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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 300. 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 20 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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