Type Ia supernova
Type Ia supernovae occur in binary systems (two stars orbiting one another) in which one of the stars is a white dwarf while the other can vary from a giant star to an even smaller white dwarf. A white dwarf is the remnant of a star that has completed its normal life cycle and has ceased nuclear fusion. However, white dwarfs of the common carbon-oxygen variety are capable of further fusion reactions that release a great deal of energy if their temperatures rise high enough.
Physically, carbon-oxygen white dwarfs with a low rate of rotation are limited to below 1.38 solar masses (M☉). Beyond this, they re-ignite and in some cases trigger a supernova explosion. Somewhat confusingly, this limit is often referred to as the Chandrasekhar mass, despite being marginally different from the absolute Chandrasekhar limit where electron degeneracy pressure is unable to prevent catastrophic collapse. If a white dwarf gradually accretes mass from a binary companion, the general hypothesis is that its core will reach the ignition temperature for carbon fusion as it approaches the limit. If the white dwarf merges with another star (a very rare event), it will momentarily exceed the limit and begin to collapse, again raising its temperature past the nuclear fusion ignition point. Within a few seconds of initiation of nuclear fusion, a substantial fraction of the matter in the white dwarf undergoes a runaway reaction, releasing enough energy (1–×1044 J) 2 to unbind the star in a supernova explosion.
This category of supernovae produces consistent peak luminosity because of the uniform mass of white dwarfs that explode via the accretion mechanism. The stability of this value allows these explosions to be used as standard candles to measure the distance to their host galaxies because the visual magnitude of the supernovae depends primarily on the distance.
The Type Ia supernova is a sub-category in the Minkowski-Zwicky supernova classification scheme, which was devised by American astronomer Rudolph Minkowski and Swiss astronomer Fritz Zwicky. There are several means by which a supernova of this type can form, but they share a common underlying mechanism. Theoretical astronomers long believed the progenitor star for this type of supernova is a white dwarf and empirical evidence for this was found in 2014 when a The Type Ia supernova was observed in the galaxy, Messier 82. When a slowly-rotating carbon-oxygen white dwarf accretes matter from a companion, it can exceed the Chandrasekhar limit of about 1.44 M☉, beyond which it can no longer support its weight with electron degeneracy pressure. In the absence of a countervailing process, the white dwarf would collapse to form a neutron star, as normally occurs in the case of a white dwarf that is primarily composed of magnesium, neon, and oxygen.
The current view among astronomers who model Type Ia supernova explosions, however, is that this limit is never actually attained and collapse is never initiated. Instead, the increase in pressure and density due to the increasing weight raises the temperature of the core, and as the white dwarf approaches about 99% of the limit, a period of convection ensues, lasting approximately 1,000 years. At some point in this simmering phase, a deflagration flame front is born, powered by carbon fusion. The details of the ignition are still unknown, including the location and number of points where the flame begins. Oxygen fusion is initiated shortly thereafter, but this fuel is not consumed as completely as carbon.
Once fusion has begun, the temperature of the white dwarf starts to rise. A main sequence star supported by thermal pressure would expand and cool which automatically counterbalances an increase in thermal energy. However, degeneracy pressure is independent of temperature; the white dwarf is unable to regulate the fusion process in the manner of normal stars, so it is vulnerable to a runaway fusion reaction. The flame accelerates dramatically, in part due to the Rayleigh–Taylor instability and interactions with turbulence. It is still a matter of considerable debate whether this flame transforms into a supersonic detonation from a subsonic deflagration.
Regardless of the exact details of this nuclear fusion, it is generally accepted that a substantial fraction of the carbon and oxygen in the white dwarf are converted into heavier elements within a period of only a few seconds, raising the internal temperature to billions of degrees. This energy release from thermonuclear fusion (1–×1044 J2) is more than enough to unbind the star; that is, the individual particles making up the white dwarf gain enough kinetic energy to fly apart from each other. The star explodes violently and releases a shock wave in which matter is typically ejected at speeds on the order of 5,000–000 km/s, roughly 6% of the 20speed of light. The energy released in the explosion also causes an extreme increase in luminosity. The typical visual absolute magnitude of Type Ia supernovae is Mv = −19.3 (about 5 billion times brighter than the Sun), with little variation.
