Monday, June 13, 2022

3600 - BLACKHOLES - more than we can imagine?

  -  3600 -   BLACKHOLES  -  more than we can imagine?    In was just a number of years ago that we discovered Black Holes really existed.  Now astronomers estimate that 100 million black holes roam among the stars in our own Milky Way galaxy.  And, the center of our galaxy has a mammoth blackhole.


---------------------  3600  -     BLACKHOLES  -  more than we can imagine? 

-  Astronomers had never conclusively identified an isolated smaller blackhole before. Following six years of meticulous observations, the NASA/ESA Hubble Space Telescope has, for the first time ever, provided direct evidence for a lone black hole drifting through interstellar space by a precise mass measurement of the phantom object.

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-   Until 2022, all black hole masses have been inferred statistically or through interactions in binary systems or in the cores of galaxies. “Stellar-mass Blackholes” are usually found with companion stars, making this one unusual.

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-  Stellar-mass or a single blackhole is in contrast to the giant blackholes at the center of most galaxies that are thousands, millions,  and even billions times the mass of our sun.  The blackhole at the center of our galaxy is 4,000,000 solar mass.

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-  This newly detected wandering black hole lies about 5,000 light-years away, in the Carina-Sagittarius spiral arm of our galaxy. However, its discovery allows astronomers to estimate that the nearest isolated stellar-mass black hole to Earth might be as close as 80 light-years away. The nearest star to our solar system, Proxima Centauri, is a little over 4 light-years away.

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-  Black holes roaming our galaxy are born from rare, monstrous stars and are less than one-thousandth of the galaxy's stellar population.   They are at least 20 times more massive than our Sun ( that is 20 solar mass).

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-  These stars explode as supernovae, and the remnant core is crushed by gravity into a black hole. Because the self-detonation is not perfectly symmetrical, the black hole may get a kick, and go careening through our galaxy like a blasted cannonball.

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-  Telescopes can't photograph a wayward black hole because it doesn't emit any light. However, a black hole warps space, which then deflects and amplifies starlight from anything that momentarily lines up exactly behind it.

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-  Ground-based telescopes, which monitor the brightness of millions of stars in the rich star fields toward the central bulge of our Milky Way, look for a tell-tale sudden brightening of one of them when a massive object passes between us and the star. Then Hubble follows up on the most interesting of these events.

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-   The warping of space due to the gravity of a foreground object passing in front of a star located far behind it will momentarily bend and amplify the light of the background star as it passes in front of it. Astronomers use the phenomenon, called “gravitational microlensing“, to study stars and exoplanets in the approximately 30,000 events seen so far inside our galaxy.

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-  The signature of a foreground black hole stands out as unique among other microlensing events. The very intense gravity of the black hole will stretch out the duration of the lensing event for over 200 days.

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-   If the intervening object was instead a foreground star, it would cause a transient color change in the starlight as measured because the light from the foreground and background stars would momentarily be blended together. But no color change was seen in this black hole event.

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-  Hubble measures the amount of deflection of the background star's image by the black hole. Hubble is capable of the extraordinary precision needed for such measurements. The star's image was offset from where it normally would be by about a milliarcsecond. That’s equivalent to measuring the height of an adult human lying on the surface of the moon from the Earth.

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-   This “astrometric microlensing” technique provided information on the mass, distance, and velocity of the black hole. 

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-    Reports of a slightly lower mass range means that the object may be either a neutron star or a small black hole. This estimate is that the mass of the invisible compact object is between 1.6 and 4.4 times that of the Sun. At the high end of this range the object would be a black hole; at the low end, it would be a neutron star.

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-   Whatever it is, the object is the first dark stellar remnant discovered wandering through the galaxy, unaccompanied by another star.

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-  This was a particularly difficult measurement for the team because there is another bright star that is extremely close in angular separation to the source star.   It’s like trying to measure the tiny motion of a firefly next to a bright light bulb. Astronomers had to meticulously subtract the light of the nearby bright star to precisely measure the deflection of the faint source.

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-   The isolated black hole is traveling across the galaxy at 160,000 kilometers per hour (fast enough to travel from Earth to the Moon in less than three hours). That's faster than most of the other neighboring stars in that region of our galaxy.

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-  “Astrometric microlensing” is conceptually simple but observationally very tough.  Microlensing is the only technique available for identifying isolated black holes. When the black hole passed in front of a background star located 19,000 light-years away in the galactic bulge, the starlight coming toward Earth was amplified for a duration of 270 days as the black hole passed by. 

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-  It took several years of Hubble observations to follow how the background star's position appeared to be deflected by the bending of light by the foreground black hole.

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-  The existence of stellar-mass black holes has been known since the early 1970s, but all of their mass measurements have been in binary star systems. Gas from the companion star falls into the black hole and is heated to such high temperatures that it emits X-rays. 

