Editing Black body (section)

== Realizations ==
A ''realization'' of a black body refers to a real world, physical embodiment. Here are a few.

=== Cavity with a hole ===
In 1898, [[Otto Lummer]] and [[Ferdinand Kurlbaum]] published an account of their cavity radiation source.<ref>{{harvnb|Lummer|Kurlbaum|1898}}</ref> Their design has been used largely unchanged for radiation measurements to the present day. It was a hole in the wall of a platinum box, divided by diaphragms, with its interior blackened with iron oxide. It was an important ingredient for the progressively improved measurements that led to the discovery of Planck's law.<ref name=Rechenberg/><ref>{{harvnb|Kangro|1976|page=159}}</ref> A version described in 1901 had its interior blackened with a mixture of chromium, nickel, and cobalt oxides.<ref>{{harvnb|Lummer|Kurlbaum|1901}}</ref> See also [[Hohlraum]].

=== Near-black materials ===
There is interest in blackbody-like materials for [[camouflage]] and [[radiation-absorbent material]]s for radar invisibility.<ref name=Lewis/><ref name=Quinn/> They also have application as solar energy collectors, and infrared thermal detectors. As a perfect emitter of radiation, a hot material with black body behavior would create an efficient infrared heater, particularly in space or in a vacuum where convective heating is unavailable.<ref name=Mizuno/> They are also useful in telescopes and cameras as anti-reflection surfaces to reduce stray light, and to gather information about objects in high-contrast areas (for example, observation of planets in orbit around their stars), where blackbody-like materials absorb light that comes from the wrong sources.

It has long been known that a [[carbon black]] coating will make a body nearly [[black]]. An improvement on lamp-black is found in manufactured [[carbon nanotube]]s. Nano-porous materials can achieve [[Refractive index|refractive indices]] nearly that of vacuum, in one case obtaining average reflectance of 0.045%.<ref name=Chun/><ref name=Yang/> In 2009, a team of Japanese scientists created a material called nanoblack which is close to an ideal black body, based on vertically aligned single-walled [[carbon nanotube]]s. This absorbs between 98% and 99% of the incoming light in the spectral range from the ultra-violet to the far-infrared regions.<ref name=Mizuno/>

Other examples of nearly perfect black materials are [[super black]], prepared by chemically etching a [[nickel]]–[[phosphorus]] [[alloy]],<ref name=Brown/> [[vertically aligned carbon nanotube arrays]] (like [[Vantablack]]) and flower carbon nanostructures;<ref>{{Cite journal |doi = 10.1021/acsanm.9b01950|title = Dandelion-Like Carbon Nanotubes for Near-Perfect Black Surfaces|year = 2019|last1 = Ghai|first1 = Viney|last2 = Singh|first2 = Harpreet|last3 = Agnihotri|first3 = Prabhat K.|journal = ACS Applied Nano Materials|volume = 2|issue = 12|pages = 7951–7956| s2cid=213017898 }}</ref> all absorb 99.9% of light or more.

=== Stars and planets <span class="anchor" id="stars_and_planets_anchor"></span> ===
{{For| more about the UBV color index|Photometric system}}
[[File:Black Body Curves.png|thumb|349x349px|Diagram comparing the response curves of the red, green, and blue light receptors in human eyes against the approximate black body curves of a number of stars: [[Antares]] (a [[red supergiant]]), the Sun (a [[G-type main-sequence star|yellow dwarf]]), [[Sirius]] (a [[A-type main-sequence star|white main-sequence star]]), [[Spica]] (a blue star), and [[Gamma Velorum]].]]

A star or planet often is modeled as a black body, and electromagnetic radiation emitted from these bodies as [[black-body radiation]]. The figure shows a highly schematic cross-section to illustrate the idea. The [[photosphere]] of the star, where the emitted light is generated, is idealized as a layer within which the photons of light interact with the material in the photosphere and achieve a common temperature ''T'' that is maintained over a long period of time. Some photons escape and are emitted into space, but the energy they carry away is replaced by energy from within the star, so that the temperature of the photosphere is nearly steady. Changes in the core lead to changes in the supply of energy to the photosphere, but such changes are slow on the time scale of interest here. Assuming these circumstances can be realized, the outer layer of the star is somewhat analogous to the example of an enclosure with a small hole in it, with the hole replaced by the limited transmission into space at the outside of the photosphere. With all these assumptions in place, the star emits black-body radiation at the temperature of the photosphere.<ref name="Green" />