The theory of this type of supernovae is similar to that of novae, in which a white dwarf accretes matter more slowly and does not approach the Chandrasekhar limit. In the case of a nova, the in-falling matter causes a hydrogen fusion surface explosion that does not disrupt the star. This type of supernova differs from a core-collapse supernova, which is caused by the cataclysmic explosion of the outer layers of a massive star as its core implodes.
Single degenerate progenitors
One model for the formation of this category of supernova is a close binary star system. The progenitor binary system consists of main sequence stars, with the primary possessing more mass than the secondary. Being greater in mass, the primary is the first of the pair to evolve onto the asymptotic giant branch, where the star's envelope expands considerably. If the two stars share a common envelope then the system can lose significant amounts of mass, reducing the angular momentum, orbital radius and period. After the primary has degenerated into a white dwarf, the secondary star later evolves into a red giant and the stage is set for mass accretion onto the primary. During this final shared-envelope phase, the two stars spiral in closer together as angular momentum is lost. The resulting orbit can have a period as brief as a few hours. If the accretion continues long enough, the white dwarf may eventually approach the Chandrasekhar limit.
The white dwarf companion could also accrete matter from other types of companions, including a subgiant or (if the orbit is sufficiently close) even a main sequence star. The actual evolutionary process during this accretion stage remains uncertain, as it can depend both on the rate of accretion and the transfer of angular momentum to the white dwarf companion.
It has been estimated that single degenerate progenitors account for no more than 20% of all Type Ia supernovae.
Double degenerate progenitors
A second possible mechanism for triggering a Type Ia supernova is the merger of two white dwarfs whose combined mass exceeds the Chandrasekhar limit. The resulting merger is called a super-Chandrasekhar mass white dwarf. In such a case, the total mass would not be constrained by the Chandrasekhar limit.
Collisions of solitary stars within the Milky Way occur only once every - 107; far less frequently than the appearance of novae. 1013 years Collisions occur with greater frequency in the dense core regions of globular clusters. (Cf. blue stragglers) A likely scenario is a collision with a binary star system, or between two binary systems containing white dwarfs. This collision can leave behind a close binary system of two white dwarfs. Their orbit decays and they merge through their shared envelope. However, a study based on SDSS spectra found 15 double systems of the 4,000 white dwarfs tested, implying a double white dwarf merger every 100 years in the Milky Way. Conveniently, this rate matches the number of Type Ia supernovae detected in our neighborhood.
A double degenerate scenario is one of several explanations proposed for the anomalously massive (2 M☉) progenitor of the SN 2003fg. It is the only possible explanation for SNR 0509-67.5, as all possible models with only one white dwarf have been ruled out. It has also been strongly suggested for SN 1006, given that no companion star remnant has been found there. Observations made with NASA's Swift space telescope ruled out existing supergiant or giant companion stars of every Type Ia supernovae studied. The supergiant companion's blown out outer shell should emit X-rays, but this glow wasn't detected by Swift's XRT (X-Ray telescope) in the 53 closest supernova remnants. For 12 Type Ia supernovae observed within 10 days of the explosion, the satellite's UVOT (Ultraviolet/Optical Telescope) showed no ultraviolet radiation originating from the heated companion star's surface hit by the supernova shock wave, meaning there were no red giants or larger stars orbiting those supernova progenitors. In the case of SN 2011fe, the companion star must have been smaller than the Sun, if it existed. The Chandra X-ray Observatory revealed that the X-ray radiation of five elliptical galaxies and the bulge of the Andromeda galaxy is 30-50 times fainter than expected. X-ray radiation should be emitted by the accretion discs of Type Ia supernova progenitors. The missing radiation indicates that few white dwarfs possess accretion discs, ruling out the common, accretion-based model of Ia supernovae. Inward spiraling white dwarf pairs must be strong sources of gravitational waves, but this can't be detected as of 2012.