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-  About two dozen black holes have had their masses measured in X-ray binaries through their gravitational effect on their companions. Mass estimates range from 5 to 20 solar masses.

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-   Black holes detected in other galaxies by gravitational waves from mergers between black holes and companion objects have been as high as 90 solar masses.

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-   In his 1916 paper on general relativity, Albert Einstein predicted that his theory could be tested by observing the offset in the apparent position of a background star caused by the Sun’s gravity. This was tested by a collaboration led by astronomers Arthur Eddington and Frank Dyson during a solar eclipse on 29 May 1919. 

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-  Eddington and his colleagues measured a background star being offset by 2 arc seconds, validating Einstein’s theories. These scientists could hardly have imagined that over a century later this same technique would be used with a then-unimaginable thousand fold improvement in precision to look for black holes across our galaxy.

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-  Earth just got 7 kilometers / second faster and about 2,000 light-years closer to the supermassive black hole in the center of the Milky Way Galaxy.  This doesn't mean that our planet is plunging towards this black hole. Instead the changes are results of a better model of the Milky Way Galaxy based on new observation data, including a catalog of objects observed over the course of more than 15 years by radio astronomy project VERA.

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-  VERA (VLBI Exploration of Radio Astrometry, by the way "VLBI" stands for Very Long Baseline Interferometry) started in year 2000 to map three-dimensional velocity and spatial structures in the Milky Way. 

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-  VERA uses a technique known as interferometry to combine data from radio telescopes scattered across the Japanese archipelago in order to achieve the same resolution as a 2,300 km diameter telescope would have. Measurement accuracy achieved with this resolution, 10 micro-arcseconds, is sharp enough in theory to resolve a US penny placed on the surface of the Moon.

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-  Because Earth is located inside the Milky Way Galaxy, we can't step back and see what the Galaxy looks like from the outside. Astrometry, accurate measurement of the positions and motions of objects, is a vital tool to understand the overall structure of the Galaxy and our place in it. The First VERA Astrometry Catalog was published containing data for 99 objects.

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-   The center of the Galaxy, and the supermassive black hole which resides there, is located 25,800 light-years from Earth. This is closer than the official value of 27,700 light-years adopted by the International Astronomical Union in 1985. The velocity component of the map indicates that Earth is traveling at 227 km/s as it orbits around the Galactic Center. This is faster than the official value of 220 km/s.

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-  In the past century, black holes have transformed from being a mere curiosity into a key element of modern astronomy.  Our understanding of black holes is now central to our understanding of the cosmos. The next generation Very Large Array (ngVLA) will help astronomers study these mysterious objects.

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-  Astronomers have long known that Einstein's theory of gravity allowed for an object to be so massive that light itself could not escape, but they initially doubted that black holes existed in the Universe. Today black holes are recognized as a standard result of the death of very massive stars.

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-   A century ago, astronomers thought that the Universe consisted mostly of stars. They shine with the colors of light that our human eyes can see, and to most of us, the picture of an astronomer includes a telescope turned to the heavens. Today, however, we now recognize that a variety of objects shine at wavelengths that our eyes cannot see, from long wavelength radio waves to extremely high-energy gamma rays.

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-  We now know that there are a variety of other messengers carrying to us information about the Universe. Cosmic rays are energetic sub-atomic particles, with energies well above those that particle accelerators such as the Large Hadron Collider can produce. 

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-  In the most extreme cases, a sub-atomic particle can hit the Earth's atmosphere with as much energy as a fast-pitch baseball. Billions of neutrinos rain upon us every second. They are born from nuclear fusion in the Sun, from distant exploding stars, from the regions near supermassive black holes. 

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-  And,  gravitational waves constantly wash over the Earth and the Solar System. These distortions of spacetime itself are generated by colliding black holes, and potentially by the expansion of the Universe.

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-  The detection of gravitational waves, along with their direct link to merging compact objects, marks one of the major breakthroughs in astrophysics over the past 10 years. The “ngVLA” will be able to resolve and observe the motion of mergers of supermassive black holes and neutron stars, both sources of gravitational waves.

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-   New facilities can detect these merging stellar remnants in galaxies up to 600 million light-years away through the gravitational wave and neutrino events they produce, and the ngVLA will be able to detect the radio emission to the same distance, permitting us to determine the physical conditions at the location of neutrino production.

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-  Astronomers still have much to learn from black holes. In the near future, the ngVLA provide astronomers with a central tool for understanding black holes and multi-messenger astronomy.  The more we learn the more we see we need to learn.  

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June 12, 2022     -     BLACKHOLES  -  more than we can imagine?                3600                                                                                                                                           

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