Stars
1) How far away are the stars? The closest star to the Sun is about 4 light years away. All
other stars are beyond that. There are about 100 billion stars in our galaxy which is about
160,000 light years across.
2) What evidence do astronomers have that the Sun is a typical star? The Sun is typical
among stars in its luminosity and in its size and in its mass.
3) What is meant by a “first magnitude” star? What is meant by a “second magnitude”
star? First magnitude stars are the brightest in the sky (besides the Sun). Second
magnitude stars are the group that is second brightest—they are about half as bright as
first magnitude stars. A star that is 5 magnitudes greater than another star is 100 times
dimmer—the greater the magnitude, the dimmer the star.
3A) Why is the magnitude scale called a “backward” scale? The magnitude scale is used
for describing the apparent brightness of stars. Because the brighter stars have smaller
(or more negative) magnitudes, and the dimmer stars have greater magnitudes. (So a star
of magnitude -1 is brighter than a star of magnitude +3.)
4) Why are some stars red and others blue? A star appears a certain color because of its
temperature and the blackbody curve. A cooler star emits more red light than any other
color so it appears red. A hot star emits more blue light so it appears blue.
5) What are stars made of? Stars are made of mostly hydrogen and Helium.
6) As stars go, is our Sun especially large or small? No. There are stars much smaller
than the Sun and there are stars much larger than the Sun.
7) What are giants, supergiants, and white dwarfs? These are stars that have left the main
sequence (they are at the end of their “lives”). Giants are stars that are 10 to 100 times the
diameter of the Sun. Supergiants are stars that are up to 1000 times the diameter of the
Sun (or more). White dwarfs are very small, often the size of the Earth.
8) How do we know the distances to remote stars? Parallax measurements do not work
for distant stars. Astronomers use the spectrum of a star to estimate its luminosity. Then
they measure its apparent brightness, then they use the luminosity and the apparent
brightness to calculate the distance to the star from the equation:
L
b
b= apparent brightness, L=luminosity, d=distance to star
4 d 2
9) Why are binary star systems important in astronomy? Binary stars are used to
calculate the masses of stars from the orbiting motions of the stars in the binary system.
10) What is an H-R diagram?
An H-R diagram is a Hertzsprung-Russell diagram in which luminosity is plotted versus
spectral type or temperature. Look at the examples on page 513-516 of your book.
Stars on the left side of an H-R diagram are hottest, stars at the top of the diagram are the
most luminous.
11) About what fraction of stars we see at night are main sequence stars? About 90%.
12) Of stars on the main sequence in an H-R diagram (Fig 15.10 on p 514), which stars
are the most massive, and which are the lease massive? Most massive are in the upper
left, least massive are in the lower right.
12a) of the main sequence stars, in what part of the H-R diagram are the stars with the
longest lifetimes? Shortest lifetimes?
13) What are the spectral classes of a star? OBAFGKM.
O stars are the hottest, about 60,000 F on the “surface,” M stars are the coolest about
6000F on the “surface.”
14) Explain the difference between a star’s apparent brightness and its luminosity. The
Luminosity of a star is the total power it emits. The apparent brightness (also called
simply the brightness) is how bright it looks in our sky. A 60 watt bulb at a distance of
10 miles has a luminosity of 60 watts, but its apparent brightness is much smaller than
that.
15) Why are measurements of stellar parallax difficult to make? The nearest stars have a
parallax of less than 1 arcsecond—this is a very tiny angle and a difficult measurement to
make.
16) Which are more common, stars more luminous than the Sun, or stars less luminous
than the Sun? Stars that are less luminous than the Sun are more common.
17) What are the 2 types of star clusters? Open clusters, which occur in the disk of the
galaxy, and globular clusters which occur in the halo around the center of the galaxy
(outside the disk). All stars in the same cluster are the same age, and at about the same
distance.
18) How can you tell the age of a star cluster? If you plot all the stars in a cluster on an
HR diagram, the stars that are just leaving the main sequence are about the same age as
the cluster.
19) What is a variable star? It is a star that has left the main sequence and has a varying
luminosity.
20) How are variable stars used by astronomers? There is a relationship between the
period of variation and the luminosity of a variable star, so we can use them to determine
the distance to galaxies. In fact variable stars were used to show that the Milky Way is
not the only galaxy.
Keywords: Apparent brightness, brightness, double star, eclipsing binary, giant, H-R
diagram, luminosity, magnitude scale, OBAFGKM , spectral class, red giant, parallax,
supergiant, white dwarf.