Stars/Supernovas

At right is an image of supernova SN 1987A, one of the brightest stellar explosions since the invention of the telescope more than 400 years ago.

A star that suddenly increases greatly in brightness because of a catastrophic explosion that ejects most of its mass may be a supernova.

SN 1987A
"On February 23.316 UT, 1987, [blue] light and neutrinos from the brightest supernova in 383 years arrived at Earth ... it has been observed ... at all wavelengths from radio through gamma rays, SN 1987A is the only object besides the Sun to have been detected in neutrinos."

"Astronomers using ESO’s Very Large Telescope have for the first time obtained a three-dimensional view [on the left] of the distribution of the innermost material expelled by a recently exploded star. The original blast was not only powerful, according to the new results. It was also more concentrated in one particular direction. This is a strong indication that the supernova must have been very turbulent, supporting the most recent computer models."

"New observations making use of a unique instrument, SINFONI [1], on ESO’s Very Large Telescope (VLT) have provided even deeper knowledge of this amazing event, as astronomers have now been able to obtain the first-ever 3D reconstruction of the central parts of the exploding material."

"This view shows that the explosion was stronger and faster in some directions than others, leading to an irregular shape with some parts stretching out further into space."

"The first material to be ejected from the explosion travelled at an incredible 100 million km per hour, which is about a tenth of the speed of light or around 100 000 times faster than a passenger jet. Even at this breakneck speed it has taken 10 years to reach a previously existing ring of gas and dust puffed out from the dying star. The images also demonstrate that another wave of material is travelling ten times more slowly and is being heated by radioactive elements created in the explosion."

"Such asymmetric behaviour was predicted by some of the most recent computer models of supernovae, which found that large-scale instabilities take place during the explosion. The new observations are thus the first direct confirmation of such models."

"SINFONI is the leading instrument of its kind, and only the level of detail it affords allowed the team to draw their conclusions. Advanced adaptive optics systems counteracted the blurring effects of the Earth's atmosphere while a technique called integral field spectroscopy allowed the astronomers to study several parts of the supernova’s chaotic core simultaneously, leading to the build-up of the 3D image."

“Integral field spectroscopy is a special technique where for each pixel we get information about the nature and velocity of the gas. This means that besides the normal picture we also have the velocity along the line of sight. Because we know the time that has passed since the explosion, and because the material is moving outwards freely, we can convert this velocity into a distance. This gives us a picture of the inner ejecta as seen straight on and from the side.”

Theoretical supernovas
Def. a "star which explodes, increasing its brightness to typically a billion times that of our sun, though attenuated by the great distance from our sun" is called a supernova.

Analytical models such as polytropes to approximate the behaviors of a star and computational numerical simulations give insight into the heart of what is going on [or] can reveal the existence of phenomena and effects that would otherwise not be seen.

At right is a computer simulation of a supernova explosion.

"Our primary scientific and computational focus is on tera- to exa-scale simulation of supernovae of both classes in the Universe."

"The stream lines in this image [at right] show the two counter rotating flows that may be established below the supernova shock wave (the surface in the image) by the instability of the shock in a core collapse supernova explosion. The innermost flow accretes onto the central object, known as the proto-neutron star, spinning it up. This may be the mechanism whereby pulsars (spinning neutron stars) are born."

"[T]he core collapse supernova shock wave is likely reenergized to initiate an explosion at much later times than previously anticipated. The shock wave must exit the iron core and enter the oxygen layer before shock revival can occur. In the oxygen layer, the density of the star drops off dramatically, which gives the shock less to plow through. In addition, in the oxygen layer, nuclear burning can occur, aiding the shock energetically. The delay to explosion is naturally set by the time it takes for the shock to reach the oxygen layer. The previously discovered stationary accretion shock instability (SASI) causes large-scale distortions of the shock, causing it to reach the oxygen layer sooner in certain directions, thereby precipitating the onset of explosion. In this new picture, we have obtained explosions over a range of stellar progenitors, between 10 and 20 Solar masses."

"Stars from about 8 to about 15 Mʘ explode as supernovae, but do not have a strong stellar wind, and so explode into the interstellar medium".

Neutrinos
"In the 1980s two early water-Cherenkov experiments were built. The Irvine-Michigan-Brookhaven detector in an Ohio salt mine and the Kamiokande detector in a Japanese zinc mine were tanks containing thousands of tons of purified water, monitored with phototubes. The two detectors launched the field of neutrino astronomy by detecting some 20 low-energy (about 10 MeV) neutrinos from Supernova 1987A—the first supernova since the 17th century that was visible to the naked eye."

