THE USM SOUTHWORTH PLANETARIUM

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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?    

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.