1. 1. absolute brightness - the brightness a star would have if it were 10 parsecs from Earth

2. 2. apparent brightness - brightness as observed from Earth

3. 3. binary- star system consisting of two stars orbiting their common center of mass most star systems are binary

4. 4. blue giant - large, hot, bright star upper left of Hertzsprung-Russell diagram

5. 5. blue supergiant - the very largest of the hot, bright stars

6. 6. color index - quantifying a star’s color by comparing its apparent brightness through different filters

7. 7. color-magnitude diagram - absolute magnitude plotted vs. color index

8. 8. dwarf - a star as small as, or smaller than our Sun

9. 9. eclipsing binary - a binary system aligned in such a way that we see one star pass in front of the other

10. 10. energy flux - energy emitted per unit time per unit area from a hot body

11. 11. giant - a star 10 to 100 times the size of the Sun

12. 12. globular cluster - spherical collection of 100,000’s to millions of stars about 50 parsecs across older stars because no type O or B

13. 13. HR diagram - plot of luminosity vs. temperature for a group of stars

14. 14. light curve - variation in brightness of a star with time

15. 15. luminosity class - groups stars according to width of spectral lines

16. 16. main sequence - well-defined band on HR diagram where most stars fall top left to bottom right

17. 17. mass-luminosity relation - luminosity increases roughly like the mass raised to the third power

18. 18. mass-radius relation - radius rises roughly in proportion to the mass

19. 19. open cluster - loosely bound collection of 10’s to 100’s of stars a few parsecs across generally “young” because O and B type stars are present

20. 20. optical double - pair of stars that appear very close as observed from Earth, but are actually unrelated and not close to each other

21. 21. parsec - distance at which a star must lie so that its measured parallax is exactly one arc second

22. 22. photometry - intensity measurements made through a set of standard filters

23. 23. proper motion - angular movement of a star through the sky, measured from year to year represents the actual motion of the star through space.

24. 24. radial velocity - component of a star’s motion that is along our line of sight and therefore does not contribute to the star’s proper motion

25. 25. radius-luminosity- temperature relation - a proportionality that lets astronomers find a star’s radius once its luminosity and temperature are known

26. 26. red dwarf - small, cool, faint star at lower-right end of the main sequence of an HR diagram the most plentiful stars in the universe

27. 27. red giant - giant star with a relatively low surface temperature off the main sequence

28. 28. red giant region - upper right hand corner of the HR diagram

29. 29. speckle interferometry - many short-exposure images of a star are patched together to form a composite image

30. 30. spectral class - classification based on length of spectral lines, indicates temperature of star

31. 31. spectroscopic binary - appears from Earth as a single star spectral lines show back and forth Doppler shifts as they orbit

32. 32. spectroscopic parallax - determining a star’s distance from Earth by measuring its temperature and determining its absolute brightness by comparing to a standard HR diagram. Absolute and apparent brightness compare to give distance.

33. 33. star cluster - group of stars that formed at the same time from the same materials. All roughly the same age, composition, and distance from Earth.

34. 34. supergiant - Star’s radius between 100 and 1000 times that of the Sun.

35. 35. transverse velocity - component of star’s motion perpendicular to our line of sight,giving rise to the observed proper motion (no Doppler shift).

36. 36. UVB system - photometry with filters in ultraviolet (U), blue (B), and visual (V) portion of the spectrum.

37. 37. visual binary - binary star system in which both members are resolvable from Earth.

38. 38. white dwarf - a hot star the size of the Sun or smaller off the main sequence

39. 39. white dwarf region - bottom left corner of the HR diagram

40. 1. Explain two ways in which a star’s real space motion translates into motion observable from Earth?

41. Change in location from one year to the next is proper motion. This is the component called transverse velocity. Radial velocity is along the line of sight, measured by the Doppler shift.

42. 2. What is the difference between the absolute and apparent brightness of stars? Apparent brightness is the brightness as observed from Earth. Absolute brightness is the brightness that would be observed if the star were 10 parsecs from Earth.

43. 3. What is the inverse-square law, and how does it affect our ability to see distant stars? The apparent brightness is inversely proportional to the square of the distance. 2 X as far, 4 X dimmer 3 X as far, 9 X dimmer 4 X as far, 16 X dimmer

44. 4. How do astronomers measure the temperatures of stars? Temperature is found by measuring the star’s radiation at different frequencies and matching the frequencies to the appropriate Planck curve.

45. 5. Briefly describe the system of classification of stars according to their spectral characteristics. They are classified according to the spectral lines observed, originally the amount of Hydrogen the lines seemed to indicate. Today they are ranked in order of surface temperature. O, B, A, F, G, K, M from hottest to coolest.

46. 6. Explain how stars are plotted on the Hertzsprung-Russell diagram. The vertical axis is luminosity (in solar units); the horizontal axis is surface temperature from hottest to coolest. Generally, hotter stars are larger and brighter.

47. 7. What is the main sequence? The general trend of hot, bright stars (upper left) to cooler, dimmer stars (lower right). Most stars fit on this line.

48. 8. Why does an HR diagram tend to be biased? The most easily seen stars are the brighter blue and white stars, so they are greatly over-represented on an HR diagram. The dimmer red and yellow stars are underrepresented.

49. 9. What is the most important factor influencing a star’s ability to radiate? Radiation is primarily determined by a star’s mass.

50. 10. What is the difference between an open cluster and a globular cluster? Open cluster - scattered stars, 10’s to 100’s of stars, few parsecs across, less than 20 million years old (comparatively young) Globular cluster - tightly packed stars, 10,000’s to millions of stars, about 50 parsecs across, at least 10 billion years old (comparatively old).