The water-based detectors Kamiokande II and IMB detected 11 and 8 antineutrinos of thermal origin, respectively, while the scintillator-based Baksan detector found 5 neutrinos (lepton number = 1) of either thermal or electron-capture origin, in a burst lasting less than 13 seconds.

"In 1987, astronomers counted 19 neutrinos from an explosion of a star in the nearby Large Magellanic Cloud, 19 out of the billion trillion trillion trillion trillion neutrinos that flew from the supernova."

Gamma rays
On June 19, 1988, from Birigüi (50° 20' W 21° 20' S) at 10:15 UTC a balloon launch occurred which carried two NaI(Tl) detectors (600 cm2 total area) to an air pressure altitude of 5.5 mb for a total observation time of 6 hr.

The supernova SN1987A in the Large Magellanic Cloud (LMC) was discovered on February 23, 1987, and its progenitor is a blue supergiant (Sk -69 202) with luminosity of 2-5 x 1038 erg/s.

The 847 keV and 1238 keV gamma-ray lines from 56Co decay have been detected.

"The image [on the right] shows a colour composite of three images obtained with the EMMI multi-mode instrument at the ESO 3.58-m New Technology Telescope (NTT) at La Silla on May 4, 1998. The short exposures were obtained through V (green), R (red) and I (near-infrared) filtres. The new supernova, SN 1998bw [120 million light years away], is the very bright, bluish star at the center (indicated with an arrow), located on an arm of spiral galaxy ESO 184-G82. There are several other galaxies in the field."

A "Type Ic supernova, SN 1998bw [at (J2000) RA 19 35 03.30 Dec -52 50 45.9, from SIMBAD in Telescopium in the image on the right], was discovered coincident with a gamma-ray burst, GRB 980425 [detected on 25 April 1998]."

"For spherically symmetric models, both fail to produce a GRB of even the low luminosity inferred for GRB 980425."

A "more likely explanation for what was seen is a highly asymmetric explosion in which the GRB was produced by a relativistic jet, perhaps viewed obliquely, and only a fraction of the total stellar mass was ejected, the remainder accreting into a black hole. The ejected mass (but not the 56Ni mass), explosion energy, and velocities may then be smaller."

X-rays
"February 24, 2007 marks the 20th anniversary of one of the most spectacular events observed by astronomers in modern times, Supernova 1987A. The destruction of a massive star in the Large Magellanic Cloud, a nearby galaxy, spawned detailed observations by many different telescopes, including NASA's Chandra X-ray Observatory and Hubble Space Telescope. The outburst was visible to the naked eye, and is the brightest known supernova in almost 400 years."

"This composite image [second down on the right] shows the effects of a powerful shock wave moving away from the explosion. Bright spots of X-ray and optical emission arise where the shock collides with structures in the surrounding gas. These structures were carved out by the wind from the destroyed star. Hot-spots in the Hubble image (pink-white) now encircle Supernova 1987A like a necklace of incandescent diamonds. The Chandra data (blue-purple) reveals multimillion-degree gas at the location of the optical hot-spots. These data give valuable insight into the behavior of the doomed star in the years before it exploded."

Supernova progenitor systems
How “many different type 1a progenitor systems exist, and how common each of them is.“

The progenitor-supernova map on the right presents associations of supernovas with progenitor stars based on direct observations of the progenitors in pre-explosion images.

Wolf-Rayet stars
Notation: let N-rich (and usually also He-rich) Wolf-Rayet stars be represented by WN.

Notation: let C-rich W-R stars be represented by WC.

Notation: let O-rich W-R stars be represented by WO.

Notation: let red supergiant stars be represented by RSG.

"Let us consider a simplistic massive-star evolutionary scheme, for single stars with approximately solar metallicity, broadly following, e.g., Maeder & Conti ( 1994)."

"40 M⊙ < M < 80 M⊙: O → LBV → WN → WC/WO → SN Ic"

"25 M⊙ < M < 40 M⊙: O → LBV → (early) WN → SN Ib"

"15 M⊙ < M < 25 M⊙: O → RSG → (late) WN → SN II - L/IIb"

"This image of part of the Carina Nebula was created from images taken through red, green and blue filters with the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile. It is centred on the unusual hot massive young star WR 22 [7500 light years away in Carina at (J2000) RA 10 41 27.53 Dec -59° 40' 16.81"], a member of the rare class of Wolf–Rayet stars. The field of view is 0.55 x 0.55 degrees, covering a 72 x 72 light-year region at the distance of the nebula."

Luminous blue variables
On the left an upper portion of the H-R Diagram that shows the location of the S Doradus instability strip where quiescent luminous blue variables (LBVs) are found, and the region where LBVs in outburst are found.

Several well-known LBVs are marked at both their quiescent and outburst locations. Some are only known in one state. The non-conserving outburst of HD 5980A and the Great Eruption of Eta Carinae, as well as several Yellow Hypergiants are also marked.

