Look at the center of most galaxies and you will find a black hole, gobbling up material from around space. Many of the mysteries of black holes have eluded astronomers since they were first discovered. However, scientists are now on the cusp of solving a huge cosmic riddle. Why is the Milky Way’s black hole, called Sagittarius A-Star (A.K.A. Sagittarius A*), unable to “eat” its space meals properly. Is Sagittarius A-Star on a cosmic diet?

Although the inception of these monstrous space vacuums is not exactly pinned down, many prominent astronomers suggest they are the result of a star’s death, squeezing matter into a small volume. The resulting density of this matter gives rise to huge gravitational forces that even light is unable to escape from. Upon reflection of this phenomenon, scientists postulated that black holes indiscriminately devour everything in their path.

This theory has recently been denounced by a group of astronomers who have been observing the behavior of the Milky Way’s own supermassive black hole, Sagittarius A-Star. This black hole encompasses a mass four million times greater than that of our sun.

Surprisingly, this black hole was emitting low levels of radiation, suggested by the faint levels of X-ray detected from Sagittarius A-Star’s accretion disk.

The black holes have a gravitational force that pulls in matter, which begins to “queue up” in a particular region around its center. This region is called the accretion disk, where matter revolves around the black hole, slowly creeping towards its interior. As the matter is sucked ever-closer, frictional and gravitational forces cause the particulate to release energy in the form of radiation.

The gravitational force exerted by a black hole is typically measured by X-ray emissions of the matter within this accretion disk. This is an indirect measure of the size of a black hole, as the center of a black hole cannot be observed.

The fact that astronomers were only able to detect faint radiation emissions suggests that the black hole is not becoming any bigger. Scientists were perplexed by this unusual observation, prompting further investigation.

Much debate has raged over the destiny of the matter within these accretion disks. Initial theories suggested that most of the matter eventually becomes consumed. However, Q. Daniel Wang, an astrophysicist leading the research effort at the University of Massachusetts Amherst, has collected scientific evidence that challenges this theory, revealing that such matter may actually face expulsion. He maintains that his studies represent “… the first direct evidence for outlflow in the accretion process.”

Contrary to popular scientific understanding, it seems that Sagittarius A-Star, which remains at a fixed point within the Milky Way, does indeed spit out more matter than eat consumes and would appear to be on a cosmic diet. Wang briefly elaborates on the black hole’s capacity to uptake matter:

“We found that when matter flows toward a black hole, more than 99% of it is ejected.”

This data was collected from NASA’s Chandra X-Ray Observatory, an X-ray telescope, and amounted to approximately five weeks worth of observation time. Chandra is able to “break up” X-ray waves into their constituent wavelengths to achieve a more comprehensive picture of how black holes influence nearby gases. Wang and his colleagues published the results in the journal Science, on Thursday.

The group work able to differentiate between high and low energy gases within the accretion disk. The ratio of high temperature to low temperature gas was very low, inferring a loss of gas over time. This finding has led to the ejection “process” hypothesis that Wang had previously described.

This also explains the unusually low levels of radiation around the black hole. Much of this gas cloud is emitted from the accretion disk, consequently reducing the level of radiation perceived. But where does the gas originally come from?

According to Wang, the gas is derived from a cluster of stars of the Milky Way, which have aligned around the black hole. These stars propel jets of gaseous material into space, which collide with one another to form high-temperature plasma gas.

Although this discovery is very exciting, Wang admits that more detailed work needs to be conducted to truly understand all the intricacies of Sagittarius A-Star. We are still unaware of the mechanism of gaseous ejection, where the gases are dispatched or why they are expelled in the first place.

Indeed, according to Space.com, the team are looking forward to when a cold stream of gas (G2) becomes sucked into the black hole, around September of this year. This will represent an opportunity to witness an event that has not taken place for almost 40 years, and will enable scientists to acquire a better understanding of the accretion process. In line with Wang’s latest conclusions, as G2 interacts with the ambient gases around the black hole, it should demonstrate a lower than expected luminosity.

According to NBC News, astrophysicist Jeremy Schnittman, based at NASA’s Goddard Space Flight Center, recently expounded upon the applicability of Wang’s research conclusions, relative to other “low-luminosity” black holes within the universe:

“… we now have a better understanding of their radiative efficiency, i.e., how to relate the light that we see to the amount of gas actually getting accreted onto the black hole.”

Although it does appear that Sagittarius A-Star is currently on a space diet, astronomers are still unraveling the exact mechanisms behind accretion and expulsion of surrounding material. What we do know, however, is that there is a black hole at the heart of the Milk Way that won’t be gaining too many pounds in the near future.

By: James Fenner

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