2. CHARACTERISTICS OF STARS 21-2

3. STARS Stars “twinkle” because our atmosphere causes them to shimmer and blur. A star is a very large ball of mostly hydrogen and helium gas that shines extremely brightly by nuclear fusion. A star is basically an element creating factory! Our universe is full of stars. Our Milky Way galaxy contains hundreds of millions of stars. Our universe contains billions of galaxies!

4. THE UNIVERSE All of space and everything in it. Most of the universe seems to be empty space. The observable universe is over 10 billion light years across. It could be much larger! About 13.7 billion years old. Formed at the Big Bang.

5. DISTANCES TO STARS Stars are so far apart that it is not practical to use kilometers. Light travels at 300,000 km/s, or 9.5 trillion km a year! So we use light years to measure distances in space. A light year is the distance light travels in one year which is about 5.88 trillion miles. Light from the closest star to us takes 4.2 years to reach us, so Proxima Centari is 4.2 light years away.

6. PARALLAX Parallax is one method to measure distances to stars that are somewhat close. Parallax is the apparent change in position of an object when you look at it from different places. Scientists look at a star when the Earth is on one side of the sun. Six months later they again look at that star. They measure the apparent shift of the background stars. The greater the parallax shift, the closer the star is. This method is only good for nearby stars, less than 1000 light years away.

7. PARALLAX

8. CLASSIFYING STARS Stars are classified by temperature. The brightness of stars is affected by its size and temperature. Even though they are all sphere’s of glowing gas that are powered by fusion, they can be very different from one another.

9. STARS AND COLOR A star’s color indicates its temperature. Reddish color stars are fairly cool, about 3200°C. Our sun appears almost white and is about 5500°C. Rigel is bluish white, and is over 15,000°C. They are classified by temperature, which puts a star in a spectral class.

10. SIZES OF STARS From Earth, all the stars appear to be the same size. But they range from 10’s of km to super giants that are the size of our solar system.

11. BRIGHTNESS OF STARS Brightness (luminosity) is the amount of light a star gives off. How bright a star appears from Earth depends on how far the star is from Earth and the amount of light it emits. The actual brightness of a star depends on its size and temperature.

12. APPARENT MAGNITUDE The brightness of a star as seen from Earth. You cannot tell how much light a star gives off by its apparent magnitude. A small dim star close by can look as bright as a really bright star much further away. If two stars are equally bright, one closer to us will appear brighter than one further away. In this system, the smaller the number the brighter the star! So negative numbers are brighter than positive numbers How bright it seems.

13. ABSOLUTE MAGNITUDE The brightness of a star as if it were a standard distance from us (10 parsecs=32.6 light years). Much more difficult to compute, as you need to know how far the star is from you. This method allows us to compare how bright stars are as if they were all the same distance from us. This way dimmer stars would appear dimmer, and brighter stars would appear brighter. How bright the star really is.

14. HERTZSPRUNG-RUSSELL DIAGRAM Graph of stars brightness and temperature. This makes a pattern with 4 areas that show the main classes of stars. Actually shows the life cycle of stars.