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Friday June 23rd 2017

Black Holes

Gravitational distortions caused by a black hole in front of the Large Magellan cloud.
Gravitational distortions caused by a black hole in front of the Large Magellan cloud.

Black holes, the enigmatic cosmic objects, are actually not so rare in the universe. All it needs is a star with the mass of at least 10 times that of our sun. After its Hydrogen has run out and Hydrogen-fusion is no longer possible, things start to change. Without fusion, the equilibrium between gravity and fusion pressure ceases to exist. The result: a supernova explosion that sheds the outer layers of the star. With only gravitational forces left, remnant material collapses. If the remaining mass is about 1.5 to 2 times that of the sun, the force will push electrons into protons, which become neutrons, hence the name neutron star. If more mass remains, gravitational forces squeeze even neutrons into each other. This results is an area so massive and so dense that even light cannot escape its gravitational pull. The process is quite normal; black holes are simply evolutionary endpoints in the life of a large star.

Milky Way’s center has a much higher star density than the outer regions; subsequently it would also be populated with a greater number of black holes. As a matter of fact, so many black holes were concentrated in the center, that they have merged to one super massive black hole, with a mass equal to 4.3 million suns. Due to its position in the constellation Sagittarius, this black hole  is called Sagittarius A*, or short SGR A* (pronunciation: Sagittarius A star). It is thought that most galaxies contain both, super massive black holes in their centers and stellar mass size black holes floating in their outer regions.

Why are black holes black?

The speed required to overcome the gravitational pull of any celestial body is called escape velocity (EV) . It depends on the mass of the body and the distance of the escaping body to the center of gravity. An space ship must exceed a speed of  11.2 km/s to escape the earth’s gravity.   Escape velocity in the proximity of the black whole is higher than the speed of light (299,792 km/s). Since relativity theory predicts that nothing can exceed the speed of light, even photons (light) are caught and cannot escape. Due to that characteristic, black holes are indeed perfectly black.

Black hole regions.

When discussing black holes we look at three different active regions: Singularity, Event Horizon and Ergosphere.

Singularity

Matter and electromagnetic waves in the vicinity of a black hole are sucked into it.  As closer they come, as higher the gravitational forces. These forces increase also with the amount of matter sucked in. But where is the matter going?  This point is called singularity. With equation at hand it is not possible for scientists to describe location and ruling conditions of a singularity mathematically. All equations trying to describe this point result in infinity, like infinitely small or infinitely dense. 

Singularity is the entity of matter and energy compressed into one point.

Black Hole Regions - Singularity, Event Horizon, ErgosphereEvent Horizon & Schwarzschild Radius

The event horizon of a black hole marks the dark region in space where the escape velocity is higher than the speed of light. This region is typically considered the “black hole”. Nothing can escape it, both photons and matter is inevitably pulled into the singularity. The distance from the singularity to the event horizon is also known as the Schwarzschild radius.

Ergosphere

Ergosphere is a region outside the event horizon, where gravitational forces start to influence objects movements. Objects here can no longer remain stationary in space. Depending on the distance between object and to the event horizon, the influence can be extremely strong or very weak. Close to the event horizon, where escape velocity is nearly the speed of light gravitational will rip objects apart and eventually draw most of their material in. Far away the effects are virtually non-existent. Objects in the Ergosphere can escape the forces of a black hole if their speed is higher than the appropriate escape velocity.

General relativity theory predicts that any rotating mass drags surrounding space-time with it. This makes the ergosphere not just a characteristic of black holes, but it is present with all regular cosmic objects of mass, including earth, planets or the sun. 

Size of a Black hole

As discussed above, a black hole is understood as the region within the event horizon. With 30 km in diameter, stellar black holes are actually quite small, approximately the size of an asteroid. Super massive black holes are significantly larger but still relatively small in cosmic scale. The event horizon of SGR A* has an estimated size that it would fit in the orbit of Mercury. The active ergosphere of SGR A* is thought to be only 10 light days in diameter.  

How can we detect black holes?

Object movement in the vicinity

Due to their perfect absorption of light, it is impossible to see black holes directly. Their large mass allows however to observe their influence on other bodies in the vicinity. An analogy might be air movement, we can’t see the wind but because leaves are moving we are certain about its existence.

Researchers started in 1998 to record the movement of 90 objects in the neighborhood of SGR A*. By applying Kepler’s laws, this information provided very accurate information about location and mass of Milky Way’s central black hole.

X-Ray Emissions

M87 with jet caused by the black hole in the center of this galaxy. Credit: NASA, ESA.
M87 with jet caused by the black hole in the center of this galaxy. Credit: NASA, ESA.

Interstellar matter that is drawn towards a black hole accelerates. The closer it comes the higher the gravitational pull. This acceleration heats it up, and with high enough temperature ionizes its atoms. Once the temperature reaches several million Kelvin, atoms start emitting X-Rays, which can be observed with x-Ray telescopes or indirectly, through emissions of objects that are excited by them.

Typical for a black hole is that X-Ray jets are not constant. There is simply no steady input of matter. Black holes nurture more randomly, and are depending on how much matter is available in their ergosphere.

SGR A* is currently very quite simply because most of the matter in its vicinity has already been attracted and is used up.  Other galaxies are more active and show intense X-Rays emitting from their center, a famous example is M87.

Gravitational Lensing

As any body with a mass, black holes bent space-time fabric. Since they employ huge mass in a very small region the effects of black holes in the vicinity are very strong.  Gravitational lensing could be used to determine black holes but due to the very small active area of stellar black holes, it might only be practicable with super massive black holes.   

A word on singularity

Mathematically, a singularity is a condition where equations do not provide valid values. Our current equations, based on general relativity theory, are not sufficient to describe conditions of the singularity. Equations and scale used by relativity theory are simply too coarse to present satisfactory results. Quantum theory equations would be more appropriate, but unfortunately the mathematical bridge between both theories has yet to be made.

Stephen Hawking radiation equation shows that black holes can indeed radiate energy and conserve entropy (disorder). According to the second law of thermodynamics entropy implies heat and therefore temperature. Hawking’s calculations predict a black body temperature of a couple nano-Kelvin; low indeed but an energy emission nevertheless. This means that black holes do not exist infinitely, they rather evaporate.

Further sources

Hubble-Cast: Black holes

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