Double degenerate scenarios raise questions about the applicability of Type Ia supernovae as standard candles, since total mass of the two merging white dwarfs varies significantly, meaning luminosity also varies.
It has been proposed that a group of sub-luminous supernovae that occur when helium accretes onto a white dwarf should be classified as Type Iax. This type of supernova may not always completely destroy the white dwarf progenitor.
Unlike the other types of supernovae, Type Ia supernovae generally occur in all types of galaxies, including ellipticals. They show no preference for regions of current stellar formation. As white dwarf stars form at the end of a star's main sequence evolutionary period, such a long-lived star system may have wandered far from the region where it originally formed. Thereafter a close binary system may spend another million years in the mass transfer stage (possibly forming persistent nova outbursts) before the conditions are ripe for a Type Ia supernova to occur.
A long-standing problem in astronomy has been the identification of supernova progenitors. Direct observation of a progenitor would provide useful constraints on supernova models. As of 2006, the search for such a progenitor had been ongoing for longer than a century. Observation of the supernova SN 2011fe has provided useful constraints. Previous observations with the Hubble Space Telescope did not show a star at the position of the event, thereby excluding a red giant as the source. The expanding plasma from the explosion was found to contain carbon and oxygen, making it likely the progenitor was a white dwarf primarily composed of these elements. Similarly, observations of the nearby SN PTF 11kx, discovered January 16, 2011 (UT) by the Palomar Transient Factory (PTF), lead to the conclusion that this explosion arises from single-degenerate progenitor, with a red giant companion, thus suggesting there is no single progenitor path to SN Ia. Direct observations of the progenitor of PTF11kx were reported in the August 24 edition of Science and support this conclusion, and also show that the progenitor star experienced periodic nova eruptions before the supernova - another surprising discovery.  However, later analysis revealed that the CSM is too massive for the single-degenerate scenario, and fits better the core-degenerate scenario.
Type Ia supernovae have a characteristic light curve, their graph of luminosity as a function of time after the explosion. Near the time of maximum luminosity, the spectrum contains lines of intermediate-mass elements from oxygen to calcium; these are the main constituents of the outer layers of the star. Months after the explosion, when the outer layers have expanded to the point of transparency, the spectrum is dominated by light emitted by material near the core of the star, heavy elements synthesized during the explosion; most prominently isotopes close to the mass of iron (or iron peak elements). The radioactive decay of nickel-56 through cobalt-56 to iron-56 produces high-energy photons which dominate the energy output of the ejecta at intermediate to late times.
The use of Type Ia supernovae to measure precise distances was pioneered by a collaboration of Chilean and US astronomers, the Calán/Tololo Supernova Survey. In a series of papers in the 1990s the survey showed that while Type Ia supernovae do not all reach the same peak luminosity, a single parameter measured from the light curve can be used to correct unreddened Type Ia supernovae to standard candle values. The original correction to standard candle value is known as the Phillips relationship  and was shown by this group to be able to measure relative distances to 7% accuracy. The cause of this uniformity in peak brightness is related to the amount of nickel-56 produced in white dwarfs presumably exploding near the Chandrasekhar limit.
The similarity in the absolute luminosity profiles of nearly all known Type Ia supernovae has led to their use as a secondary standard candle in extragalactic astronomy.  Improved calibrations of the Cepheid variable distance scale  and direct geometric distance measurements to NGC 4258 from the dynamics of maser emission  when combined with the Hubble diagram of the Type Ia supernova distances have led to an improved value of the Hubble constant.
Two distinct types
Recently it has been discovered that type Ia supernovae which were considered the same are in fact different, moreover a form of the type Ia supernova which is relatively infrequent today was for more common earlier in the history of the universe. This could have far reaching cosmological significance and could lead to revision of estimation of the rate of expansion of the universe and the prevalence of dark energy. More research is needed.
