Milky Way black hole lights up brighter than ever

A black hole can shine bright when it feasts on stars or gas.

The GIF speaks for itself: Something’s going on in the center of the Milky Way.

If youd didn’t know, the supermassive black hole (SMBH) at the center of our galaxy is called Sagittarius A Star, or Sgr A* for short, and it has shown some unusual activity during observations this year. It is normal for its brightness to vary but during these observations the brightness reached a whole new level.

Tuan Do, UCLA astronomer and the author of the tweet above, is also the author of a paper that was published recently on Arxiv.com, and according to him and his co-authors, this is the brightest we have ever seen the black hole to be. The observations were made with the Keck Telescope at near-infrared wavelengths.

“That night, I totally mistook it for the star S0-2 at first because it was brighter than I had ever seen,” Tuan Do wrote in a response on Twitter.

Page 3 in the paper by Tuan Do et al. Relative brightness is measured in magnitude – the lower the magnitude the brighter the object. The data stems from the same observation that the GIF above is based on. arXiv:1908.01777

Now, some of you out there might ask yourself:

Isn’t a black hole, like, black. How can it be bright and how can we even see that?

You’re quite right – no light escapes a black hole itself and therefore we can’t see them directly. The light that comes from Sgr A*, though, stems from the area around the black hole, the disk of matter that gathers around it like a gravitational whirlpool, what astronomers call an accretion disk.

When astronomers for the first time took a direct picture of a black hole, the one at the center of the galaxy M 87, the accretion disk was clearly visible.

The supermassive black hole at the center of the galaxy M 87. Event Horizon Telescope [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)%5D

In the paper, Tuan Do and his co-authors write, that the change in brightness from Sgr A* “may be from changes in the accretion-flow as a result of the star S0-2’s closest passage to the black hole in 2018 or from a delayed reaction to the approach of the dusty object G2 in 2014.”

In other words: A star or a cloud of gas may have come too close to Sgr A* and lit up the accretion disk. When matter gets caught by a black hole, it is ionized and therefore lights up.

The center of the Milky Way is a busy place and several stars orbit close to Sgr A*.

So, I recently learned a new word – Active Galactic Nucleus, or AGN for short. AGNs are supermassive black holes at the center of galaxies that are constantly fed huge loads of mass. This makes them shine brightly and consistently and they are some of the brightest phenomena in the universe. M 87 is one example of a an active galaxy.

I asked on Twitter if the recent activity around Sgr A* might indicate that the Milky Way might turn active.

“It is very unlikely that Sgr A* will become an AGN because there is not enough gas in that region right now. However, we think a few hundred years ago, the black hole was maybe briefly 10,000 times brighter,” Tuan Do responded on Twitter.

This story has left me with several more questions that I would like to write about, so I will return to the subject as I get more information and with possible corrections if I’ve got some of the facts wrong (I am but a journalist, not an astronomer). For now I am just amazed by the thought that we might have seen the light from a black hole gobbling up a star or a gas cloud.

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