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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).

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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