What Is the Center of Our Galaxy Like?

Zoom into the Center of Our Galaxy
This video zooms into the Hubble Space Telescope view of the galactic core. Hubble's infrared vision pierced the dusty heart of our Milky Way galaxy to reveal more than half a million stars at its core. Except for a few blue, foreground stars, the stars are part of the Milky Way's nuclear star cluster, the most massive and densest stellar cluster in our galaxy. Located 27,000 light-years away, this region is so packed with stars, it is equivalent to having a million suns crammed into the volume of space between us and our closest stellar neighbor, Alpha Centauri, 4.3 light-years away. At the very hub of our galaxy, this star cluster surrounds the Milky Way's central supermassive black hole, which is about 4 million times the mass of our Sun.

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Hubble's infrared vision reveled more than half a million stars at the core of the Milky Way.
Our solar system resides in one of the spurs off the spiral arms of the Milky Way galaxy, and we tend to think of our experience of the Milky Way as typical. Even in our science fiction films, when the heroes travel between stars, every sky looks the same.

But the Milky Way isn't quite so uniform. If you lived in the center of the Milky Way, you could look up on a sky thick with stars, a thousand to a million times more than we're used to seeing, depending on how close you were to the core. For Earth's inhabitants, the next closest star to our Sun is about 4 light-years away. For our central Milky Way cousins, that star would be around 0.4–0.04 light-years distant.

The center of the Milky Way consists of the region where the galaxy's spiral arm structure has broken down and transformed into a "bulge" of stars, or roughly the inner 10,000 light-years. At its heart — and the dominant force in that area of the galaxy — is a million-solar-mass black hole we call Sagittarius A*.

It would be an inhospitable area for humanity, rife with radiation emanating from a surplus of massive stars and material being torn apart by the black hole. Plus it would take us more than 25,000 years to reach it, even if we could travel close to the speed of light. Fortunately, the Webb telescope is designed to do our exploring of the galactic center for us.

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This simulated image shows a supermassive black hole at the core of a galaxy. The black region in the center represents the event horizon, where no light can escape the massive object's gravitational pull. The black hole's powerful gravity distorts the space around it, stretching light from background stars.

Sleeping Giant

Our central, supermassive black hole is relatively quiet when compared to its counterparts in other galaxies, flaring only occasionally with X-rays and infrared light as objects fall into it. It could be that there's simply not that much material around Sagittarius A*. Webb will investigate our strangely calm central black hole, providing a more accurate measurement of its mass, as well as how much material is falling into it, and when. Furthermore, the mass of our black hole ranges on the low end of normal for galaxies of our size. Webb will examine why that is and the relationship between a black hole and the matter surrounding it.

While Webb helps reveal why we have the kind of black hole that we do, it'll also be shedding light on central, supermassive black holes in other galaxies. Active galactic nuclei (AGN) are a type of extremely bright galaxy core seemingly fueled by powerful black holes actively gobbling large amounts of material. Astronomers would like to know what, exactly, AGN are and if they are triggered by events occurring in the centers of galaxies or by mergers between galaxies.

Webb's investigations of our own black hole and the relationship between black holes and galaxy evolution could help solve a cosmic chicken-and-egg problem: Did black holes come first and galaxies form around them, did galaxies form first and develop black holes, or did the galaxies and black holes develop together?

Updated: May 06, 2016