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When Our Galaxy's Dark Heart Exploded

When Our Galaxy’s Dark Heart Exploded
When Our Galaxy’s Dark Heart Exploded

Voracious, bizarre, and bewitching, supermassive black holes are thought to haunt the mysterious hearts of perhaps every large galaxy in the observable Universe–including our own Milky Way. Our Galaxy’s resident dark heart is named Sagittarius A*–Sgr A*, for short (pronounced saj-a-star), and it is dormant now, but still manages to awaken from its slumber now and then to dine on an unlucky star or cloud of gas that has wandered too close to its gravitational embrace. Sgr A*, despite its currently quiet nature, still has enough “life” left to surprise astronomers with a sudden and dramatic episode of turbulent activity. In October 2019, a team of astronomers announced that they had found evidence of a recent cataclysmic flare that exploded so far out of the Milky Way that its destructive influence was felt 200,000 light-years from its origins.

This enormous, expanding beam of energy erupted close to Sgr A* only a “mere” 3.5 million years ago–which is only a blink of the eye in cosmic time. The flare shot out a cone-shaped burst of radiation through both poles of our Galaxy, that then rushed screaming into deep space.

This new finding is based on research conducted by a team of scientists led by Dr. Joss Bland-Hawthorn from Australia’s ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ARSTRO 3D). Their work is published in The Astrophysical Journal.

Our Milky Way’s Bewitching Heart

Sgr A* is a brilliant and compact astronomical radio source. It is situated at the very heart of our large spiral Galaxy, close to the border of the constellations Sagittarius and Scorpius. As supermassive black holes go, Sgr A* is relatively light in weight. Similar dark hearts inhabiting other galaxies sport millions to billions of times more than our Sun. Sgr A* sports “only” millions of solar-masses.

Observations of many stars circling around Sgr A* have been used to provide important evidence for the presence of, as well as providing data about, our Milky Way’s supermassive black hole. These observations have caused some astronomers to conclude that Sgr A* is a black hole beyond a shadow of a doubt.

The shredded remains of what were once stars, as well as clouds of unfortunate and disrupted gas, create the wreckage that eventually tumbles down into the waiting maw of a supermassive gravitational beast like Sgr A*. This terrible banquet of swirling material creates a massive disk encircling the black hole. The disk itself, termed an accretion disk, grows hotter and hotter with the passage of time–particularly when it is pulled into the bizarre vortex close to the dreaded point of no return, called the event horizon. Nothing can return to the outside once it has entered this point. Not even light can return once it has been swallowed. The event horizon is situated at the innermost region of the brilliant accretion disk.

Supermassive black holes, along with their surrounding glaring accretion disks, can be as large as our Solar System–at least. These mysterious entities are described by their greedy appetites, large masses, and messy table manners. When Sgr A* was a young black hole, billions of years ago, it tripped the light fantastic as a searing-hot young, dazzling quasar in the ancient Universe. However, it has grown quiet in its old age, with only a shadow of the appetite it once had when it was in its flaming youth. Quasars lit up the ancient Universe, but like Sgr A* many of them have also lost their sparkle in their old age.

All black holes are dense, compact regions of space. Sgr A* sports the compressed mass of approximately 4.5 million suns, and this hefty mass is squeezed into a relatively small area of space.

In a paper published on October 31, 2018, astronomers announced their discovery of conclusive evidence that Sgr A* is a black hole. Using the GRAVITY interferometer and a quartet of telescopes of the Very Large Telescope (VLT) to form a virtual telescope 130 meters in diameters, the scientists spotted clumps of gas traveling at approximately 30% the speed of light. Emission from highly energetic electrons very close to Sgr A* was visible as a trio of brilliant flares. The flares precisely matched theoretical predictions for hot spots circling close to a black hole of about four milliion times solar mass. The flares are believed to originate from magnetic interactions in the extremely hot gas circling close to our Galaxy’s resident dark heart.

What has been seen, so far, of Sgr A*, is not the black hole itself. However, the observations are consistent only if there really is a black hole lurking near Sgr A*. In the case of such a black hole, the observed radio and infrared energy emanates from the gas and dust that is heated to millions of degrees while falling to its doom into the maw of the black hole. But the dark-hearted beast itself is believed to emit only Hawking radiation at a negligible temperature.

