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 11:03 AM EDT, Wed April 9, 2014
Planetary nebula Abell 33 has taken on romantic proportions.
updated 11:55 AM EDT, Tue April 8, 2014
You can't see it happening on Earth, but space itself is stretching. Ever since the Big Bang happened 13.8 billion years ago, the universe has been getting bigger.
updated 4:59 PM EDT, Wed March 26, 2014
Scientists have added another celestial body to the short list of objects in our solar system that have rings around them.
updated 1:59 PM EDT, Thu March 27, 2014
Astronomers have discovered a dwarf planet that's even farther away than Pluto.
updated 7:59 AM EST, Fri February 28, 2014
Our galactic neighborhood just got a lot bigger. NASA announced the discovery of 715 new planets.
updated 10:37 AM EDT, Tue March 18, 2014
Scientists have made a breakthrough in understanding how our world as we know it came to be.
updated 3:27 PM EST, Tue February 25, 2014
From a sheep ranch in Western Australia comes the oldest slice of Earth we know.
updated 2:02 PM EST, Wed February 19, 2014
Cassiopeia A was a star more than eight times the mass of our sun before it exploded in the cataclysmic, fiery death astronomers call a supernova.
updated 5:07 PM EST, Mon February 10, 2014
Researchers have found clues that water could be flowing in the present, at least during warm seasons.
updated 11:02 AM EST, Sat February 15, 2014
The "jelly doughnut" rock that seemed to appear out of nowhere on Mars last month did not fall out of an extraterrestrial pastry box.
updated 10:56 PM EST, Thu February 6, 2014
It's a dot in the sky.
updated 2:44 AM EST, Thu February 13, 2014
Reports of Jade Rabbit's demise may have been premature.
updated 8:58 AM EST, Thu January 16, 2014
It's rare for astronomers to spot a planet in a star cluster. That's partly why a cluster called Messier 67 is so special: We now know that it has three planets orbiting stars.
updated 7:03 AM EST, Thu December 19, 2013
What do you need to map a billion stars? A billion-pixel camera certainly helps.
updated 2:50 PM EST, Tue December 10, 2013
NASA's rover Curiosity has now given scientists the strongest evidence to date that the environment on the Red Planet could have supported life billions of years ago.
updated 12:45 PM EST, Sat December 7, 2013
NASA's Cassini spacecraft has provided your multicolored space distraction of the day: images of a swirling, six-sided weather feature on the surface of Saturn.
updated 3:23 PM EST, Mon December 9, 2013
Imagine the delight at unwrapping your Christmas present in 2043 and discovering you've been gifted a trip around the Moon.
updated 5:06 PM EST, Tue December 10, 2013
A Dutch company says it is moving along with its plan to send four lucky Earthlings to colonize the Red Planet. The catch: They won't ever come back.
updated 12:11 PM EST, Tue November 19, 2013
You may have heard it before: billions of years ago Mars probably looked more like Earth does now, with clouds and oceans and a much thicker atmosphere.
updated 10:52 AM EST, Wed November 13, 2013
NASA has given the people of Earth a rare treat: A color mosaic that captures not only Saturn, but also the tiny dots of Earth and other planets in the background.
updated 12:39 PM EST, Tue November 5, 2013
Ever have one of those days where you just wanna be alone, maybe have the planet to yourself?
ADVERTISEMENT