The Distance to the Galactic Center
M. Colleen Gino
Astronomers have been striving to accurately measure the distance to the Galactic Center since the early 20th century, when it was discovered that our Solar System is not located in the center of the Milky Way Galaxy. While the first estimates of this distance were widely varied and woefully incorrect, technological advancements have enabled astronomers to gauge the distance to the galactic center with increasing accuracy. The distance between the Sun and the center of our Galaxy is used as a reference for a multitude of other distance calculations in astronomy, making the determination of an accurate figure a matter of extreme importance.
In 1914, Harlow Shapley began a survey of globular clusters using the 60-inch telescope at Mt. Wilson Observatory. He found that a large percentage of the globular clusters he observed contained a type of Cepheid variable, a star in the late stages of life that varies in brightness in a regular pattern. The relationship between the period of variability and the intrinsic luminosity of an RR Lyrae variable allows its distance to be accurately determined. Armed with this information, Shapley measured the distances to 93 globular clusters. Based upon these distance estimates, Shapley calculated the diameter of the Galaxy to be 300,000 light years. More importantly, he determined the Galactic Center to be located in the constellation of Sagittarius, by mapping out the three dimensional distribution of the clusters. According to Shapley, we were no longer at the center of our Galaxy, but 40,000 light years away (1).
Jan Oort came to the same conclusion regarding the location of the Galactic Center when conducting a study on the motion of stars in the vicinity of the Sun. As he expected, the stars exhibited differential rotation stars closer to the center of the galaxy traveled at higher velocities than stars farther away from the center. By determining the center of rotation of the stars he pinpointed the center of the Galaxy, which was within 2° of Shapley's estimate. In his paper published in 1927 Oort reported the distance to the center of the Galaxy as 19,000 light years, a figure much lower than Shapley's (2).
Shapley and Oorts disparate distance estimates were due in large part to their lack of knowledge of interstellar extinction, the dimming of starlight as it passes through the gas and dust of the interstellar medium. Discovered in the 1930's by Robert Trumpler of Lick Observatory, interstellar extinction is responsible for the dimming of light by one magnitude for every 5000 light years of distance. When Shapley's distance estimate of 40,000 light years to the center of the Galaxy is corrected for interstellar extinction, the resulting figure, 25,000 light years, is extremely close to the approximate distance agreed upon today of 26,000 light years (3).
Directly observing the Galactic Center has been difficult for optical astronomers, as it is shrouded in gas and dust that is impenetrable by visible light. However, since the introduction of radio astronomy in the 1930's by Karl Jansky, and the successive improvements in instrument technology such as cooling systems for the receivers and faster computer processors for data acquisition and analysis, it is now possible to directly observe the Galactic core and thereby measure its distance from us. Radio astronomer Mark Reid is currently working on a project to directly determine the distance to the Galactic Center using trigonometric parallax (4). Using the Very Long Baseline Array (VLBA), a set of 10 identical radio telescopes spread across the US from Hawaii to St. Croix in the US Virgin Islands, Reid has been able to measure the motion of the Sun around the Galactic Center and the motion of Sgr A* (believed to be massive black hole at the center of the Galaxy) using distant quasars as reference points. As expected for an object with the mass of a black hole (2.6 billion solar masses), Sgr A* remains virtually motionless. Therefore, any detected change in the location of Sgr A* is due to our own motion in the Galaxy. By measuring this difference in position of the Galactic Center when observed six months apart in time, allowing the Earth to move halfway around in its orbit, the distance to the Galactic Center can be calculated.
The distance between the Sun and the Galactic Center, referred to as Ro, is an important one. Many of the measured parameters of galactic objects such as distance, mass and luminosity are directly related to Ro, which has an estimated value of 8.0 kilo parsecs (~26,000 light years), with a standard error of about 0.5 kilo parsecs. Furthermore, many of the estimates of both the gravitational mass and luminosity of objects within our Galaxy, and even our Galaxy itself, are proportionate to Ro. As the estimate of this distance changes, so does the estimated distance, mass and luminosity of scores of galactic and extra-galactic objects. For this reason, astronomers continue to put a concerted effort into establishing an accurate value for Ro.
Samir Salim and Andrew Gould from Ohio State University are two such astronomers. They are using a technique to directly measure the distance to the Galactic Center with the highest precision yet achieved (5). They plan to solve for the Keplerian orbit of individual stars in orbit around the massive black hole presumed to be at the center of our Galaxy. A Keplerian orbit is a closed ellipse that occurs when an object orbits a point mass, such as a planet orbiting the Sun. In general, the orbits of stars in a galaxy are not Keplerian, since they are not in orbit around a point mass. In the case of the stars in the Galactic Center, however, they are clearly in orbit around a point mass, a black hole.
Salim and Gould expect to be able to determine Ro to an accuracy within 4% as early as the year 2002. While this is indeed an improvement over the current standard of error, it is a slight one. Nonetheless, the astronomical community awaits their results.
More than eight decades after the realization that we are not located at the center of our Galaxy, astronomers are still trying to come up with an accurate distance measurement to the Galactic Center. This measurement, Ro, has great impact on the field of astronomy. This is important because as the distance estimate for Ro changes, so do the estimates for galactic rotation, galactic size, and extra galactic distances. Although advancements in technology have allowed scientists to narrow the margin of error, we will likely not have a definitive answer until well into the 21st century.
(1), (2) Editors, Galaxies, Time-Life Books
(3) Reid, M., 1993, The Distance to the Center of the Galaxy, Annual Review of Astronomy and Astrophysics, Vol. 31 (A94-12726 02-90), p. 345-372
(4) Reid, M., et al, 1999, The Proper Motion of Sagittarius A*, First VLBA Results, The Astrophysical Journal, Volume 524, Issue 2, p. 816-823
(5) Salim, S., Gould, A., Sgr A* "Visual Binaries": A Direct Measurement of the Galactocentric Distance, submitted to Astrophysical Journal 2001, http://www.mpifr-bonn.mpg.de/gcnews/gcnews/Vol.firstname.lastname@example.org_ap9812292.abs.shtml
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