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The Distance to the Galactic Center
By Tim Hunter |
Introduction
The Milky Way is a luminous band that encircles the sky and is
readily visible to the naked eye from a dark sky location.
Through a small telescope the Milky Way consists of thousands of
faint stars. Because it completely encircles the sky, early
observers concluded we live within it (Moore, 2002). It is widest
in Sagittarius where its width is more than 30 degrees. It was not
realized until the twentieth century that the Solar System does
not lie at the center of the Milky Way. According to Bok (1981):
“There is much evidence to show that the galactic center lies in
the direction of Sagittarius...[The irregular distribution of
globular clusters first noted by Shapley]…is strong evidence for
the existence of a distant center in Sagittarius.” There is a
concentration toward Sagittarius of novae, distant variable
stars, and planetary nebulae. Galactic rotation studies and
strong radio emissions from the same part of the sky also
support the presence of the Galactic center in Sagittarius (Bok,
1981).
Shapley and Oort
In 1914 Shapley began a survey of globular clusters and
estimated the distance to 93 of them using RR Lyrae stars. By
calculating the center of the distribution of the globular
clusters surrounding the Milky Way, he determined the Galactic
center was in Sagittarius, and the Solar System was 13
kiloparsecs from the Galactic center (Reid, 1993; Gino, 2003).
Oort came to the same conclusion regarding the location of the
Galactic center by studying the motion of stars near the Sun. He
found stars closer to the center of the galaxy had higher
rotation speeds than those further from the center. This
differential rotation was best explained by locating the center
of the galaxy in Sagittarius. His Galactic center location was
within 2 degrees of that of Shapley (Gino, 2003).
Work by Trumpler in the 1930’s showed interstellar gas and dust
causes dimming of visual starlight as it passes through the dust
and gas. Bok and others have shown that in the direction of the
Galactic center there is much obscuring interstellar matter
within two or three kiloparsecs of the Sun. Bok and Bok (1981)
state there is “…25 to 30 magnitudes for the overlying
obscuration in visible light between us and the galactic
center.” When Shapley’s estimate for the distance to the
Galactic center is revised to account for interstellar
extinction, it becomes the basis for our modern value of 8
kiloparsecs (Reid, 1993).
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Observations of the Galactic Core |
Visual Observations
Walter Baade started using the 18-in Schmidt telescope at Mount
Palomar in 1937 soon after it was completed. He attempted to
penetrate the heavy interstellar extinction near the Galactic
center by using panchromatic films and deep red filters (Osterbrock,
2001). Baade was unable to observe the Galactic center itself,
but he found regions of less obscuration close to the heart of
the Milky Way, including “Baade’s window,” which is centered
around NGC 6522 in Sagittarius (Osterbrock, 2001; Moore, 2002).
These regions of less obscuration permit visual and near
infrared observation of RR Lyrae stars, Mira variables, and
other types of stars in the Galactic bulge relatively close to
the Galactic core. Observation of such stars provides another
method besides those of Shapley and Oort for measuring the
distance to the Galactic center. The observed brightness of
these stars near the Galactic center can be corrected for
interstellar extinction and then compared to their known
intrinsic luminosities, thus permitting the distance to the
Galactic center to be estimated.Infrared and Radio
Observations; The Black Hole at the Galactic Center
Observations of the Galactic center at infrared and radio
wavelengths show a very complex structure. These observations
are possible because long wavelengths readily penetrate through
the obscuring dust and gas between the Solar System and the
Galactic center. Radio and infrared observations are also
complemented by observations of the Galactic center at very
short wavelengths in X-ray and gamma ray frequencies (NRL, 2003;
Baganoff, 2003).
At the very center of the Milky Way, there is a complex of hot
dense molecular clouds 14 parsecs in diameter which partially
surrounds an inner most star cluster (Spark and Gallagher, 2000).
This complex of molecular clouds, Sagittarius A (Sgr A), is the
strongest radio source in the Galactic plane near the Galactic
center. In 1974 Balick and Brown reported “(t)he detection of
strong radio emission from [a] bright…unresolved…structure in
the inner 1 pc core of the galactic nucleus…” This is now called
Sagittarius A* (Sgr A*) to distinguish it from Sagittarius A
that surrounds it (Balick and Brown, 1974; Science NASA, 2002).
Sagittarius A* is contiguous to the inner most star cluster. |
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