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Another alternative to calibrating the Cepheid variable
yardstick began unfolding when in 1917, Ritchey discovered a
nova in the galaxy NGC6946. At that time, it was estimated that
a nova should have peak brightness about 10,000 times that of
our Sun. Ritchey looked through the entire photographic plate
collection at Mount Wilson, and discovered two other faint
novae. After hearing about Ritchey’s search, H. D. Curtis
examined the Lick Observatory 36-inch reflector plate collection
and found another three novae in other galaxies. Additionally,
other astronomers found five more novae in galaxies. Based on
this work, Lundmark in 1919 estimated the distance to M31 as
650,000 light years.
Curtis felt that novae proved galaxies were separate groups of
stars like the Milky Way. There were spirited debates at
astronomical conferences from 1917 to 1924 as to what galaxies
were. That debate ended dramatically at an astronomical
conference on January 1, 1925. Edwin Hubble, unable to make this
conference, had sent a note detailing the first observations
with the 100-inch telescope at Mt. Wilson. He had resolved stars
in the M31, M32 and NGC6822 galaxies and discovered that some of
them were Cepheid variables whose approximate period-luminosity
relationship was now known.
Hubble’s preliminary distance to M31 was 900,000 light years
(modern value 2.5 million light years) and the zero point to
calibrating the Cepheid variable yardstick improved. Galaxies
were found to be island continents of stars, the largest having
several hundred billions stars, and most galaxies were millions
of light years distant.
The period-luminosity (P-L) relation of Cepheid variable stars
states that those Cepheids with longer periods have greater mean
luminosity. The zero point of this relationship was defined as
the absolute magnitude of a Cepheid variable with a period of
one day. The work in the 1920’s and 1930’s seemed to indicate
this value was 0.0; i.e., a Cepheid variable star with a period
of one day has an absolute magnitude of 0.0.
This belief was held until approximately 1950. By then, it had
become apparent that the P-L relation needed major revision. RR
Lyrae stars are periodic variable stars with periods under one
day. Because of their regularity, they were assumed to follow
the Cepheid P-L relation. However, Shapley and his colleagues
could not detect any RR Lyrae stars in the Magellanic Clouds,
even though they should have been detectable at the clouds then
presumed distance of 100,000 light years. Also, Lundmark noted
the discrepancy between distances to M31 as estimated by the
Cepheid P-L relation versus those derived from other methods,
such as using ordinary nova brightness.
Milky Way globular clusters contain large numbers of short
period RR Lyrae variable stars. The distance and absolute
magnitude of nearby RR Lyrae stars was estimated using their
proper motions and spectral characteristics. The data for RR
Lyrae stars was then used to estimate the distances to other
Milky Way globular clusters. It was found that the largest and
brightest globular clusters were similar to each other, and it
was then assumed that the brightness of globular clusters could
be used to estimate the distance to M31 using the data for its
globular clusters. When Lundmark compared the globular cluster
brightness method with the novae brightness method in 1948, he
found the two agreed with each other and gave a distance to M31
approximately twice as far as had been thought up to that time,
1.8 million light years.
It wasn’t until 1941-1942, when Los Angeles was blacked out
during the nights of World War II, that Walter Baade using the
100-inch telescope at Mt. Wilson, resolved M31’s Cepheid
variables as well as luminous normal stars in the galaxy. Baade
found that the brighter stars in the nucleus differed in
apparent luminosity and periodicity from those in the spiral
arms, although they were obviously more or less at the same
distance. Baade had discovered that M31 was composed of two
distinct populations of stars and two different types of Cepheid
variable stars. Up to that point, it had been assumed there was
only one type of Cepheid variable star.
Baade identified Population II stars with galactic nucleii and
globular clusters. These are “metal poor” stars, and Baade’s
Population I stars are found in a galaxy’s spiral arms.
Population I stars are relatively “metal rich” stars formed
later in time than Population II stars.
Population II pulsating variables are really RR Lyrae and other
types of cluster variable stars and are not classical Cepheid
variables. RR Lyrae type stars are identified with galactic
nuclei and are numerous in globular star clusters. RR Lyrae
stars are low mass stars which burn helium in their cores. They
are red giants about 50 times more luminous than the Sun.
Cepheid variables on the other hand are massive helium burning
stars with luminosity up to 30,000 times that of the Sun. Many
of them are supergiants and among the most luminous stars
known. Cepheids in the disk of the Milky Way have a higher metal
content than Cepheids in the Large Magellanic cloud. Cepheids
with a higher metal content are brighter than Cepheids with the
same period that have a lower metal content. A “metal” is
defined as any element heavier than hydrogen or helium. By mass,
the surface layers of the Sun are 70% hydrogen, 30% helium, and
2% heavier elements (“metals”). The Sun is considered to have a
high metal content in comparison to most stars.
In general, Population I Cepheid variables (true “Cepheids”) are
about 1.5 magnitudes more luminous than Population II Cepheids.
