Nova
A nova (pl. novae or novas) is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star (hence the name "nova", 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 stars.
Classical nova eruptions are the most common type. This type is likely created in a close binary star system consisting of a white dwarf and either a main sequence, subgiant, or red giant star. If the orbital period of the system 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 its surface, creating a dense but shallow atmosphere. This atmosphere, mostly consisting of hydrogen, is heated by the hot white dwarf and eventually reaches a critical temperature, causing ignition of rapid runaway fusion.
The sudden increase in energy expels the atmosphere into interstellar space, creating the envelope seen as visible light during the nova event. In past centuries such an event was thought to be a new star. A few novae produce short-lived nova remnants, lasting for perhaps several centuries. A recurrent nova involves the same processes as the classical nova, except that the fusion ignition is 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 supernovae, averaging about ten per year in the Milky Way. Most are found telescopically, perhaps only one every 12–18 months reaching naked-eye visibility. Novae reaching first or second magnitude occur only a few times per century. The last bright nova was V1369 Centauri, which reached 3.3 magnitude on 14 December 2013.[1]
Etymology edit
During the sixteenth century, astronomer Tycho Brahe observed the supernova 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.[2] 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 on white dwarfs, which are remnants of extremely old stars.
Stellar evolution of novae edit
Evolution of potential novae begins with two main sequence stars in a binary system. One of the two 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 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.[3]
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 reaction,[2] 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; after the peak, the brightness declines steadily.[4] The time taken for a nova to decay by around 2 or 3 magnitudes from maximum optical brightness is used for grouping novae into speed classes. Fast novae typically will take fewer than 25 days to decay by 2 magnitudes, while slow novae will take more than 80 days.[5]
Despite its violence, usually the amount of material ejected in a nova is only about 1⁄10,000 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.[2] 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 times solar.[2][6] In 2010 scientists using NASA's Fermi Gamma-ray Space Telescope discovered that a nova also can emit gamma rays (>100 MeV).[7]
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, which is known to have flared seven times (in 1898, 1933, 1958, 1967, 1985, 2006, and 2021). Eventually, the white dwarf can explode as a Type 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 Nova Cygni 1975. This nova appeared on 29 August 1975, in the constellation Cygnus about 5 degrees north of Deneb, and reached magnitude 2.0 (nearly as bright as Deneb). The most recent were V1280 Scorpii, which reached magnitude 3.7 on 17 February 2007, and Nova Delphini 2013. Nova Centauri 2013 was discovered 2 December 2013 and so far is the brightest nova of this millennium, reaching magnitude 3.3.
Helium novae edit
A helium nova (undergoing a helium flash) 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.[8] Since then, four other novae have been proposed as helium novae.[9]
Occurrence rate and astrophysical significance edit
Astronomers estimate that the Milky Way experiences roughly 30 to 60 novae per year; a recent examination refined the estimate to 50±27.[10] The number of novae actually observed in the Milky Way each year is much lower, about 10,[11] probably because distant novae are obscured by gas and dust absorption.[11] Roughly 25 novae brighter than about 20th magnitude are discovered in the Andromeda Galaxy each year and smaller numbers are seen in other nearby galaxies.[12] As of 2019, 407 probable novae had been recorded in the Milky Way.[11]
Spectroscopic observation of nova ejecta nebulae has shown that they are enriched in elements such as helium, carbon, nitrogen, oxygen, neon, and magnesium.[2] Classical nova explosions are galactic producers of the element lithium.[13][14] The contribution of novae to the interstellar medium is not great; novae supply only 1⁄50 as much material to the galaxy as do supernovae, and only 1⁄200 as much as red giant and supergiant stars.[2]
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.[15] 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.[2] Consequently, the interval is shorter for high-mass white dwarfs.[2]
V Sagittae is unusual in that we can predict now when it will go nova—in approximately 2083, plus or minus about 11 years.[16]
Subtypes edit
Novae are classified according to the light curve development speed:
- NA: fast novae, with a rapid brightness increase, followed by a brightness decline of 3 magnitudes—to about 1⁄16 brightness—within 100 days.[17]
- NB: slow novae, with magnitudes of 3, declining in 150 days or more.
- NC: very slow novae, also known as symbiotic novae, 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 10–80 years have been observed.[18]
Remnants edit
Some novae leave behind visible nebulosity, material expelled in the nova explosion or in multiple explosions.[19]
Novae as distance indicators edit
Novae have some promise for use as standard candle measurements of distances. For instance, the distribution of their absolute magnitude is bimodal, 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). Nova-based distance estimates to various nearby galaxies and galaxy clusters have been shown to be of comparable accuracy to those measured with Cepheid variable stars.[20]
Recurrent novae edit
A recurrent nova (RNe) is an object that has been seen to experience repeated nova eruptions. The recurrent nova typically brightens by about 9 magnitudes, whereas a classic nova may brighten by more than 12 magnitudes.[21]
Although it is estimated that as many as a quarter of nova systems experience multiple eruptions, only ten recurrent novae (listed below) have been observed in the Milky Way.[22]
Several extragalactic recurrent novae have been observed in the Andromeda Galaxy (M31) and the Large Magellanic Cloud. One of these extragalactic novae, M31N 2008-12a, erupts as frequently as once every 12 months.
