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By the 1820’s, Joseph Fraunhofer, famous for his identification
of the elemental lines in the Sun’s spectrum, had built
telescopes with such superb pointing accuracy that they were at
last capable of making parallax measurements. Using a Fraunhofer
6-inch refractor, the German astronomer Friedrich Bessel is
credited with having determined the first definitive parallax.
Bessel assuming that a high proper motion indicated that a star
was close to Earth, determined the distance to 61 Cygni. In
October 1838, he announced a parallax value for 61 Cygni as
0.314 arc seconds based upon hundreds of position measurements
from March 1829 to October 1838. He determined this star was
some 99 trillion miles distant, equivalent to 10.3 years of
travel by light. He thus was the first astronomer to introduce
the concept of light years. The modern value for the distance to
61 Cygni is 11.4 light years.
Independently, in 1835 Wilhelm Struve, another German astronomer
who was working in Russia using a Fraunhofer 9-½ inch refractor,
began measurements of the first magnitude star Vega. In 1837
Struve reported that he had obtained a preliminary parallax
value for Vega of 0.261 arc seconds or 12.5 light years. In late
1839, Struve published a greatly improved distance value for
Vega of 26 light years (modern value is 25.3 light years), which
was about twice as distant as his earlier reported value.
The third astronomer making the earliest successful parallax
observations independently was a Scotsman named Thomas
Henderson. Henderson also assumed that brighter stars indicated
that they were closer and observed Alpha Centauri from the Cape
of Good Hope. Henderson obtained a parallax displacement for
Alpha Centauri of more than 1 arc second indicating that it was
about 3 light years away (modern value is 4.39 light years).
Although Henderson made his observations much earlier than
Bessel and Struve, he didn’t reduce and publish his results
until two months after Bessel did.
For succeeding years, visual trigonometric parallax measurements
were considered only accurate for stars out to a distance of 30
light years. Beyond that they became increasingly more
uncertain. With the advent of photography in the late 1800’s,
the accuracy and ease of parallax measurement was significantly
increased. However, even by the 1980’s, the accuracy of these
measurements from the Earth’s surface was no better than about
0.008 arc seconds. Distances beyond 100 light years were
problematic, though some estimates were made out to
approximately 400 light years, which is too small a distance to
give us a good picture of our stellar neighborhood. For example,
the closest open (galactic) star cluster to the Earth was found
to be the Hyades, which was estimated to be 120 light years away
(modern value 150 light years). The next closest open star
cluster is the Pleiades, which was estimated to be 400 light
years away (modern value is 380 light years).
Because of the limitations of ground based parallax measurements
to provide an accurate picture of the stars around us, a
satellite system was developed to measure parallaxes from space
where the distortion of the Earth’s atmosphere would not
interfere with the measurements. This would extend an accurate
parallax distance scale out much further to more accurately map
remote celestial objects. The European Space Agency (ESA)
launched the High Precision Parallax Collecting Satellite (Hipparcos)
in August 1989, and it ceased functioning in August 1993.
Hipparcos was named for Hipparchus (190-120 B.C.) who compiled
the first “precise” star catalog in antiquity.
Due to a malfunction in the rocket booster’s final stage, the
satellite did not achieve its intended 22,000 mile geostationary
circular orbit. Instead it settled in a long elliptical orbit,
which greatly increased the effort needed to communicate with
the satellite. Nevertheless, it produced an incredible wealth of
information (Sky & Telescope June 1999, pages 40-50).
The main Hipparcos catalog has parallaxes and other information
for 118,218 stars. Its precision is approximately 0.001 arc
seconds. Distances to stars within 30 light years of the Sun are
precise to 1% accuracy. The Hyades was found to be 151 light
years away and the Pleiades 375 light years away. Distances to
stars out to 300 light years away were measured with an error
rate of 10% or less. Hipparchos also collected lower precision
parallax data on 2.5 million stars, the Tyco Catalog.
Because of the success of Hipparcos, ESA and NASA have several
parallax satellites on the drawing board to provide very precise
parallax data on millions of stars out to distances of several
thousand light years.
Recently, G. Fritz Benedict of MacDonald Observatory and
eighteen other astronomers used one of the HST’s fine guidance
sensors to obtain an even more accurate parallax distance for
the classic variable star Delta Cepheus of 3.66 ± 0.15
milliseconds or 890 ± 36 light years (Sky & Telescope December
2002, page 26). The importance of highly accurate variable star
measurements for Cepheid Variable Stars is described in detail
below.