Sgr A* is invisible to the eyes of observers. Like all others of its kind, it sends no energy out into space, and it is completely dark. Our Galaxy’s elderly, quiet resident supermassive black hole shows very little of the greedy appetite of its youth, when it was still in its brilliant quasar stage. Indeed, at least in the case of Sgr A*, it’s been a very long time between dinners. It is thought to have feasted on its last big buffet about six million years ago, when it dined on an unfortunate large cloud of messed up gas. Afterwards, the sloppy black hole, its hunger now quenched, dispatched a huge bubble of gas into interstellar space that was equivalent to millions of solar masses. This gas bubble now bounces both below and above Sgr A*. These after-dinner bubbles are termed Fermi Bubbles, and they were first discovered by NASA’s Fermi Gamma-ray Space Telescope back in 2010.

Black holes are indisputably weird denizens of the cosmic zoo. Our Galaxy’s own resident beast is encircled by a twinkling cluster of baby stars, some of which have had the misfortune of falling to within only a few million miles of where Sgr A* was lying in terrible wait for its dinner to come tumbling down to it.

Not all black holes are of the supermassive kind. Indeed, there are smaller black holes of only stellar-mass inhabiting the Cosmos. These much less hefty black holes form when an especially massive star has finished burning its necessary supply of nuclear-fusing fuel, and it blasts itself into oblivion when its core collapses. This results in a Type II (core-collapse) supernova explosion. There is also a likely population of intermediate-size black holes that weigh-in between the mass of black holes of only stellar-mass and the supermassive variety. It has also been suggested that a population of hypothetical primordial black holes were born in the Big Bang birth of the Universe almost 14 billion years ago. Indeed, if enough mass is squashed into a small enough space, a black hole will invariably form.

Seyfert Flare

The violent flare that broke our Galaxy’s heart is known as a Seyfert Flare. It created two immense ionization cones that ripped through our Milky Way–starting with a relatively small diameter near Sgr A*, and then ballooning greatly in size as they rushed out of of our Galaxy.

This particular Seyfert Flare was so powerful that it ran into the Magellanic Stream, which is a long trail of gas extending from the Large and Small Magellanic Clouds–which are two dwarf galaxies close to our own. The Magellanic Stream is situated at an average distance of 200,000 ligh
t years from our Milky Way.

The catastrophic explosion was so powerful, according to the Australian team of astronomers, that it could not have been ignited by anything other than nuclear activity originating from our Galaxy’s supermassive black hole.

“The flare must have been a bit like a lighthouse beam,” commented Dr. Bland-Hawthorne in an October 2, 2019 ScienceinPublic Press Release. Dr. Bland-Hawthorne is also of the University of Sydney in Australia.

“Imagine darkness, and then someone switches on a lighthouse beacon for a brief period of time,” he added.

Using data collected by the Hubble Space Telescope (HST), the scientists determined that the enormous explosion occurred only a little more than three million years ago–a mere wink of the eye on cosmological time scales. Indeed, in Galactic terms, that is amazingly recent. On our own planet, at that time, the asteroid that was the final, fatal blow to the dinosaurs was 63 million years in the past. Humanity’s ancient ancestors already existed. Called the Australopithecines, these ancestors of human beings were already roaming the African continent.

“This is a dramatic event that happened a few million years ago in the Milky Way’s history,” noted Dr. Lisa Kewley in the October 2, 2019 ScienceinPublic Press Release. Dr. Kewley is Director of ASTRO 3D.

“A massive blast of energy and radiation came right center and into the surrounding material. This shows that the center of the Millky Way is a much more dynamic place than we had previously thought. It is lucky we’re not residing there,” she added.

The explosion, the astronomers calculate, lasted for a mere 300,000 years. This is an extremely brief period in Galactic terms.

In conducting the research, Dr. Bland-Hawthorne was joined by colleagues from the Australia National University and University of Sydney, and, in the US, the University of North Carolina (Chapel Hill), University of Colorado (Boulder), and the Space Telescope Science Institute in Baltimore, Maryland.

The new paper follows earlier research, also conducted by Dr. Bland-Hawthorne, that was published in 2013. The previous work studied evidence of a tremendous explosive event originating in the center of our Galaxy, and it ruled out a nuclear starburst as the trigger. Instead, the earlier study tentatively tied the explosion to activity in Sgr A*.

“These results dramatically change our understanding of the Milky Way,” commented study co-author Dr. Magda Guglielmo of the University of Sydney (Australia).

“We always thought about our Galaxy as an inactive galaxy, with a not so bright center. These new results instead open the possibility of a complete reinterpretation of its evolution and nature. The flare event that occurred three million years ago was so powerful that it had consequences on the surroundings of our Galaxy. We are the witness to the awakening of the sleeping beauty,” she continued.

The recent research considers Sgr A* as a prime suspect. However, the researchers concede that there is still much more work to be done. How black holes evolve, influence and interact with galaxies, they conclude, “is an outstanding problem in astrophysics.”

