What's Happening in the Milky Way's Core?

What's Happening in the Milky Way's Core?

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Something's happening at the heart of the Milky Way galaxy - something intriguing and really fascinating. Whatever it is, the events they've seen there have astronomers focused on understanding just how it works. What they learn will go a long way to helping our understanding of such black holes at the hearts of other galaxies, too.

All the activity is related to the galaxy's supermassive black hole - named Sagittarius A* (or Sgr A* for short) - and it lies right at the center of our galaxy. Normally, this black hole has been pretty quiet, for a black hole. Sure, it periodically feasts on stars or gas and dust that stray into its event horizon. But, it doesn't have strong jets as other supermassive black holes do. Instead, it's pretty quiet, for a supermassive black hole.

What's it Eating?

Astronomers began to notice in recent years that Sgr A* is sending out "chatter" that's visible to x-ray telescopes. So, they began to ask, "What kind of activity would cause it to suddenly wake up and start sending out emissions?" and they began looking at possible causes. Sgr A* seems to produce about one bright x-ray flare every ten days or so, as picked up by long-term monitoring done by the Chandra X-ray Observatory, Swift, and XMM-Newton spacecraft (which all perform x-ray astronomy observations). Suddenly, in 2014, the black hole kicked up its messaging - producing a flare every day.

A Close Approach Starts Sgr A* Chattering

What could have irritated the black hole? The uptick in x-ray flares happened soon after the
close approach to the black hole by a mysterious object astronomers named G2. They long thought G2 was an extended cloud of gas and dust in motion around the central black hole. Could it be the source of material for the black hole's feeding uptick? In late 2013, it passed very close to Sgr A*. The approach didn't tear apart the cloud (which was one possible prediction of what might happen). But, the black hole's gravitational pull did stretch the cloud a bit.

What's Happening?

That posed a mystery. If G2 was a cloud, it very likely would have been stretched quite a bit by the gravitational tug it experienced. It didn't. So, what could G2 be? Some astronomers suggest it might be a star with a dusty cocoon wrapped around it. If so, the black hole may have pulled some of that dusty cloud away. When the material encountered the black hole's event horizon, it would have been heated enough to give off x-rays, which were reflected by the clouds of gas and dust and picked up by the spacecraft.

The increased activity at Sgr A* is giving scientists another look at how the material is funneled into our galaxy's supermassive black hole and what happens to it once it gets close enough to feel the gravitational pull of the black hole. They know that it is heated as it spins around, partly from friction with other materials, but also by magnetic field activity. All of that can be detected, but once the material is beyond the event horizon, it's lost forever, as is any light it is emitting. At that point, it's all trapped by the black hole and can't escape.

Also of interest in our galaxy's core is the action of supernova explosions. Along with strong stellar winds from hot young stars, such activity blows "bubbles" through interstellar space. The solar system is moving through one such bubble, located far from the center of the galaxy, called the Local Interstellar Cloud. Bubbles such as these may help protect young planetary systems from stronger, harsher radiation for periods of time.

Black Holes and Galaxies

Black holes are ubiquitous throughout the galaxy, and supermassive ones exist at the hearts of most galactic cores. In recent years, astronomers have figured out that central supermassive black holes are an integral part of a galaxy's evolution, affecting everything from star formation to the shape of a galaxy and its activities.

Sagittarius A* is the closest supermassive black hole to us - it lies at a distance of about 26,000 light-years from the Sun. The next closest one lies at the heart of the Andromeda Galaxy, at a distance of 2.5 million light-years. These two provide astronomers with "up-close" experience with such objects and help develop an understanding of how they form and how they behave in their galaxies.

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