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Chapter 8: Stars By Sydney Bullock & Abby Swanager Stars • The different colors of stars tells how hot the stars are. • All stars are made up of different elements in the form of gases. • Some gases can be hotter than others making the stars different colors. Differences in Brightness • The brightest stars in the sky are called firstmagnitude stars. • The dimmest stars are called sixth-magnitude stars. • Positive numbers represent dim Motion of stars • Daytime and nighttime are both caused by Earth’s rotation • The Earth’s tilt and revolution around the Sun cause the seasons. • During each season, the Earth faces a different part of the sky at night. • Because of Earth’s rotation, the Sun appears to move across the sky. • In fact, at night you can observe that the whole sky is rotating above us. The Actual Motion of Stars • Because of stars being distant, their actual motion is hard to see. • If you could put thousands of years into one hour, a star’s movement would be obvious. The Life Cycle of Stars The Beginning and End of Stars • A star enters the first stage of it’s life cycle as a ball of gas and dust. • Gravity pulls the gas and dust into a sphere. • As the sphere becomes denser, it gets hotter and the hydrogen changes to helium in a process called nuclear fusion Different types of Stars • Stars can be classified by their size, mass, brightness, color, temperature, spectrum, and age. • After a star forms, it enters the second and longest stage of its life cycle known as the main sequence. • After the main-sequence stage, a star can enter the third stage of their life cycle. A Tool for Studying Stars • In 1911, a Danish astronomer named Ejnar Hertzsprung compared the brightness and temperatures of stars on a graph. • Two years later, American astronomer named Henry Norris Russell made some similar graphs. Although these astronomers used different data, they had similar results. A Tool for Studying Tools (continued) • The combination of their ideas is now called the Hertzsprung-Russell Diagram, or HR Diagram. • The HR Diagram is a graph that shows the relationship between a stars’ surface temperature and its absolute magnitude. • The modern HR Diagram is shown below. HR Diagram • This is the modern HR Diagram. When Stars Get Old • Although stars may stay on the main sequence for a long time, they don’t stay their forever. • Average stars like the Sun, become red giants and then white dwarfs. • Massive stars use their hydrogen much faster than stars like the sun do. Types of Galaxies • There are many different types of galaxies. Edwin Hubble, the astronomer for whom The Hubble Space Telescope is named, began to classify galaxies, mostly by their shapes in the 1920s. • Here are a couple of galaxies, spiral galaxies, Milky Way, elliptical galaxies, and irregular galaxies. The Big Bang Theory • With the discovery that the universe is expanding, scientists started wondering what it would be like to watch the formation of the universe in reverse. • The universe would appear to be contracting, not expanding. • All matter would eventually come together at a single point.