They have a heavier metal content and occur in the galactic
disk, while the fainter type II Cepheids have similar periods
but have a lower metal content and tend to occur in globular
clusters and the galactic halo. The classic Cepheids are
relatively massive stars about 100 million years old, while the
type II Cepheids are far less massive and much older, being many
billions of years of age. The two types of Cepheids can be
distinguished by their spectral characteristics and their
slightly differing light curves.
The Milky Way’s globular star clusters were found to be some
seven to forty thousands light years away surrounding the
nucleus of the Milky Way Galaxy. The 100-inch Mt. Wilson
telescope and then the 200-inch Mount Palomar telescope
stretched the Cepheid “standard candle” measuring rod distance
capability out to an estimated 3 million light years,
encompassing about 20 galaxies in what is now known as the Local
Group of galaxies. Today, using the HST we can monitor other
galaxy Cepheid and RR Lyrae variables as well as other very
bright stars in galaxies out to about hundred million light
years (Sky &Telescope February 2002, pages 18 – 19). However, it
takes a great deal of observing time to make this type of
measurement for each galaxy.
It should be remembered that a zero point for adjusting the
Cepheid variable “standard candle” was based on averaging
estimated distances of nearer Cepheids. This means that if such
distances need further adjustment, the distance scale for
measurements to globular star clusters and galaxies would need
to be shifted. Cepheids are quite rare, and not one of them was
close enough to show a parallax before Hipparcos! Only a few of
them were within Hipparcos’s outer reach. Thus, the Cepheid
standard candle, the jumping off point for measuring the
distances to the galaxies, remains more uncertain than we would
like, thought there are many ongoing programs to examine
Cepheids in the Milky Way, the Magellanic Clouds and nearby
galaxies to ever better “standardize” this standard candle.
ECLIPSING BINARY TECHNIQUES
Recent work on eclipsing binary stars has yielded an accurate
distance to the globular cluster Omega Centauri, and it has
promise for finding distances to more remote Milky Way and
Magellanic Cloud star clusters (Sky & Telescope March 2002, page
20). An eclipsing binary star near the center of Omega Centauri
was carefully studied by Ian B. Thompson, Janusz Kaluzny, and
their colleagues using large Southern Hemisphere telescopes and
infrared imaging. They measured the binary system’s light curve,
orbital speed, and the spectrum of each star. From these
measurements, they were able to determine the two stars’ masses,
their radii, and their luminosities.
The stars’ luminosities enabled them to estimate Omega
Centauri’s (NGC 5139) modern distance at 17, 800 light years
with a precision of 4% where it had been previously estimated at
16000 light years. The orbital speeds of the stars were measured
by the Doppler effects on their spectra as they rotated around a
common center of mass, and this in turn was used with their
orbital period to calculate the stars’ orbital diameter in
kilometers. The sizes of the stars were calculated from the
system’s light curve, because the shape of the light curve tells
how large the stars are relative to the size of their orbit.
Once the stars’ diameters in kilometers was calculated, the
stars’ spectra provided information concerning their light
output per square kilometer. This then led to determining the
total light output for each star, which was compared to their
observed magnitudes to calculate the distance to Omega Centauri.
Hopefully, other binary star systems in more remote clusters in
the Milky Way and the Magellanic Clouds can be used to similarly
precisely measure distances to these clusters.
DISTANCE BY (SUPER)NOVA CHARACTERISTICS
As mentioned earlier for the calibration of the Cepheid variable
yardstick, Ritchey, Curtis and others found novae (now called
supernovae) in galaxies using photographic plate collections at
several observatories. Novae are stars that temporarily brighten
to approximately 10,000 times brighter than the Sun, and
supernovae are even more than 10,000 times brighter than
ordinary novae. In fact, supernovae at their peak are among the
brightest objects in the Universe. A supernova may brighten 12
magnitudes and can temporarily outshine the brightness of all
the rest of the stars in the galaxy in which it occurs. Novae
are much more common than supernovae. Several occur in the Milky
Way every year, while supernovae are very rare in the Milky Way,
the last being in 1604. In 1919, Lundmark estimated the distance
to M31 from a nova in it as approximately 660,000 light years,
and in 1925 he revised this to 1,250,000 light years (modern
value 2,500,000 light years) when it was appreciated novae and
supernovae are different from each other.
Supernova 1940c in NGC 4725 had a spectrum different than those
of prior supernovae. Because its spectrum contained strong
hydrogen lines and was unlike previous supernovae observed,
Rudolph Minkowski suggested there were at least two basically
different types of supernovas (Sky & Telescope, December 1983, pages
485-490). What is now know as a Type I supernova is a Sun-sized
star that is at the end of its long life and has exhausted its
hydrogen, at which point its core contracts, the hydrogen left
ignites and begins the process of helium burning. The star then
temporarily swells into a red giant, but it is eventually left
with a contracting core of carbon and oxygen, and it becomes a
white dwarf star. If it has a nearby stellar companion, it may
be able to pull matter from the companion. In many cases, the
excess matter is blown off periodically as a nova. More rarely,
the star may continue to accumulate matter until it gets so
massive that it passes Chandrasekhar’s limit (1.44 times the
sun’s mass).
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