- HubbleSite - Dark Energy - Type Ia Supernovae
- Yoon, S.-C.; Langer, L. (2004). "Presupernova Evolution of Accreting White Dwarfs with Rotation". Astronomy and Astrophysics 419 (2): 623. arXiv:astro-ph/0402287. Bibcode:2004A&A...419..623Y. doi:10.1051/0004-6361:20035822. Retrieved 2007-05-30.
- Mazzali, P. A.; Röpke, F. K.; Benetti, S.; Hillebrandt, W. (2007). "A Common Explosion Mechanism for Type Ia Supernovae". Science 315 (5813): 825–828. arXiv:astro-ph/0702351. Bibcode:2007Sci...315..825M. doi:10.1126/science.1136259. PMID 17289993.
- Khokhlov, A.; Müller, E.; Höflich, P. (1993). "Light curves of Type IA supernova models with different explosion mechanisms". Astronomy and Astrophysics 270 (1–2): 223–248. Bibcode:1993A&A...270..223K.
- Staff (2006-09-07). "Introduction to Supernova Remnants". NASA Goddard/SAO. Retrieved 2007-05-01.
- Matheson, Thomas; Kirshner, Robert; Challis, Pete; Jha, Saurabh et al. (2008). "Optical Spectroscopy of Type Ia Supernovae". Astronomical Journal 135 (4): 1598–1615. arXiv:0803.1705. Bibcode:2008AJ....135.1598M. doi:10.1088/0004-6256/135/4/1598.
- da Silva, L. A. L. (1993). "The Classification of Supernovae". Astrophysics and Space Science 202 (2): 215–236. Bibcode:1993Ap&SS.202..215D. doi:10.1007/BF00626878.
- Type 1a Supernovae: Why Our Standard Candle Isn’t Really Standard
- Lieb, E. H.; Yau, H.-T. (1987). "A rigorous examination of the Chandrasekhar theory of stellar collapse". Astrophysical Journal 323 (1): 140–144. Bibcode:1987ApJ...323..140L. doi:10.1086/165813.
- Canal, R.; Gutiérrez, J. (1997). "The possible white dwarf-neutron star connection". Astrophysics and Space Science Library. Astrophysics and Space Science Library 214: 49. arXiv:astro-ph/9701225. Bibcode:1997astro.ph..1225C. doi:10.1007/978-94-011-5542-7_7. ISBN 978-0-7923-4585-5.
- Fryer, C. L.; New, K. C. B. (2006-01-24). "2.1 Collapse scenario". Gravitational Waves from Gravitational Collapse. Max-Planck-Gesellschaft. Retrieved 2007-06-07.
- Wheeler, J. Craig (2000-01-15). Cosmic Catastrophes: Supernovae, Gamma-Ray Bursts, and Adventures in Hyperspace. Cambridge, UK: Cambridge University Press. p. 96. ISBN 0-521-65195-6.
- Hillebrandt, W.; Niemeyer, J. C. (2000). "Type IA Supernova Explosion Models". Annual Review of Astronomy and Astrophysics 38 (1): 191–230. arXiv:astro-ph/0006305. Bibcode:2000ARA&A..38..191H. doi:10.1146/annurev.astro.38.1.191.
- "Science Summary". ASC / Alliances Center for Astrophysical Thermonuclear Flashes. 2001. Retrieved 2006-11-27.
- Röpke, F. K.; Hillebrandt, W. (2004). "The case against the progenitor's carbon-to-oxygen ratio as a source of peak luminosity variations in Type Ia supernovae". Astronomy and Astrophysics 420 (1): L1–L4. arXiv:astro-ph/0403509. Bibcode:2004A&A...420L...1R. doi:10.1051/0004-6361:20040135.
- Gamezo, V. N.; Khokhlov, A. M.; Oran, E. S.; Chtchelkanova, A. Y.; Rosenberg, R. O. (2003-01-03). "Thermonuclear Supernovae: Simulations of the Deflagration Stage and Their Implications". Science 299 (5603): 77–81. doi:10.1126/science.1078129. PMID 12446871. Retrieved 2006-11-28.