On 20 April 2016, the Sky & Telescope website reported a sustained brightening of T Coronae Borealis from magnitude 10.5 to about 9.2 since February 2015. A similar event was reported in 1938, followed by another outburst in 1946.[23] By June 2018, the star had dimmed slightly but still remained at an unusually high level of activity. In March or April 2023, it dimmed to magnitude 12.3.[24] A similar dimming occurred in the year before the 1945 outburst, indicating that it will likely erupt between March and September 2024.[25]
| Full name | Discoverer | Magnitude range | Days to drop 3 magnitudes from peak | Known eruption years | Time span (years) | Years since latest eruption |
|---|---|---|---|---|---|---|
| CI Aquilae | K. Reinmuth | 8.6–16.3 | 40 | 1917, 1941, 2000 | 24–59 | 23 |
| V394 Coronae Australis | L. E. Erro | 7.2–19.7 | 6 | 1949, 1987 | 38 | 36 |
| T Coronae Borealis | J. Birmingham | 2.5–10.8 | 6 | 1217, 1787, 1866, 1946 | 80 | 78 |
| IM Normae | I. E. Woods | 8.5–18.5 | 70 | 1920, 2002 | ≤82 | 22 |
| RS Ophiuchi | W. Fleming | 4.8–11 | 14 | 1898, 1907, 1933, 1958, 1967, 1985, 2006, 2021 | 9–26 | 2 |
| V2487 Ophiuchi | K. Takamizawa (1998) | 9.5–17.5 | 9 | 1900, 1998 | 98 | 25 |
| T Pyxidis | H. Leavitt | 6.4–15.5 | 62 | 1890, 1902, 1920, 1944, 1967, 2011 | 12–44 | 12 |
| V3890 Sagittarii | H. Dinerstein | 8.1–18.4 | 14 | 1962, 1990, 2019 | 28–29 | 4 |
| U Scorpii | N. R. Pogson | 7.5–17.6 | 2.6 | 1863, 1906, 1917, 1936, 1979, 1987, 1999, 2010, 2022, | 8–43 | 1 |
| V745 Scorpii | L. Plaut | 9.4–19.3 | 7 | 1937, 1989, 2014 | 25–52 | 10 |
Extragalactic novae edit
Novae are relatively common in the Andromeda Galaxy (M31); several dozen novae (brighter than about apparent magnitude +20) are discovered in M31 each year.[12] The Central Bureau for Astronomical Telegrams (CBAT) has tracked novae in M31, M33, and M81.[26]
See also edit
References edit
- ^ "Nova Centauri 2013: Another bright, naked-eye nova | aavso.org". www.aavso.org. Retrieved 2 November 2020.
- ^ a b c d e f g h Prialnik, Dina (2001). "Novae". In Paul Murdin (ed.). Encyclopedia of Astronomy and Astrophysics. Institute of Physics Publishing/Nature Publishing Group. pp. 1846–1856. ISBN 978-1-56159-268-5.
- ^ M.J. Darnley; et al. (10 February 2012). "On the Progenitors of Galactic Novae". The Astrophysical Journal. 746 (61): 61. arXiv:1112.2589. Bibcode:2012ApJ...746...61D. doi:10.1088/0004-637x/746/1/61. S2CID 119291027.
- ^ AAVSO Variable Star Of The Month: May 2001: Novae 6 November 2003 at the Wayback Machine
- ^ Warner, Brian (1995). Cataclysmic Variable Stars. Cambridge University Press. ISBN 978-0-521-41231-5.
- ^ Zeilik, Michael (1993). Conceptual Astronomy. John Wiley & Sons. ISBN 978-0-471-50996-7.
- ^ JPL/NASA (12 August 2010). "Fermi detects 'shocking' surprise from supernova's little cousin". PhysOrg. Retrieved 15 August 2010.
- ^ Kato, Mariko; Hachisu, Izumi (December 2005). "V445 Puppis: Helium Nova on a Massive White Dwarf". The Astrophysical Journal. 598 (2): L107–L110. arXiv:astro-ph/0310351. Bibcode:2003ApJ...598L.107K. doi:10.1086/380597. S2CID 17055772.
- ^ Rosenbush, A. E. (17–21 September 2007). Klaus Werner; Thomas Rauch (eds.). "List of Helium Novae". Hydrogen-Deficient Stars. 391. Eberhard Karls University, Tübingen, Germany (published July 2008): 271. Bibcode:2008ASPC..391..271R.
- ^ Shafter, A.W. (January 2017). "The Galactic Nova Rate Revisited". The Astrophysical Journal. 834 (2): 192–203. arXiv:1606.02358. Bibcode:2017ApJ...834..196S. doi:10.3847/1538-4357/834/2/196. S2CID 118652484.
