Green star (astronomy)

WD J1330+6435, the first known star with a greenish hue

In astronomy, a green star is a star whose light is perceived as green or greenish, or that appears greenish in some viewing conditions.

Ordinary stars do not appear green because their spectra broadly resemble blackbody radiation, which produces a range of colours that humans generally perceive as orange, white, or blue.

However, a 2026 study found that several white dwarfs may have greenish colours resulting from unusually broad absorption by sodium and calcium in their dense atmospheres.

Observation

Blackbody radiation and stellar colour

A star's perceived colour depends on the distribution of wavelengths in its light and on the response of human vision. The spectra of most main-sequence stars broadly resemble those of blackbodies, with their colours following the Planckian locus in a chromaticity diagram. Along this locus, stars range broadly from orange at lower temperatures, through white, to pale blue at higher temperatures.

The Planckian locus on a chromaticity diagram. The colours associated with blackbody radiation follow the curved line through red, orange, yellow, white and blue regions; it does not pass through the green region.

For a star whose spectrum closely follows blackbody radiation, a peak in the green part of the visible range does not make the star look green: it still emits substantial amounts of red and blue light, and the combined light is generally perceived as whitish.

The peak wavelength of a spectrum therefore does not, by itself, determine the colour seen by an observer. This explains why the Sun, whose visible spectrum peaks in the green region, appears approximately white rather than green.

This general pattern is not a universal rule for every type of star. A star's atmosphere can absorb or emit light at particular wavelengths, and in some objects these effects can alter the overall spectrum enough to produce unusual perceived colours.

White dwarfs, whose atmospheres can be much denser than those of ordinary stars, provide examples of this effect.

Examples of green stars

WD J1330+6435

WD J1330+6435 is a white dwarf in the constellation Draco, approximately 280 light-years from Earth. In a 2026 study of white-dwarf colours, spectral modelling indicated that it would appear chartreuse green if viewed closely enough for its disc to be resolved. The researchers identified it as the first known star reported to have this colour.

The colour is associated with the white dwarf's unusual atmospheric composition. Material from a planetary system has enriched its outer layers with elements including sodium, calcium and lithium.

WD J1351+2645

Spectrum of WD J1351+2645

WD J1351+2645 is a white dwarf located in the constellation Boötes. Classified as a DZ white dwarf, it exhibits a modeled pale green hue attributed to strong absorption by ionized calcium (Ca II) at blue wavelengths, combined with a steep Rayleigh–Jeans tail at red wavelengths.

It was identified as a green-hued object in the same study that reported the more vividly green WD J1330+6435.

WD J0851+1543

Spectrum of WD J0851+1543

WD J0851+1543 is a white dwarf located in the constellation Cancer. Its modelled appearance is pale green, with a hue between sRGB green and cyan.

The authors considered its green classification reliable because its atmospheric model was based on a spectrum with a high signal-to-noise ratio. Its modelled colour also varies across the stellar disc, with subtle differences in hue toward the outer limb.

Its green appearance is attributed to strong calcium ion (Ca II) absorption at blue wavelengths and a steep Rayleigh–Jeans tail at red wavelengths.

There have been other astronomical objects and effects that sometimes appeared to be green stars.

Beta Librae

The star Beta Librae (Zubeneschamali, β Lib) is usually described as appearing white to modern observers, viewed through modern telescopes. However, its color is controversial, and many earlier observers stated that they saw it as green. There seems to be no consensus about what its color really is, and no generally accepted explanation for why some observers have seen it as green.

Complementary color contrast

There are a few stars in double or multiple star systems that appear greenish, even though they are blue or white under typical viewing conditions. This can happen if the star system contains a large red or orange star. Color contrast effects cause objects close to the reddish star to appear slightly greenish (and vice versa: complementary colors).

Antares (red) and its companion star, Antares B

Antares

The classical example is Antares (Alpha Scorpii or α Sco), the brightest star in the constellation Scorpius: The primary of the double star system is the red supergiant primary α Sco A; it has a blue companion α Sco B. When viewed at low-enough power so that the light of the two stars is seen combined, some observers describe the color as greenish.

Almach

Other examples include the star system Almach (γ¹ and γ² Andromedae) a golden-yellow star next to a dimmer, indigo-blue star about 350 light years away.

The green planetary nebula NGC 6826

σ Cassiopeiae

Another is the Sigma Cassiopeiae (σ Cas) star system, a binary star 1,500 light-years from Earth. σ Cas‑A is a greenish-hued magnitude 5.0 primary, next to magnitude 7.3 blue-hued secondary σ Cas‑B.

Planetary nebulae

Some planetary nebulae themselves glow with the green OII emission lines, especially if the nebula is enriched oxygen, well irradiated by its central late-stage star or white dwarf. These nebulae, or the stars within them (usually bluish or white Wolf-Rayet stars) may appear to be green when viewed in combination at low power. Examples include the planetary nebulae NGC 6572, NGC 6826, and NGC 7009.[citation needed]

Uranus

Uranus

The planet Uranus occasionally appeared in ancient star catalogs, and in more recent catalogs before William Herschel discovered that it is a planet—most recently as 34 Tauri in Flamsteed's catalog. It appears as a greenish or blue-green dot in binoculars or a small telescope. Uranus appears more green during its solstices and more blue near its equinoxes, because the composition of its atmosphere varies somewhat between its poles and its equator.

