Editing Black (section)

===Astronomy===
* A [[black hole]] is a region of [[spacetime]] where gravity prevents anything, including [[light]], from escaping.<ref>{{Cite book |last=Wald |first=Robert M. |title=General relativity |date=1984 |isbn=0-226-87032-4 |location=Chicago |publisher=University of Chicago Press |oclc=10018614}}</ref> The theory of [[general relativity]] predicts that a sufficiently compact [[mass]] will deform spacetime to form a black hole. Around a black hole there is a mathematically defined boundary called an [[event horizon]] that marks the point of no return. It is called "black" because it absorbs all the light that hits the horizon, reflecting nothing, just like a perfect [[black body]] in [[thermodynamics]].<ref>{{cite book
|title=Gravity from the ground up
|first1=Bernard F.
|last1=Schutz
|publisher=Cambridge University Press
|year=2003
|isbn=0-521-45506-5
|page=110
|url=https://books.google.com/books?id=P_T0xxhDcsIC
|access-date=24 October 2016
|archive-date=2 December 2016
|archive-url=https://web.archive.org/web/20161202222711/https://books.google.com/books?id=P_T0xxhDcsIC
|url-status=live
}}</ref><ref>{{Cite journal
 |last=Davies
 |first=P. C. W.
 |title=Thermodynamics of Black Holes
 |url=http://cosmos.asu.edu/publications/papers/ThermodynamicTheoryofBlackHoles%2034.pdf
 |journal=[[Reports on Progress in Physics]]
 |volume=41
 |year=1978
 |issue=8
 |pages=1313–55
 |doi=10.1088/0034-4885/41/8/004
 |bibcode=1978RPPh...41.1313D
 |s2cid=250916407
 |url-status=dead
 |archive-url=https://web.archive.org/web/20130510184530/http://cosmos.asu.edu/publications/papers/ThermodynamicTheoryofBlackHoles%2034.pdf
 |archive-date=10 May 2013

  | issn = 0034-4885}}</ref> Black holes of stellar mass are expected to form when very massive stars collapse at the end of their life cycle. After a black hole has formed it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, [[supermassive black hole]]s of millions of solar masses may form. There is general consensus that supermassive black holes exist in the centers of most [[galaxy|galaxies]]. Although a black hole itself is black, infalling material forms an [[accretion disk]], one of the brightest types of object in the universe.
* [[Black-body radiation]] refers to the radiation coming from a body at a given temperature where all incoming energy (light) is converted to heat.
* Black sky refers to the appearance of space as one emerges from Earth's atmosphere.

<gallery mode="packed" heights="150px">
File:NGC 406 Hubble WikiSky.jpg|Image of the NGC 406 galaxy from the Hubble Space Telescope
File:Spirit Rover-Mars Night Sky.jpg|The night sky seen from Mars, with the two moons of Mars visible, taken by the NASA Spirit Rover.
File:Top of Atmosphere.jpg|Outside Earth's atmosphere, the sky is black day and night.
File:Olber's Paradox - All Points.gif|An illustration of [[Olbers' paradox]] (see below)
File:Black hole - Messier 87 crop max res.jpg|Image of the central black hole of Messier 87 taken by the Event Horizon Telescope.
</gallery>

====Why the night sky and space are black – Olbers' paradox====
The fact that [[outer space]] is black is sometimes called [[Olbers' paradox]]. In theory, because the universe is full of stars, and is believed to be infinitely large, it would be expected that the light of an infinite number of stars would be enough to brilliantly light the whole universe all the time. However, the background color of outer space is black. This contradiction was first noted in 1823 by German astronomer [[Heinrich Wilhelm Matthias Olbers]], who posed the question of why the night sky was black.

The current accepted answer is that, although the universe may be infinitely large, it is not infinitely old. It is thought to be about 13.8 billion years old, so we can only see objects as far away as the distance light can travel in 13.8 billion years. Light from stars farther away has not reached Earth, and cannot contribute to making the sky bright. Furthermore, as the universe is expanding, many stars are moving away from Earth. As they move, the wavelength of their light becomes longer, through the [[Doppler effect]], and shifts toward red, or even becomes invisible. As a result of these two phenomena, there is not enough starlight to make space anything but black.<ref>{{cite web|url=https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question52.html|title=Why is space black?|website=starchild.gsfc.nasa.gov|access-date=30 December 2016|archive-date=31 December 2016|archive-url=https://web.archive.org/web/20161231214621/http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question52.html|url-status=live}}</ref>

The daytime sky on Earth is blue because light from the Sun strikes molecules in Earth's atmosphere scattering light in all directions. Blue light is scattered more than other colors, and reaches the eye in greater quantities, making the daytime sky appear blue. This is known as [[Rayleigh scattering]].

The nighttime sky on Earth is black because the part of Earth experiencing night is facing away from the Sun, the light of the Sun is blocked by Earth itself, and there is no other bright nighttime source of light in the vicinity. Thus, there is not enough light to undergo Rayleigh scattering and make the sky blue. On the Moon, on the other hand, because there is virtually no atmosphere to scatter the light, the sky is black both day and night. This also holds true for other locations without an atmosphere, such as [[Mercury (planet)|Mercury]].