Sagittarius - A-Star's Last Banquet

Secretive, sinister, and gluttonous, supermassive black holes hide in the hearts of perhaps every large galaxy in the Universe--including our own barred-spiral Milky Way. Such supermassive hearts-of-darkness feed on ill-fated wandering stars, as well as on doomed clouds of floating gas, that have traveled too close to their inescapable gravitational embrace--never to return. Our Galaxy's resident supermassive beast, named Sagittarius A* (pronounced Sagittarius-A-star)--or Sgr A*, for short--is itself invisible to the prying eyes of curious astronomers because, like all black holes, it sends forth no energy of any kind into space--and is completely dark. In March 2017, a team of astronomers using NASA's Hubble Space Telescope (HST) announced their discovery that, in the case of our own Galaxy's supermassive black hole, its been a long time between banquets. In fact, Sgr A* ate its last big feast about six million years ago, when it hungrily consumed a large and very unfortunate blob of tumbling, infalling gas. After dinner, the messy and engorged black hole spewed out an enormous bubble of gas weighing-in at the equivalent of millions of Suns--which now billows above and below our Galaxy's hungry heart.

These big after-dinner bubbles, dubbed the Fermi Bubbles, were first discovered by NASA's Fermi Gamma-ray Space Telescope in 2010. However, recent HST observations of the northern bubble have enabled astronomers to calculate a more accurate age for the bubbles and how they managed to form.

"For the first time, we have traced the motion of cool gas throughout one of the bubbles, which allowed us to map the velocity of the gas and calculate when the bubbles formed. What we find is that a very strong energetic event happened 6 million to 9 million years ago. It may have been a cloud of gas flowing into the black hole, which fired off jets of matter, forming the twin lobes of hot gas seen in X-ray and gamma-ray observations. Ever since then, the black hole has just been eating snacks," commented lead researcher, Dr. Rongmon Bordoloi in a March 9, 2017 Hubblesite Press Release.

Black holes are dense and compact regions of space with gravitational snatching claws so immensely powerful that nothing, nothing, nothing at all--not even light--can flee from their gravitational kiss of death, and escape to freedom. Sgr A* has compressed the mass of 4.5 million Suns into an extremely tiny region of space.

Unfortunate material that wanders too close to a supermassive heart-of-darkness is snared by its powerful gravity--only to swirl down, down, down into the merciless whirling vortex, surrounding its waiting maw. Ultimately, the doomed shredded erstwhile star, or the fragmented gas cloud, falls in--and is forever gone. However, some more fortunate morsels of the beast's dinner manage to grow so searing-hot that they do escape along the black hole's spin axis. This material forms an outflow that reaches high above and far below the plane of a galaxy.

Astronomers actually have managed to learn quite a bit about Sgr A*. It weighs-in at approximately four million solar-masses--which makes it a relative light-weight, as supermassive black holes go. Voracious hearts-of-darkness can weigh billions of times the mass of our Sun. Sgr A* is surrounded by a cluster of sparkling baby stars, some of which have had the misfortune of diving in to within a few billion miles of where the hungry beast lies in wait. Our Galaxy's supermassive black hole is a quiet beast now--but this apparently was not the case a century ago when it sloppily devoured a blob of matter that had wandered too close to it. This feast formed a brilliant, multicolored fireworks display that lit up our Milky Way Galaxy's hungry heart.

Strange, Sinister, Gluttonous And Marvelous

Supermassive black holes are some of the weirdest denizens of our indisputably weird Universe. These bizarre, bewitching objects grow by eating their surroundings, and they can be very greedy. They are also very messy. These strange beasts are so insatiable and voracious that they attempt to swallow more than they can chew--and wind up spewing some of it out.

Sgr A* is enjoying a peaceful old age now, but it was much more active and hungry in its glory-days, billions of years ago, when our Milky Way was also a young and active galaxy.

Black holes do not come in only one size. In addition to the supermassive kind, smaller black holes of "only" stellar-mass also inhabit the Cosmos. These relatively tiny gravitational beasts are born from the funeral pyre of an extremely massive star that has reached the end of the stellar road, and has collapsed in the fantastic fires of a supernova explosion that has blown the progenitor star into oblivion. The supernova blast marks the dramatic end of a massive star's brilliant life on the hydrogen-burning main-sequence of the Hertzsprung-Russell Diagram of Stellar Evolution. After a stellar-mass black hole has emerged from the incinerated wreckage of its progenitor star, it can go on to gain even more weight as it greedily and voraciously devours whatever doomed object has catastrophically wandered too close to its gravitational kiss of death.

The material of unlucky stars and clouds of doomed gas whirl around into the maelstrom of the turbulent vortex surrounding gigantic supermassive dark hearts.This crashing banquet swirls down, creating a gigantic accretion disk. This terrible feast becomes increasingly hotter and hotter, and emits an enormous amount of radiation, as it approaches the infamous point of no return, where all hope must be abandoned. This terrible region of inescapable doom is called the event horizon, and it is located at the innermost region of the accretion disk.