- Gilmore, Gerry (2004). "The Short Spectacular Life of a Superstar". Science 304 (5697): 1915–1916. doi:10.1126/science.1100370. PMID 15218132. Retrieved 2007-05-01.
- Paczynski, B. (July 28 – August 1, 1975). "Common Envelope Binaries". Structure and Evolution of Close Binary Systems. Cambridge, England: Dordrecht, D. Reidel Publishing Co. pp. 75–80. Bibcode:1976IAUS...73...75P.
- Postnov, K. A.; Yungelson, L. R. (2006). "The Evolution of Compact Binary Star Systems". Living Reviews in Relativity. Retrieved 2007-01-08.
- Langer, N.; Yoon, S.-C.; Wellstein, S.; Scheithauer, S. (2002). "On the evolution of interacting binaries which contain a white dwarf". In Gänsicke, B. T.; Beuermann, K.; Rein, K. The Physics of Cataclysmic Variables and Related Objects, ASP Conference Proceedings. San Francisco, California: Astronomical Society of the Pacific. p. 252. Bibcode:2002ASPC..261..252L.
- González Hernández, J. I.; Ruiz-Lapuente, P.; Tabernero, H. M.; Montes, D.; Canal, R.; Méndez, J.; Bedin, L. R. (2012). "No surviving evolved companions of the progenitor of SN 1006". Nature 489 (7417): 533–536. doi:10.1038/nature11447. PMID 23018963. See also lay reference: John Matson (December 2012). "No Star Left Behind". Scientific American 307 (6). p. 16
- Staff. "Type Ia Supernova Progenitors". Swinburne University. Retrieved 2007-05-20.
- "Brightest supernova discovery hints at stellar collision". New Scientist. 2007-01-03. Retrieved 2007-01-06.
- Whipple, Fred L. (1939). "Supernovae and Stellar Collisions". Proceedings of the National Academy of Sciences of the United States of America 25 (3): 118–125. Bibcode:1939PNAS...25..118W. doi:10.1073/pnas.25.3.118.
- Rubin, V. C.; Ford, W. K. J. (1999). "A Thousand Blazing Suns: The Inner Life of Globular Clusters". Mercury 28: 26. Bibcode:1999Mercu..28d..26M. Retrieved 2006-06-02.
- Middleditch, J. (2004). "A White Dwarf Merger Paradigm for Supernovae and Gamma-Ray Bursts". The Astrophysical Journal 601 (2): L167–L170. arXiv:astro-ph/0311484. Bibcode:2003astro.ph.11484M. doi:10.1086/382074.
- "Important Clue Uncovered for the Origins of a Type of Supernovae Explosion, Thanks to a Research Team at the University of Pittsburgh". University of Pittsburgh. Retrieved 23 March 2012.
- "The Weirdest Type Ia Supernova Yet". Lawrence Berkeley National Laboratory. 2006-09-20. Retrieved 2006-11-02.
- "Bizarre Supernova Breaks All The Rules". New Scientist. 2006-09-20. Retrieved 2007-01-08.
- Schaefer, Bradley E.; Pagnotta, Ashley (2012). "An absence of ex-companion stars in the type Ia supernova remnant SNR 0509-67.5". Nature 481 (7380): 164–166. Bibcode:2012Natur.481..164S. doi:10.1038/nature10692. PMID 22237107.
- "NASA'S Swift Narrows Down Origin of Important Supernova Class". NASA. Retrieved 24 March 2012.
- "NASA's Chandra Reveals Origin of Key Cosmic Explosions". Chandra X-Ray Observatory website. Retrieved 28 March 2012.
- Bo Wang; Stephen Justham; Zhanwen Han (2013). "Double-detonation explosions as progenitors of Type Iax supernovae". v1. arXiv:1301.1047 [astro-ph.SR].
