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Black Holes Encyclopedia

NGC 3842

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In the constellation Leo

Finder Chart


320 million light-years (98 megaparsecs)


9.7 billion times the mass of the Sun


Diameter roughly 6 times the size of the orbit of Neptune

Discovery Methods


NGC 3842

Records are made to be broken -- a rule that applies not only to football games and pie-eating contests but to supermassive black holes as well.

In 2010, for example, astronomers announced a new record in the black-hole "weight" contest: a monster at the core of the galaxy M87 tipping the scales at more than six billion times the mass of the Sun. Then, in late 2011, they topped that record -- not once, but twice. One of the new entries was in the center of NGC 3842, a giant elliptical galaxy in the Coma Cluster, a collection of hundreds of galaxies in the constellation Coma Berenices. It weighed in at almost 10 billion times the mass of the Sun.

Not only did the discoveries mess with the record books, they seemed to mess up one of the ways in which astronomers determine the masses of these giant black holes.

The most accurate method for measuring the mass of a supermassive black hole is to clock the speeds of stars in orbit around it. By measuring the speeds at different distances from the center of the galaxy, astronomers can determine how much mass is confined to the galaxy's central region. The closer astronomers can see into the galaxy's center, the more accurate their measurement of the black hole's mass.

In the case of NGC 3842, for example, astronomers used the Gemini North and Keck 2 telescope in Hawaii and the Harlan J. Smith at McDonald Observatory to measure the velocities of stars at 82 different points in the galaxy. They then entered those measurements into mathematical models of how stars orbit black holes in the centers of galaxies, and computed the results with the supercomputer facilities at the Texas Advanced Computing Center. The calculations showed that the galaxy's central black hole is roughly 9.7 billion times as massive as the Sun.

Most galaxies are so far away, however, that it is difficult to accurately measure the velocities of their stars, so astronomers must estimate the size of the galaxy's supermassive black hole by comparing the mass and brightness of the galaxy itself.

This relationship is applied particularly to elliptical galaxies like NGC 4889, which look like fat, fuzzy footballs. These galaxies are much larger than our own galaxy, the spiral-shaped Milky Way, and almost all of their stars are old and relatively faint.

The mass-luminosity relationship, however, predicted much smaller black holes for NGC 3842, NGC 4889 (the other record breaker), and M87. The larger masses for the black holes suggests that the relationship doesn't work well for the largest ellipticals, which means it could be more difficult to estimate the sizes of black holes in distant galaxies.

NGC 3842's black hole may be a dead quasar -- an object that once shined more brightly than all of the stars in the galaxy combined. When the black hole was born it was surrounded by vast clouds of gas and dust and millions of stars. Much of this matter was funneled toward the black hole, forming a broad, thin, superhot disk.

Strong "winds" of particles from the disk, as well as the pressure of the radiation itself, soon blew away most of the remaining matter close to the black hole. When the last of the matter in the accretion disk was either ingested by the black hole or blown back out into space, the quasar shut down.

Today, the black hole is quiet. An occasional star or gas cloud strays too close to the black hole and is gobbled up in a blaze of energy, however, adding a tiny bit to the black hole's record-breaking mass.


New Heavyweight Champions (McDonald Observatory)
An international team of astronomers has measured the masses of the heaviest black holes yet confirmed: two galactic behemoths that top the scales at about 10 billion times the mass of the Sun.



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This document was last modified: March 15, 2012.


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