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A nova (plural novae or novas) is a
transient astronomical event The asterisk ( ), from Late Latin , from Ancient Greek , ''asteriskos'', "little star", is a typographical symbol. It is so called because it resembles a conventional image of a heraldic star. Computer scientists and mathematicians often voc ...
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 dramatic appearance of a nova vary, depending on the circumstances of the two progenitor stars. All observed novae involve white dwarfs in close binary systems. The main sub-classes of novae are classical novae, recurrent novae (RNe), and dwarf novae. They are all considered to be
cataclysmic variable star In astronomy, cataclysmic variable stars (CVs) are stars which irregularly increase in brightness by a large factor, then drop back down to a quiescent state. They were initially called novae (), since ones with an outburst brightness visible to ...
s. Classical nova eruptions are the most common type. They are likely created in a close binary star system consisting of a white dwarf and either a
main sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...
, subgiant, or red giant star. When the orbital period falls in the range of several days to one day, the white dwarf is close enough to its companion star to start drawing accreted matter onto the surface of the white dwarf, which creates a dense but shallow atmosphere. This atmosphere, mostly consisting of hydrogen, is thermally heated by the hot white dwarf and eventually reaches a critical temperature causing ignition of rapid
runaway Runaway, Runaways or Run Away may refer to: Engineering * Runaway reaction, a chemical reaction releasing more heat than what can be removed and becoming uncontrollable * Thermal runaway, self-increase of the reaction rate of an exothermic proce ...
fusion. The sudden increase in energy expels the atmosphere into interstellar space creating the envelope seen as visible light during the nova event. Such were taken in past centuries to be a new star. A few novae produce short-lived nova remnants, lasting for perhaps several centuries. Recurrent nova processes are the same as the classical nova, except that the fusion ignition may be repetitive because the companion star can again feed the dense atmosphere of the white dwarf. Novae most often occur in the sky along the path of the Milky Way, especially near the observed Galactic Center in Sagittarius; however, they can appear anywhere in the sky. They occur far more frequently than galactic
supernova A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...
e, averaging about ten per year. Most are found telescopically, perhaps only one every 12–18 months reaching naked-eye visibility. Novae reaching first or second magnitude occur only several times per century. The last bright nova was V1369 Centauri reaching 3.3 magnitude on 14 December 2013.


Etymology

During the sixteenth century, astronomer Tycho Brahe observed the
supernova A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...
SN 1572 in the constellation Cassiopeia. He described it in his book ''De nova stella'' ( Latin for "concerning the new star"), giving rise to the adoption of the name ''nova''. In this work he argued that a nearby object should be seen to move relative to the fixed stars, and that the nova had to be very far away. Although this event was a supernova and not a nova, the terms were considered interchangeable until the 1930s. After this, novae were classified as ''classical novae'' to distinguish them from supernovae, as their causes and energies were thought to be different, based solely in the observational evidence. Although the term "stella nova" means "new star", novae most often take place as a result of white dwarfs, which are remnants of extremely old stars.


