V476 Cygni or Nova Cygni 1920 was a

nova A nova (plural novae or novas) is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star (hence the name "nova", which is Latin for "new") that slowly fades over weeks or months. Causes of the dramati ...

which occurred in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

Cygnus in 1920. It was discovered by

William Frederick Denning William Frederick Denning (25 November 1848 – 9 June 1931) was a British amateur astronomer who achieved considerable success without formal scientific training. He is known for his catalogues of meteor radiants, observations of Jupiter's re ...

, an English amateur astronomer, at 09:30

GMT Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon; as a cons ...

on 20 August 1920, at which time it had a

magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...

of 3.7. It reached a peak brightness of magnitude 1.7 on 23 August 1920. Its quiescent brightness is magnitude 17.09. V476 Cygni's

light curve In astronomy, a light curve is a graph of light intensity of a celestial object or region as a function of time, typically with the magnitude of light received on the y axis and with time on the x axis. The light is usually in a particular frequ ...

is quite unusual, showing a rapid decline from maximum brightness, followed by a slow nearly linear fading. It is classified as a very fast nova, with a very long tail. The light curve shows a "dust dip", a fast decline to a local brightness minimum, then a small increase in brightness, and finally a long, slow decline. This dust dip is believed to is believed to coincide with the formation of dust in the material ejected from the nova as it moves away from the star and cools. All novae consist of a close binary star pair, with a

white dwarf A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes fro ...

star and a "donor" star orbiting each other. The stars are so close together that the less dense donor star transfers matter to the white dwarf. In the case of V476 Cygni, modeling indicates that the white dwarf has a mass of 1.18 M_⊙, and it is receiving M_⊙ of material from the donor star, per year. V476 Cygni has transitioned into a recurrent

dwarf nova A U Geminorum-type variable star, or dwarf nova (pl. novae) is one of several types of cataclysmic variable star, consisting of a close binary star system in which one of the components is a white dwarf that accretes matter from its companion. Dw ...

, although it has done so much earlier than expected. A small

emission nebula An emission nebula is a nebula formed of ionized gases that emit light of various wavelengths. The most common source of ionization is high-energy ultraviolet photons emitted from a nearby hot star. Among the several different types of emissio ...

(

shell Shell may refer to: Architecture and design * Shell (structure), a thin structure ** Concrete shell, a thin shell of concrete, usually with no interior columns or exterior buttresses ** Thin-shell structure Science Biology * Seashell, a hard o ...

) is visible around the star, which resembles a planetary nebula. Santamaria ''et al.'' examined images of the nebula taken in 1944, 1993 and 2018 and found that the shell is clearly expanding. It is slightly elliptical, with major and minor axes of 14.6×13.4 arc seconds (as of 2018) expanding at a rate of 0.073×0.067 arc seconds per year, implying a physical expansion rate of 230×200 km/sec. Surprisingly, a 1997 attempt to image the shell using the

Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...

, was unsuccessful.

Gallery

V476CygLocation.png, The location of V476 Cygni (circled in red) V476CygLightCurve.png, The light curve of V476 Cygni, plotted from

AAVSO The American Association of Variable Star Observers (AAVSO) is an international nonprofit organization, founded in 1911, focused on coordinating, analyzing, publishing, and archiving variable star observations made largely by amateur astronomers. ...

data V476CygNebulaExpanding.png, Two images of the shell surrounding V476 Cygni taken 25 years apart, showing the nebula's expansion. Both were taken with

Hα H-alpha (Hα) is a specific deep-red visible spectral line in the Balmer series with a wavelength of 656.28 nm in air and 656.46 nm in vacuum; it occurs when a hydrogen electron falls from its third to second lowest energy level. H-alpha ...

filters Filter, filtering or filters may refer to: Science and technology Computing * Filter (higher-order function), in functional programming * Filter (software), a computer program to process a data stream * Filter (video), a software component tha ...

, left at the

William Herschel Telescope The William Herschel Telescope (WHT) is a optical/near-infrared reflecting telescope located at the Roque de los Muchachos Observatory on the island of La Palma in the Canary Islands, Spain. The telescope, which is named after William Herschel, ...

, and right with the

Nordic Optical Telescope The Nordic Optical Telescope (NOT) is an astronomical telescope located at Roque de los Muchachos Observatory, La Palma in the Canary Islands. The telescope saw first light in 1988, and was officially inaugurated during September 1989. Regular o ...

References

External links

* https://web.archive.org/web/20051018142030/http://www.aqua.co.za/assa_jhb/Canopus/Can2003/c038LitU.htm * http://www.otticademaria.it/astro/Costellazioni/st_cyg.html * http://www.iop.org/EJ/article/1538-3881/120/4/2007/200096.html {{Novae Novae Cygnus (constellation) 1920 in science Cygni, V476