1. Stars and Galaxies Chapter 12

2. Stars Definition: a large ball of gas that emits energy produced by nuclear reactions in the star’s interior Planets, comets, and asteroids shine by reflecting light from the Sun (which is an average sized star). Distances between stars Distance between stars is measured in Light Years (LY), which equals the distance that light travels in one year. Light travels at 300,000 km/s 1 LY = 9.5 trillion km

3. Formation of a Star Stars are formed in a NEBULA, which is a large cloud of gas and dust Also called interstellar clouds Space between stars is called interstellar space Nebula can be so dense that they can block light (interstellar cloud) 1) Gravitational force causes clumps of matter to form in nebula 2) As they move closer, they move faster and heat up 3) As the clump contracts, it becomes spherical 4) When mass gets large enough it become a protostar 5) Temperature continues to increase and sphere becomes disk-like (elliptical) 6) Continues to become denser and hotter and boom! Nuclear Fusion! A new star is born!

4. Types of Stars By analyzing the light emitted by a star, you can learn about the star’s motion, temperature, and chemical elements it contains. The color of a star depends on its temperature (just like when you heat metal): Red (3500 ° K) longest wavelength Yellow (5000 ° K) Yellow (5000 ° K) White (7000 ° K) White (7000 ° K) Blue (25000 ° K) shortest wavelength The brightness of stars depends on two things: The amount of energy in the star The distance the star is from Earth Things far away look smaller!

5. Three sizes of stars can be born in a Stellar Nursery (nebula) – their lifecycle is below:

6. Types of Stars Red Giant: sun-sized stars that use up their fuel Hydrogen converts to helium and the core collapses As it expands, it cools and becomes a red giant White Dwarf: the dense core of a red-giant star Red-giant stars lose mass from their surfaces Fusion in the core has ceased and gravity causes it to contract until it is about the size of Earth Some become so hot they emit a blue light The Sun will become a dwarf star in billions of years Supernova: the explosion of a supergiant star A supergiant star can explode before it dies The debris is still visible as an interstellar cloud

7. Kuiper Belt Similar to the astroid belt (between Mars and Jupiter) 20 times as wide and 20-200 times as massive Extending from the orbit of Neptune to about 50AU from the sun. Only discovered in 1992 Pluto is the largest known member of the Kuiper Belt This discovery is what helped Pluto to be reclassified as a “dwarf planet”

8. Galaxies Definition: massive systems of stars, dust, and gas held together by gravity are called galaxies Some contain billions of stars Shapes and Sizes: Spiral The Milky Way Elliptical Oval Shape Irregular No apparent shape The Big Bang Theory 14 billion years ago the universe was the size of a tiny point that was extremely hot. It began to expand rapidly and after several hundred thousand years, it cooled, and hydrogen and helium atoms began to form. Our solar system is about 4.5 billion years old