Slumbering at the Heart of the Milky Way

We live in a galaxy with a black hole at its heart. Such a thing is not unheard of the universe. Many galaxies have at least one black hole at their cores, some have more than one. The Milky Way’s black hole is called Sagittarius A*, or Sag A* (pronounced “SADGE A-star”) for short. Sag A* lies in the constellation Sagittarius, just off the spout of the Teapot shape. The region where the black hole lives is about 26,000 light-years away from us, and is orbited by a swarm of stars. Astronomers have measured the paths of those stars and that information let them determine the mass of Sag A*:  four million times the mass of the Sun. Unfortunately, the visible light from the region is blocked by clouds of gas and dust. However, the black hole itself and its neighboring stars give off many different wavelengths of light, ranging from radio and infrared to ultraviolet, x-ray, and gamma-rays. Detectors sensitive to those wavelengths give us a whole new window on goings-on in the core of the Milky Way, including measurements of stellar orbits and determining the temperatures of gases that appear to stream out from the core.

Sag A* is something of a quiescent black hole right now. That is, it’s not pouring out as much of a torrent of radiation from its accretion disk area and jets as other, more massive and busy black holes do from the hearts of their galaxies. it does glow in various wavelengths of light, but it’s not as active as some of its cousins.

However, our galactic black hole does occasionally swallow up a cloud of gas and dust, and in return belches out hot gases through its jets. On very rare occasions in galactic history, Sag A* undergoes huge eruptions. The last one took place about 2 million years ago, and it shot filaments of gas out to well beyond the galaxy. The trail of gas from this cataclysm is called the Magellanic Stream, and its glow caught the attention of astronomers.  This stream is evidence of a major cataclysm. Infrared and x-ray satellites have detected a powerful outflow of material (sometimes referred to as a “wind”) from the center of the galaxy. There are also “Fermi bubbles”, large globes of gas billowing out from the region around Sag A* and visible in gamma-rays and radio frequency emissions. According to Dr. Philip Maloney of the University of Colorado in Boulder, these point to something big. “All these point to a huge explosion at the center of our galaxy,” he said, referring to what astronomers call a Seyfert Flare.

You may have heard of Seyfert galaxies—they have radio-loud centers, indicating the presence of something very energetic, like a black hole accretion disk and jet. They’re named after Carl Seyfert, the astronomer who first characterized these objects. The idea that our galaxy was once more Seyfert-like is rather interesting, and tells astronomers the Milky Way’s history is more active than expected.

Right now, the stream is emitting light in a manner that is familiar to anyone who has seen northern or southern lights here on Earth.  In the stream, ultraviolet light from nearby stars and active regions splits hydrogen atoms into protons and electrons. When those components recombine, the electrons give off what is called “H-alpha” emission—a specific wavelength of light that is given off by excited hydrogen gas molecules. The brightest glow in the stream comes from the region nearest the galactic center.

Astronomers know that the galaxy’s stars don’t produce enough UV light to account for this glow, and they very likely didn’t in the past. This is because the galactic center never formed stars at high enough rates. There simply aren’t enough stars to do the job.  So, the logical hypothesis is that our galaxy’s black hole swallowed something pretty big in the past, and the resulting “eruption” belched out the glowing stream we see today.

Is it possible Sag A* will do it again?  Absolutely.  Think of it as a dormant volcano sleeping until there’s enough material available to erupt again. There’s a large supply of “star stuff” and gas clouds available to feed the black hole. It’s not a question of “if” but “when”.  In fact, there’s a gas cloud called G2 that will be passing through the galactic center over the next year. Not only could some of it fall into the black hole and set off a new round of explosive interactions, but it could also feed any smaller black holes hiding in the neighborhood. Telescopes sensitive to radio, ultraviolet, x-ray, infrared, and gamma-rays will be able to detect the uptick of activity once G2 hits the scene. So, stay tuned.  Things could get interesting as G2 passes through.