Astronomy Picture of the Day

    Discover the cosmos! Each day a different image or photograph of our
      fascinating universe is featured, along with a brief explanation
                    written by a professional astronomer.

                              2020 September 8

                  GW190521: Unexpected Black Holes Collide
       Illustration Credit: Raśl Rubio (Virgo Valencia Group, The Virgo
                               Collaboration)

   Explanation: How do black holes like this form? The two black holes
   that spiraled together to produce the gravitational wave event GW190521
   were not only the most massive black holes ever seen by LIGO and VIRGO
   so far, their masses -- 66 and 85 solar masses -- were unprecedented
   and unexpected. Lower mass black holes, below about 65 solar masses are
   known to form in supernova explosions. Conversely, higher mass black
   holes, above about 135 solar masses, are thought to be created by very
   massive stars imploding after they use up their weight-bearing
   nuclear-fusion-producing elements. How such intermediate mass black
   holes came to exist is yet unknown, although one hypothesis holds that
   they result from consecutive collisions of stars and black holes in
   dense star clusters. Featured is an illustration of the black holes
   just before collision, annotated with arrows indicating their spin
   axes. In the illustration, the spiral waves indicate the production of
   gravitational radiation, while the surrounding stars highlight the
   possibility that the merger occurred in a star cluster. Seen last year
   but emanating from an epoch when the universe was only about half its
   present age (z ~ 0.8), black hole merger GW190521 is the farthest yet
   detected, to within measurement errors.

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       Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)
            NASA Official: Phillip Newman Specific rights apply.
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                             & Michigan Tech. U.


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