- ^ a b c "CBAT List of Novae in the Milky Way". IAU Central Bureau for Astronomical Telegrams.
- ^ a b "M31 (Apparent) Novae Page". IAU Central Bureau for Astronomical Telegrams. Retrieved 24 February 2009.
- ^ Arizona State University (1 June 2020). "Class of stellar explosions found to be galactic producers of lithium". EurekAlert!. Retrieved 2 June 2020.
- ^ Starrfield, Sumner; et al. (27 May 2020). "Carbon–Oxygen Classical Novae Are Galactic 7Li Producers as well as Potential Supernova Ia Progenitors". The Astrophysical Journal. 895 (1): 70. arXiv:1910.00575. Bibcode:2020ApJ...895...70S. doi:10.3847/1538-4357/ab8d23. S2CID 203610207.
- ^ Seeds, Michael A. (1998). Horizons: Exploring the Universe (5th ed.). Wadsworth Publishing Company. p. 194. ISBN 978-0-534-52434-0.
- ^ "Binary star V Sagittae to explode as very bright nova by century's end". phys.org. Retrieved 20 January 2020.
- ^ "Ritter Cataclysmic Binaries Catalog (7th Edition, Rev. 7.13)". High Energy Astrophysics Science Archive Research Center. 31 March 2010. Retrieved 25 September 2010.
- ^ GCVS' vartype.txt at VizieR
- ^ Liimets, T.; Corradi, R.L.M.; Santander-García, M.; Villaver, E.; Rodríguez-Gil, P.; Verro, K.; Kolka, I. (2014). "A Dynamical Study of the Nova Remnant of GK Persei / Stella Novae: Past and Future Decades.". Stellar Novae: Past and Future Decades. ASP Conference Series. Vol. 490. pp. 109–115. arXiv:1310.4488. Bibcode:2014ASPC..490..109L.
- ^ Robert, Gilmozzi; Della Valle, Massimo (2003). "Novae as Distance Indicators". In Alloin, D.; Gieren, W. (eds.). Stellar Candles for the Extragalactic Distance Scale. Springer. pp. 229–241. ISBN 978-3-540-20128-1.
- ^ Schaefer, Bradley E. (2010). "Comprehensive Photometric Histories of All Known Galactic Recurrent Novae". The Astrophysical Journal Supplement Series. 187 (2): 275–373. arXiv:0912.4426. Bibcode:2010ApJS..187..275S. doi:10.1088/0067-0049/187/2/275. S2CID 119294221.
- ^ Pagnotta, Ashley; Schaefer, Bradley E. (2014). "Identifying and Quantifying Recurrent Novae Masquerading as Classical Novae". The Astrophysical Journal. 788 (2): 164. arXiv:1405.0246. Bibcode:2014ApJ...788..164P. doi:10.1088/0004-637X/788/2/164. S2CID 118448146.
- ^ "Is T CrB About to Blow its Top?". Sky & Telescope website. 20 April 2016. Retrieved 6 August 2017.
- ^ Schaefer, B.E.; Kloppenborg, B.; Waagen, E.O. "Announcing T CrB pre-eruption dip". AAVSO. American Association of Variable Star Observers. Retrieved 18 January 2024.
- ^ Todd, Ian. "T Coronae Borealis nova event guide and how to prepare". Sky at Night Magazine. BBC. Retrieved 18 March 2024.
- ^ Bishop, David. "Extragalactic Novae". International Supernovae Network. Retrieved 11 September 2010.
Further reading edit
- Payne-Gaposchkin, C. (1957). The Galactic Novae. North Holland Publishing Co.
- Hernanz, M.; Josè, J. (2002). Classical Nova Explosions. American Institute of Physics.
- Bode, M.F.; Evans, E. (2008). Classical Novae. Cambridge University Press.
- Schaefer (2010). "Comprehensive Photometric Histories of All Known Galactic Recurrent Novae". The Astrophysical Journal Supplement Series. 187 (2): 275–373. arXiv:0912.4426. Bibcode:2010ApJS..187..275S. doi:10.1088/0067-0049/187/2/275. S2CID 119294221.
- Shafter; et al. (2011). "A Spectroscopic and Photometric Survey of Novae in M31". The Astrophysical Journal. 734 (1): 12. arXiv:1104.0222. Bibcode:2011ApJ...734...12S. doi:10.1088/0004-637X/734/1/12. S2CID 119114867.
- Massimo Della Valle; Luca Izzo (30 April 2020). "Observations of Galactic and Extragalactic Novae". Astronomy and Astrophysics Review. 28 (1): 3. arXiv:2004.06540. Bibcode:2020A&ARv..28....3D. doi:10.1007/s00159-020-0124-6. S2CID 215754507.
External links edit
- General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow
- AAVSO Variable Star of the Month. Novae: May 2001 6 November 2003 at the Wayback Machine