Black holes can be large or small. These strange objects can be defined as a region of Spacetime where the pull of gravity has become so intense that not even light can flee from its grasp. The pull of gravity becomes this powerful because matter has been crushed into a very small space. Squeeze enough matter into a small enough space, and a black hole will be born every time.

Most supermassive black holes, such as Sgr A* accrete at a very slow rate--and this makes it difficult for astronomers to distinguish them from the dark galactic hearts in which they dwell. Sgr A* provides a precious and instructive exception to this frustrating rule. This is because astronomers can obtain a close view of its somewhat gentle X-ray emission.

"This is the best case we have for a supermassive black hole anywhere in our Universe. What's happening in other galaxies made us ask the question in the first place, but in fact we have the best evidence for the existence of these incredibly exotic objects from the center of the Galaxy," explained Dr. Andrea Ghez in The Black Hole Encyclopedia. Dr. Ghez is a professor of physics and astronomy at the University of California, Los Angeles (UCLA), and a leading expert on our Milky Way's mysterious center.

Sagittarius-A-Star's Last Banquet

The new study, released in March 2017, is a follow-up of earlier HST observations that determined the enormous bubbles are about 2 million years old. The team's conclusions are based on observations conducted by HST's Cosmic Origins Spectrograph (COS) which analyzed ultraviolet light emanating from 47 remote quasars. Quasars are extremely brilliant Active Galactic Nuclei (AGN) lurking in the hearts of distant galaxies in the early Universe. They are the bright accretion disks swirling around supermassive black holes haunting the cores of ancient galaxies.

Imprinted on the quasars' revealing light--as it makes its long journey through the Milky Way's bubble--is important information about the speed, temperature, and composition of the gas within the expanding bubble.

The COS observations indicate that the gas within the bubble is approximately 17,700 degrees Fahrenheit. However, even at these searing-hot temperatures, the gas is actually much cooler than most of the super-hot gas in the outflow--which is about 18 million degrees Fahrenheit--and is observed as gamma rays. The cooler gas observed by COS could be interstellar gas originating from our Milky Way's disk that is being swept up and entrained into the super-hot outflow. COS also detected carbon and silicon as two of the atomic elements being carried away within the gaseous cloud. These common elements are seen in most galaxies, and they represent the fossil remnants of stellar evolution. This is because, through the process of stellar nucleosynthesis, stars fuse the atomic nuclei of light atomic elements into heavier things. The only atomic elements produced in the Big Bang birth of the Universe almost 14 billion years ago were hydrogen, helium, and scant quantities of lithium and beryllium. All of the other, heavier atomic elements were forged within the nuclear-fusing hearts of the stars into elements like carbon and silicon. All atomic elements heavier than helium--termed metals by astronomers--were either created in the searing-hot ovens of the stars or in the supernovae blasts marking the explosive deaths massive stars.

The "cooler" gas is zipping through the bubble at the breathtaking pace of 2 million miles per hour. By mapping the movement of this gas throughout the structure, the team of astronomers calculated that the minimum mass of the streaming cool gas in both bubbles is approximately equal to 2 million solar-masses. The edge of the northern bubble extends 23,000 light-years above the Galaxy.

"We have traced the outflows of other galaxies, but we have never been able to actually map the motion of the gas. The only reason we could do it here is because we are inside the Milky Way. This vantage point gives us a front-row seat to map out the kinematic structure of the Milky Way outflow," Dr. Bordoloi explained in the March 9, 2017 Hubblesite Press Release.

These recent COS observations build and expand on the earlier discoveries of 2015 conducted by the same team of astronomers. In the 2015 study, the astronomers analyzed the light traveling from one quasar that pierced the base of the bubble.

"The Hubble data open a whole new window on the Fermi Bubbles. Before, we knew how big they were and how much radiation they emitted; now we know how fast they are moving and which chemical elements they contain. That's an important step forward," commented study co-author, Dr. Andrew Fox, in the March 9, 2017 Hubblesite Press Release. Dr. Fox is of the Space Telescope Science Institute (STSI) in Baltimore, Maryland.

The more recent HST study also provides an independent confirmation of the nature of the bubbles and their mysterious origin, as detected by X-ray and gamma-ray observations.

Dr. Bordolai continued to explain that "This observation would be almost impossible to do from the ground because you need ultraviolet spectroscopy to detect the fingerprints of these elements, which can only be done from space. Only with COS do you have the wavelength coverage, the sensitivity, and the spectral resolution coverage to make this observation."

The results of this study appeared in the January 10, 2017 edition of The Astrophysical Journal.

Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various journals, magazines, and newspapers. Although she has written on a variety of topics, she particularly loves writing about astronomy, because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, "Wisps, Ashes, and Smoke," will be published soon.


 By Judith E Braffman-Miller


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