"80 M⊙ < M < 150 M⊙: O → LBV → SN IIn(?)".

On the right is an image of the luminous blue variable AG Carinae as seen by the Hubble Space Telescope.

Blue hypergiants
"The progenitor of supernova SN 2005gl was [a massive blue hypergiant]. This very massive star was likely a luminous blue variable that standard stellar evolution predicts should not have exploded in that state."

Blue supergiants
"Four days after the event was recorded, the progenitor star was tentatively identified as Sanduleak -69° 202, a blue supergiant.

This was an unexpected identification, because at the time a blue supergiant was not considered a possibility for a supernova event in existing models of high mass stellar evolution. Many models of the progenitor have attributed the color to its chemical composition, particularly the low levels of heavy elements, among other factors.

In the image on the right, "Alnitak, Alnilam, and Mintaka, are the bright bluish stars from east to west (left to right) along the diagonal in this gorgeous cosmic vista. Otherwise known as the Belt of Orion, these three blue supergiant stars are hotter and much more massive than the Sun. They lie about 1,500 light-years away, born of Orion's well-studied interstellar clouds. In fact, clouds of gas and dust adrift in this region have intriguing and some surprisingly familiar shapes, including the dark Horsehead Nebula and Flame Nebula near Alnitak at the lower left. The famous Orion Nebula itself lies off the bottom of this star field that covers about 4.5x3.5 degrees on the sky. This image was taken last month with a digital camera attached to a small telescope in Switzerland, and better matches human color perception than a more detailed composite taken over 15 years ago."

Red supergiants
The explosion expels most or all of a star's material at a velocity of up to 30000 km/s (10% of light speed, powering a shock wave into the interstellar medium.

“Less than 1 percent of all 1a supernovae could be from a companion red giant star.“

"8 M⊙ < M < 15 M⊙: B/O → RSG → SN II-P".

On the right is a VLT Survey Telescope image of Westerlund 1. This is a false colour optical and near infrared image where nebulosity (hydrogen alpha emission) is highlighted in green. The inset shows the red supergiant W1-26 at the centre with the associated ionised hydrogen triangular nebula above and left (the O9 supergiant W1-25 appears between the two in green).

Red giants
"The type Ia supernova SN 2011fe was recently detected in a nearby galaxy7. [...] The luminosity of the progenitor system [on the right] (especially the companion star) is 10–100 times fainter than previous limits on other type Ia supernova progenitor systems8, 9, 10, allowing us to rule out luminous red giants and almost all helium stars as the mass-donating companion to the exploding white dwarf."

White dwarfs
Type “1as [may] result from a pair of two white dwarf stars (a double-degenerate system). So, instead of being locked in a deadly dance with a red giant, the white dwarf is dancing with another white dwarf.“

Surveys “look at the gas outflows produced by type 1a supernovae and find only scarce evidence for the expected bits of red giant; and observations of exploding supernovae that show no evidence for what astronomers call a “shock breakout,” or a kind of flare produced by a big star getting in the way of the billowing debris cloud.“

In the centre of the image on the right is a small white dwarf star that has undergone a helium flash, the encircling smudge of red.

"The double degenerate model posits two white dwarfs with an orbital period of a few hours which merge because of the loss of gravitational wave radiation. Two drawbacks with this model have been a poor understanding of how such a detonation might be initiated and the lack of observed progenitors. There are now many double degenerates known (Marsh, Dhillon & Duck 1995; Moran, Marsh & Bragaglia 1997; Moran, Maxted & Marsh 2000) but none has both a sufficiently short orbital period and a total mass in excess of 1.4 M⊙. However, at least one good candidate for the progenitor of an edge-lit detonation is known (WD 1704 +481.2; Maxted et al. 2000)."

Subdwarf stars
Notation: Let sdB stand for subdwarf B stars.

The "pulsating sdB star KPD 1930 +2752 [...] is a binary. The radial velocities measured from the Hα and He I 6678-Å spectral lines vary sinusoidally with the same period (2 h 17 min) as the ellipsoidal variability seen by Billéres et al. The amplitude of the orbital motion (349.3 ± 2.7 km s-1) combined with the canonical mass for sdB stars (0.5 M⊙) implies a total mass for the binary of 1.47 ± 0.01 M⊙. The unseen companion star is almost certainly a white dwarf star. The binary will merge within ~200 million years because of gravitational wave radiation. The accretion of helium and other elements heavier than hydrogen on to the white dwarf, which then exceeds the Chandrasekhar mass (1.4 M⊙), is a viable model for the cause of Type Ia supernovae."

O stars
"80 M⊙ < M < 150 M⊙: O → LBV → SN IIn(?)".

