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:  2458584.35
2018-19:   CXX
           "Out of night, out of mind."

THE DAILY ASTRONOMER
Thursday, April 11, 2019
About That Black Hole Photograph

[image: wS5ytyMNaRpRMuCp62yjKn-970-80.jpg]

Yes, that one.

It has certainly generated a tsunami of interest in all things astronomical
these last couple of days.  This behemoth of a black hole has also elicited
a deluge of questions about itself and about black holes in general.
So, today and on Monday -we think- we'll strive to answer as many of these
queries as we can possibly address.     We apologize for not being able to
offer specific attributions for these questions.   The following is,
instead, a collection of the questions we've received over the last
forty-eight hours

WHERE IS IT AND HOW BIG IS IT?
This supermassive black hole, which, if put in the Sun's place would extend
beyond Neptune's orbit, is in the center of galaxy M87, located about 55
million light years from our solar system.     This black hole is
approximately 6.5 million times more massive than the Sun.  (The black hole
occupying the center of the Milky Way is about 4 million times more massive
than our parent star.)

HOW CAN WE PHOTOGRAPH A BLACK HOLE IF THERE IS NOTHING TO SEE?
True enough.    A black hole's gravity is so powerful that not even light
can escape from it.  However, an accretion disk will often form around a
black hole.  Such a disk produces copious amounts of high energy radiation
due to the differential rotation. (The gases close to the black hole move
faster than those farther away:  the resultant frictional heating produces
this radiation.)   Also, the extreme spacetime curvature around a black
hole causes light to shift around it, thereby producing a photon ring.

WHY BOTHER PHOTOGRAPHING IT IF WE ALREADY KNOW ABOUT BLACK HOLES?
There is no substitute for visual confirmation.    The black hole image is
consistent with astrophysical models developed about black holes.    While
most researchers were confident that these descriptions were accurate, they
are pleased to have this added confirmation.

WE'RE SEEING THE BLACK HOLE AS IT WAS 55 MILLION YEARS AGO. IS IT STILL
THERE?
Oh, absolutely!  Black holes do "evaporate" due to the emission of Hawking
radiation, but such evaporation is a laboriously long process.  In fact,
supermassive black holes are basically immortal, as their life expectancy
is on the order of ten to the power of 66 Earth years:   so long as to be
meaningless, even on cosmological time scales.

Do you have any questions about the black hole photo or black holes, in
general?   Please send them.  We'll address them on Monday.