Stellar evolution of novae

Evolution of potential novae begins with two main sequence stars in a binary system. One of the two evolves into a
red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...
, leaving its remnant white dwarf core in orbit with the remaining star. The second star—which may be either a main sequence star or an aging giant—begins to shed its envelope onto its white dwarf companion when it overflows its Roche lobe. As a result, the white dwarf steadily captures matter from the companion's outer atmosphere in an accretion disk, and in turn, the accreted matter falls into the atmosphere. As the white dwarf consists of degenerate matter, the accreted hydrogen does not inflate, but its temperature increases. Runaway fusion occurs when the temperature of this atmospheric layer reaches ~20 million K, initiating nuclear burning, via the CNO cycle. Hydrogen fusion may occur in a stable manner on the surface of the white dwarf for a narrow range of accretion rates, giving rise to a super soft X-ray source, but for most binary system parameters, the hydrogen burning is unstable thermally and rapidly converts a large amount of the hydrogen into other, heavier chemical elements in a
runaway Runaway, Runaways or Run Away may refer to: Engineering * Runaway reaction, a chemical reaction releasing more heat than what can be removed and becoming uncontrollable * Thermal runaway, self-increase of the reaction rate of an exothermic proce ...
reaction, liberating an enormous amount of energy. This blows the remaining gases away from the surface of the white dwarf surface and produces an extremely bright outburst of light. The rise to peak brightness may be very rapid, or gradual. This is related to the speed class of the nova; yet after the peak, the brightness declines steadily. The time taken for a nova to decay by around 2 or 3 magnitudes from maximum optical brightness is used for classification, via its speed class. Fast novae typically will take fewer than 25 days to decay by 2 magnitudes, while slow novae will take more than 80 days. Despite their violence, usually the amount of material ejected in novae is only about of a
solar mass The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...
, quite small relative to the mass of the white dwarf. Furthermore, only five percent of the accreted mass is fused during the power outburst. Nonetheless, this is enough energy to accelerate nova ejecta to velocities as high as several thousand kilometers per second—higher for fast novae than slow ones—with a concurrent rise in
luminosity Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a st ...
from a few times solar to 50,000–100,000 times solar. In 2010 scientists using NASA's Fermi Gamma-ray Space Telescope discovered that a nova also can emit gamma-rays (>100 MeV). Potentially, a white dwarf can generate multiple novae over time as additional hydrogen continues to accrete onto its surface from its companion star. An example is
RS Ophiuchi RS Ophiuchi (''RS Oph'') is a recurrent nova system approximately 5,000 light-years away in the constellation Ophiuchus. In its quiet phase it has an apparent magnitude of about 12.5. It has been observed to erupt in 1898, 1933, 1958, 1967 ...
, which is known to have flared seven times (in 1898, 1933, 1958, 1967, 1985, 2006, and 2021). Eventually, the white dwarf could explode as a Type 
Ia supernova A Type Ia supernova (read: "type one-A") is a type of supernova that occurs in binary systems (two stars orbiting one another) in which one of the stars is a white dwarf. The other star can be anything from a giant star to an even smaller white ...
if it approaches the Chandrasekhar limit. Occasionally, novae are bright enough and close enough to Earth to be conspicuous to the unaided eye. The brightest recent example was
Nova Cygni 1975 __NOTOC__ V1500 Cygni or Nova Cygni 1975 was a bright nova occurring in 1975 in the constellation Cygnus. It had the second highest intrinsic brightness of any nova of the 20th century, exceeded only by CP Puppis in 1942. V1500 Cygni ...
. This nova appeared on 29 August 1975, in the constellation Cygnus about five degrees north of Deneb, and reached magnitude 2.0 (nearly as bright as Deneb). The most recent were
V1280 Scorpii V1280 Scorpii (or Nova Scorpii 2007 Number 1) is the first of two novae that occurred in the constellation Scorpius during February 2007 (the second nova was the fainter V1281 Scorpii, which was discovered on 19 February 2007). Announced by ...
, which reached magnitude 3.7 on 17 February 2007, and
Nova Delphini 2013 V339 Delphini or Nova Delphini 2013 (PNV J20233073+2046041) is a bright nova star in the constellation Delphinus. It was discovered on 14 August 2013 by amateur astronomer Koichi Itagaki in Japan and confirmed by the Liverpool Tel ...
.
Nova Centauri 2013 V1369 Centauri also known as Nova Centauri 2013 was a bright nova in the constellation Centaurus that occurred in 2013. It was discovered on December 2, 2013 by amateur astronomer John in Australia with a magnitude of 5.5. On December ...
was discovered 2 December 2013 and so far, is the brightest nova of this millennium, reaching magnitude 3.3.


Helium novae

A helium nova (undergoing a
helium flash A helium flash is a very brief thermal runaway nuclear fusion of large quantities of helium into carbon through the triple-alpha process in the core of low mass stars (between 0.8 solar masses () and 2.0 ) during their red giant phase (the Sun is ...
) is a proposed category of nova events that lacks hydrogen lines in its spectrum. This may be caused by the explosion of a helium shell on a white dwarf. The theory was first proposed in 1989, and the first candidate helium nova to be observed was V445 Puppis in 2000. Since then, four other novae have been proposed as helium novae.


