Laurence A. Marschall in the department of physics at Gettysburg College in Gettysburg, Pa., answers: "There is no short answer to this question, because astronomers have followed many lines of evidence to determine the location of the solar system in the Milky Way. But some of the general techniques can be outlined briefly. "Finding one's location in a cloud of a hundred billion stars--when one can't travel beyond one's own planet--is like trying to map out the shape of a forest while tied to one of the trees. One gets a rough idea of the shape of the Milky Way galaxy by just looking around--a ragged, hazy band of light circles the sky. It is about 15 degrees wide, and stars are concentrated fairly evenly along the strip. That observation indicates that our Milky Way Galaxy is a flattened disk of stars, with us located somewhere near the plane of the disk. Were it not a flattened disk, it would look different. For instance, if it were a sphere of stars, we would see its glow all over the sky, not just in a narrow band. And if we were above or below the disk plane by a substantial amount, we would not see it split the sky in half--the glow of the Milky Way would be brighter on one side of the sky than on the other. "The position of the sun in the Milky Way can be further pinned down by measuring the distance to all the stars we can see. In the late 18th century, astronomer William Herschel tried to do this, concluding that the earth was in the center of a 'grindstone'-shaped cloud of stars. But Herschel was not aware of the presence of small particles of interstellar dust, which obscure the light from the most distant stars in the Milky Way. We appeared to be in the center of the cloud because we could see no further in all directions. To a person tied to a tree in a foggy forest, it looks like the forest stretches equally away in all directions, wherever one is. "A major breakthrough in moving the earth from the center of the galaxy to a point about 3/5 away from the edge came in the early decades of this century, when Harlow Shapley measured the distance to the large clusters of stars called globular clusters. He found they were distributed in a spherical distribution about 100,000 light-years in diameter, centered on a location in the constellation Sagittarius. Shapley concluded (and other astronomers have since verified) that the center of the distribution of globular clusters is the center of the Milky Way as well, so our galaxy looks like a flat disk of stars embedded in a spherical cloud, or 'halo,' of globular clusters. "In the past 75 years, astronomers have refined this picture, using a variety of techniques of radio, optical, infrared and even x-ray astronomy, to fill in the details: the location of spiral arms, clouds of gas and dust, concentrations of moleculesand so on. The essential modern picture is that our solar system is located on the inner edge of a spiral arm, about 25,000 light-years from the center of the galaxy, which is in the direction of the constellation of Sagittarius. "For details on how we know all this, there are, fortunately, two excellent and complementary books: The Discovery of Our Galaxy, by Charles A. Whitney (Knopf, 1971), and The Alchemy of the Heavens: Searching for Meaning in the Milky Way , by Ken Croswell (Anchor Books, 1995).
The Halo The Halo consists of the oldest stars known, including about 146 Globular Clusters, believed to have been formed during the early formation of the Galaxy with ages of 10-15 billion years from their H-R Diagrams. The halo is also filled with a very diffuse, hot, highly-ionized gas. The very hot gas in the halo produces a gamma-ray halo. Neither the full extent nor the mass of the halo is well known. Investigations of the gaseous halos of other spiral galaxies show that the gas in the halo extends much further than previously thought, out to hundreds of thousands of light years. Studies of the rotation of the Milky Way show that the halo dominates the mass of the galaxy, but the material is not visible, now called dark matter. The Disk The disk of the Galaxy is a flattened, rotating system which contains the Sun and other intermediate-to-young stars. The sun sits about 2/3 of the way from the center to the edge of the disk (about 25,000l.y. by the most modern estimates). The sun revolves around the center of the galaxy about once every 250 million years. The disk also the galaxy about contains atomic (HI) and molecular (H2) gas and dust. Here is an excellent tutorial on the Shape of the Milky Way by Rick Arendt.  Credit & Copyright: John P. Gleason, Celestial Images The Optical View (above) is dominated by emission from stars and extinction by dust; we can see only a thousand light years or so in the plane. The infrared views from IRAS, shown below demonstrate that the shape of the Milky Way is more regular Stars in the Milky Way at Mid-Infrared Wavelengths (free from dust extinction - NASA/IRAS Image).
Warm Dust in the Milky Way (NASA/IRAS Image)
Multiwavelength Milky Way Request a free copy of the Multiwavelength Milky Way poster. COBE Infrared view. Spiral Structure Spiral Arms
The Center of the Galaxy What lies at the center of our Galaxy? Again, dust obscures the visible light from us and we must use radio and infrared observations to elicit the nuclear properties of the Galaxy. A census shows us that the Galactic Center region is an unusually crowded place, even in this visible-light Map of Central region. At radio wavelengths, where we can peer down to the very center, we see the complex strctures shown in the 1-meter wavelength radio map made by NRL astronomers which is shown below. The map shows a region about 2000 light-years on a side; the center of the Milky Way coincides with the source marked Sag A (or Sagittarius A), which is actually three sources, a yound supernova remnant on the east side, an unusual ionized hydrogen region on the west side, and a very compact source called Sagittarius A* at the very center.
Although there is no lack of fascinating questions about the Galactic Center, recent interest has been focused on the question of the possibility that a massive black hole exists at the center of the central star cluster. The presence of very high velocities in the stars and gas near the center of the galaxy has suggested to astronomers for a long time that a massive black hole might be present, providing a strong enough gravitational pull to keep the stars and gas in orbit. Andrea Ghez, a professor at UCLA, used the Keck 10-meter telescope at infrared wavelengths to measure the velocities of 20 stars that lie close to the galactic center over a three-year period. She found the stars are orbiting at speeds up to 1000 km/s (3 million miles per hour)! Observation done by scientists at the Max-Planck Institut in Germany have confirmed these results. This large gravitational acceleration requires an object with a mass 2.5 million times that of our Sun. The stars are located near Sagittarius A*, the radio source lying near the center of the galaxy. From its radio signal alone, Sgr A* did not have to be particularly massive, since its emission is not very powerful. Using the VLBA (Very Long Baseline Array) radio telescope, astronomers studied the motion of Sgr A*; they found a velocity of less than 20 km/sec for Sgr A* itself. This means it is very unlikely that Sgr A* is a single star or group of stars. Only a very massive object could remain stationary under the conditions exisiting at the center of the galaxy. The evidence is mounting that Sag A* is indeed a black hole of 2-3 million times the mass of the sun. Astronomers speculate that the Black Hole is being "fed" by gas from the molecular ring, or supernova remnant. By consuming less than about 1% of the mass of a star each year, releasing gravitational potential energy, Sag A* can easily account for the high-energy phenomena near the galactic center.
Comments? You may send email to Prof. H. E. (Gene) Smith CASS 0424 UCSD 9500 Gilman Drive La Jolla, CA 92093-0424 Last updated: 28 April 1999 |
This photo shows an object located in the halo of our milky way galaxy. what kind of object is it?
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