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Nova (plural novae) comes from the Latin word meaning "new", and refers to the phenomenon where a "new" star suddenly becomes visible where no star was visible before. In fact, these are not newly formed stars, but stars which suddenly become very much brighter, for a variety of reasons. The most spectacular are the supernova explosions, which in the most extreme cases can completely destroy the star leaving only an expanding gaseous nebula. In other cases, some form of remnant object - a neutron star or possibly a black hole - may be left behind. In yet other cases, as yet poorly understood mechanisms can give rise to recurrent novae, in which the star brightens then fades more than once.
Keywords: nova, supernova, pulsar, black hole, white dwarf
Nova (plural novae) comes from the Latin word meaning new. To the ancients, a nova was a new star – the Chinese called them ‘guest stars’ – which suddenly appeared where no star was visible before. In fact, these are not newly formed stars (star formation requires many thousands, if not millions, of years, and is usually hidden from our view inside a molecular cloud) but stars which suddenly become very much brighter. The star was there before, only too faint to be seen with the naked eye.
Stars brighten suddenly in response to a number of phenomena, the most spectacular being supernova explosions, which completely disrupt the star. Other types of outburst do not, however. For example, the famous Southern Hemisphere object, Eta Carinae has undergone several giant outbursts in the last centuries. A Sumerian recording of a ‘new star’ in 3000 B.C. is possibly attributable to Eta Carinae (Naeye 1997). In 1837, Eta Carinae flared up, peaking second only to Sirius at magnitude -0.8, in 1843. It remained at first magnitude for around 20 years, but has since settled back around 6 to 8.
“Once every second, somewhere in the universe a massive star is disrupted in a supernova explosion” (Janka 2002, p. 1134). They represents a huge outpouring of energy, which can be powered either by the gravitational energy released during the core collapse of a massive star or by the nuclear energy released by explosive thermonuclear reaction, colloquially known as ‘burning’ although there is no combustion involved.
Ideally, astronomers would like to identify supernova progenitors on pre-supernova images. Unfortunately, supernova progenitors are extremely elusive. In 2003, Van Dyk noted that at that time no supernova has been definitely discovered by eye in our Galaxy since 1604 A.D., and only 6 progenitors had been identified for the ~2900 known extragalactic supernovae. Moreover, “5 of these 6 supernovae are somewhat peculiar; 2 may not be supernovae at all. … [F]or the vast majority of supernovae, the progenitor’s nature can only be inferred indirectly from observed characteristics of individual supernovae, global properties of supernovae, or their relationship to their environment in the host galaxy” (Van Dyk 2003, p. 1161).
Supernova explosions are visible even when they occur in distant galaxies. The largest supernovae, sometimes called hypernovae, are believed to be responsible for the formerly enigmatic gamma ray bursts sometimes observed. This link was strongly established when a particularly nearby example, known as 2003dh, appeared in Leo on 29 March 2003 (Schilling 2003, p. 1860).
Classification of Supernovae
Type Ia Supernovae
Type Ia supernovae are characterised by the absence of hydrogen lines in their optical spectra, the presence of a deep Si II absorption line near wavelength 6150 Å, and their late-time spectra being dominated by strong blends of Fe emission lines.
Type Ia SNe are extremely bright; bright enough to be observed out to a red shift of about 1. They also have a predictable maximum brightness (it varies with the rate of decline in their light curves; brighter type Ia SNe decline more slowly). These two characteristics make them useful as “standard candles”.
Type Ia supernovae are thought to result from the explosion of massive binary stars. There are spectroscopic and photometric indications that the progenitor stars of type Ia supernovae are white dwarfs that are composed of C + O with strongly degenerate electrons. Such stars are formed from intermediate mass stars, < 8 M¤.
“Type Ia supernovae are believed to arise from the destruction of a white dwarf. White dwarfs, with typical masses of ~1 solar mass, are the bare cores of dead stars that were originally less massive than 8 solar masses. When a white dwarf reaches ~1.4 solar masses – near the maximum that nature allows for these stars (the Chandrasekhar limit) – a wave of nuclear burning moves through the star, leading to its complete incineration. The carbon and oxygen that make up most of the white dwarf is converted almost entirely to radioactive 56Ni.... The decay of 56Ni to 56Co (and then 56Fe) causes gamma-ray emission, making Type Ia supernovae the most luminous supernovae” (Van Dyk 2003).
“White dwarfs composed of C+O are formed from intermediate mass stars (M < 8 M☉, where M☉ is the mass of our sun), undergo cooling, and eventually become dark matter as they evolve towards fainter luminosities. … In a close binary system, the white dwarf evolves differently because the companion star expands to transfer matter to the white dwarf; the accreting white dwarfs are rejuvenated and, in certain cases, undergo thermonuclear explosions to give rise to SNe Ia. Theoretically, the Ch [Chandrasekhar mass] white dwarf models and the sub-Ch models have been considered to explain the origin of SNe Ia [Branch et al. 1995; Renzini 1996]” (Nomoto et al. 1997, p. 1378-1379).
