Download 1. absolute brightness -

1. absolute brightness • the brightness a star would have if it were
10 parsecs from Earth
2. apparent brightness • brightness as observed from Earth
3. binary• star system consisting of two stars orbiting
their common center of mass
• most star systems are binary
4. blue giant • large, hot, bright star
• upper left of Hertzsprung-Russell diagram
5. blue supergiant • the very largest of the hot, bright stars
6. color index • quantifying a star’s color by comparing its
apparent brightness through different filters
7. color-magnitude diagram • absolute magnitude plotted vs. color index
8. dwarf • a star as small as, or smaller than our Sun
9. eclipsing binary • a binary system aligned in such a way that
we see one star pass in front of the other
10. energy flux • energy emitted per unit time per unit area
from a hot body
11. giant • a star 10 to 100 times the size of the Sun
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. HR diagram • plot of luminosity vs. temperature for a
group of stars
14. light curve • variation in brightness of a star with time
15. luminosity class • groups stars according to width of spectral
lines
16. main sequence • well-defined band on HR diagram where
most stars fall
• top left to bottom right
17. mass-luminosity relation • luminosity increases roughly like the mass
raised to the third power
18. mass-radius relation • radius rises roughly in proportion to the
mass
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. optical double • pair of stars that appear very close as
observed from Earth, but are actually
unrelated and not close to each other
21. parsec • distance at which a star must lie so that its
measured parallax is exactly one arc second
22. photometry • intensity measurements made through a set
of standard filters
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. 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. radius-luminositytemperature relation • a proportionality that lets astronomers find a
star’s radius once its luminosity and
temperature are known
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. red giant • giant star with a relatively low surface
temperature
• off the main sequence
28. red giant region • upper right hand corner of the HR diagram
29. speckle interferometry • many short-exposure images of a star are
patched together to form a composite image
30. spectral class • classification based on length of spectral
lines, indicates temperature of star
31. spectroscopic binary • appears from Earth as a single star
• spectral lines show back and forth Doppler
shifts as they orbit
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. 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. supergiant • Star’s radius between 100 and 1000 times
that of the Sun.
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. UVB system • photometry with filters in ultraviolet (U),
blue (B), and visual (V) portion of the
spectrum.
37. visual binary • binary star system in which both members
are resolvable from Earth.
38. white dwarf • a hot star the size of the Sun or smaller
• off the main sequence
39. white dwarf region • bottom left corner of the HR diagram
1. Explain two ways in
which a star’s real space
motion translates into
motion observable from
Earth?
• 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.
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.
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
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.
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.
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.
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.
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.
9. What is the most important
factor influencing a star’s ability
to radiate?
• Radiation is primarily determined by a
star’s mass.
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).