"40 M⊙ < M < 80 M⊙: O → LBV → WN → WC/WO → SN Ic"

"25 M⊙ < M < 40 M⊙: O → LBV → (early) WN → SN Ib"

"15 M⊙ < M < 25 M⊙: O → RSG → (late) WN → SN II - L/IIb"

"8 M⊙ < M < 15 M⊙: B/O → RSG → SN II-P".

Luminosities
Stars including Os can be put into classes based on luminosity, size or apparent mass. These may be 0 - hypergiants, I - supergiants, II - bright giants, III - giants, IV - subgiants, V - main sequence, VI - dwarfs and VII - subdwarfs. But, these are seldom rigidly followed. For example, class Ia may be supergiants, while class Ib may be bright giants. Another example, class V may be main sequence dwarfs, class VI may be subdwarfs, while class VII may be white dwarfs.

For simplicity and emphasis on size and special characteristics, star classes may be by diameter, when known.

Temperatures
Each star class is subdivided into temperature ranges using numbers: hottest - 0 to coolest - 9.9. For example, O00 or O0 0 is an O hypergiant star having the hottest known photospheric temperature.

Gamma-ray stars have surface temperatures starting at 300,000,000 K (300 MK) corresponding to a peak wavelength of 0.010 nm (10 pm) for the beginning of super soft gamma-ray sources.

A "new subclass, O1, has been created".

X-ray stars have surface temperatures starting at 300,000 K corresponding to a peak wavelength of 10 nm for the beginning of super soft X-ray sources.

"Clegg and Walsh (1989) determined Teff = 120 000 K for the central star NGC 7293 from the nebular lines. This is in reasonable agreement with the temperature determined from Hδ (Teff = 110 000 K)."

An O2 VIII [BD+28 4211, sdO2VIII:He5, on the right] can have an effective temperature of 82,000 K and an O9 V can have an effective temperature of 38,000 K.

"For BD+28° 4211 [...] the He I 5876 Å line in a high-resolution spectrum taken at the McDonald Observatory [has a] best fit [of] Teff = 82000 K in good agreement with Hε [on the right at the bottom]."

The "effective temperatures [are] derived from [spectral] lines".

If "the [helium] lines are significantly wider than the average [class VII], we use luminosity class “VIII”."

Subdwarf O stars
US 708 is a high-velocity star of spectral type sdOHe, where sd stands for subdwarf, O stands for an O-type star, and He stands for helium star.

"Hypervelocity stars (HVSs) travel with velocities so high that they exceed the escape velocity of the Galaxy."

"A multinational team of astronomers led by Dr Stephan Geier from the European Southern Observatory in Garching, Germany, has determined that a hypervelocity star known as US 708 is traveling at about 1,200 km per second."

"By measuring the velocity, trajectory and rotation of the star, known as US 708, researchers at the European Southern Observatory determined that it started life as one half of a close binary pair — two stars that closely orbited one other."

"Scientists using the W. M. Keck Observatory and Pan-STARRS1 telescopes on Hawaii have discovered a star that breaks the galactic speed record, traveling with a velocity of about 2.7 million mph (1,200 km/s). This velocity is so high, the star will escape the gravity of our galaxy. In contrast to the other known unbound stars, the team showed that this compact star was ejected from an extremely tight binary by a thermonuclear supernova explosion."

"A helium star is an O star or B star in which the absorption lines of helium are abnormally strong and those of hydrogen are absent or weak. Extreme helium stars (also called hydrogen-deficient stars) show no trace of hydrogen, while intermediate helium-rich stars have hydrogen lines that are visible but weaker than in normal stars. The loss or depletion of the star's hydrogen envelope, leaving essentially an exposed helium core, may have happened because of a powerful stellar wind, as in Wolf-Rayet stars, or because of mass transfer to a close binary companion."

Dwarf O stars
Ton S 61 (2MASS J22361663-3142130, PHL 334) is a spectral type OVI.

Main sequence O stars
Spectral-class "or temperature classification criteria in [the O3-B0 main sequence] are He II λ4541/He I λ4471 and He II λ4200/He I(+II) λ4026; at the later types He II λ4541/He I λ4387 and He II λ4200/He I λ4144 are useful checks, since the former ratios become very small, but with care since the latter ratios are also sensitive to luminosity."

"The definition of type 03 in the 60 Å mm-1 photographic classification was that He I is not seen, but λ4471 appears to have been detected in the present observation of HDE 303308, so that the distinction between the 03 V and 04 V spectra is not clear in [the] digital data."

"Type 07 is defined by He II λ4541 = He I λ4471."

By "definition, the O-type luminosity class V spectra all have strong He II λ4686 absorption; the notation ((f)) signifies that in addition weak Ν III λλ4634-4640-4642 emission is present."