It is unclear how the white dwarfs approach the Chandrasekhar limit. Two main scenarios have been proposed: In the first scenario, the white dwarf is in a mass-transferring close binary system with a less evolved low-mass “donor” star. The donor star dumps primarily hydrogen via an accretion disk onto the white dwarf, at just the right rate for total incineration. The supernova occurs when the white dwarf has accreted sufficient mass from its companion to trigger an explosion. However, the progenitor systems and hydrodynamical models are still controversial.
In the second scenario, two white dwarfs in a close binary merge due to loss of orbital angular momentum via gravitational radiation, resulting in the explosion (Iben & Tutukov 1984, Webbink 1984). Examples of both types of systems exist in the Milky Way, possibly in sufficient numbers for one or both scenarios to be plausible. In both cases, some quantity of matter – mostly hydrogen-rich – should exist in the supernova environment at the time of explosion. The shock wave propagating outward should interact with, and energize, this circumstellar matter, leading to characteristic emission at several wavelengths. No evidence for any such emission had been found in the spectra of nearby Type Ia supernovae (Cumming et al. 1996). Recently, however, Type Ia supernova 2002ic has been shown to exhibit fairly strong spectral hydrogen emission, providing the first evidence for interaction with circumstellar matter (Hamuy et al. 2003). Livio and Riess (2003) concluded that the SN2002ic observations could be most consistent with the double white dwarf scenario. Unfortunately, no other Type Ia supernova shows evidence for this interaction. At least one other example must be discovered, hopefully somewhat closer to us than SN2002ic, to allow detailed study by radio, x-ray (particularly the Chandra Observatory), and optical telescopes. Such additional evidence would greatly increase confidence in the physics of Type Ia supernovae and their cosmological implications. (After Nomoto et al. 1997 and Van Dyk 2003).
Thermonuclear reactions power the expansion of the core and eventual disruption of the star, but not the luminosity of the expanding gas. The energy source for the latter is provided by the slow radioactive decay sequence 56Ni → 56Co → 56Fe (Gamezo et al. 2002, p. 77).
In the early 1990s, David Branch and colleagues (Branch & Tammann 1992, Branch & Miller 1993) argued that this high intrinsic brightness, with relatively low dispersion, would make Type Ia supernovae ideal “standard candles” – objects with known absolute brightness, detectable out to hundreds of millions of lightyears. By comparing the apparent brightness of a standard candle with its absolute brightness, one can determine its exact distance from Earth. Although it turned out that Type Ia supernovae are not all of the same absolute brightness, their brightnesses can be adjusted to a common value (after Van Dyk 2003).
Type Ib and Ic Supernovae
Like all type I supernovae, subtypes b and c lack hydrogen lines in their spectra. Types b and c differ from type Ia in lacking the deep Si II absorption line near wavelength 6150 Å. They differ from one another in that SNe Ib exhibit moderately strong He I lines, especially at 5876 Å, in their early-time spectra, whereas the helium lines are weak or absent in type Ic SNe (after Nomoto et al. 1997).
Size Range and Distribution
Thought to probably result from core-collapse events (Van Dyk 2003, p. 1161). “[A]ssociated with parent stars of of more than about 30 solar masses” (Gilmore 2004).
Type II Supernovae
Type II supernovae are characterised by hydrogen emission in their spectra, and light curve shapes that differ significantly from those of Type I supernovae.
Type II supernovae are subdivided into two major classes, depending on how their light curves decay over time following maximum brightness. Type II-L supernovae show a linear (steady) decline in luminosity whereas Type II-P light curves exhibit a “plateau phase” in which the luminosity decays at a slower rate – and may remain nearly constant – for an extended period of time before resuming a more “normal” decline.
However, Type II SNe are highly variable, and two additional types – Type IIn and Type IIb – have also been proposed. It is not intended to discuss these here (some information is available on Wikipedia).
The peak brightness of all Type II supernovae is several magnitudes fainter than that of a Type Ia supernova, but while SNII-P show a large dispersion in their maximum brightness, the peak brightnesses of SNII-L are nearly uniform at 2.5 magnitudes fainter than a Type Ia supernova.
Size Range and Distribution
Type II supernovae are believed to be produced by the explosion following the core-collapse of massive stars that have retained most of their hydrogen envelope at the time of explosion.
“Astronomers are confident that Type II supernovae occur when all the nuclear fuel in the core of a massive (>8 to 10 [but less than about 30; Gilmore 2004] solar masses) supergiant star has been burned. The supernova arises when the iron-rich core within the hydrogen-rich envelope of such a star … collapses to form a neutron star. The released energy is carried away primarily by neutrinos. A fraction is carried by a shock wave of kinetic energy, which rips apart the envelope” (Van Dyk 2003, p. 1161).
These stars produce, first, helium cores from hydrogen “burning”, followed by carbon-oxygen cores from the helium reactions. Once the carbon-oxygen core reaches ~1.4 M☉, electron pressure is no longer sufficient to support the weight of the star. The core contracts and density increases rapidly, briefly halted at stages, by the establishment – first in the core, then in outwardly moving spherical shells – by fusion reactions producing successively heavier elements: sulphur, silicon, magnesium and iron.