Occurrence rate and astrophysical significance

Astronomers estimate that the Milky Way experiences roughly 30 to 60 novae per year, but a recent examination has found the likely improved rate of about 50±27. The number of novae discovered in the Milky Way each year is much lower, about 10, probably due to distant novae being obscured by gas and dust absorption. Roughly 25 novae brighter than about the twentieth magnitude are discovered in the Andromeda Galaxy each year and smaller numbers are seen in other nearby galaxies. As of 2019, 407 probable novae are recorded in the Milky Way. Spectroscopic observation of nova ejecta nebulae has shown that they are enriched in elements such as helium, carbon, nitrogen, oxygen, neon, and magnesium. The contribution of novae to the
interstellar medium In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...
is not great; novae supply only as much material to the Galaxy as do supernovae, and only as much as
red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...
and supergiant stars. Recurrent novae such as
RS Ophiuchi RS Ophiuchi (''RS Oph'') is a recurrent nova system approximately 5,000 light-years away in the constellation Ophiuchus. In its quiet phase it has an apparent magnitude of about 12.5. It has been observed to erupt in 1898, 1933, 1958, 1967 ...
(those with periods on the order of decades) are rare. Astronomers theorize, however, that most, if not all, novae are recurrent, albeit on time scales ranging from 1,000 to 100,000 years. The recurrence interval for a nova is less dependent on the accretion rate of the white dwarf than on its mass; with their powerful gravity, massive white dwarfs require less accretion to fuel an eruption than lower-mass ones. Consequently, the interval is shorter for high-mass white dwarfs. V Sagittae is unusual in that we can predict now that it will go nova in approximately 2083, plus or minus about 11 years. On 27 May 2020, astronomers reported that classical novae explosions are galactic producers of the element lithium.


Subtypes

Novae are classified according to the light curve development speed, so in * NA: fast novae, with a rapid brightness increase, followed by a brightness decline of 3 magnitudes — to about brightness — within 100 days. * NB: slow novae, with magnitudes of 3, decline in 150 days or more. * NC: very slow novae, also known as
symbiotic nova Symbiotic novae are slow irregular eruptive variable stars with very slow nova-like outbursts with an amplitude of between 9 and 11 magnitudes. The symbiotic nova remains at maximum for one or a few decades, and then declines towards its original l ...
e, staying at maximum light for a decade or more and then fading very slowly. * NR/RN: recurrent novae, novae with two or more eruptions separated by 10–80 years have been observed.


Remnants

Some novae leave behind visible
nebulosity A nebula ('cloud' or 'fog' in Latin; pl. nebulae, nebulæ or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regio ...
, material expelled in the nova explosion or in multiple explosions.


Novae as distance indicators

Novae have some promise for use as standard candle measurements of distances. For instance, the distribution of their absolute magnitude is
bimodal In statistics, a multimodal distribution is a probability distribution with more than one mode (statistics), mode. These appear as distinct peaks (local maxima) in the probability density function, as shown in Figures 1 and 2. Categorical, ...
, with a main peak at magnitude −8.8, and a lesser one at −7.5. Novae also have roughly the same absolute magnitude 15 days after their peak (−5.5). Comparisons of nova-based distance estimates to various nearby
galaxies A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...
and galaxy clusters with those measured with Cepheid variable stars, have shown them to be of comparable accuracy.


Recurrent novae

Recurrent novae (RNe) are objects that have been seen to experience multiple nova eruptions. As of 2009, there are ten known galactic recurrent novae, as well as several
extragalactic Extragalactic astronomy is the branch of astronomy concerned with objects outside the Milky Way galaxy. In other words, it is the study of all astronomical objects which are not covered by galactic astronomy. The closest objects in extragalactic ...
ones (in the Andromeda Galaxy (M31) and the
Large Magellanic Cloud The Large Magellanic Cloud (LMC), or Nubecula Major, is a satellite galaxy of the Milky Way. At a distance of around 50 kiloparsecs (≈160,000  light-years), the LMC is the second- or third-closest galaxy to the Milky Way, after the ...
). One of these extragalactic novae, M31N 2008-12a, erupts as frequently as once every 12 months. The recurrent nova typically brightens by about 8.6 magnitudes, whereas a classic nova may brighten by more than 12 magnitudes. The ten known galactic recurrent novae are listed below.


Extragalactic novae

Novae are relatively common in the Andromeda Galaxy (M31). Approximately several dozen novae (brighter than about apparent magnitude 20) are discovered in M31 each year. The Central Bureau for Astronomical Telegrams (CBAT) tracked novae in M31, M33, and M81.


See also


References


Further reading

67.48.28.33 * * *


External links

* *
General Catalog of Variable Stars
Sternberg Astronomical Institute, Moscow
AAVSO Variable Star of the Month. Novae: May 2001


{{Authority control Stellar phenomena Astronomical events