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Supermassive black holes, that weigh millions to billions of times more than our Sun, probably lurk in the dark and secretive hearts of most, if not all, galaxies floating around in our observable Universe. Our resident Beast, residing in the mysterious center of our own Milky Way Galaxy, is named Sagittarius A-Star, or Sgr A-star, for short (pronounced Saj-a-star), and it is a relative light-weight as these hefty, hungry supermassive gravitational Beasts go, weighing “merely” millions, as opposed to billions, of solar-masses. Black holes, as their name implies, are quite black! However, in the August 30, 2013 issue of the journal Science, a group of astronomers present new findings that shed light on the weird behavior of our own resident Beast–and by inference, on the other supermassive black holes that haunt the dark hearts of other galaxies that dance around in our Cosmos.
Over the past twenty years, astronomers have gathered strong evidence supporting the concept that a supermassive Beast hides in secret at the very heart of our large, barred-spiral, Milky Way Galaxy. Because this mysterious entity dwells relatively close to our own planet, it provides precious information concerning current theories about the weird, and little understood, workings of extreme gravity, and also about General Relativity. Because black holes are so extremely black, astronomers must try to understand their properties by observing the light emanating from the hot gas immediately circling around them.
Such supermassive Beasts are certainly some of the weirdest entities haunting the Cosmos. These strange and bewitching objects grow by devouring their surroundings, and they are very hungry, dining greedily on gas and star-stuff with amazingly voracious appetites! They also have no table manners whatsoever, and are very messy as they greedily gulp down unfortunate stars and blobs of gas, attempting to bite off considerably more than they can chew. Sgr A-star is a quiet old Beast now; it was much more active, brilliant, and hungry in its glory-days billions of years ago when our elderly Galaxy was young.
Smaller black holes, of “only” stellar-mass, also prance around the Cosmos. These relatively tiny gravitational monsters are born from the funeral pyre of a very massive star that has “died,” after having collapsed in the incandescent fires of a supernova blast that has torn the doomed star to shreds. The supernova explosion heralds the end of a massive star’s furiously incandescent life as a main-sequence (hydrogen-burning) star. After a black hole has risen like a Phoenix Bird, from the fiery ashes of the supernova, it can continue to put on more and more weight as it dines on whatever unfortunate object is unlucky enough to wander too close to its gravitational embrace.
The stuff of doomed stars and blobs of hapless gas whirl around into the turbulent vortex of gigantic supermassive Beasts, and this infalling banquet swirls around and around and down, creating an enormous disk, called an accretion disk. This doomed dinner becomes increasingly hotter and hotter, and emits a staggering quantity of radiation, as it comes ever-closer to that dreaded, hell-like point where it must abandon all hope–entering that infamous region of no return called the event horizon. The event horizon is situated at the innermost portion of the accretion disk.
Albert Einstein’s Theory of General Relativity predicts the existence of black holes, which he conceived to be entities sporting such deep gravitational wells that absolutely nothing, not even light, could soar away to freedom, and escape from their diabolically strong gravitational grip. Anything that is unlucky enough to wander too close to one of these insatiably hungry Beasts will inevitably, invariably be eaten. However, the real existence in Nature of these gravitational monstrosities seemed so weird at the time, that even Einstein doubted his own prediction. Eventually, however, he went on to characterize them in this way: “Black holes are where God divided by zero.”
Black holes can be big or small. Such a weird entity can be defined as a region in Spacetime where the lure of gravity is so unimaginably powerful that not even light can escape from its pull. The pull of gravity is this powerful because matter has been squeezed into a very tiny space. Pack enough matter into a small enough space, and you will get a black hole every time!
Most supermassive black holes, such as Sgr A-star, accrete very slowly, and are difficult to distinguish from the dark galactic hearts they inhabit. Sgr A-star provides a valuable and instructive exception to the rule, because astronomers can obtain an up close and personal view of its gentle X-ray emission. The authors of the August 30, 2013 Science paper titled Dissecting X-ray-Emitting Gas Around the Center of Our Galaxy by Dr. Q.D. Wang et al. write: “The nucleus of our Galaxy offers a multitude of opportunities for observing the interplay between a supermassive black hole… and its immediate surroundings… It is believed that Sgr A-star feeds off the winds from surrounding massive stars.” Dr. Wang is of the University of Cambridge in the UK, and University of Massachusetts at Amherst.