Like Type Ib and Type Ic supernovae, Type II supernovae are found in regions of star formation.
A supernova explosion can completely disrupt the star or, famously, leave behind a small, dense object: a neutron star or possibly a black hole. These are topics for another day.
Either way, the supernova also leaves behind a shock wave: expanding shells of photons, neutrinos, and gas. The ejected material, particularly of Type II supernovae, is rich in heavy elements synthesised during the explosion. This is one of the principal sources for heavy elements in the Universe. Moreover, “shock waves from supernova explosions have played a key role in the evolution of the Milky Way Galaxy by depositing energy and material enriched in heavy elements into interstellar space.... The passage of a shock wave past an interstellar molecular cloud contributes to the cloud’s eventual dispersal, but the compression of the cloud’s gas during the interaction may lead to star formation” (Wardle & Yusef-Zadeh 2002, p. 2350).
Unlike Type Ia supernovae, where nothing except ejected stellar material remains after the explosion, Type II supernovae tend to leave behind remnants, either neutron stars (of which pulsars are a subset), or, possibly, black holes. “Exactly which progenitor stars lead to which remnant is only moderately well understood, although the general principles are clear” (Gilmore 2004).
Stellar nucleosynthesis proceeds by converting lighter elements, beginning with hydrogen and helium, into progressively heavier elements, such as oxygen, nitrogen, sulphur, silicon, magnesium and, eventually, iron. As it does so, energy is produced and radiated away; the reactions are exothermic. Iron has a minimum energy nuclear configuration, however. To convert iron to any other element, whether lighter or heavier, requires energy to be input. When an iron core begins to build up within a star, energy is no longer being produced to hold up against the gravitational pressure from above. The core begins to contract and, consequently, to heat up from the release of gravitational energy. Once the core temperature reaches ~1010 K, the black body radiation produced (i.e., radiating away the heat) is of sufficiently short wavelength to interact with the iron nuclei, and convert them back into He4. This process absorbs the energy which would otherwise oppose and slow down the core contraction. Instead we have a positive feedback process which accelerates core contraction, leading to runaway collapse.
Core pressure continues to increase, until electrons are forced into the helium nuclei. The helium atom ceases to exist, leaving instead four neutrons and releasing two neutrinos. The conversion of helium to neutrons absorbs more energy, and the neutrinos are able to radiate energy away more effectively than photons. Both act to cool the core and further accelerate its collapse. Moreover, the loss of the electrons reduces the electron degeneracy pressure that was previously acting against contraction.
e.g. Pulsar B1620-26 in the Messier 4 globular cluster
Other Types of Novae
at least something about Eta Car in here
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Branch, D.; Livio, M.; Yungelson, L.R.; Boffi, F.R.; Baron, E. 1995:
Branch, D.; Miller, D.L. 1993: Type IA supernovae as standard candles. Astrophysical Journal, Part 2 - Letters 405 (1): L5-L8.
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Cristiani, S. Cappellaro, E.; Turatto, M.; Bergeron, J.; Bues, I. et al. 1992:
Cumming, R.J. 1996:
Filippenko, A.V. 1997: Optical spectra of supernovae. Annual Review of Astronomy and Astrophysics 35: 309-355.
Gamezo, V.N.; Khokhlov, A.M.; Oran, E.S.; Chtchelkanova, A.Y.; Rosenberg, R.O. 2002: Thermonuclear supernovae: Simulations of the deflagration stage and their implications. Science 299: 77-81.
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Iben, I; Tutukov, A. 1984:
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Kirshner, R.P.; Oke, J.B. 1975:
Kirshner, R.P.; Oke, J.B.; Penston, M.V.; Searle, L. 1973:
Livio, M.; Riess, A. 2003:
Meikle, W.P.S.; Cumming, R.J.; Geballe, T.R.; Lewis, J.R.; Walton, N.A. et al. 1996:
Nomoto, K.; Iwamoto, K.; Kishimoto, N. 1997: Type Ia supernovae: Their origin and possible applications in cosmology. Science 276: 1378-1382.
Patat, F.; Benetti, S.; Cappellaro, E.; Danziger, I.J.; Della Valle, M. et al. 1996:
Phillips, M.M.; Phillips, A.C.; Heathcote, S.R.; Blanco, V.M.; Geisler, D. et al. 1987:
Renzini, A. 1996:
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Turatto, M.; Cappellaro, E.; Benetti, S.; Danziger, I.J. 1993a:
Turatto, M.; Cappellaro, E.; Danziger, I.J.; Benetti, S.; Gouiffes, C.; Della Valle, M. 1993b:
Uomoto, A.; Kirshner, R.P. 1986:
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Wardle, M.; Yusef-Zadeh, F. 2002: Supernova remnant OH masers: Signposts of cosmic collision. Science 296: 2350-2354.
Webbink, R. 1984:
Wegner, G.; Swanson, S.R. 1996:
Wells, L.A.; Phillips, M.M.; Suntzeff, N.B.; Heathcote, S.R.; Hamuy, M.; et al. 1994:
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