[[File:Idealized photosphere.png|thumb|An idealized view of the cross-section of a star. The [[photosphere]] contains [[photon]]s of light nearly in thermal equilibrium, and some escape into space as near-black-body radiation. ]]

Using this model the [[effective temperature]] of stars is estimated, defined as the temperature of a black body that yields the same surface flux of energy as the star. If a star were a black body, the same effective temperature would result from any region of the spectrum. For example, comparisons in the ''B'' (blue) or ''V'' (visible) range lead to the so-called ''B-V'' [[color index]], which increases the redder the star,<ref name=Kelley/> with the Sun having an index of +0.648 ± 0.006.<ref name=Gray/> Combining the ''U'' (ultraviolet) and the ''B'' indices leads to the ''U-B'' index, which becomes more negative the hotter the star and the more the UV radiation. Assuming the Sun is a type G2 V star, its ''U-B'' index is +0.12.<ref name=Golay/> The two indices for two types of most common star sequences are compared in the figure (diagram) with the effective surface temperature of the stars if they were perfect black bodies. There is a rough correlation. For example, for a given ''B-V'' index measurement, the curves of both most common sequences of star (the main sequence and the supergiants) lie below the corresponding black-body ''U-B'' index that includes the ultraviolet spectrum, showing that both groupings of star emit less ultraviolet light than a black body with the same ''B-V'' index. It is perhaps surprising that they fit a black body curve as well as they do, considering that stars have greatly different temperatures at different depths.<ref name=Aller/> For example, the [[Sun]] has an effective temperature of 5780 K,<ref name=Lang/> which can be compared to the temperature of its [[photosphere]] (the region generating the light), which ranges from about 5000 K at its outer boundary with the [[chromosphere]] to about 9500 K at its inner boundary with the [[convection zone]] approximately {{convert|500|km|abbr=on}} deep.<ref name=Bertotti/>

[[File:Effective temperature and color index.png|thumb|Effective temperature of a black body compared with the ''B-V'' and ''U-B'' color index of main sequence and super giant stars in what is called a [[color-color diagram]].<ref name="UBV" />]]

=== Black holes ===
{{See also|Hawking radiation}}
A [[black hole]] is a region of [[spacetime]] from which nothing escapes. Around a black hole there is a mathematically defined surface 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, making it almost an ideal black body<ref name=Schutz/> (radiation with a wavelength equal to or larger than the diameter of the hole may not be absorbed, so black holes are not perfect black bodies).<ref name=Davies/> Physicists believe that to an outside observer, black holes have a non-zero temperature and emit [[black-body radiation]], radiation with a nearly perfect black-body spectrum, ultimately [[Black hole#Evaporation|evaporating]].<ref name=Wald/> The mechanism for this emission is related to [[vacuum fluctuations]] in which a [[virtual particles|virtual pair]] of particles is separated by the gravity of the hole, one member being sucked into the hole, and the other being emitted.<ref name=Carr/> The energy distribution of emission is described by [[Planck's law]] with a temperature ''T'':
:<math>T=\frac {\hbar c^3}{8\pi Gk_\text{B}M} \ ,</math>
where ''c'' is the [[speed of light]], ℏ is the [[reduced Planck constant]], ''k''<sub>B</sub> is the [[Boltzmann constant]], ''G'' is the [[gravitational constant]] and ''M'' is the mass of the black hole.<ref name=Frolov/> These predictions have not yet been tested either observationally or experimentally.<ref name=Wald2/>

=== Cosmic microwave background radiation ===
{{See also|Big Bang|Cosmic microwave background radiation}}
The Big Bang theory is based upon the [[cosmological principle]], which states that on large scales the Universe is homogeneous and isotropic.  According to theory, the Universe approximately a second after its formation was a near-ideal black body in thermal equilibrium at a temperature above 10<sup>10</sup> K. The temperature decreased as the Universe expanded and the matter and radiation in it cooled.  The cosmic microwave background radiation observed today is "the most perfect black body ever measured in nature".<ref name=White/> It has a nearly ideal Planck spectrum at a temperature of about 2.7&nbsp;K.  It departs from the perfect isotropy of true black-body radiation by an observed anisotropy that varies with angle on the sky only to about one part in 100,000.