Subgiant O stars
HD 93250 is a class O4 IV(fc).

Giant O stars
The "distinction between giant and supergiant spectra at class O3 may not correspond to a significant physical luminosity difference."

Supergiant O stars
According to SIMBAD, HD 93129 in the image on the right is a double or multiple star, where the two stars are of spectral types: O2If*+O3.5V.

For 03-08f supergiant spectra the "notation f* signifies Ν IV λ4058 emission stronger than Ν III λ4640, which is a characteristic of 03 spectra, while f+ denotes Si IV λλ4089,4116 in emission as well as the Of features."

09-09.7 supergiant spectra [include] "several of types ON and OC. The CNO anomalies are very well-defined [...], particularly by Ν III λ4097 and the ratio Ν III λ4640/C III λ4650."

"The primary defining criterion for the [...] interpolated type 09.7 is He II λ4541 ≈ Si III λ4552 (Walborn 1971c)."

A "luminosity sequence at spectral class 03 [for] the prototype O3 supergiant (Walborn 1971&); [has] the strong, narrow Ν IV λ4058 emission feature and Ν V λλ4604,4620 absorption lines [as] the outstanding spectral characteristics of the type."

"On the main sequence [for spectral classes 06-06.5, 07, and 08] one finds strong He II λ4686 absorption often accompanied by weak Ν III λλ4634-4640-4642 emission, a combination denoted ((f)); in the intermediate luminosity classes the λ4686 absorption weakens and may become neutralized while the Ν III emission strength increases—the (f) category; and finally, the Of supergiants have both of these selective He II and Ν III features strongly in emission."

There is "a correlative increase with luminosity in the strengths of the Si IV absorption lines flanking Ηδ, [...] the Si IV absorption strength is a primary luminosity criterion at types O9-B0".

Neutron stars
It is a type of stellar remnant (a compact star) that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons.

Neutron stars are theorized as the radiation source for anomalous X-ray pulsars (AXPs), binary pulsars, high-mass X-ray binaries, intermediate-mass X-ray binaries, low-mass X-ray binaries (LMXB), pulsars, and soft gamma-ray repeaters (SGRs).

"The [image on the right] shows two observations of [the] neutron star [RX J0822-4300] obtained with the Chandra X-ray Observatory over the span of five years, between December 1999 [on the left] and April 2005 [on the right]. By combining how far it has moved across the sky with its distance from Earth [at about 7,000 light years], astronomers determined the cosmic cannonball is moving at over 3 million miles per hour, one of the fastest moving stars ever observed. At this rate, RX J0822-4300 [at (J2000) RA 08h 23m 08.16s Dec -42° 41' 41.40" in Puppis] is destined to escape from the Milky Way after millions of years, even though it has only traveled about 20 light years so far."

Planetary nebulas
"Hubble astronomers have found an unexpected surprise while surveying more than 100 planetary nebulae in the central bulge of our Milky Way galaxy. Those nebulae that are butterfly-shaped or hourglass-shaped tend to be mysteriously aligned such that their rotation axis is perpendicular to the plane of our galaxy."

"Astronomers have used the NASA/ESA Hubble Space Telescope and ESO's New Technology Telescope to explore more than 100 planetary nebulae in the central bulge of our galaxy. They have found that butterfly-shaped members of this cosmic family tend to be mysteriously aligned — a surprising result given their different histories and varied properties."

"Planetary nebulae are the expanding gaseous shrouds encircling dying stars. A subset of this population has bipolar outflows that align to the star's rotation axis. Such nebulae formed in different places and have different characteristics and so it is a puzzle why they should always point on the same sky direction, like bowling pins set up in an alley."

"All these nebulae formed in different places and have different characteristics. Neither the individual nebulae, nor the stars that formed them, interact with other planetary nebulae. However, a new study by astronomers from the University of Manchester, UK, now shows surprising similarities between some of these nebulae: many of them line up in the sky in the same way. The "long axis" of a bipolar planetary nebula slices though the wings of the butterfly, whilst the "short axis" slices through the body."

"The astronomers looked at 130 planetary nebulae in the Milky Way's central bulge. They identified three different types, and peered closely at their characteristics and appearance. The shapes of the planetary nebula images were classified into three types, following conventions: elliptical, either with or without an aligned internal structure, and bipolar."

"This really is a surprising find and, if it holds true, a very important one, [...] Many of these ghostly butterflies appear to have their long axes aligned along the plane of our galaxy. By using images from both Hubble and the NTT we could get a really good view of these objects, so we could study them in great detail."

