Cepheid Variables as Cosmic Yardsticks
Cepheid stars oscillate between two states:
In one of the states, the star is compact and large temperature and
pressure gradients build up in the star. These large pressures cause the
star to expand. When the star is in its expanded state, there is a much weaker
pressure gradient in the star. Without the pressure gradient to
support the star against gravity, the star contracts and
the star returns to its compressed state.
Cepheid variable stars have masses between five and twenty times the mass
of our Sun.
The more massive stars are more luminous and have more extended envelopes
(the outer layers of gas in a star are sometimes called its "envelope").
Because these envelopes are more extended and the density in their envelopes is
lower, their variability period, which is proportional to the inverse square
root of the density in the layer, is longer.
Difficulties in Using Cepheids to Determine the Size of the
There have been a number of difficulties associated with using Cepheids
as distance indicators. Until recently, astronomers used photographic
plates to measure the fluxes from stars. The plates were highly non-linear and
often produced faulty flux measurements. Since massive stars are
short-lived, they are always located near their dusty birthplaces. Dust absorbs
light, particularly at blue wavelengths where most photographic images were
taken, and if not properly corrected for, this dust absorption can lead
to erroneous luminosity determinations. Finally, it has been very difficult
to use ground-based telescopes
to detect Cepheids in distant galaxies: Earth's fluctuating
atmosphere makes it impossible to separate these stars from the diffuse
light of their host galaxies.
Another difficulty with using Cepheids as distance indicators
has been the problem of determining the distance to a sample of nearby
Cepheids. In recent years, this problem has lessened. Astronomers have developed several very reliable and
independent methods of determining the distances to the Large Magellanic
Cloud and Small Magellanic Cloud, two of the satellite galaxies of
our own Milky Way Galaxy. Since both of the Magellanic Clouds contain
large numbers of Cepheids, they can be used to calibrate the distance scale.
Recent technological advances enabled astronomers to overcome a
number of the other past difficulties. New detectors called CCDs (charge
coupled devices) made more accurate flux measurements possible. These new detectors
are also sensitive in the infrared wavelengths. Dust is much more
transparent at these wavelengths. By measuring fluxes at multiple wavelengths,
astronomers were able to correct for the effects of dust and make much more
accurate distance determinations.
These advances enabled accurate study of the nearby galaxies that
comprise the "Local Group" (the group of galaxies including our own
Milky Way galaxy and our neighbor the Andromeda galaxy). Astronomers
observed Cepheids in both the metal
rich inner region of M31 (Andromeda) and its metal poor outer region. This work
showed that the properties of Cepheids did not depend sensitively on chemical
abundances. Despite these advances, astronomers, limited by the Earth's
atmosphere, could only measure the distances to the nearest galaxies. In
addition to the motion due to the expansion of the Universe, galaxies
have "relative motions" due to the gravitational pull of neighboring
Because of these peculiar motions, astronomers need to measure the distances to
distant galaxies so that they can determine the Hubble constant.
Over the past few decades, astronomers, using different data
sets and methods, have reported values for the Hubble constant which range
between 50 km/s/Mpc and 100 km/s/Mpc. Resolving this discrepancy is one of
the most important outstanding problems in observational cosmology.
Thank you to the MAP project for contributing to this article. Find out
about the Microwave Anisotropy Probe at http://map.gsfc.nasa.gov/.