Chapter 16--Properties of Stars

apparent brightness
period
0 25 50 75 100 125 150 175 200
Pulsating Variable Stars
time (days)
Figure 16.12 A typical light curve for a Cepheid variable star.
Cepheids are giant, whitish stars whose luminosities regularly pulsate
over periods of a few days to about a hundred days. The pulsation
period of this Cepheid is about 50 days.
Not all stars shine steadily like our Sun. Any star that significantly
varies in brightness with time is called a variable
star. A particularly important type of variable star has a peculiar
problem with achieving the proper balance between
the power welling up from its core and the power being
radiated from its surface. Sometimes the upper layers of
such a star are too opaque, so energy and pressure build up
beneath the photosphere and the star expands in size. However,
this expansion puffs the upper layers outward, making
them too transparent. So much energy then escapes that
the underlying pressure drops, and the star contracts again.
In a futile quest for a steady equilibrium, the atmosphere
of such a pulsating variable star alternately expands
and contracts, causing the star to rise and fall in luminosity.
Figure 16.12 shows a typical light curve for a pulsating
variable star, with the star’s brightness graphed against time.
Any pulsating variable star has its own particular period
between peaks in luminosity, which we can discover easily
from its light curve. These periods can range from as short
as several hours to as long as several years.
Most pulsating variable stars inhabit a strip (called the
instability strip) on the H–R diagram that lies between the
main sequence and the red giants (Figure 16.13). A special
category of very luminous pulsating variables lies in the
upper portion of this strip: the Cepheid variables,or
Cepheids (so named because the first identified star of
this type was the star Delta Cephei).
Cepheids fluctuate in luminosity with periods of a few
days to a few months. In 1912, another woman astronomer
at Harvard, Henrietta Leavitt, discovered that the periods of
these stars are very closely related to their luminosities: The
longer the period, the more luminous the star. This period–
luminosity relation holds because larger (and hence more
luminous) Cepheids take longer to pulsate in and out in size.
Once we have measured the period of a Cepheid variable,
we can use the period–luminosity relation to determine
its luminosity. We can then calculate its distance with
536 part V • Stellar Alchemy
luminosity (solar units)
10 6
10 5
10 4
10 3
10 2
10
10 2
Figure 16.13 An H–R diagram with the instability strip highlighted.
the luminosity–distance formula. In fact, as we’ll discuss in
Chapter 20, Cepheids provide our primary means of measuring
distances to other galaxies and thus teach us the true
scale of the cosmos. The next time you look at the North
Star, Polaris, gaze upon it with renewed appreciation. Not
only has it guided generations of navigators in the Northern
Hemisphere, but it is also one of these special Cepheid
variable stars.
astronomyplace.com
1
0.1
10 3
10 4
10 5
10 Solar Radii
1 Solar Radius
0.1 Solar Radius
10 2 Solar Radius
10 3 Solar Radius
10 2 Solar Radii
Stellar Evolution Tutorial, Lessons 1, 4
16.6 Star Clusters
Cepheids with
periods of days
instability strip
All stars are born from giant clouds of gas. Because a single
interstellar cloud can contain enough material to form many
stars, stars almost inevitably form in groups. In our snapshot
of the heavens, many stars still congregate in the groups
in which they formed. These groups are of two basic types:
modest-size open clusters and densely packed globular
clusters.
Open clusters of stars are always found in the disk of
the galaxy (see Figure 1.18). They can contain up to several
thousand stars and typically span about 30 light-years
(10 parsecs). The most famous open cluster is the Pleiades, a
prominent clump of stars in the constellation Taurus (Figure
16.14). The Pleiades are often called the Seven Sisters,
although only six of the cluster’s several thousand stars are
easily visible to the naked eye. Other cultures have other
names for this beautiful group of stars. In Japanese it is
called Subaru, which is why the logo for Subaru automobiles
is a diagram of the Pleiades.
Globular clusters are found primarily in the halo of
our galaxy, although some are in the disk. A globular clus-
Sun
Polaris
surface temperature (Kelvin)
10 3 Solar Radii
variable stars with
periods of hours
(called RR Lyrae
variables)
30,000 10,000 6,000 3,000