Way and black holes. - Compare the types of galaxies. - Relate the life of a star with the formation of black holes (e.g. Dwarf, Red Giant,...

## Our Universe ### Students' Learning Outcomes After studying this chapter, students will be able to: - Explore and understand the terms star, galaxy, Milky Way and black holes. - Compare the types of galaxies. - Relate the life of a star with the formation of black holes (e.g. Dwarf, Red Giant, White Dwarf). - Discuss the birth and eventual death of our Sun. - Show how information is collected from space by using telescopes (e.g. Hubble Space Telescope) and space probes (e.g. Galileo). - Describe advancements in space technology and analyze the benefits generated by the technology of space exploration. ### 12.1 Galaxies The word galaxy comes from the Greek word "gala" meaning "milk". Our galaxy looks like a milky cloud in the night sky. Hence, it is popularly known as Milky Way Galaxy. Its top as well as side view is shown in Fig. 12.1. It is a spiral type galaxy with mostly old stars at its centre, surrounded by a halo of newly generated stars. The estimation is that there are more than a billion stars in this galaxy. This galaxy circles the whole sky. Our Sun along with its solar system is situated in one of its spiral arms. The stars which we see during the night are all parts the Milky Way Galaxy. There is still enough gas and other matter in its arms for many more generations of stars. ### Interesting Information - Milky Way Galaxy | Age | 13.6 billion years | |---|---| | Diameter | 106,000 light years | | Thickness of the center | 10,000 - 15,000 light years | | Speed in space | 2,100,000 kilometer per hour | | Revolving speed | Once cycle in 225,000,000 years | | Approximate distance of the Sun from the center of the galaxy | 21,000 light years | The astronomers through radio-telescope collect information about astronomical objects. ### Interesting Information Astronomy is a very old science. Overtimes, it was about the study of objects seen in the night sky. Now it is the study of the whole universe which includes many mysterious and unseen objects. ### Do You Know? Stars are large distances apart. A unit of distance has been defined known as a light year. It is the distance that light travels in one year. This distance is 9.5 trillion kilometers. ### Andromeda Andromeda is our neighboring galaxy which we can see without a telescope during the clear night. It is bigger than our galaxy and situated about 2.5 million light years away from us. ### 12.1.1 Types of Galaxies There are many types of galaxies. Some major types are given below: 1. **Spiral Galaxies:** A galaxy that has a flat disklike shape with a bulge in the middle is called a Spiral Galaxy. It rotates about its center. Milky Way and Andromeda galaxies are spiral galaxies. 2. **Elliptical Galaxies:** They look like flattened balls. They are oval shaped. They contain mostly old stars and do not rotate around their centers. They are the most abundant type of galaxies in the universe. They are dim and old. 3. **Jelly Fish Galaxies:** Another amazing type is like a Jelly fish swimming across a sea of stars known as Jelly Fish Galaxy. They have giant tails looking as blue ribbons of young stars extended away from the galaxy disk. More than 30 such galaxies have been discovered. 4. **Irregular Galaxies:** They do not have any distinct regular shape. They are