Editing Nova (section)

== Occurrence rate and astrophysical significance ==

Astronomers have estimated that the [[Milky Way]] experiences roughly 25 to 75 novae per year.<ref name="Shafter2017">{{cite journal
|last1=Shafter 
|first1=A.W.
|title=The Galactic Nova Rate Revisited
|journal=The Astrophysical Journal | volume=834 | issue=2 | pages=192–203
|date=January 2017
|doi=10.3847/1538-4357/834/2/196
|bibcode=2017ApJ...834..196S
|arxiv=1606.02358|s2cid=118652484
|doi-access=free
}}</ref> The number of novae actually observed in the Milky Way each year is much lower, about 10,<ref name=nova_list>
{{cite web
 |title=CBAT List of Novae in the Milky Way
 |url=http://cbat.eps.harvard.edu/nova_list.html
 |publisher=IAU [[Central Bureau for Astronomical Telegrams]]
}}</ref> probably because distant novae are obscured by gas and dust absorption.<ref name="nova_list"/> As of 2019, 407 probable novae had been recorded in the Milky Way.<ref name="nova_list" /> In the [[Andromeda Galaxy]], roughly 25 novae brighter than about 20th magnitude are discovered each year, and smaller numbers are seen in other nearby galaxies.<ref name="M31-Nova">
{{cite web
 |title=M31 (Apparent) Novae Page
 |url=http://cbat.eps.harvard.edu/CBAT_M31.html
 |publisher=IAU [[Central Bureau for Astronomical Telegrams]]
 |access-date=2009-02-24
}}</ref>

[[Spectroscopy|Spectroscopic]] observation of nova ejecta [[nebulae]] has shown that they are enriched in elements such as helium, carbon, nitrogen, oxygen, neon, and magnesium.<ref name="encyc" /> Classical nova [[explosion]]s are galactic producers of the element [[lithium]].<ref name="EA-20200601">{{cite news |author=[[Arizona State University]] |date=1 June 2020 |title=Class of stellar explosions found to be galactic producers of lithium |url=https://www.eurekalert.org/pub_releases/2020-06/asu-cos060120.php |access-date=2 June 2020 |work=[[EurekAlert!]]}}</ref><ref name="AJ-20200527">{{cite journal |author=Starrfield, Sumner |author1-link=Sumner Starrfield |display-authors=et al. |date=27 May 2020 |title=Carbon–Oxygen Classical Novae Are Galactic 7Li Producers as well as Potential Supernova Ia Progenitors |journal=[[The Astrophysical Journal]] |volume=895 |page=70 |arxiv=1910.00575 |bibcode=2020ApJ...895...70S |doi=10.3847/1538-4357/ab8d23 |s2cid=203610207 |doi-access=free |number=1}}</ref> The contribution of novae to the [[interstellar medium]] is not great; novae supply only {{frac|50}} as much material to the galaxy as do supernovae, and only {{frac|200}} as much as [[red giant]] and [[supergiant]] stars.<ref name="encyc" />

Observed recurrent novae such as [[RS Ophiuchi]] (those with periods on the order of decades) are rare. Astronomers theorize, however, that most, if not all, novae recur, albeit on time scales ranging from 1,000 to 100,000 years.<ref>
{{cite book
 |last=Seeds |first=Michael A.
 |date=1998
 |title=Horizons: Exploring the Universe
 |page=194 |edition=5th
 |publisher=[[Wadsworth Publishing Company]]
 |isbn=978-0-534-52434-0
}}</ref> 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.<ref name="encyc" /> Consequently, the interval is shorter for high-mass white dwarfs.<ref name="encyc" />

[[V Sagittae]] is unusual in that the time of its next eruption can be predicted fairly accurately; it is expected to recur in approximately 2083, plus or minus about 11 years.<ref>{{Cite web|url=https://phys.org/news/2020-01-binary-star-sagittae-bright-nova.html|title=Binary star V Sagittae to explode as very bright nova by century's end|website=phys.org|language=en-us|access-date=2020-01-20}}</ref>

=== Subtypes ===
Novae are classified according to the [[light curve]] decay speed, referred to as either type A, B, C and R,<ref>{{Cite web |title=Overview: Long-term visual light curves {{!}} aavso |url=https://www.aavso.org/overview-long-term-visual-light-curves |access-date=2024-07-14 |website=www.aavso.org}}</ref> or using the prefix "N": 
* '''NA''': fast novae, with a rapid brightness increase, followed by a brightness decline of 3 magnitudes—to about {{frac|16}} brightness—within 100 days.<ref name="heasarc">
{{cite web
 |date=31 March 2010
 |title=Ritter Cataclysmic Binaries Catalog (7th Edition, Rev. 7.13)
 |publisher=[[High Energy Astrophysics Science Archive Research Center]]
 |url=http://heasarc.gsfc.nasa.gov/W3Browse/all/rittercv.html
 |access-date=2010-09-25
}}</ref>
* '''NB''': slow novae, with a brightness decline of 3 magnitudes in 150 days or more.
* '''NC''': very slow novae, also known as [[symbiotic nova]]e, staying at maximum light for a decade or more and then fading very slowly.
* '''NR'''/'''RN''': recurrent novae, where two or more eruptions separated by 80 years or less have been observed.<ref>{{Cite web |title=GCVS Variability Types and Distribution Statistics of Designated Variable Stars According to their Types of Variability |url=https://cdsarc.u-strasbg.fr/ftp/cats/B/gcvs/vartype.txt |website=VizieR archive server, Strasbourg astronomical Data Center (CDS)}}</ref> These are generally also fast.