- Ryan J. Foley; P. J. Challis; R. Chornock; M. Ganeshalingam; W. Li; G. H. Marion; N. I. Morrell; G. Pignata; M. D. Stritzinger; J. M. Silverman; X. Wang; J. P. Anderson; A. V. Filippenko; W. L. Freedman; M. Hamuy; S. W. Jha; R. P. Kirshner; C. McCully; S. E. Persson; M. M. Phillips; D. E. Reichart; A. M. Soderberg (2012). "Type Iax Supernovae: A New Class of Stellar Explosion". v2. arXiv:1212.2209 [astro-ph.SR].
- "Hubble finds supernova star system linked to potential 'zombie star'". SpaceDaily. 6 August 2014.
- van Dyk, Schuyler D. (1992). "Association of supernovae with recent star formation regions in late type galaxies". Astronomical Journal 103 (6): 1788–1803. Bibcode:1992AJ....103.1788V. doi:10.1086/116195.
- Hoeflich, N.; Deutschmann, A.; Wellstein, S.; Höflich, P. (1999). "The evolution of main sequence star + white dwarf binary systems towards Type Ia supernovae". Astronomy and Astrophysics 362: 1046–1064. arXiv:astro-ph/0008444. Bibcode:2000astro.ph..8444L.
- Kotak, R. (December 2008). "Progenitors of Type Ia Supernovae". Written at Keele University, Keele, United Kingdom. In Evans, A.; Bode, M. F.; O'Brien, T. J.; Darnley, M. J. RS Ophiuchi (2006) and the Recurrent Nova Phenomenon, proceedings of the conference held 12–14 June 2007. ASP Conference Series 401. San Francisco: Astronomical Society of the Pacific, 2008. p. 150. Bibcode:2008ASPC..401..150K.
- Nugent, Peter E.; Sullivan, Mark; Cenko, S. Bradley; Thomas, Rollin C.; Kasen, Daniel; Howell, D. Andrew; Bersier, David; Bloom, Joshua S.; Kulkarni, S. R.; Kandrashoff, Michael T.; Filippenko, Alexei V.; Silverman, Jeffrey M.; Marcy, Geoffrey W.; Howard, Andrew W.; Isaacson, Howard T.; Maguire, Kate; Suzuki, Nao; Tarlton, James E.; Pan, Yen-Chen; Bildsten, Lars; Fulton, Benjamin J.; Parrent, Jerod T.; Sand, David; Podsiadlowski, Philipp; Bianco, Federica B.; Dilday, Benjamin; Graham, Melissa L.; Lyman, Joe; James, Phil et al. (December 2011). "Supernova 2011fe from an Exploding Carbon-Oxygen White Dwarf Star". Nature 480 (7377): 344–347. arXiv:1110.6201. Bibcode:2011Natur.480..344N. doi:10.1038/nature10644. PMID 22170680
- Dilday, B.; Howell, DA; Cenko, SB; Silverman, JM; Nugent, PE; Sullivan, M; Ben-Ami, S; Bildsten, L; Bolte, M; Endl, M; Filippenko, A. V.; Gnat, O; Horesh, A; Hsiao, E; Kasliwal, MM; Kirkman, D; Maguire, K; Marcy, GW; Moore, K; Pan, Y; Parrent, J. T.; Podsiadlowski, P; Quimby, RM; Sternberg, A; Suzuki, N; Tytler, DR; Xu, D; Bloom, JS; Gal-Yam, A et al. (2012). "PTF11kx: A Type-Ia Supernova with a Symbiotic Nova Progenitor". Science 337 (6097): 942–5. arXiv:1207.1306. Bibcode:2012Sci...337..942D. doi:10.1126/science.1219164. PMID 22923575.
- Dilday, B.; Howell, DA; Cenko, SB; Silverman, JM; Nugent, PE; Sullivan, M; Ben-Ami, S; Bildsten, L; Bolte, M; Endl, M; Filippenko, A. V.; Gnat, O; Horesh, A; Hsiao, E; Kasliwal, MM; Kirkman, D; Maguire, K; Marcy, GW; Moore, K; Pan, Y; Parrent, J. T.; Podsiadlowski, P; Quimby, RM; Sternberg, A; Suzuki, N; Tytler, DR; Xu, D; Bloom, JS; Gal-Yam, A et al. (24 August 2012). "PTF 11kx: A Type Ia Supernova with a Symbiotic Nova Progenitor". Science 337 (6097): 942–945. arXiv:1207.1306. Bibcode:2012Sci...337..942D. doi:10.1126/science.1219164. PMID 22923575.