Dr. Wang and his colleagues present X-ray observations of Sgr A-star that help astronomers to constrain some of the most important theoretical models describing the bizarre behavior of material accreting onto the gigantic, hungry Beast.
By keeping track of the orbits of individual stars circling around our resident supermassive black hole, using the Keck Telescope, astronomers previously calculated its mass–with the best estimate now being approximately 4 million solar-masses, which is quite light in comparison to some others of its beastly kind. The W.M. Keck Observatory is comprised of two 10-meter ‘scopes, situated at an elevation of 13,600 feet near the summit of Mauna Kea in Hawaii.
Using a global network of radio telescopes, astronomers were also able to determine the size of the radio source emanating from Sgr A-star, calculating it to sport a diameter of less than 40 million kilometers. Due to additional observations of the very speedy X-ray flares flying from Sgr A-star, its X-ray emitting component was determined to be of similar size.
Dr. Jeremy D. Schnittman, in an accompanying commentary to the paper published in the August 30, 3013 Science, noted that “Combined, these vital stats can only be explained with a massive black hole.” Dr. Schnittman is at NASA Goddard Space Flight Center in Greenbelt, Maryland.
Strange Things Happen In The Galactic Center
The observations conducted by Dr. Wang and his colleagues are important for several reasons. First, the center of our Milky Way is well-hidden by a thick and enticing veil composed of heavy dust. This obscuring veil stops any visible light from escaping to freedom–and,therefore, it is not possible to observe the Milky Way’s dark heart with conventional optical telescopes. Infrared telescopes and radio telescopes, though representing an improvement, also present serious shortcomings. Dr. Schnittman explained in the August 30, 2013 Science that “X-rays, by contrast, have extremely short wavelengths, so should be ideal for taking high-resolution pictures.” He added that “The all-time champion of angular resolution among X-ray telescopes is the Chandra X-ray Observatory, which can resolve images with better than 1 arc sec… detail.”
Dr. Wang and his team used the Chandra X-ray Space Observatory to make major discoveries in their efforts to understand why starry-stuff and hot gas whirling around Sgr A-star are surprisingly dim at X-ray wavelengths. The new findings result from one of the largest observing endeavors conducted by the highly successful Chandra mission. During the year 2012, Chandra gathered approximately five weeks worth of observations to obtain by far the best X-ray images and energy signatures of multi-million degree gas whirling, in doomed splendor, around our Galaxy’s resident Beast. At a “mere” 26,000 light-years from our planet, Sgr A-Star is one of a very small number of black holes–among billions that haunt the Cosmos–that astronomers can observe, in order to witness the motion of nearby matter. The diffuse X-ray emission from Sgr A-star comes from the searing-hot gas that it has ensnared, and gravitationally sucked inwards towards its waiting maw. The very hot gas is caused by winds churned out by a disk-shaped population of bouncy, young, and massive stars seen in infrared observations.
Dr. Wang and his colleagues suggest that less than 1% of the material, that is within the supermassive Beast’s deadly reach, actually makes it to the event horizon. This is because a great quantity of it is apparently hurled back out. As a result, the X-ray emission emanating from material close to Sgr A-star is very dim–like that of the majority of supermassive black holes haunting the Universe.
The doomed material must lose some of its heat and angular momentum before it can take the fateful plunge into the supermassive black hole, never to return. The ejection of material enables this to happen.
Dr. Wang and his colleagues’ important work should help those astronomers, using radio telescopes, to watch and understand the mysterious “shadow” that is cast into Space by the event horizon of Sgr A-star. This weird shadow is cast against the background of glowing, surrounding material. This work should also prove useful in enabling astronomers to understand the impact that orbiting stars and gas clouds have on the matter traveling towards and away from the supermassive Beast.
Dr. Schnittman noted in his article that “(T)he future looks bright, quite literally. In the next few months a large cloud of gas is on course to collide with the black hole, which could potentially brighten by a factor of a million or more, greatly enhancing our ability to observe this unique region where gravity rules supreme”.
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Source by Judith E Braffman-Miller
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