Editing Nova (section)

== Stellar evolution of novae ==
[[File:Nova-Eridani-2009-LB4.jpg|thumb|right|[[KT Eridani|Nova Eridani 2009]] ([[apparent magnitude]] ~8.4)]]
Evolution of potential novae begins with two [[main sequence]] stars in a binary system. One of the two [[Stellar evolution|evolves]] into a [[red giant]], leaving its remnant white dwarf core in orbit with the remaining star. The second star—which may be either a [[Main sequence|main-sequence star]] or an aging [[Giant star|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 [[Accretion (astrophysics)|accreted]] [[hydrogen]] is unable to expand even though its temperature increases. Runaway fusion occurs when the temperature of this atmospheric layer reaches ~20&nbsp;million [[Kelvin|K]], initiating nuclear burning via the [[CNO cycle]].<ref>{{cite journal |title= On the Progenitors of Galactic Novae |journal= The Astrophysical Journal |date= 10 February 2012 |volume= 746 |issue= 61 |pages= 61 |author= M.J. Darnley |display-authors= etal |doi=10.1088/0004-637x/746/1/61 |bibcode=2012ApJ...746...61D|arxiv = 1112.2589 |s2cid= 119291027 }}</ref>

If the accretion rate is just right, hydrogen fusion may occur in a stable manner on the surface of the white dwarf, giving rise to a [[super soft X-ray source]], but for most binary system parameters, the hydrogen burning is thermally unstable and rapidly converts a large amount of the hydrogen into other, [[Weight|heavier]] [[chemical element]]s in a [[thermal runaway|runaway]] reaction,<ref name="encyc">{{cite book|chapter=Novae|date=2001|editor=Paul Murdin|first=Dina|isbn=978-1-56159-268-5|last=Prialnik|pages=1846–1856|publisher=[[Institute of Physics Publishing]]/[[Nature Publishing Group]]|title=Encyclopedia of Astronomy and Astrophysics}}</ref> liberating an enormous amount of energy. This blows the remaining gases away from the surface of the white dwarf and produces an extremely bright outburst of light.

The rise to peak brightness may be very rapid, or gradual; after the peak, the brightness declines steadily.<ref>[[AAVSO]] Variable Star Of The Month: [http://www.aavso.org/vstar/vsots/0501.shtml May 2001: Novae] {{Webarchive|url=https://web.archive.org/web/20031106083823/http://www.aavso.org/vstar/vsots/0501.shtml |date=6 November 2003 }}</ref> The time taken for a nova to decay by 2 or 3&nbsp;magnitudes from maximum optical brightness is used for grouping novae into speed classes. Fast novae typically will take less than 25&nbsp;days to decay by 2&nbsp;magnitudes, while slow novae will take more than 80&nbsp;days.<ref>
{{cite book
 | first= Brian | last = Warner
 | date = 1995
 | title = Cataclysmic Variable Stars
 | isbn = 978-0-521-41231-5
 | publisher = [[Cambridge University Press]]
}}</ref>

Despite its violence, usually the amount of [[material]] ejected in a nova is only about {{frac|10,000}} of a [[solar 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.<ref name="encyc" /> 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]] from a few times solar to 50,000–100,000&nbsp;times solar.<ref name="encyc" /><ref>

{{cite book
 |last=Zeilik |first=Michael
 |date=1993
 |title=Conceptual Astronomy
 |publisher=[[John Wiley & Sons]]
 |isbn=978-0-471-50996-7
}}</ref> In 2010 scientists using NASA's [[Fermi Gamma-ray Space Telescope]] discovered that a nova also can emit [[gamma ray]]s (>100&nbsp;MeV).<ref>
{{cite web
 |author=[[JPL]]/[[NASA]]
 |date=12 August 2010
 |title=Fermi detects 'shocking' surprise from supernova's little cousin
 |url=http://www.physorg.com/news200849593.html
 |work=[[PhysOrg]]
 |access-date=15 August 2010
}}</ref>

Potentially, a [[white dwarf]] can generate multiple novae over time as additional [[hydrogen]] continues to accrete onto its surface from its [[Binary star|companion star]]. Where this repeated flaring is observed, the object is called a recurrent nova. An example is [[RS Ophiuchi]], which is known to have flared seven times (in 1898, 1933, 1958, 1967, 1985, 2006, and 2021). Eventually, the [[white dwarf]] can [[Explosion|explode]] as a [[Type Ia supernova|Type&nbsp;Ia supernova]] 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 [[V1500 Cygni|Nova Cygni 1975]]. This nova appeared on 29&nbsp;August 1975, in the constellation [[Cygnus (constellation)|Cygnus]] about 5 degrees north of [[Deneb]], and reached [[Apparent magnitude|magnitude]]&nbsp;2.0 (nearly as bright as [[Deneb]]). The most recent were [[V1280 Scorpii]], which reached magnitude&nbsp;3.7 on 17&nbsp;February 2007, and [[Nova Delphini 2013]]. [[Nova Centauri 2013]] was discovered 2&nbsp;December 2013 and so far is the brightest nova of this [[millennium]], reaching magnitude&nbsp;3.3.

=== Helium novae ===
A [[helium]] nova (undergoing a [[helium flash]]) is a proposed category of nova event that lacks [[Hydrogen spectral series|hydrogen lines]] in its [[Spectral line|spectrum]]. The absence of [[hydrogen]] lines 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.<ref>{{cite journal
 | author=Kato, Mariko
 | author2=Hachisu, Izumi
 | title=V445 Puppis: Helium Nova on a Massive White Dwarf
 | journal=The Astrophysical Journal | volume=598 | issue=2 | pages=L107–L110
 |date=December 2005
 | doi=10.1086/380597
 | bibcode=2003ApJ...598L.107K
|arxiv = astro-ph/0310351 | s2cid=17055772
 }}</ref> Since then, four other novae have been proposed as [[helium]] novae.<ref>{{cite journal
 | title=List of Helium Novae
 | last=Rosenbush | first=A. E.
 | journal=Hydrogen-Deficient Stars
 | volume=391| pages=271 | date=17–21 September 2007
 | location=Eberhard Karls University, Tübingen, Germany
 | editor=Klaus Werner |editor2=Thomas Rauch
 | publication-date=July 2008
 | bibcode=2008ASPC..391..271R
}}</ref>