The Sun

Green flash

The Sun can sometimes appear as a green spot for a second or two as it is rising or setting: this is known as a green flash. Roughly speaking, the red light from the Sun is blocked by Earth, the blue light is scattered by the atmosphere, and the green light is refracted by the atmosphere to the observer.

A similar effect can occasionally be seen with other astronomical objects such as the moon and bright planets.

Furthermore, the Sun emits more green photons than any other color; i.e., it peaks in the green part of the visible spectrum.

References and Notes

  1. 1 2 3 4 5 6 7 8 9 10 11 Hollands, Mark A.; Gänsicke, Boris T.; Tremblay, Pier-Emmanuel; Koester, Detlev (15 October 2026). "Any colour you like: the diverse chromaticities of white dwarfs". RAS Techniques and Instruments. 5 rzag069. doi:10.1093/rasti/rzag069.
  2. 1 2 "Colors of Stars". Astronomy. Lumen Learning / OpenStax. Retrieved October 18, 2026.
  3. 1 2 3 Cain, Fraser (10 February 2009). "Are there Green Stars?". Universe Today.
  4. 1 2 3 Dunham, Will (16 October 2026). "Newly identified green star leaves astronomers amazed". Reuters.
  5. ↑ "Color". Encyclopaedia Britannica.
  6. 1 2 Hollands, M. A.; Gänsicke, B. T. (2018). "Cool DZ white dwarfs II: Compositions and evolution of old remnant planetary systems". arXiv:1801.07714 [astro-ph.SR].
  7. 1 2 3 "A white dwarf's unusual atmosphere gives it a ghostly green glow". The Brighter Side of News. Retrieved 23 October 2026.
  8. 1 2 Andrei, Mihai (October 22, 2026). "Astronomers Found Something They Thought Couldn't Exist: A Green Star". ZME Science. Retrieved October 23, 2026.
  9. ↑ "* bet Lib – variable star". SIMBAD. Basic data. Centre de Données astronomiques de Strasbourg. Retrieved 20 May 2015.
  10. ↑ Burnham, Robert (1978). Burnham's Celestial Handbook: An observer's guide to the universe beyond the solar system. Vol. 2 (Reprint ed.). New York, NY: Dover. p. 1105. ISBN 978-0-486-23568-4. Retrieved 26 August 2017 – via Google Books.
  11. ↑ Kaler, James B. (2006). "Zubeneschamali". Astronomy (personal academic site). Stars. University of Illinois. Archived from the original on 14 July 2006. Retrieved 26 August 2017.
  12. ↑ Loomis, Elias (1870). A Treatise on Astronomy. New York, NY: Harper and Brothers. p. 299 – via Internet Archive (archive.org).
  13. ↑ Schaaf, Fred (1983). Wonders of the Sky: Observing rainbows, comets, eclipses, the stars, and other phenomena. Courier Corporation. p. 213. ISBN 9780486244020. Retrieved 7 September 2017 – via Google Books.
  14. ↑ Burnham, Robert (1 January 1978). "Andromeda-Cetus". Burnham's Celestial Handbook: An observer's guide to the universe beyond the solar system. Courier Corporation. ISBN 978-0-486-23567-7.{{cite book}}: CS1 maint: date and year (link)
  15. ↑ Mason, Brian D.; Wycoff, Gary L.; Hartkopf, William I.; Douglass, Geoffrey G.; Worley, Charles E. (2001). "The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog". The Astronomical Journal. 122 (6): 3466. Bibcode:2001AJ....122.3466M. doi:10.1086/323920.
  16. ↑ Plait, Phil (2008-08-28). "Followup: Green objects in space". Slate. Retrieved 2023-01-13.
  17. ↑ Lawrence, Pete (December 2023). "See Uranus through binoculars and telescope in December 2023". Sky at Night Magazine. BBC. Retrieved 2023-01-13.
  18. ↑ King, Bob (2021-10-27). "Uranus Queues Up For Opposition". Sky & Telescope. Retrieved 2023-01-13.
  19. ↑ Ferreira, Becky (4 January 2024). "Uranus and Neptune Reveal Their True Colors - Neptune is not as blue as you've been led to believe, and Uranus's shifting colors are better explained, in new research". The New York Times. Retrieved 5 January 2024.{{cite news}}: CS1 maint: deprecated archival service (link)
  20. ↑ Young, Andrew T. "Explaining green flashes". San Diego, CA: San Diego State University. Retrieved 26 August 2017.
  21. ↑ Maunder, Michael (2007). Lights in the Sky: Identifying and Understanding Astronomical and Meteorological Phenomena. Springer. pp. 72–73. ISBN 978-1846287619.
  22. ↑ Nave, C. R. "Red Sunset, Green Flash". Georgia State University. HyperPhysics. Retrieved 2023-01-13.
  23. ↑ O'Connell, D. J. K. (1958). "The green flash and other low sun phenomena". Castel Gandolfo: Vatican Observatory, Ricerche Astronomiche. 4. Harvard: 7. Bibcode:1958RA......4.....O.
  1. ↑ The star's identifier signifies J2000 coordinates of Right Ascension 13h 51m and Declination +26° 45', placing it within Bootes.
  2. ↑ The star's identifier signifies J2000 coordinates of Right Ascension 08h 51m and Declination +15° 43', placing it within Cancer.