Stars Unit 1-2: Stars

1. Stars Unit 1-2: Stars

2. How Far to a Star? • The closest star to Earth is the Sun. • The average distance from the Earth to the Sun (since it varies greatly) is approximately 150 million kilometers. • This distance is designated as 1 Astronomical Unit, or AU. • The next closest star is Alpha Centauri, which is 4x1013km away, or 270000AU.

3. How Far to a Star? • As you can imagine, if our neighboring star is almost 300000AU away, this unit doesn’t work too well for interstellar distances. • A second unit is the light-year, measures the distance that light travels in one year. • One light-year is approximately 9500000000000km.

4. How Far to a Star • Using light-years, Alpha Centauri can be expressed as being 4.3 light-years away. • Sirius is 9 light-years away. • Vega, the brightest summertime star, is 27 light-years away. • Polaris, the North Star, is 680 light-years away. • The Milky Way Galaxy is about 100,000 light-years across.

5. Physical Properties of Stars • The physical properties of a star include: • Size • Density • Mass • Color &Temperature • Brightness & Luminosity

6. Size • Stars vary in size a great deal. • The sun has a diameter of approximately 1,380,000km. • Some stars are smaller than the Earth! • The largest observed stars have diameters some 2,000 time the size of the sun! • That’s approximately 2,760,000,000km. • For the record, there are just about 11 football fields in one km.

7. Density • Although it seems that stars vary greatly in size, they vary even more in density! • Our sun has a density about 1.4 times greater than water. • Betelgeuse (don’t say it two moretimes!) is one-millionth the densityof the sun. • Sirius is so dense, that oneteaspoon of it would weighmore than a ton on earth!

8. Mass • The difference between mass in stars in significantly less than the difference in density. • The sun’s mass is about 300,000 times that of Earth. Approx 2x1030kg. • Most stars are fairly close to the mass of the sun.

9. Color & Temperature • The color of a star tells us the star’s approximate temperature. • Cooler stars are more red. • Hotter stars are more blue. • The sun is a yellow star, so it’s actually on the cooler side of the color spectrum.

10. Brightness & Luminosity • Brightness is the perceived strength of the star’s light when observed from earth. • This is more commonly called apparent magnitude. • The actual brightness of a star is called its luminosity.

11. Brightness & Luminosity • Luminosity depends on the stars temperature and size. • Apparent magnitude depends on the distance from the star to the Earth. • Example: • A 100W light bulb has a higher luminosity than a 20W flashlight bulb. • But if that flashlight bulb is right in front of your eye, it will appear brighter than the 100W bulb far away.

12. Brightness & Luminosity • Both the brightness and luminosity of the stars is used to categorize them. • This is the purpose of studying star magnitudes.

13. Elements in Stars • Scientists use spectral analysis to determine the composition of stars. • Spectral analysis is a process that uses a tool called a spectroscope to separate the different wavelengths of light coming from a star. • By observing which wavelengths are stronger, missing, or weaker, scientists can determine the composition of the star.

14. Elements in Stars • Through spectral analysis, scientists have found stars are primarly made of the following elements: • Hydrogen • Helium • Rarely: • Iron • Titanium • Calcium • Sodium • Etc.

15. Elements in Stars • Spectral analysis of the sun: • 70% Hydrogen • 28% Helium • 2% heavier elements • Every star has slightly different spectra, and every star has slightly different composition. • Almost every star powers itself by fusing hydrogen atoms together and making helium.