"While two of these populations were completely randomly aligned in the sky, as expected, we found that the third — the bipolar nebulae — showed a surprising preference for a particular alignment, [...] While any alignment at all is a surprise, to have it in the crowded central region of the galaxy is even more unexpected."

"Planetary nebulae are thought to be sculpted by the rotation of the star system from which they form. This is dependent on the properties of this system — for example, whether it is a binary [A binary system consists of two stars rotating around their common centre of gravity.], or has a number of planets orbiting it, both of which may greatly influence the form of the blown bubble. The shapes of bipolar nebulae are some of the most extreme, and are thought to be caused by jets blowing mass outwards from the star system perpendicular to its orbit."

"The alignment we're seeing for these bipolar nebulae indicates something bizarre about star systems within the central bulge, [...] For them to line up in the way we see, the star systems that formed these nebulae would have to be rotating perpendicular to the interstellar clouds from which they formed, which is very strange."

"While the properties of their progenitor stars do shape these nebulae, this new finding hints at another more mysterious factor. Along with these complex stellar characteristics are those of our Milky Way; the whole central bulge rotates around the galactic centre. This bulge may have a greater influence than previously thought over our entire galaxy — via its magnetic fields. The astronomers suggest that the orderly behaviour of the planetary nebulae could have been caused by the presence of strong magnetic fields as the bulge formed."

"Researchers suggest that there is something bizarre about star systems within the central hub of our galaxy. They would all have to be rotating perpendicular to the interstellar clouds from which they formed. At present, the best guess is that the alignment is caused by strong magnetic fields that were present when the galactic bulge formed billions of years ago."

"As such nebulae closer to home do not line up in the same orderly way, these fields would have to have been many times stronger than they are in our present-day neighbourhood. Very little is known about the origin and characteristics of the magnetic fields that were present in our galaxy when it was young, so it is unclear how they have changed over time."

"We can learn a lot from studying these objects, [...] If they really behave in this unexpected way, it has consequences for not just the past of individual stars, but for the past of our whole galaxy."

Nova-likes
In nova-like stars the binary system is visible.

These stars exhibit "only irregular small-scale brightness changes or occasional drops in luminosity".

They have accretion disks.

"There exist two sub-classes of nova-like stars, the DQ Herculis stars and the AM Herculis stars, whose white dwarfs possess magnetic fields of appreciable strength which dominate the accretion disk and basically all phenomena related to it."

"The most reliable means of determining white dwarf masses in recurrent novae is dynamically, via radial-velocity and rotational-broadening measurements of the companion star. Such measurements require the system to be both eclipsing and to show absorption features from the secondary star. [For the eclipsing recurrent nova CI Aquilae (CI Aql), the] mass of the white dwarf [is] 1.00 ± 0.14 M⊙ and the mass of the secondary star [is] 2.32 ± 0.19 M⊙. [The] radius of the secondary [is estimated] to be 2.07 ± 0.06 R⊙, implying that it is a slightly evolved early A-type star. The high mass ratio of q = 2.35 ± 0.24 and the high secondary-star mass implies that the mass transfer occurs on a thermal time-scale. [...] CI Aql [may be] rapidly evolving into a supersoft X-ray source, and ultimately may explode as a Type Ia supernova within 10 Myr."

Dwarf novas
"The first known detection of a dwarf nova [U Geminorum] was recorded by Hind (1856), who describes how on 1855 December 15 he discovered a ninth-magnitude star in a field which he knew well and which he had been monitoring for 5 (!) years."

Novas
A nova is a star showing a sudden large increase in brightness and then slowly returning to its original state over a few months.

Type Ia supernovas
"Type Ia supernovae occur in a binary system — two stars orbiting one another. One of the stars in the system must be a white dwarf star, the dense, carbon remains of a star that was about the size of our Sun. The other can be a giant star or even a smaller white dwarf."

"When a runaway thermonuclear explosion rips through a white dwarf star and blows the star to bits, it’s called a type 1a supernova."

"The star doing the exploding is a white dwarf with a fairly standard mass, so the supernova’s brightness is predictable."

"Chemical signatures [gamma-rays are formed from the synthesis of iron, cobalt, and nickel] in the billowing debris cloud revealed that supernova 2014J, as it’s called, [was a white dwarf star, with about 1.4 solar masses] is a type 1a supernova."

“Fusion happens in a flash."

“A thermonuclear flame rips through the white dwarf, fusing carbon into heavier elements with a sudden release of energy that tears the star apart.”

“The Chandrasekhar limit is a red herring. It’s a physical thing that’s very, very important, but for type 1a supernovae, it’s not the most important thing.”

“You have to have the carbon ignition, somewhere near the center of the star.”

