Standard Candles - Cepheid Variables
Astronomers have struggled to find the exact distance to stars that are beyond
our relatively local neighborhood. Using the method of Parallax measurements,
astronomers can accurately determine the distance of stars out to several hundred
light years. The completion of the HR Diagram allowed the determination of stellar
types, and with the magnitude-distance formula, astronomers could more accurately
map larger portions of the Milky Way. However, to determine stars that are several
thousand light years away, better tools needed to be developed. With the launch
of the satellite Hipparchus, astronomers could make very precise measurements
without the blurring effect of the Earth's atmosphere. Hipparchus was extremely
valuable in helping to map much of the Milky Way.
At issue now is determining the distance to other galaxies. With the discovery
of galactic red-shifts by Edwin Hubble and the clear demonstration that the
Universe is expanding, it became crucial to pin point the exact distance to
far-off galaxies. Knowing the distance, astronomers could then use that value
and insert it into the Hubble Law to measure the rate of the expansion of the
Universe and perhaps determine the ultimate fate of everything.
Henrietta Leavitt (pictured above to the left) provided the first tool with
the discovery of a variable star whose brightness changed over a period of days,
and also whose spectral class was very closely related to the period of brightening.
These particular stars were first found in the contellation Cepheus and later
became known as Cepheid variable stars. The Cepheids are giant stars in the
latter stages of their lives, and whose brightnesses change by a factor of 1
or more magnitudes over a period of 3-30 days. Since these are giant branch
stars, they are visible in distant galaxies while the majority of distant galactic
stars appear to be part of the fuzzy cloud of stars. One example of a Cepheid
is seen in the HST image set below taken over a period of days looking into
the galaxy M100.
Below is a hypothetical light curve of a Cepheid variable. When I began preparing
this page, I more fully appreciated the work of my Uncle Bill and the members
of the AAVSO (American Association of Variable Star Observers). These astronomers
measure the fluctuations in light intensity among thousands of galactic stars,
turning in their measurements to the AAVSO office out east. There the results
of many individual's work is compiled in such light curves as you see below.
What makes Cepheid variables so important is that their light curves are consistently
within a high and low magnitude range, and the period of brightening and dimming
is consistently the same length for any given star.
The real value of Cepheid variables lies in the interesting relationship between
the period of brightening and dimming and the absolute magnitude of the star.
As seen in the graph below, the longer the period, the more luminous the Cepheid.
Putting these pieces of information together, one can do a systematic search
for Cepheid variables in more distant galaxies. Once one is found, then its
period can be measured. With the period known, and the apparent magnitude (m)
known, it is a simple matter of plugging these numbers into the magnitude-distance
formula and determining the distance to the galaxy that holds the Cepheid.
Using Cepheid Variables of known periods, astronomers have been able to more
precisely determine the distance to neighboring galaxies and many beyond our
local group. However, as astronomers probe deeper into space, it becomes increasingly,
if not impossible, to resolve individual stars within deep space galaxies, even
giant stars like Cepheids. Another method was needed to determine the distance
to some of the more remote galaxies. Once again, members of the AAVSO provided
the clues and tools. Please more forward to Type Ia Supernovae for a look at
these interesting phenomenon and their usefulness as "standard candles."
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