Voracious, bizarre, and bewitching, supermassive black holes are thought to haunt the mysterious hearts of perhaps every large galaxy in the observable Universe–including our own Milky Way. Our Galaxy’s resident dark heart is named Sagittarius A*–Sgr A*, for short (pronounced saj-a-star), and it is dormant now, but still manages to awaken from its slumber now and then to dine on an unlucky star or cloud of gas that has wandered too close to its gravitational embrace. Sgr A*, despite its currently quiet nature, still has enough “life” left to surprise astronomers with a sudden and dramatic episode of turbulent activity. In October 2019, a team of astronomers announced that they had found evidence of a recent cataclysmic flare that exploded so far out of the Milky Way that its destructive influence was felt 200,000 light-years from its origins.

This enormous, expanding beam of energy erupted close to Sgr A* only a “mere” 3.5 million years ago–which is only a blink of the eye in cosmic time. The flare shot out a cone-shaped burst of radiation through both poles of our Galaxy, that then rushed screaming into deep space.

This new finding is based on research conducted by a team of scientists led by Dr. Joss Bland-Hawthorn from Australia’s ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ARSTRO 3D). Their work is published in The Astrophysical Journal.

Our Milky Way’s Bewitching Heart

Sgr A* is a brilliant and compact astronomical radio source. It is situated at the very heart of our large spiral Galaxy, close to the border of the constellations Sagittarius and Scorpius. As supermassive black holes go, Sgr A* is relatively light in weight. Similar dark hearts inhabiting other galaxies sport millions to billions of times more than our Sun. Sgr A* sports “only” millions of solar-masses.

Observations of many stars circling around Sgr A* have been used to provide important evidence for the presence of, as well as providing data about, our Milky Way’s supermassive black hole. These observations have caused some astronomers to conclude that Sgr A* is a black hole beyond a shadow of a doubt.

The shredded remains of what were once stars, as well as clouds of unfortunate and disrupted gas, create the wreckage that eventually tumbles down into the waiting maw of a supermassive gravitational beast like Sgr A*. This terrible banquet of swirling material creates a massive disk encircling the black hole. The disk itself, termed an accretion disk, grows hotter and hotter with the passage of time–particularly when it is pulled into the bizarre vortex close to the dreaded point of no return, called the event horizon. Nothing can return to the outside once it has entered this point. Not even light can return once it has been swallowed. The event horizon is situated at the innermost region of the brilliant accretion disk.

Supermassive black holes, along with their surrounding glaring accretion disks, can be as large as our Solar System–at least. These mysterious entities are described by their greedy appetites, large masses, and messy table manners. When Sgr A* was a young black hole, billions of years ago, it tripped the light fantastic as a searing-hot young, dazzling quasar in the ancient Universe. However, it has grown quiet in its old age, with only a shadow of the appetite it once had when it was in its flaming youth. Quasars lit up the ancient Universe, but like Sgr A* many of them have also lost their sparkle in their old age.

All black holes are dense, compact regions of space. Sgr A* sports the compressed mass of approximately 4.5 million suns, and this hefty mass is squeezed into a relatively small area of space.

In a paper published on October 31, 2018, astronomers announced their discovery of conclusive evidence that Sgr A* is a black hole. Using the GRAVITY interferometer and a quartet of telescopes of the Very Large Telescope (VLT) to form a virtual telescope 130 meters in diameters, the scientists spotted clumps of gas traveling at approximately 30% the speed of light. Emission from highly energetic electrons very close to Sgr A* was visible as a trio of brilliant flares. The flares precisely matched theoretical predictions for hot spots circling close to a black hole of about four milliion times solar mass. The flares are believed to originate from magnetic interactions in the extremely hot gas circling close to our Galaxy’s resident dark heart.

What has been seen, so far, of Sgr A*, is not the black hole itself. However, the observations are consistent only if there really is a black hole lurking near Sgr A*. In the case of such a black hole, the observed radio and infrared energy emanates from the gas and dust that is heated to millions of degrees while falling to its doom into the maw of the black hole. But the dark-hearted beast itself is believed to emit only Hawking radiation at a negligible temperature.

