Content deleted Content added
→See also: +Spectral index +{{div col}} |
No edit summary |
||
| (2 intermediate revisions by 2 users not shown) | |||
Line 1:
{{Short description|
{{About||the colorant reference database|Colour Index International|the term in geology|Color index (geology)}}
Line 22:
|}
In [[astronomy]], the '''color index''' is a simple [[Numerical analysis|numerical]] [[Expression (mathematics)|expression]] that determines the [[color]] of an object, which in the case of a [[star]] gives its [[temperature]]. The lower the color index, the more [[blue]] (or hotter) the object is. Conversely, the larger the color index, the more [[red]] (or cooler) the object is. This is a consequence of the inverse [[Logarithmic scale|logarithmic magnitude scale]], in which brighter objects have smaller (more negative) magnitudes than dimmer ones. For comparison, the whitish [[Sun]] has a B−V index of {{nowrap|0.656 ± 0.005}},<ref name=sun/> whereas the bluish [[Rigel]] has a B−V of −0.03 (its B magnitude is 0.09 and its V magnitude is 0.12, B−V = −0.03).<ref name=rigel/> Traditionally, the color index uses [[Vega]] as a [[Zero Point (photometry)|zero point]]. The [[blue supergiant]] [[Theta Muscae]] has one of the lowest B−V indices at −0.41,<ref>{{Cite book |title=Encyclopedia of Astronomy & Astrophysics |chapter=Tycho Star Catalogs: The 2.5 Million Brightest Stars |date=2001 |publisher=CRC Press |isbn=978-1-003-22043-5 |editor-last=Murdin |editor-first=P. |___location=Boca Raton |doi=10.1888/0333750888/2862 |chapter-url=https://archive.org/details/paul-murdin-encyclopedia-of-astronomy-and-astrophysics-vol.-1-a-gel.-institute-o/page/n4971/mode/2up<!--?q=%22Tycho+Star+Catalogs%22--> |url=https://archive.org/details/paul-murdin-encyclopedia-of-astronomy-and-astrophysics-vol.-1-a-gel.-institute-o/mode/2up}}</ref> while the [[red giant]] and [[carbon star]] [[R Leporis]] has one of the largest, at +5.74.<ref>{{Cite web |title=VizieR |url=http://webviz.u-strasbg.fr/viz-bin/VizieR-5?-out.add=.&-source=V/50/catalog&recno=1607 |access-date=2024-04-02 |website=webviz.u-strasbg.fr}}</ref>
To measure the index, one observes the [[Magnitude (astronomy)|magnitude]] of an object successively through two different [[Astronomical filter|filter]]s, such as U and B, or B and V, where U is sensitive to [[ultraviolet]] rays, B is sensitive to blue light, and V is sensitive to visible (green-yellow) light (see also: [[UBV system]]). The set of passbands or filters is called a [[photometric system]]. The difference in magnitudes found with these filters is called the U−B or B−V color index respectively.
In principle, the temperature of a star can be calculated directly from the B−V index, and there are several formulae to make this connection.<ref name=Sekiguchi>{{cite journal |author1=Maki Sekiguchi |author2=Masataka Fukugita |name-list-style=and |date=2000 |title=A Study of the ''B−V'' Color-Temperature Relation |journal=Astrophysical Journal |volume=120 |number=2 |page=1072 |doi=10.1086/301490 |doi-access=free|arxiv=astro-ph/9904299 }}</ref> A good approximation can be obtained by considering stars as [[black body|black bodies]], using Ballesteros' formula<ref name=Ballesteros>{{cite journal |last=Ballesteros |first=F. J. |date=2012 |title=New insights into black bodies |journal=EPL |volume=97 |number=3 |at=34008 |arxiv=1201.1809}}</ref> (also implemented in the PyAstronomy package for Python):<ref name=PyAstronomy>BallesterosBV_T API http://www.hs.uni-hamburg.de/DE/Ins/Per/Czesla/PyA/PyA/index.html.</ref>
:<math>T = 4600\,\mathrm{K} \left( \frac{1}{0.92\;(B\text{-}\!V) + 1.7} + \frac{1}{0.92\;(B\text{-}\!V) + 0.62} \right). </math>
| |||