15.4: Milky Way's Center

The Galactic Core

The core of the Milky Way contains a large concentration of mass. We now have evidence that it contains a black hole with a mass equivalent to 4.6 million Suns and that all this mass fits within a sphere less than the diameter of Mercury's orbit. This type of black hole is called a supermassive black hole by astronomers, indicating that the mass it contains is far greater than that of the typical black hole created by the death of a single star.

Recall that we cannot see a black hole directly because by definition it radiates no energy. In addition, we cannot see into the center of the Galaxy using 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 (10¹²) by all this dust. Fortunately, we can observe 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 can reach our telescopes with hardly any dimming. In fact, the very bright radio source in the nucleus of the Galaxy, now known as Sagittarius A*, pronounced "Sagittarius A-star" and abbreviated Sgr A*, was the first cosmic radio source astronomers discovered.

A Journey Toward the Center

Let's take a voyage to the mysterious heart of our Galaxy and see what's there. Figure \(\PageIndex{1}\) is a radio image of a region about 1500 light-years across, centered on Sagittarius A, a bright radio source that contains the smaller Sagittarius A*. Much of the radio emission comes from hot gas heated either by clusters of hot stars or by supernova blast waves. 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. The bright thin arcs and threads on the figure show us where this type of emission is produced.

Figure \(\PageIndex{1}\): This Very Large Array (VLA) radio map reveals the complex structure at the Milky Way's galactic center, including the Sagittarius A* region, star-forming regions Sagittarius B1 and B2, thread-like filaments, and several supernova remnant shells. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.). Accessible description of Figure \(\PageIndex{1}\).

Stars Near the Galactic Center

Now let's focus in on the central region using a more energetic form of electromagnetic radiation. Figure \(\PageIndex{2}\) shows the X-ray emission from a smaller region 400 light-years wide and 900 light-years across centered on Sagittarius A^*. In this picture there 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. The colors indicate X-ray energy bands: red (low energy), green (medium energy), and blue (high energy)

Figure \(\PageIndex{2}\): Galactic Center in X-Rays. This Chandra X-ray mosaic, centered on Sagittarius A*, reveals point sources such as white dwarfs, neutron stars, and stellar black holes, along with a diffuse haze of 10 million K gas flowing outward from the galactic center. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.). Accessible description of Figure \(\PageIndex{2}\).

As we approach the center of the Galaxy, we find the supermassive black hole Sagittarius A*. There are also thousands of stars within a few lightyears of Sagittarius A*. Most of these are old, reddish main-sequence stars. There are also about one hundred hot O-B stars that must have formed within the last few million years. As of yet, there is no good explanation for how stars could have formed so recently, being this 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. However, 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 \(\PageIndex{3}\) is a radio map that shows these gas streamers as lines slanting across the top of the image.

Figure \(\PageIndex{3}\): This Very Large Array radio image maps hot, ionized gas near the Milky Way's center, showing streamers of gas and the filamentary structure known as The Arc above the bright source Sagittarius A*. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.). Accessible description of Figure \(\PageIndex{3}\).

The Radius and Mass of Sagittarius A*

Just what is Sagittarius A*, which lies right at the center our Galaxy? 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. 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. They have to find 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 about 17 times the size of the Sun, or 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. Therefore, we must measure both the diameter of Sagittarius A* and its mass. Both radio and infrared observations are required to give us the necessary evidence.

First, let's look at how the mass can be measured. 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 \(\PageIndex{4}\) includes measurements by the Nobel Prize winning astronomer, Andrea Ghez. She used the Keck Telescope to track stars that are orbiting Sgr A* from 1995 to 2014. The stars are moving around the center very fast, and their tracks are all consistent with a single massive object that resides in the center of this image.

Figure \(\PageIndex{4}\): Galactic Center in Near Infrared. Tracks of eight stars orbiting the galactic center, measured from 1995 to 2014 with the Keck Telescope, all point to a single massive object concentrated within this small region at the center of the Milky Way. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.). Accessible description of Figure \(\PageIndex{4}\).

If we combine observations of their periods and the size of their orbits with Kepler's third law, we can estimate the mass of the object that keeps them in their orbits. One of the stars has been observed for its full orbit of 15.6 years. Its closest approach takes it to a distance of only 124 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 4.6 million M_Sun must be concentrated inside the orbit.

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 Milky Way. As matter spirals inward toward the event horizon of a black hole, it is heated in a whirling accretion disk and produces radio radiation. Measurements of the size of the accretion disk with the Very Long Baseline Array (VLBA), which provides very high spatial resolution, show that the diameter of the radio source Sagittarius A^* is no larger than about 0.3 AU, or about the size of Mercury's orbit. In light units, that's only 2.5 light-minutes!

The observations thus show that 4.6 million solar masses are crammed into a volume that has a diameter that is no larger than the orbit of Mercury. If this were anything other than a supermassive black hole, such as low-mass stars that emit very little light or neutron stars or a very large number of small black holes, calculations show that these objects would be so densely packed that they would collapse to a single black hole within a hundred thousand years. That is a very short time compared with the age of the Milky Way, which probably began forming more than 13 billion years ago. Since it seems very unlikely that we would have caught such a complex cluster of objects just before it collapsed, the evidence for a supermassive black hole at the center of the Galaxy is very convincing.

Origin and Growth of Sagittarius A*

Where did our galactic black hole come from? The origin of supermassive black holes in galaxies like ours is currently an active field of research. One possibility is that a large cloud of gas near the center of the Milky Way collapsed directly to form a black hole. Since we find large black holes at the centers of most other large galaxies, even ones that are very young, this collapse probably would have taken place when the Milky Way was just beginning to take shape. The initial mass of this black hole might have been only a few tens of solar masses. Another way it could have started is that a massive star might have exploded to leave behind a seed black hole, or a dense cluster of stars might have collapsed into a black hole.

Once a black hole exists at the center of a galaxy, it can grow over the next several billion years by devouring nearby stars and gas clouds in the crowded central regions. It can also grow by merging with other black holes. At the present time, we observe clouds of gas and dust falling into the galactic center at the rate of about 1 M_Sun per thousand years. The density of stars near the galactic center is high enough that we would expect a star to pass near the black hole and be swallowed by it every ten thousand years or so. As this happens, some of the energy of infall is released as radiation. As a result, the center of the Galaxy might flare up and even briefly outshine all the stars in the Milky Way. Other objects might also venture too close to the black hole and be pulled in. How great a flare we observe would depend on the mass of the object falling in.

In 2013, the Chandra X-ray satellite detected a flare from the center of our Galaxy that was 400 times brighter than the usual output from Sagittarius A*. A year later, a second flare, only half as bright, was also detected. This is much less energy than swallowing a whole star would produce. There are two theories to account for the flares. First, an asteroid might have ventured too close to the black hole and been heated to a very high temperature before being swallowed up. Alternatively, the flares might have involved interactions of the magnetic fields near the galactic center in a process similar to the one described for solar flares. Astronomers continue to monitor the galactic center area for flares or other activity. Although the black hole in the center of the Galaxy is not close enough to us to represent any danger, we still want to keep our eyes on it.

Further Exploration:

• Watch an animation of stars orbiting Sagittarius A*, the compact object at the center of the Milky Way. The observed stellar motions provide strong evidence for the presence of a supermassive black hole.
• Watch Andrea Ghez's TED Talk to learn how observations of stars orbiting Sagittarius A*, the object at the center of the Milky Way, were used to measure its mass and provide evidence for the presence of a super massive black hole.