What Is the Center of Our Galaxy Like?

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Even if humans could explore there, it would take us more than 25,000 years to reach it.

We tend to think of our experience in the Orion Spur of the Milky Way as typical. Even in science fiction films, when traveling 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 would look up at a sky thick with stars, one thousand to 1 million times more dense 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 four light-years away. In the center of the galaxy, stars are only 0.4–0.04 light-years apart.

 

Zoom into the Hubble Space Telescope view of the Milky Way’s dense galactic core. Hubble’s near-infrared vision pierced the dusty heart of the galaxy to reveal more than half a million stars in its nuclear star cluster, the most massive and densest stellar cluster in the galaxy. This region is so packed with stars, it is equivalent to having 1 million suns crammed into the volume of space between us and our closest stellar neighbor, Alpha Centauri, 4.3 light-years away. The star cluster surrounds the Milky Way’s central supermassive black hole, which is about 4 million times the mass of the Sun. CREDIT: NASA, ESA, and G. Bacon (STScI).

 

Milky Way's galactic core
Hubble's infrared vision reveled more than half a million stars at the core of the Milky Way. CREDIT: NASA, ESA, and the Hubble Heritage Team (STScI, AURA).

The center of the Milky Way, roughly the inner 10,000 light-years, consists of the region where the galaxy’s spiral arm structure has broken down and transformed into a “bulge” of stars. At its heart—and the dominant force in that area of the galaxy—is a 1 million-solar-mass black hole named Sagittarius A*. The center of the galaxy would be an inhospitable region for humanity, rife with radiation from the stars and material being torn apart by the powerful gravity of Sagittarius A*. Even if humans could explore the region, it would take us more than 25,000 years to reach it, traveling at close to the speed of light. Fortunately, the James Webb Space Telescope is designed to explore the galactic center for us.

Sleeping Giant

Black hole-powered Jet in galaxy M87
A black-hole-powered stream of material is being ejected from the core of elliptical galaxy M87. The galaxy is the nearest example of an active galactic nucleus with a bright optical jet. CREDIT: NASA, ESA, and the Hubble Heritage Team (STScI, AURA).

Sagittarius A* is relatively quiet compared to central supermassive black holes in other galaxies, flaring only occasionally with X-rays and infrared light as objects fall into it. Webb can 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.

Previous research has indicated that the mass of Sagittarius A* ranges on the low end of normal for galaxies the size of the Milky Way. Webb can examine why that is, and the relationship between a black hole and the matter surrounding it in part by studying 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 consuming large amounts of cosmic material. Astronomers plan to test their hypotheses about the nature of AGN, and if they are triggered by events occurring in the centers of galaxies or by mergers between galaxies.

Webb’s investigations of our own galaxy’s central 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, or did galaxies form first and develop black holes? Or did the galaxies and black holes develop together?

Last Updated: May 31, 2018

Keywords:  Black HolesGalaxiesMilky Way Center

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