Where is galactic center

At the beginning of this chapter, we hinted that the core of our Galaxy contains a large concentration of mass. In fact, we now have evidence that the very center contains a black hole with a mass equivalent to 4. Such monster black holes are called supermassive black holes by astronomers, to indicate that the mass they contain is far greater than that of the typical black hole created by the death of a single star.

It is amazing that we have very convincing evidence that this black hole really does exist. After all, recall from the chapter on Black Holes and Curved Spacetime that we cannot see a black hole directly because by definition it radiates no energy.

And we cannot even see into the center of the Galaxy in visible light because of absorption by the interstellar dust that lies between us and the galactic center. Light from the central region of the Galaxy is dimmed by a factor of a trillion 10 12 by all this dust. Fortunately, we are not so blind at other wavelengths. Infrared and radio radiation, which have long wavelengths compared to the sizes of the interstellar dust grains, flow unimpeded past the dust particles and so reach our telescopes with hardly any dimming.

Most of the hollow circles visible on the radio image are supernova remnants. The other main source of radio emission is from electrons moving at high speed in regions with strong magnetic fields. Figure 1. Brighter regions are more intense in radio waves. The galactic center is inside the region labeled Sagittarius A. Sagittarius B1 and B2 are regions of active star formation.

Many filaments or threadlike features are seen, as well as a number of shells labeled SNR , which are supernova remnants. The scale bar at the bottom left is about light-years long. Notice that radio astronomers also give fanciful animal names to some of the structures, much as visible-light nebulae are sometimes given the names of animals they resemble.

Kassim, D. Briggs, T. Lazio, T. LaRosa, and J. Seen in this picture are hundreds of hot white dwarfs, neutron stars, and stellar black holes with accretion disks glowing with X-rays. The diffuse haze in the picture is emission from gas that lies among the stars and is at a temperature of 10 million K.

Figure 2. The X-ray-emitting point sources are white dwarfs, neutron stars, and stellar black holes. This hot gas is flowing away from the center out into the rest of the Galaxy. The colors indicate X-ray energy bands: red low energy , green medium energy , and blue high energy. Wang et al. Most of these are old, reddish main-sequence stars.

But there are also about a hundred hot OB stars that must have formed within the last few million years. There is as yet no good explanation for how stars could have formed recently so close to a supermassive black hole. Perhaps they formed in a dense cluster of stars that was originally at a larger distance from the black hole and subsequently migrated closer.

There is currently no star formation at the galactic center, but there is lots of dust and molecular gas that is revolving around the black hole, along with some ionized gas streamers that are heated by the hot stars. Figure 3 is a radio map that shows these gas streamers.

Figure 3. Sagittarius A: This image, taken with the Very Large Array of radio telescopes, shows the radio emission from hot, ionized gas in the center of the Milky Way. The lines slanting across the top of the image are gas streamers. To establish that there really is a black hole there, we must show that there is a very large amount of mass crammed into a very tiny volume. As we saw in Black Holes and Curved Spacetime , proving that a black hole exists is a challenge because the black hole itself emits no radiation.

What astronomers must do is prove that a black hole is the only possible explanation for our observations—that a small region contains far more mass than could be accounted for by a very dense cluster of stars or something else made of ordinary matter. To put some numbers with this discussion, the radius of the event horizon of a galactic black hole with a mass of about 4 million M Sun would be only about 17 times the size of the Sun—the equivalent of a single red giant star.

The corresponding density within this region of space would be much higher than that of any star cluster or any other ordinary astronomical object. Both radio and infrared observations are required to give us the necessary evidence.

If we zero in on the inner few light-days of the Galaxy with an infrared telescope equipped with adaptive optics, we see a region crowded with individual stars Figure 4. These stars have now been observed for almost two decades, and astronomers have detected their rapid orbital motions around the very center of the Galaxy. Figure 4. Keck Observatory Laser Team. One of the stars has been observed for its full orbit of Its closest approach takes it to a distance of only AU or about 17 light-hours from the black hole.

This orbit, when combined with observations of other stars close to the galactic center, indicates that a mass of 4. Even tighter limits on the size of the concentration of mass at the center of the Galaxy come from radio astronomy, which provided the first clue that a black hole might lie at the center of the Galaxy.

As matter spirals inward toward the event horizon of a black hole, it is heated in a whirling accretion disk and produces radio radiation.

Such accretion disks were explained in Black Holes and Curved Spacetime. We live in the Milky Way Galaxy, which is a collection of stars, gas, dust, and a supermassive black hole at it's very center. Our Galaxy is a spiral galaxy, which are rotating structures that are flat disk-like like a DVD when looked upon edge-on. There is also a bulge in the middle that consists of mostly old stars. When you look at a spiral galaxy face-on, you can see beautiful spiral arms where stars are being born.

Our solar system is in the Orion arm, and we are about 25, light years 2. Schematic of the Milky way Credit: Oglethorpe University. Since our solar system lies in one of the spiral arms, we live in the flat plane of the Milky Way. We can actually see the dense plane of the Milky Way stretch across the sky in dark places that do not have a lot of surrounding light pollution.

The diffuse light is the combined light from millions of stars.