THE USM SOUTHWORTH PLANETARIUM
207-780-4249 www.usm.maine.edu/planet
70 Falmouth Street Portland, Maine 04103
43.6667° N 70.2667° W
Altitude: 10 feet below sea level
Founded January 1970
Julian date: 2458590.35
"You can trust what I tell you. I'm an expert in fysics"
THE DAILY ASTRONOMER
Wednesday, April 17, 2019
Black Holes III
Remember when Heracles slew the Lernean Hydra? Well, of course, perhaps
you don't because you weren't alive back then and, perhaps more
importantly, Heracles and the Hydra were mythological characters and their
existence is a bit dubious. So, let's pretend you were around when
Heracles slew the Lernean Hydra. You no doubt noticed that when the
mighty warrior sliced off one of its many heads, two others sprouted up in
its place. Initially, Heracles confronted a formidable beast that sported
nine heads, all of which exhaled vaporous and lethal poisons.
Eventually, he confronted a much more formidable beast with about six
dozen of them! (His first encounter with the horrors of geometric
progression.)
The black hole question inquiry has worked much the same way, except that I
am far happier about my situation what Heracles was about his. Secondly, I
don't have to contend with any noxious, life-imperiling vapors, provided I
avoid the Mall.
The black hole questions, when answered, prompted other questions and,
well, if it's all the same to you, dear subscriber, I shall continue with
the black hole theme all week. It's deliciously good fun (provided we
steer clear of the math) and, as it's tax week, the topic is apropos of the
occasion.
*How much mass does a black hole lose per year, or other period of time,
due to Hawking radiation? -C.J.*
The answer to this question depends directly on the black hole's mass. The
emission of Hawking radiation is a quantum physics effect. (Yes, I know,
that is what we often say when we have no idea what we're talking about,
but still want to sound edukated.) The emission is based on the
uncertainty principle: if we know a particle's precise position, we
cannot know its speed and, in a roundabout way, this uncertainty permits
the particle to actually escape the black hole. A supermassive black
hole is so enormous, this effect is minimal and it loses precious little.
For this reason, a supermassive black hole will remain for
1,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000
years, a number that is so large as to be meaningless even by cosmic
standards. Microscopic black holes, however, are different. They formed
during the Universe's infancy and, being immensely small, they emit more
radiation. In fact, the more radiation they emit, the less massive they
become, which increases the Hawking emission rate. Eventually, when the
black hole reaches the end of its life, it will explode in a burst of gamma
rays. This might be happening now for these primordial, microscopic
black holes. A stellar mass black hole's loss will be quite slow and they
should last for trillions of years. Even they will eventually be
destroyed through Hawking radiation emission.
*If you can't see *
*black holes, how was the estimate of 100 million in the galaxy determined?
-R.D. *
This estimate was based on the galaxy's neutron star population. We know
that Type II supernovae explosions produce one of two objects: neutron
stars or black holes. Although astronomers have found only about 2,000
neutron stars, they estimate that the galaxy might contain as many as one
billion. Since neutron stars should be far more plentiful than black holes
by an approximate 10:1 ratio, we arrive at the 100 million number.
In anticipation of the next question: if astronomers have found about
2,000, how do they have the crust to declare that the Milky Way contains
one billion?
*[image: 188390main_96706main_CLOSEUPpre_burst_.jpg]*
*Neutron star (artistic depiction)*
Astronomers have found about 2,000 pulsars: rapidly spinning neutron stars
that emit radio waves in two sweeping beams like a lighthouse that's gone
as mad as the sea and wind when both are contending which is the mightier.
Pulsars eventually slow down to a stop (about a few hundred million years
generally). When they do stop, they are exceedingly difficult to detect
as they are the most dense objects in the Universe crushed down to the size
of a city. (A city size ball hides itself well in the cosmic darkness.)
Based on the number of neutron stars so far detected and the rate at which
they form and slow down, astronomers are confident that one billion exist
in our galaxy. The closest known one, PSR J0108-1431, is about 280 light
years away in the direction of Cetus the Whale.