Skip to main content

Why our galaxy's black hole is a picky eater

This image shows the X-Ray close-up of Sagittarius A*, the Milky Way's central black hole.
This image shows the X-Ray close-up of Sagittarius A*, the Milky Way's central black hole.
STORY HIGHLIGHTS
  • Black hole at center of Milky Way ejects more than 99% of material for the 1% it captures
  • This has to do with temperature and angular momentum of gas in its surroundings
  • In the early universe, there was a greater abundance of cold, dense gas

(CNN) -- You might think of black holes as indiscriminate eaters, hungrily gobbling up everything in their vicinity.

But the black hole at the center of our Milky Way galaxy, Sagittarius A*, is not exactly like this, new research suggests. Instead, this black hole -- and likely other black holes in the centers of galaxies -- must spit out a lot in order to swallow a little.

It's been a mystery why black holes at the centers of galaxies in the present universe appear so much dimmer than quasars, extremely bright objects from the early universe that have black holes at their centers, too.

As Albert Einstein noted in his famous formula E=mc², energy is equivalent to mass times the speed of light squared. In a black hole, crushed mass gets converted into energy. Black holes in quasars eat a lot, creating the spectacular brightness associated with them. But we don't find as much radiation emanating from Sagittarius A*, or other black holes in the centers of galaxies in the present universe.

So what's going on? Is the hot gas that Sagittarius A* is eating just not radiating as much as the colder gas that quasars capture?

To find out, researchers used NASA's Chandra X-ray Observatory to take X-ray images and capture other signature of energy. The study is published in the journal Science, and led by Q. Daniel Wang, astrophysicist at the University of Massachusetts at Amherst.

Sagittarius A* has about 4 million times the mass of the sun. It is located 26,000 light years from Earth. That's still super far, as one light year is about 5.9 trillion miles, but it's close enough that human technology can help us see what's happening to the matter around it.

The gas swirling around this black hole has a temperature of millions of degrees.

Based on these new observations, researchers suggest less than 1% of the material that the black hole's gravity pulls near actually gets sucked in to the "point of no return," which is called the event horizon. Instead, a lot of it gets spat back out. That's why the X-ray emission from the black hole is faint; in theory, the radiation output would be stronger if the black hole were swallowing more.

"Less than 1% of matter will be actually sacrificed for the freedom of 99% of gas," Wang said. "So, 99% of gas can escape from the capture of the black hole."

Why is that the case?

It appears that in order to be gobbled for good by the black hole, material must lose heat and angular momentum, which is a measurement of how an object or system rotates around a particular axis.

The temperature is important because hotter material is harder to pin down, even for a black hole. Wang uses the analogy of a sink: You can pour cold water in and watch it spiral down a drain, but if it's steam, far less will actually go in; the water particles are more diffuse and energetic.

According to Wang and colleagues, the black hole needs to throw out more than 99% of the material in order to accomplish this. That ejected 99%, in turn, heats up the environment around it, which affects the evolution of the galaxy as a whole.

Cold and dense gas goes down easier into the black hole, though, and a black hole may sometimes capture a lot of it. This is the gas that tends to form a disk, called an accretion disk, around the black hole. In the accretion disk, the gas's energy and angular momentum dissipate, so more of it is swallowed up by the black hole.

There was more cold and dense gas in the early universe, so black holes at that time were better at accumulating material this way. That's why we find quasars in the early universe that are so much brighter than Sagittarius A*.

Quasar -- galactic beauty, deadly beast -- discovered 50 years ago

The research is important because a galaxy such as ours is intimately linked with the black hole at its center. The more massive the black hole, the more massive the surrounding galaxy is, scientists have found.

"Understanding how the black hole grows with time and how the black hole ejects matter and energy back into the galaxy has strong implications for understanding how galaxies form and evolve," Wang said. "That, of course, directly affects how stars form and evolve."

Dieters, take note: Our galaxy may have gotten to be the way it is by consuming small portions.

Follow Elizabeth Landau on Twitter at @lizlandau

ADVERTISEMENT
Part of complete coverage on
Space
updated 4:19 PM EST, Sat November 15, 2014
It's hard to top the tricky, first-ever landing on a comet but we'll try. Here are 11 other space missions to know about.
updated 5:27 PM EST, Wed November 12, 2014
November 2014 may well be remembered as the time when humanity first landed a robotic probe on the nucleus of a comet.
updated 6:21 PM EST, Fri November 7, 2014
Add another entry to the growing list of crazy footage captured by GoPro cameras.
updated 1:00 PM EDT, Sat November 1, 2014
It is in our DNA to explore the unknown. But pushing boundaries and exploring space is far from easy.
updated 11:42 AM EDT, Fri October 24, 2014
If there's one thing we've learned about the CNN iReport community, it's that you all love to capture celestial events.
updated 3:58 PM EDT, Mon October 20, 2014
It was the closest comet near-miss known to astronomers, but everything is alright.
updated 9:04 AM EDT, Wed October 15, 2014
It wasn't a trick. But for space geeks, it sure was a treat.
updated 8:25 PM EDT, Sun October 12, 2014
Want to ride an elevator into space? A breakthrough in nanotechnology could mean we will be riding into space on a cable made of diamonds.
updated 1:52 PM EDT, Tue October 7, 2014
Astronauts lie motionless in a row of compartments with medical monitoring cables connected to their bodies, as their space ship cuts through the silent blackness.
updated 3:29 PM EDT, Sat September 20, 2014
This image from the Hubble Space Telescope indicates that a huge ring of dark matter likely exists surrounding the center of CL0024+17 that has no normal matter counterpart.
Scientists are closer to seeing a vast, invisible universe as a spectrometer in Earth orbit picks up possible clues of dark matter.
updated 9:21 AM EDT, Wed September 10, 2014
The Soviets sent stray dogs up to conquer space. This is what happened next
updated 5:20 AM EDT, Thu August 28, 2014
Scientists believe that a hot gas bubble was formed by multiple supernovas.
updated 11:47 AM EDT, Wed August 27, 2014
Robonaut is the next generation dexterous robot
Life aboard the International Space Station.
updated 9:53 PM EDT, Tue August 26, 2014
NASA's New Horizons mission hurtles toward Pluto in historic 3 billion mile expedition.
updated 11:56 PM EDT, Mon July 14, 2014
Scientists looking for signs of life in the universe -- as well as another planet like our own -- are a lot closer to their goal than people realize.
updated 11:51 AM EDT, Sun June 29, 2014
If you think you saw a flying saucer over Hawaii, you might not be crazy -- except what you saw didn't come from outer space, though that may be its ultimate destination.
updated 10:21 AM EDT, Fri June 13, 2014
When I first poked my head inside Virgin Galactic's newest spaceship, I felt a little like I was getting a front-row seat to space history.
updated 3:27 PM EST, Tue February 25, 2014
From a sheep ranch in Western Australia comes the oldest slice of Earth we know.
ADVERTISEMENT