Sgr A* is invisible to the eyes of observers. Like all others of its kind, it sends no energy out into space, and it is completely dark. Our Galaxy’s elderly, quiet resident supermassive black hole shows very little of the greedy appetite of its youth, when it was still in its brilliant quasar stage. Indeed, at least in the case of Sgr A*, it’s been a very long time between dinners. It is thought to have feasted on its last big buffet about six million years ago, when it dined on an unfortunate large cloud of messed up gas. Afterwards, the sloppy black hole, its hunger now quenched, dispatched a huge bubble of gas into interstellar space that was equivalent to millions of solar masses. This gas bubble now bounces both below and above Sgr A*. These after-dinner bubbles are termed Fermi Bubbles, and they were first discovered by NASA’s Fermi Gamma-ray Space Telescope back in 2010.

Black holes are indisputably weird denizens of the cosmic zoo. Our Galaxy’s own resident beast is encircled by a twinkling cluster of baby stars, some of which have had the misfortune of falling to within only a few million miles of where Sgr A* was lying in terrible wait for its dinner to come tumbling down to it.

Not all black holes are of the supermassive kind. Indeed, there are smaller black holes of only stellar-mass inhabiting the Cosmos. These much less hefty black holes form when an especially massive star has finished burning its necessary supply of nuclear-fusing fuel, and it blasts itself into oblivion when its core collapses. This results in a Type II (core-collapse) supernova explosion. There is also a likely population of intermediate-size black holes that weigh-in between the mass of black holes of only stellar-mass and the supermassive variety. It has also been suggested that a population of hypothetical primordial black holes were born in the Big Bang birth of the Universe almost 14 billion years ago. Indeed, if enough mass is squashed into a small enough space, a black hole will invariably form.

Seyfert Flare

The violent flare that broke our Galaxy’s heart is known as a Seyfert Flare. It created two immense ionization cones that ripped through our Milky Way–starting with a relatively small diameter near Sgr A*, and then ballooning greatly in size as they rushed out of of our Galaxy.

This particular Seyfert Flare was so powerful that it ran into the Magellanic Stream, which is a long trail of gas extending from the Large and Small Magellanic Clouds–which are two dwarf galaxies close to our own. The Magellanic Stream is situated at an average distance of 200,000 ligh
t years from our Milky Way.

The catastrophic explosion was so powerful, according to the Australian team of astronomers, that it could not have been ignited by anything other than nuclear activity originating from our Galaxy’s supermassive black hole.

“The flare must have been a bit like a lighthouse beam,” commented Dr. Bland-Hawthorne in an October 2, 2019 ScienceinPublic Press Release. Dr. Bland-Hawthorne is also of the University of Sydney in Australia.

“Imagine darkness, and then someone switches on a lighthouse beacon for a brief period of time,” he added.

Using data collected by the Hubble Space Telescope (HST), the scientists determined that the enormous explosion occurred only a little more than three million years ago–a mere wink of the eye on cosmological time scales. Indeed, in Galactic terms, that is amazingly recent. On our own planet, at that time, the asteroid that was the final, fatal blow to the dinosaurs was 63 million years in the past. Humanity’s ancient ancestors already existed. Called the Australopithecines, these ancestors of human beings were already roaming the African continent.

“This is a dramatic event that happened a few million years ago in the Milky Way’s history,” noted Dr. Lisa Kewley in the October 2, 2019 ScienceinPublic Press Release. Dr. Kewley is Director of ASTRO 3D.

“A massive blast of energy and radiation came right center and into the surrounding material. This shows that the center of the Millky Way is a much more dynamic place than we had previously thought. It is lucky we’re not residing there,” she added.

The explosion, the astronomers calculate, lasted for a mere 300,000 years. This is an extremely brief period in Galactic terms.

In conducting the research, Dr. Bland-Hawthorne was joined by colleagues from the Australia National University and University of Sydney, and, in the US, the University of North Carolina (Chapel Hill), University of Colorado (Boulder), and the Space Telescope Science Institute in Baltimore, Maryland.

The new paper follows earlier research, also conducted by Dr. Bland-Hawthorne, that was published in 2013. The previous work studied evidence of a tremendous explosive event originating in the center of our Galaxy, and it ruled out a nuclear starburst as the trigger. Instead, the earlier study tentatively tied the explosion to activity in Sgr A*.

“These results dramatically change our understanding of the Milky Way,” commented study co-author Dr. Magda Guglielmo of the University of Sydney (Australia).

“We always thought about our Galaxy as an inactive galaxy, with a not so bright center. These new results instead open the possibility of a complete reinterpretation of its evolution and nature. The flare event that occurred three million years ago was so powerful that it had consequences on the surroundings of our Galaxy. We are the witness to the awakening of the sleeping beauty,” she continued.

The recent research considers Sgr A* as a prime suspect. However, the researchers concede that there is still much more work to be done. How black holes evolve, influence and interact with galaxies, they conclude, “is an outstanding problem in astrophysics.”

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