The Milky Way's huge black heart

Pick up a pencil or a fork or a little kid. In doing so, you have just overcome the gravitational might of the entire planet Earth. You are stronger than gravity.

Yet gravity pervades the entirety of space. It is the force which keeps the moon rotating about Earth, the duo orbiting the sun, and even the whole solar system dancing around the centre of the Milky Way. Heck, gravity is what will ensure the Milky Way and Andromeda galaxies will collide some time in the distant future.

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Gravity is a mighty force. And this is nowhere more apparent than when we look into a black hole. Simply put, a black hole is a gravitational mass large enough that even light cannot reach escape velocity. Everything gets sucked into a black hole.

Last year, astronomers were able to image the supermassive black hole at the centre of M87. Using radio telescopes spread across the planet, they were effectively able to generate a telescope with a huge receiving dish and consequently able to pick up minute details. The image – which was rendered into visible colours – showed us the gravitational distortion caused by the black hole.

But the image wasn’t as important as the science behind it as it provided confirmation of a conjecture which had been floating around the astronomical community for a long time. At the heart of nearly every galaxy, including our own, is a massive black hole. Some of these monsters are millions or even billions of times more massive than our sun.

Some are thought to be the sources for quasars. These “quasi-stellar radio sources” are some of the most intense beacons of radio waves observable and maybe the most remote objects visible in the universe. Indeed, if they didn’t shine so bright, we wouldn’t be able to see them at all. Astronomers speculate there are massive black holes in the cores of very young galaxies voraciously consuming their gaseous halos. Their brightness arises from synchrotron radiation emitted by the charged particles swirling across the event horizon.

But in mature galaxies, such clouds of gases are no longer available to feed upon. The black hole at the centre of most galaxies has already swept clear its immediate surroundings. They have eaten everything nearby. These giant monsters quietly lie in wait until an innocent star gets too close and then they rip it to shreds.

The result is a tidal disruption event (TDE) which, for a brief while, can shine as bright as a super nova. As the star is slowly consumed, it is shredded into strands like spaghetti. In the first bite, the black hole swallows half of the star’s mass while the rest arcs through the surrounding space in streamers. Of course, the materials are caught by the gravitational trap and rapidly fall back into an accretion disk surrounding the black hole.

Matter in the disk is heated as it swirls into the singularity resulting in intense X-ray bursts. Satellites mapping X-rays first spotted these TDEs in the 1990s but the events are rare and instrument time is difficult to come by. You also have to have the satellite pointing in the right direction. Over the past two and a half decades, a total of 22 TDEs were detected and analyzed.

Unfortunately, by the time the astronomers had determined a TDE was happening, it was too late to capture any additional data using different instruments. A “did-you-see-that?!?” moment in astronomical terms.

However, over the past 18 months, the Zwicky Transient Facility in California has been able to capture 17 more events. Suvi Gezari of the University of Maryland College Park and her co-workers have also been able to alert other observatories, such as NASA’s Swift telescope, to make follow up observations at ultraviolet and X-ray wavelengths.

Gezari and her colleagues have determined the spectra generated during a TDE can provide a picture of the star being swallowed. They fall into three broad categories – young stars made up almost entirely of hydrogen, old stellar cores where helium is the dominant element, and occasionally a mixture of gases indicating a mature star is being consumed. The relative proportions provide some indication of the stellar neighbourhood in a galactic core. Further, if the speed of consumption could be determined, it would tell us something about the size of the black hole.

But the really intriguing part of the picture is that if a black hole gets big enough then a TDE doesn’t occur. Instead, stars are swallowed whole with nary a whimper. So far, all of the data on TDE come from smaller galaxies which fits with this picture. Mature galactic cores, like the one at the heart of the Milky Way, swallow up whole solar systems in a single gulp.

 

Just a thought to ponder. 

 

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