There are type “1a supernovae born from white dwarfs that exceeded the Chandrasekhar limit. By a lot. These type 1a explosions, called super-Chandras, are so ridiculously, anomalously bright, and kick out so much radioactive nickel, that they could only come from a bulked up, beefy dwarf star – something with 1.6 or 1.8 or even more than two solar masses of material.“

“The first of these super-Chandras was discovered in 2003; several more have been seen since then, including supernova 2007if and supernova 2009dc.“

"Type Ia supernovae (SNe Ia) are one of the most important tools for observational cosmology because there appears to be a relatively small spread in their peak optical brightness around MV = –19.6 and they can be seen out to cosmological distances (z ~ 1) so they can be used to measure cosmological parameters, e.g. the cosmological constant Λ (Riess et al. 1998; Perlmutter et al. 1999). However, the peak optical brightnesses of SNe Ia are not uniform; they are correlated with the shape of the light curve and vary by about 1mag. Meaningful measurements of cosmological parameters require this variation to be calibrated, e.g. the non-zero value of Λ measured by Perlmutter et al. is required by supernovae at z ~ 0.5 being about 0.3 mag too bright compared with the case of a non-accelerating (Λ = 0) universe. The corrections to peak brightnesses have to be empirical because it is still not yet clear what causes SNe Ia."

"Type Ia supernovae near maximum light show no hydrogen or helium lines but do show strong silicon lines. The absence in the spectrum of the two most common elements in the Universe dramatically reduces the number of potential progenitors, as does their appearance in old stellar populations, e.g. elliptical galaxies. All the most likely models for progenitors feature an accreting white dwarf (Leibundgut 2000; Branch et al. 1995) which ignites carbon in its core either because it has reached the Chandrasekhar mass (1.4 M⊙) or because ignition of accumulated helium causes compression of the core and a so-called 'edge-lit detonation'."

Type Ib supernovas
"On January 9, 2008, Soderberg and Berger were using Swift to observe a supernova known as SN 2007uy in the spiral galaxy NGC 2770, located 90 million light-years from Earth in the constellation Lynx. At 9:33 a.m. EST they spotted an extremely bright 5-minute X-ray outburst in NGC 2770."

"For years we have dreamed of seeing a star just as it was exploding, but actually finding one is a once in a lifetime event."

"Seeing the shock break-out in X-rays can give a direct view of the exploding star in the last minutes of its life and also provide a signpost to which astronomers can quickly point their telescopes to watch the explosion unfold."

Type Ic supernovas
Type Ic supernovas do not present hydrogen or helium lines in their spectra. On the right a type Ic supernova, SN 2009jf, is imaged in the nearby galaxy NGC 7479.

Type Iax supernovas
The inset panel on the right image shows a pair of NASA/ESA Hubble Space Telescope images of the galaxy that were taken before and after the appearance of Supernova 2012Z. The white X-shaped feature at the top of the image of the galaxy marks the location of the supernova.

There “are type 1a supernovae that are ridiculously, anomalously dim. These mini-supernovae, discovered in 2013, are called type 1ax explosions.“

The “progenitor system for a type 1ax [is] called SN 2012Z: A white dwarf, paired with a bright blue helium star companion. The dwarf snagged some material from its gassy blue friend [...] but didn’t gain enough mass to completely explode. So instead of a bang, the star let out a whimper.“

“The remnant of a supernova called PTF 11kx contains shells of gas that can only have come from a large, red giant companion.“

"This image [on the right] shows spiral galaxy NGC 1309. The inset panel shows a pair of NASA/ESA Hubble Space Telescope images of the galaxy that were taken before and after the appearance of Supernova 2012Z. The white X-shaped feature at the top of the image of the galaxy marks the location of the supernova."

"The inset panel from 2013 shows the supernova whilst archival Hubble data from 2005 and 2006 show the progenitor system for the supernova, thought to be a binary system containing a helium star transferring material to a white dwarf that exploded."

"The stellar blast is a member of a unique class of supernova called Type Iax. These supernovae are less energetic, and hence fainter on average, than their well-known cousins Type Ia supernovae, which also originate from exploding white dwarfs in binary systems."

"This galaxy [second down on the right] goes by the name of ESO 162-17 and is located about 40 million light-years away in the constellation of Carina. At first glance this image seems like a fairly standard picture of a galaxy with dark patches of dust and bright patches of young, blue stars. However, a closer look reveals several peculiar features."

"Firstly, ESO 162-17 is what is known as a peculiar galaxy — a galaxy that has gone through interactions with its cosmic neighbours, resulting in an unusual amount of dust and gas, an irregular shape, or a strange composition."

"Secondly, on 23 February 2010 astronomers observed the supernova known as SN 2010ae nestled within this galaxy. The supernova belongs to a recently discovered class of supernovae called Type Iax supernovae. This class of objects is related to the better known Type-Ia supernovae."

