Characteristics of stars

1. Characteristics of stars • 4. The structure and composition of the universe can be learned from studying stars • and galaxies and their evolution. As a basis for understanding this concept: • a. Students know galaxies are clusters of billions of stars and may have different • shapes. • b.Students know that the Sun is one of many stars in the Milky Way galaxy and that • stars may differ in size, temperature, and color. • c.Students know how to use astronomical units and light years as measures of • distances between the Sun, stars, and Earth. • d.Students know that stars are the source of light for all bright objects in outer space • and that the Moon and planets shine by reflected sunlight, not by their own light. • E. Students know the appearance, general composition, relative position and size, • and motion of objects in the solar system, including planets, planetary satellites, • comets, and asteroids.

2. Stars • Constellations: imaginary patterns made by stars • Example: Orion (seen in winter), Big Dipper, Little Dipper,

3. Classifying stars • Scientists classify them by these things: • Color, temperature, size, composition, and brightness • (we classify “stars” everyday • By their attractiveness…)

4. Color and temperature • Color reveals the star’s surface temperature. • Hotter – 20,000 degrees C – glows blue • Middle – about 5,500 degrees – glows yellow • Cooler – about 3200 degrees C - reddish

5. Size • Many stars are about the size of the sun, which is a medium sized star. Many are much larger than that. • Very large are called giant stars or supergiant stars. • We also have itty bitty stars, like white dwarfs and neutron stars. • What controls their size?

6. A Star is Born

7. No, seriously. Let’s talk about star birth. • Nebulas are clouds of gas (H and He) and dust what we call the Interstellar medium (the area between stuff we can see) • They look like this:

11. Oooh, pretty • Yes, super pretty nebulae (plural of nebula) collapse on themselves because of GRAVITY.

12. Once it collapses • It forms a “protostar” inside the center of the nebula. This will become a star. The other stuff around it can become MORE stars, or, as in the case of our star (the sun), the other stuff can become planets. • When the other stuff spins off into planets, we get an entire solar system.

13. Once it collapses • The nebula stars to heat up. • Heating up creates pressure, which pushes against the force of gravity. • Nuclear fusion begins (OMG WHAT IS NUCLEAR FUSION OMG) • H begins making He and releasing energy, making the star “shine”. That’s all it is. Pretty much. Sort of. • And you have a baby star. Aww.

14. Nebulae • Are different sizes themselves, so they result in different size stars. • Also, their compositions (what they’re made of) isn’t always exactly the same. • So, sometimes we get big stars, sometimes we get little stars, sometimes we get blue stars yellow stars red stars etc.

15. Chemical composition of stars • Varies, but usually about 73% Hydrogen, 25% Helium, and 2% other elements.

16. Brightness of stars • Depends on both its size and temperature. • For example, a small star might burn very hot. • A large star might burn cold. • The brightness of a star depends on BOTH factors.

17. How long do stars live? • Depends on mass. • Oddly enough, bigger stars live shorter lifetimes, because they use their fuel (Hydrogen, primarily) faster. Like an SUV. • Little stars live longer.

18. Measuring distances to stars • The light year – the distance that light (one photon of light) travels in 1 year, about 9.5 million million km. (it is distance, not time) • An astronomical unit is the distance between Earth and the Sun. It is about 93 million miles, or 1.4 x 10^11 meters

19. Some distances • 4.2 light years to the nearest star (Proximacentauri) • 30,000 light years to the center of the galaxy • 46 billion light years in radius = approximate size of observable universe

20. Our own star • The sun is at the center of our solar system. Its diameter is about 109 times that of Earth. • It formed about 4.5 billion years ago, and probably only has about 4.5 billion years worth of fuel left. Meaning it will die/blow up. When it does that, Earth will become completely inhospitable (if not a little before)

21. So, we’re stuck with this sad sack star that’s going to die in 4 billion years. • Are there more stars in the Milky Way (our galaxy?) • Whoa yeah, 300 billion. • That’s a lot.

22. So, stars • Provide all of the light in space. • Meaning, everything else (not including meteors, I guess, when they burn up) – planets, black holes, asteroids, dwarf planets, etc, only shine BECAUSE a star shines on them, and the light is reflected back.

23. The moon • Has no light of its own. Aw. • Only shines back the sun’s light. • Depending on its position relative to the sun and Earth, we have different amounts of light on the side of the moon that faces Earth.

24. Moon • Only 1 side of the moon ever faces Earth. • This is because of some old geological stuff, which resulted in: • The moon’s rotational period (how long it takes to rotate on its axis) is equal to its revolutionary period (how long it takes to revolve around Earth). • So only 1 side ever shows. • https://www.youtube.com/watch?v=jgoIP90apEs

25. Moon

26. Each one of these points is at least one star

27. That’s just OUR galaxy • It is believed/estimated that 500 billion galaxies exist. • What is a galaxy? • It’s a name we have given to the systems of stars that we see. They are held together by gravity.

28. What kind of galaxies are there? • 3 main kinds: • Spiral • Irregular • Elliptical

29. Ours is spiral

30. Questions? • Using your notes, please write a paragraph that includes a description of: • What nebulas are • What causes nebulas to collapse • How stars are formed, and why they are different sizes and colors • How color is related to surface temperature • What happens to stars when they die (run out of fuel)