Download 6. 1 Star Distances 6. 2 Apparent Brightness, Intrinsic Brightness

CHAPTER 6
THE FAMILY OF STARS
Topic Summaries
stellar parallax (p) The small
6. 1 Star Distances
apparent shift in position of a nearby
How far away are the stars?
•
Distance is critical in astronomy. Finding the luminosities, diameters, and masses
of stars requires first finding their distances.
•
Astronomers can measure the distance to nearby stars by observing their
parallaxes. Stellar distances are commonly expressed in parsecs. One parsec is
206,265 AU- the distance to an imaginary star whose parallax is 1 arc second.
star relative to distant background
objects due to Earth's o rbital motion.
parsec (pc) The distance to a
hypothetical star whose parallax is
1 second of arc. 1 pc = 206,265 AU =
3.26 ly.
Stars farther away than about 170 pc have parallaxes too small to measure from
ground-based observatories.
intrinsic brightness A measure of
the amount of light a star produces.
6. 2 Apparent Brightness, Intrinsic Brightness,
and Luminosity
flux A measure of the flow of energy
How much energy do stars make?
light.
out of a surface. Usually applied to
absolute visual magnitude
(Mu) Intrinsic brightness of a star. The
Once you know the distance to a star, you can find its intrinsic brightness, which
can be expressed as its absolute magnitude. The absolute magnitude of a star
equals the apparent magnitude it would have if it w ere 10 pc away.
apparent visual magnitude the star
would have if it were 10 pc away.
lumi osity (L) The total amount of
energ(' a star radiates per second at all
wavelengths.
6. 3 Star Temperatures
How can you tell a star's temperature using its spectrurt:~?
~
spectral class A sta r's position
in the temperature classification
system 0, B, A, F, G, K, M, based on the
The hydrogen Balmer lines are weak in cool stars because atoms are not excite(!
out of the ground state. In hot stars, the Balmer lines are weak because atoms
are excited to higher orbits or are ionized. Only at intermediate temperatures
are the Balmer lines strong.
spectral sequence The
The strength s of many spectral lines in a star's spectrum can be used to tell you
its temperature. Stars are classified in the temperature spectral sequence: 0, B,
A, F, G, K, M.
Hertzsprung-Russell (H-R)
diagram A plot of the intrinsic
Long after the spectral sequence was created, astronomers discovered LandT
objects with temperatures even cooler than th e M stars.
temperature of stars. It separates
appearance of the star's spectrum.
arrangement of spectral classes (0, B,
A, F, G, K, M) ranging from hot to cool.
brightness versus the surface
the effects of temperature and
6. 4 Star Sizes
How big are stars?
The H-R diagram is a plot of luminosity versus surface temperature. It is an
important graph in astronomy becau se it sorts the stars into categories by size.
Roughly 90 percent of normal stars, including the sun, fall on the main sequence, with the hotter main-sequence stars being more luminous. The giants
and supergiants, however, are much larger and lie above the main sequence
in the diagram. Some of the white dwarfs are hot stars, but they fall below the
main sequence because they are so small.
The large size of the giants and supergiants means their atmospheres have low
densities. Giant stars, luminosity class Ill, have narrow spectral lines, and supergiants, class I, have extremely narrow lines. Class V main-sequence stars have
relatively broad spectral lines.
surface area on stellar luminosity
and is commonly plotted as absolute
magnitude versus spectral type but
also as luminosity versus surface
temperature or color.
main sequence The region of the
H-R diagram running from upper left
to lower right, which includes roughly
90 percent of all stars generating
energy by nuclear fusion.
giant Large, cool, highly luminous
star in the upper right of the H- R
diagram, typically 10 to 100 times the
diameter of the sun.
Key Terrns
Topic Surnrnaries
supergiant Exceptionally luminous
•
star whose diameter is 100 to 1,000
Stars so far away that their parallaxes are too small to measure can have
their distances estimated by the technique of spectroscopic parallax.
times that of the sun.
red dwarf A faint, cool, low-mass,
6. 5 Star Masses-Binary Stars
main-sequence star.
How much matter do stars contain?
white dwarf Dying star at the
collapsed to the size of Earth and is
• The only direct way you can find the mass of a star is by studying binary
stars. When two stars orbit a common center of mass, astronomers find their
masses by observing the period and sizes of their orbits.
slowly cooling off.
•
Few binary star systems are easy to analyze; most are spectroscopic binaries
in which the component stars are known only by the alternating Doppler
shifts of their spectral lines.
•
Eclipsing binary star systems allow measurement not just of the component
stars' masses but also independent checks on their temperatures and
diameters.
lower left of the H-R diagram that has
luminosity class A category of
stars of similar luminosity, determined
by the widths of lines in their spectra.
spectroscopic parallax The
method of determining a star's
distance by comparing its apparent
6. 6 'TYpical Stars
magnitude with its absolute
magnitude as estimated from its
What is the typical star like?
spectrum.
binary stars Pairs of stars that orbit
around their common center of mass.
visual binary system A binary
star system in which the two stars are
separately visible in the telescope.
spectroscopic binary system A
star system in which the stars are too
close together to be visible separately.
We see a single point of light, and
only by taking a spectrum can we
determine that there are two stars.
eclipsing binary system A binary
star system in which the stars cross in
front of each other as seen from Earth.
light curve A graph of brightness
versus time commonly used in
analyzing variable stars and eclipsing
binaries.
mass-luminosity relation The
more massive a main-sequence star is,
the more luminous it is.
Review Guestions
1. Why are Earth-based parallax
measurements limited to the
nearest stars?
7. What observations would you
make to classify a star according
to its luminosity? Why does that
method work?
2. What does luminosity measure
that is different from what absolute visual magnitude measures?
8. Why does the orbital period of a
3. Why are hydrogen Balmer
lines strong in the spectra of
medium-temperature stars and
weak in the spectra of hot and
cool stars?
9. Why don't astronomers
know the inclination of a
spectroscopic binary? How do
they know the inclination of an
eclipsing binary?
4. Why does the luminosity of a
star depend on both its radius
and its temperature?
5. How can you be sure that giant
stars really are larger than mainsequence stars?
6. Why do astronomers conclude
that white dwarfs must be very
small?
binary star depend on its mass?
10. If all of the stars in the photo
to the left are members of the
same star cluster, then they all
have about the same distance.
Then why are three of the
brightest much redder than the
rest? What kind of star are they?