SPECTROSCOPIC PARALLAX
In 1902, the American astronomer W. S. Adams, who was the second
director of the Mount Wilson Observatory, and a German
astronomer A. Kohlschutter discovered the spectral criteria for
luminosity, a method by which the distance from the Earth to a
star might be estimated by its intrinsic brightness. By
examining the ratios of the intensities of the spectral lines of
elements such as hydrogen, calcium, strontium and iron, Adams
and Kohlschutter noted that the relative intensity of these
lines in F, G, K and M spectral class stars differed according
to the star’s intrinsic luminosity determined from nearby stars
whose trigonometric parallaxes were known Their study of the
slight changes in the intensity of these spectral lines resulted
in a method where the “absolute” brightness of a star could be
estimated and hence its distance computed by just comparing its
apparent brightness. This method evolved at Mount Wilson and has
been refined over the years to include stars with other spectral
types.
The absolute brightness or magnitude of a star is defined as the
brightness of the star as if it were seen from a standard
distance of 32.6 light years. This distance is known as one
parsec, because at this distance the parallax of a star is 0.01
arcsecond. The spectroscopic parallax method is less accurate
than the standard parallax method, but it extends our distance
scale much further out. It is based on the assumption that stars
of a similar mass have the same composition and the same life
cycle.
In 1911, the Danish astronomer Enjar Hertzsprung compared the
colors and luminosities of stars within several clusters by
plotting their magnitudes against their colors. In 1913, the
American astronomer Henry Norris Russell undertook a similar
investigation of stars in the solar neighborhood by plotting the
absolute magnitudes of stars of known distance against their
spectral classes. These investigations led to an extremely
important discovery concerning the relation between the
luminosities and surface temperatures of stars. Today, these
findings are exhibited graphically on a diagram known as the
Hertzsprung-Russell (HR) diagram,
which is a plot of a star’s color (spectral class or surface
temperature) versus its intrinsic luminosity.
Extensive work done in the late 1800’s and the early and mid
portion of the 1900’s, showed that a star’s position on an HR
diagram is also indicative of its approximate mass and its
approximate age. If one knows a star’s brightness (its
magnitude) and its position on an HR diagram, then one can
calculate its distance as well as its size. When stars in an
open cluster or globular cluster are measured and plotted on an
HR Diagram, it is possible to derive an estimate for the
cluster’s age and distance by looking at the distribution of its
stars amongst the various spectral classes (Sky & Telescope June
1987, page 482).
CEPHEID VARIABLE STARS- the PERIOD-LUMINOSITY RELATION
Although the spectroscopic parallax method had opened the door
to finding our place among more distant stars and nebulae in the
early 20th century, it did not show how galaxies fitted into the
picture. At this time normal F, G, K and M spectral class stars
could not be resolved even in the closest galaxies. Again, some
new signpost marker was needed.
In 1912 while studying photographic plates of the Small
Magellanic Cloud (SMC), Henrietta Leavitt at the Harvard
Observatory found about 100 variable stars, 25 of which were
Cepheids [Harvard College Observatory Circular #173, 1912].
Cepheid variable stars are named after Delta Cepheus, the first
one of this class of intrinsically pulsating variable stars to
be discovered. This type of star goes from a maximum to minimum
brightness with a predictable regularity. We now know this
change in brightness is due to these stars being in a late phase
of their life where radiative and gravitational forces are in an
oscillating state. Leavitt noted that the brighter a Cepheid
became at its maximum brightness, the longer it took the star to
go through its period of change from one maximum to the next.
Using this data, she reported her observations concerning the
relative period (of brightness change)-luminosity relationship.
Since these SMC Cepheid variable stars were all at about the
same distance, Leavitt reasoned that the period must depend upon
the absolute magnitude of these stars. If a zero point could be
deduced for the relationship between the relative and absolute
magnitude of these Cepheids, the distance to the SMC could be
determined as well as the distance to the Cepheid variable stars
in the Milky Way and its globular star clusters. However, at
this time in the early 20th century there were no Cepheid
variable stars near enough with a known distance to establish
this zero point. The nearest Cepheid star was at an uncertain
distance of about 200 light years away.
In 1913, the Danish astronomer Ejnar Hertzprung developed an
indirect method to find this zero point. Hertzprung made a
determination of the average distance (and from that the average
luminosity) of 13 Cepheids in the Milky Way system. Five years
after Hertzprung reported his zero point with respect to
relative and absolute magnitudes of Cepheid variable stars,
Harlow Shapely, later the Director of the Harvard Observatory,
recalculated the average distance to several Cepheids. Shapely
used his revised figures to map the globular clusters in our
Milky Way Galaxy’s halo and determined their distances as being
about ten to twenty thousand light years away.
Later work at a number of observatories, such as Harvard,
McCormick, and Mount Wilson resulted in further refinement of
the relationship between Cepheid period and luminosity. However,
Cepheid variable stars could not yet be used as yardsticks to
determine the distances to galaxies since such stars still could
not be resolved in them. In the early 20th century, it was not
yet known what galaxies were, and there was considerable debate
among astronomers about their composition. In fact, learned
arguments ranged as to whether galaxies were nearby nebulae or
remote groups of stars.
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