"Type Ia supernovae result when a white dwarf accumulates enough mass either from a companion or, rarely, through collision with another white dwarf, to initiate a catastrophic collapse followed by a spectacular explosion as a supernova. Type Iax supernovae also involve a white dwarf as the central star, but in this case it may survive the event. Type Iax supernovae are much fainter and rarer than Type Ia supernovae, and their exact mechanism is still a matter of open debate."

"The rather beautiful four-pointed shape of foreground stars distributed around ESO 162-17 also draws the eye. This is an optical effect introduced as the incoming light is diffracted by the four struts that support the Hubble Space Telescope’s small secondary mirror."

Type II supernovas
From the burst until it fades after some weeks or months a supernova can radiate as much energy as the Sun is expected to emit over its entire life span.

"[T]he equation of state of [hot dense matter] in the infalling core of a star undergoing the collapse ... ultimately may lead to a type II supernova (Lattimer, 1981)."

Type IIn supernovas
The light curve of a typical highly luminous type Ic supernova and a typical highly luminous type IIn supernova are shown on the right to the same scale as more normal supernovas of each type. The light curves are centred on the date of peak luminosity.

"Image [on the right] is centred on the host galaxy, circled, of one of the distant supernovae discovered here. The constant light from the galaxy is removed, revealing the supernova."

"The remnants of two massive stars that exploded about 11 billion years ago have shattered the record for the most distant supernovas in the known universe."

"Before the discovery of these distant supernovas, which belong to a category known as Type IIn, the most distant known supernovas of the same type were 6 billion light-years away, and the most distant of any supernova type were 9 billion light-years away."

"A supernova occurs when a massive star (more than eight times the mass of the sun) dies in a powerful explosion. Type IIn supernovas result from the explosive death of stars that are 50 to 100 times the mass of the sun. These stars shed most of their material before they die, and when they finally explode the remaining material is spewed out into space, plowing through the previously expelled gas. The collisions between the gas clouds make the entire stellar remnant gleam brightly for several years after the star's demise."

"The universe is about 13.7 billion years old, so really we are seeing some of the first stars ever formed."

Type III supernovas
SN 1961I is in NGC 4303 at (J2000) RA 12:22:00.44 DEC +04:28:13.30. According to SIMBAD SN 1961I is located 82" E and 12" S of the center of the apparent host galaxy NGC 4303.

Type IV supernovas
SN 1961F is in NGC 3003 at (J2000) RA 09:48:38.31 DEC +33:25:35.11. According to SIMBAD SN 1961F is located 34" E and 17" N of the center of the apparent host galaxy NGC 3003.

Type V supernovas
"In July of 1961, a star in the spiral galaxy NGC 1058 blew up, but in a very odd fashion. The time to reach its peak brightness was several months as well as a slow decline including a three year plateau. Narrow spectral lines revealed a slow expansion velocity of 2,000 km sec-1."

"The host galaxy is a beautiful face on spiral galaxy and was a tempting target for many observations well before the eruption. This has allowed astronomers to use archival images to determine properties of the parent star."

"The star [at (J2000) RA 02 43 36.42 Dec +37 20 43.6, from SIMBAD] had an absolute magnitude near -12!"

"Most estimates [for its mass] put it in the range of 100 – 200 M⊙."

"A key difference between a supernova and an eruption is the remnant. In the case of a supernova, it is expected that the result would be a neutron star or black hole. If the object were an eruption, even a large one, the star would remain intact. In this vein, many astronomers have also attempted to inspect the remnant. However, due to the shell of gas and dust created in either scenario, imaging the objects has proven to be a challenge. While prior to the event, the culprit stuck out like a sore thumb, the remnant is lost in a haze of other stars."

The Spitzer Space telescope "was unable to conclusively identify a source with sufficient intensity as to be a survivor of the SN 1961V event."

On the lower right is a photograph of the spiral galaxy NGC 1058 obtained on 22 October 1962 with the 200-inch Palomar telescope. SN 1961V is arrowed. The photograph is rotated about 45 degrees clockwise from the image above it on the right.

On the left is a direct tracing of SN 1961V spectrogram obtained with the 200-inch Palomar telescope prime focus spectrograph.

"In addition to the Balmer lines, which are indicated, many emission lines due to He I and Fe II are clearly discernible."

Hypernovas
"Chandra X-ray Image of SN 2006gy. SN 2006gy is the brightest stellar explosion ever recorded and may be a long-sought new type of supernova, according to observations by NASA's Chandra X-ray Observatory and ground-based optical telescopes. This discovery indicates that violent explosions of extremely massive stars were relatively common in the early universe. These data also suggest that a similar explosion may be ready to go off in our own Galaxy."