- "The First-Ever Direct Observations of a Type 1a Supernova Progenitor System". Scitech daily. - popular account of the discovery
- Hamuy, M. et al. (1993). "The 1990 Calan/Tololo Supernova Search". Astronomical Journal 106 (6): 2392. Bibcode:1993AJ....106.2392H. doi:10.1086/116811.
- Phillips, M. M. (1993). "The absolute magnitudes of Type IA supernovae". Astrophysical Journal Letters 413 (2): L105. Bibcode:1993ApJ...413L.105P. doi:10.1086/186970.
- Hamuy, M. et al. (1996). "The Absolute Luminosities of the Calan/Tololo Type IA Supernovae". Astronomical Journal 112: 2391. Bibcode:1996AJ....112.2391H. doi:10.1086/118190.
- Colgate, S. A. (1979). "Supernovae as a standard candle for cosmology". Astrophysical Journal 232 (1): 404–408. Bibcode:1979ApJ...232..404C. doi:10.1086/157300.
- Hamuy, M. et al. (1996). "A Hubble diagram of distant type IA supernovae". Astronomical Journal 109: 1. Bibcode:1995AJ....109....1H. doi:10.1086/117251.
- Freedman, W. et al. "Final Results from the Hubble Space Telescope Key Project to Measure the Hubble Constant". Astrophysical Journal 553 (1): 47. Bibcode:2001ApJ...553...47F. doi:10.1086/320638.
- Macri, L. M.; Stanek, K. Z.; Bersier, D.; Greenhill, L. J.; Reid, M. J. (2006). "A New Cepheid Distance to the Maser-Host Galaxy NGC 4258 and Its Implications for the Hubble Constant". Astrophysical Journal 652 (2): 1133–1149. arXiv:astro-ph/0608211. Bibcode:2006ApJ...652.1133M. doi:10.1086/508530.
- Perlmutter S, Supernova Cosmology Project, Goldhaber G, Knop RA, Nugent P, Castro PG, Deustua S, Fabbro S, Goobar A, Groom DE, Hook IM, Kim AG, Kim MY, Lee JC, Nunes NJ, Pain R, Pennypacker CR, Quimby R, Lidman C, Ellis RS, Irwin M, McMahon RG, Ruiz-Lapuente P, Walton N, Schaefer B, Boyle BJ, Filippenko AV, Matheson T, Fruchter AS et al. (1999). "Measurements of Omega and Lambda from 42 high redshift supernovae". Astrophysical Journal 517 (2): 565–86. arXiv:astro-ph/9812133. Bibcode:1999ApJ...517..565P. doi:10.1086/307221.
- Riess AG et al. (1998). "Observational evidence from supernovae for an accelerating Universe and a cosmological constant". Astronomical Journal 116 (3): 1009–38. arXiv:astro-ph/9805201. Bibcode:1998AJ....116.1009R. doi:10.1086/300499.
- Accelerating universe? Not so fast
- Falck, Bridget (2006). "Type Ia Supernova Cosmology with ADEPT". Johns Hopkins University. Retrieved 2007-05-20.
- Staff (February 27, 2007). "Sloan Supernova Survey". Sloan Digital Sky Survey. Retrieved 2007-05-25.
- "Novae and Supernovae". peripatus.gen.nz. Retrieved 2007-05-25.
- "Source for major type of supernova". Pole Star Publications Ltd. August 6, 2003. Retrieved 2007-11-25. (A Type Ia progenitor found)
- "Novae and Supernovae explosions found". peripatus.gen.nz. Retrieved 2007-05-25.
- SNFactory Shows Type Ia ‘Standard Candles’ Have Many Masses (March 4, 2014)