Thursday, December 10, 2020

BLACKHOLES - history of discovery

 -  2929  -  BLACKHOLES  -  history of discovery.  -   Blackholes were first discovered before I was born.  In 1937, an astronomically inclined electrical engineer named Grote Reber built a homemade radio dish out of lumber and sheet metal in his backyard in Wheaton, Illinois. The following year, he confirmed Karl Jansky’s 1931 discovery of radio waves emanating from the center of the Milky Way. 


---------------------------  2929  -  BLACKHOLES  -  history of discovery

-  Over the next few years after the 1937 discovery, Reber extended his research of what he called “cosmic static,” eventually publishing sky maps showing prominent radio sources in the constellations Cassiopeia and Cygnus. The first, called “Cassiopeia A“, is the brightest extrasolar radio source in the sky, now known to be a young supernova remnant.

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-  The other source, “Cygnus A“, is a galaxy more than 750 million light-years away. Reber had made the first observation of a blackhole, one more than 2 billion times the mass of the Sun.

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-  Astronomers now think such enormous “supermassive” black holes reside in the centers of most big galaxies, including our own. Recently, observations of Cygnus A using the Very Large Array in New Mexico revealed another bright radio source, possibly another giant black hole, near the galaxy’s center.

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-  Our growing understanding of black holes has unfolded along three distinct but related tracks: first through efforts to understand gravity itself, then explanations of how so-called active galaxies like Cygnus A could emit such vast amounts of energy, and finally the discovery of small stellar-mass black holes in our home galaxy and beyond.

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-  Although the term black hole wasn’t coined until 1967, when American physicist John A. Wheeler first mentioned it in a talk, the quest for these enigmatic objects began much earlier.

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-  In May 1783, the English scientist-turned-clergyman John Michell envisioned a star so large that its escape velocity equaled the speed of light. He wrote that “all light emitted from such a body would be made to return towards it, by its own proper gravity, rendering the star invisible to astronomers.

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-   Although undetectable by its own light, he noted, the object could be found by observing irregularities in any “luminous bodies” that happened to revolve around it. Next, France’s Pierre-Simon Laplace independently postulated the existence of blackholes in a book published in 1796.

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-  Fast forward to late 1915, when Albert Einstein published the defining paper on his general theory of relativity. A few weeks later, fellow German physicist Karl Schwarzschild published an exact solution to Einstein’s new relativity equations, which revealed the radius that a given mass would need to be compressed to before collapsing  into a black hole.

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-   This “Schwarzschild radius” marks the event horizon, the point of no return, from which nothing, not even light, can escape, for a non-rotating blackhole. Half a century later, the New Zealand mathematician Roy Kerr described an exact solution for rotating versions of these gravitational blackholes.

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-  In 1934, astronomers Walter Baade and Fritz Zwicky suggested that supernova explosions represented the transformation of a normal star into a “neutron star“, an object with the Sun’s mass, but crushed into a sphere the size of Manhattan. A few years later, physicists J. Robert Oppenheimer and George Volkoff, using work by physicist Richard Tolman, showed that if neutron stars become too massive, they must continue to collapse.

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-  The heaviest measured neutron star at the moment is the “pulsar” PSR J0348+0432, which tips the scales at 2.01 solar masses. We know now that when a massive star runs out of fuel and collapses under its own weight, the resulting supernova crushes the stellar core to extreme densities. 

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-  If the mass of the core falls somewhere between 2.01 and 3 solar masses (exactly where depends on unknown details of neutron star structure), then a stellar-mass blackhole is born.

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-  Confirmed stellar-mass black holes exist in 20 or so X-ray-emitting binary systems found in our galaxy and its neighbors. In these systems, a normal companion star orbits close to the blackhole.

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-   Observing the orbital motions of the companion allows astronomers to determine the masses of both members of the system. If the mass of the compact object exceeds three Suns, then it must be a blackhole.

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-  Most of these systems remain dormant for decades while gas streaming from the companion star slowly accumulates into a storage disk around the black hole. Eventually, the disk becomes unstable, and gas begins spiraling inward. Friction heats the gas up to millions of degrees so it glows in X-rays before plunging past the event horizon. Outbursts like this typically last about a year before the system returns to dormancy.

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-  A few systems persistently produce X-rays, and the most famous and best studied is Cygnus X-1. Located about 6,100 light-years away, the binary system is made up of a 15-solar-mass blackhole orbiting the common center of mass it shares with a hot, blue O-type supergiant, which is estimated at 25 or more solar masses. 

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-  The star’s surface is so hot it continuously sheds plasma into space, creating a fast-moving outflow called a stellar wind. As the blackhole orbits the star, it sweeps up a small part of this outflowing gas, which produces X-rays when the gas falls toward the blackhole.  

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-  The closest confirmed blackhole is A0620–00, about 2,800 light-years away, but it’s likely there are many others much closer that we haven’t yet found. Astronomers estimate that our Milky Way may hold as many as 100 million stellar-mass black holes. 

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-  “There is a high probability, that there is a blackhole within about 20 light-years of Earth. Unless blackholes are “fed” with matter that emits light as it heats up on the way in, they can be hard to find.

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-  Hubble’s Space Telescope Imaging Spectrograph (STIS) captures the motions of gas in the grip of M84’s black hole.   STIS found a velocity of 880,000 mph within 26 light-years of the galaxy’s center, indicating M84’s black hole weighs at least 300 million solar masses.

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-   Over the past two decades, teams using the Keck telescope in Hawaii and the European Southern Observatory’s Very Large Telescope in Chile have mapped the orbits of massive stars in the center of our galaxy. These stars loop around a common but invisible center of mass 4 million times that of the Sun. 

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-  Other evidence indicates that a radio source known as Sagittarius A* and the unseen object inferred by stellar orbits are one and the same and almost certainly a supermassive black hole.

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-  Peering into the centers of other galaxies provides even more evidence. For example, in 1997, astronomers aligned the slit of Hubble’s Space Telescope Imaging Spectrograph (STIS) with the center of galaxy M84 in the Virgo cluster, revealing a dramatic swing in gas motions over a comparatively small distance.


-   STIS measured a gas velocity of 880,000 mph  within 26 light-years of the galaxy’s center. From this motion, astronomers calculated that the blackhole at the heart of M84 contains at least 300 million solar masses.

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-  Since 2015, striking new evidence for the existence of blackholes has come from the Laser Interferometer Gravitational-wave Observatory (LIGO), which so far has detected nearly half a dozen merging black hole pairs. 

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-  Large orbiting masses lose energy by creating ripples in space-time called “gravitational waves“, yet another prediction of Einstein’s relativity. And as they lose energy, binary blackholes draw closer together over millions of years until they coalesce.

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-   How do black holes stack up to their companion stars?  The system sizes are largely determined by the orbital period, which ranges from 33.9 days to 0.2 and all are persistent X-ray sources, where the blackhole is fed by stellar winds from a massive companion star. 

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-  December 6, 2020          BLACKHOLES  -  history                     2929                                                                                                                                                             

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