1. Unit Stars and the Universe

2. Stars A star is a giant, hot ball of gas. Stars generate light and heat through nuclear reactions. They are powered by the fusion of hydrogen into helium under conditions of enormous temperature, mass, and density.

3. Stars a giant, hot ball of gas composed mainly of H and He. generate light and heat through nuclear reactions. powered by fusion of H into He under conditions of enormous temperature, mass, and density.

4. Fusion and Fission Nuclear fusion forms larger nuclei from smaller ones. This is the opposite of nuclear fission, where nuclei of larger atoms are split to form smaller nuclei.

5. Stars Astronomers classify stars according to their physical characteristics. The main characteristics used to classify stars are temperature, size and brightness.

6. Temperature of an object determines its color The hotter an object is, the brighter and lighter the color. – Example: a light blue flame is hotter than a red flame.

8. The apparent brightness of an object depends on how far away it is and how much light it actually gives off (its absolute brightness). The mathematical relationship between these variables is known as the inverse square law and is used to determine the distance to stars and galaxies. The inverse square law shows how the apparent brightness of an object decreases as you move away from it.

10. Parallax is the apparent change in position of an object when you look at it from different directions. To use parallax, astronomers determine the position of a star in the sky in relation to other stars that are too far away to show movement. Using geometry, they can determine the distance of the star from Earth.

11. Star Sizes The sun, with a diameter of 1.4 million kilometers, is a medium-sized star. The largest stars, called supergiants, have a diameter that can exceed 1,000 times that of the sun. Giants, are about 250 times the diameter of the sun. Stars that are smaller than the sun come in two categories, white dwarfs and neutron stars.

12. H-R Diagram H-R diagrams are useful because they help astronomers categorize stars into distinct groups. Stars that fall into the band that stretches diagonally from cool, dim stars to hot, bright stars are called main sequence stars.

15. Stars and Spectroscopy Spectroscopy is a tool of astronomy in which the electromagnetic radiation (including visible light) produced by a star or other object is analyzed.

16. Stars and Spectroscopy A spectrometer splits light into a spectrum of colors and displays lines of different colors along a scale. The scale measures the wavelength of each of the lines of color in nanometers (nm).

18. Galaxies and the Universe A galaxy is a huge group of stars, dust, gas, and other objects bound together by gravitational forces. The Milky Way is a typical spiral galaxy. Our solar system is in one of the “arms” of the Milky Way.

20. Galaxies and the Universe Astronomers classify galaxies according to their shape. – Spiral galaxies like the Milky Way consist of a central, dense area surrounded by spiraling arms. – Elliptical galaxies look like the central portion of a spiral galaxy without the arms. – Lenticular galaxies are lens-shaped with a smooth, even distribution of stars and no central, denser area. – Irregular galaxies exhibit peculiar shapes and do not appear to rotate like those galaxies of other shapes.

21. The Big Bang Theory The universe is thought to be 10 to 15 billion years old. According to the Big Bang theory, all of the matter and energy in the universe started out compressed into a space no bigger than the nucleus of an atom. Immediately after the explosion, the universe began to expand and cool. The universe continued as a giant cloud of gas until about 300 million years after the Big Bang. Parts of the gas cloud began to collapse and ignite to form clusters of stars—the first galaxies.

23. Evidence for Big Bang In the early 1930s, Hubble had enough evidence to prove that galaxies were moving away from a single point in the universe. THE UNIVERSE IS EXPANDING. Later scientists discovered the cosmic microwave background radiation predicted by the Big Bang theory.

25. Redshift and Blueshift Redshift: If the light from a distant star is shifted to the red end of the spectrum, the star is moving away from the astronomer. Blueshift: If the light from a distant star is shifted to the violet end of the spectrum, the star is moving toward the astronomer.

26. Doppler Effect The Doppler effect explains the change in pitch of a siren as an ambulance passes by you. It’s also used in police radar to catch those troublesome teenage speeders!