How far away are the stars?

1. How far away are the stars?

2. Parallax

4. Measuring the Distance to the Stars with Parallax Remember that 1 parsec = 3.26 light years

6. Visual magnitude scale • Originally, the brightest stars in the sky were considered first magnitude, the next brightest group second magnitude, and so on • Now, magnitude is more precisely defined, so that a m=+1.0 star is 100 times as bright as a m=+6.0 star • Each magnitude is a factor of 2.5

9. Star Names • Most of the brightest stars have names (e.g., Betelgeuse, Sirius) • Up to 24 stars in each constellation have a name with a Greek letter (e.g., β Tauri) • The rest may only be known by a catalog number (e.g., HDE 226868)

10. Absolute magnitude • Equivalent to apparent magnitude of the star if it was at a distance of 10 parsecs • The sun is M=+4.8

12. The brightest stars (M=-10) have luminosities of 106 L • The dimmest stars (M=+17) have luminosities of 10-5 L Luminosity • Measure of the amount of light a star gives off • L=3.83 x 1026 W

13. The Nature of Stars

14. “Cool”, red star Warm, yellow star Hot, blue star A star’s color reveals its surface temperature

15. Photometry • Technique used to accurately measure the color of stars using UBV filters • U - ultraviolet wavelengths • B - blue wavelengths • V - visible wavelength • astronomers compare combinations of U minus B (U-B) and B minus V (B-V) • these combinations are called color indices

16. Stars are classified by their spectra as O, B, A, F, G, K, and M spectral types

17. Stars are classified by their spectra as O, B, A, F, G, K, and M spectral types • O B A F G K M • hottest to coolest • bluish to reddish • An important sequence to remember: • Our Best Astronomers Feel Good Knowing More • Oh Boy, An F Grade Kills Me • Oh Be a Fine Girl (or Guy), Kiss Me

20. The Hertzsprung-Russell diagram identifies a definite relationship between temperature and absolute magnitude Absolute magnitude vs temperature or luminosity vs spectral type

21. Determining the Sizes of Stars from an HR Diagram • Main sequence stars are found in a band from the upper left to the lower right • Giant and supergiant stars are found in the upper right corner • Tiny white dwarf stars are found in the lower left corner of the HR diagram

22. Luminosity classes • Class I includes all the supergiants • Class V includes the main sequence stars • For example, the Sun is a G2 V

23. How do we know the mass of a star? • Binary star systems • Recently, also from extrasolar planets

25. Binary stars orbit their common center of mass

26. animation