# Stars/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.
"This infrared composite image from NASA's Spitzer Space Telescope shows the Andromeda galaxy, a neighbor to our Milky Way galaxy. The main image (top) highlights the contrast between the galaxy's choppy waves of dust (red) and smooth sea of older stars (blue). The panels below the main image show the galaxy's older stars (left) and dust (right) separately. Spiral galaxies tend to form new stars in their dusty, clumpy arms, while their cores are populated by older stars."
"The Spitzer view also shows Andromeda's dust lanes twisting all the way into the center of the galaxy, a region that is crammed full of stars. In visible-light pictures, this central region tends to be dominated by starlight."
"Astronomers used these new images to measure the total infrared brightness of Andromeda. Because the amount of infrared light given off by stars depends on their masses, the brightness measurements provided a novel method for "weighing" the Andromeda galaxy. According to this method, the mass of the stars in Andromeda is about110 billion times that of the sun, which is in agreement with past calculations. This means the galaxy contains about one trillion stars (because most stars are actually less massive than the sun). For comparison, the Milky Way is estimated to hold about 400 billion stars."
"A small, companion galaxy called NGC 205 is visible above Andromeda. Another companion galaxy called M32 can also been seen below the galaxy."
"The Andromeda galaxy, also known as Messier 31, is located 2.5 million light-years away in the constellation Andromeda. It is the closest major galaxy to the Milky Way, making it the ideal specimen for carefully examining the nature of galaxies. On a clear, dark night, the galaxy can be spotted with the naked eye as a fuzzy blob."
"Andromeda's entire disk spans about 260,000 light-years, which means that a light beam would take 260,000 years to travel from one end of the galaxy to the other. By comparison, the Milky Way is about 100,000 light-years across. When viewed from Earth, Andromeda occupies a portion of the sky equivalent to seven full moons."
"Because this galaxy is so large, the infrared images had to be stitched together out of about 3,000 separate Spitzer exposures. The light detected by Spitzer's infrared array camera at 3.6 and 4.5 microns is sensitive mostly to starlight and is shown in blue and green, respectively. The 8-micron light shows warm dust and is shown in red. The contribution from starlight has been subtracted from the 8-micron image to better highlight the dust structures."
Novae are relatively common in the Andromeda galaxy (Messier 31). Approximately several dozen novae (brighter than about apparent magnitude 20) are discovered in M31 each year. The Central Bureau for Astronomical Telegrams (CBAT) tracks novae in M31, Triangulum Galaxy (M33), and Messier 81 (M81).

## Super soft X-ray sources

There are three SSXSs with bolometric luminosity of ~10$^{38}$ erg/s that are novae: GQ Mus (BB, MW), V1974 Cyg (WD, MW), and Nova LMC 1995 (WD). "Apparently, as of 1999 the orbital period of Nova LMC 1995 if a binary was not known."

## Luminous red novas

A luminous red nova (abbr. LRN, pl. luminous red novae, pl.abbr. LRNe) is a stellar explosion thought to be caused by the merger of two stars. They are characterised by a distinct red colour, and a light curve that lingers with resurgent brightness in the infrared. Luminous red novae are not to be confused with standard novae, explosions that occur on the surface of white dwarf stars. The visible light lasts for weeks or months, and is distinctively red in colour, becoming dimmer and redder over time. As the visible light dims, the infrared light grows and also lasts for an extended period of time, usually dimming and brightening a number of times. Some astronomers believe it to be premature to declare a new class of stellar explosions based on such a limited number of observations. For instance, Pastorello et al. 2007 explained that the event may be due to a type II-p supernova and Todd et al. 2008 pointed out that supernovae undergoing a high level of extinction will naturally be both red and of low luminosity.
The first confirmed luminous red nova was the object M85 OT2006-1, in the galaxy Messier 85, first observed during the Lick Observatory Supernova Search, and its difference from known explosions such as novae and thermal pulses, and announced luminous red novae as a new class of stellar explosion.
V1309 Scorpii is a luminous red nova that followed the merger of a contact binary in 2008.
In January 2015, a luminous red nova was observed in the Andromeda Galaxy.
On February 10, 2015, a luminous red nova, known as M101 OT2015-1 was discovered in the Pinwheel Galaxy.

## Dwarf novas

The top image on the right is a 2 minute exposure of dwarf nova HT Cas as seen with a 24" telescope on 2010 November 12. HT Cas was seen to outburst on 2010 November 2.43 at magnitude 12.4-12.9. The star ~30 arcseconds south is magnitude 13.9. HT Cas is about magnitude 13.4 in this image. The last well-observed outburst of this star was on 2008 January 10.
"The first known detection of a dwarf nova [U Geminorum, first light curve on the right by the American Association of Variable Star Observers, AAVSO] 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."
A U Geminorum-type variable star, or dwarf nova is a type of cataclysmic variable star consisting of a close binary star system in which one of the components is a white dwarf that accretes matter from its companion.
The mass transfer from the donor star is less than this increased flow through the disc, so the disc will eventually drop back below the critical temperature and revert to a cooler, duller mode.
Dwarf novae are distinct from classical novae in other ways: (1) their luminosity is lower, and (2) they are typically recurrent on a scale from days to decades, (3) the luminosity of the outburst increases with the recurrence interval as well as the orbital period; recent research with the Hubble space telescope suggests that the latter relationship could make dwarf novae useful standard candles for measuring cosmic distances.
"Another dwarf nova (we now call it SS Cygni) was detected in 1886; by 1918 the number had increased to eight (Müller and Hartwig, 1918)".
There are three subtypes of U Geminorum star (UG):
* SS Cygni stars (UGSS, second light curve down on the right), which increase in brightness by 2-6 apparent magnitude (mag) in V band in 1–2 days, and return to their original brightnesses in several subsequent days.
* SU Ursae Majoris stars (UGSU, third light curve down on the right shows eclipsing dwarf nova HT Cassiopeiae during outburst, showing eclipses and superhumps), which have brighter and longer "supermaxima" outbursts, or "super-outbursts," in addition to normal outbursts. Varieties of SU Ursae Majoris star include ER Ursae Majoris stars and WZ Sagittae stars (UGWZ).
* Z Camelopardalis stars (UGZ, fourth light curve down on the right), which temporarily "halt" at a particular brightness below their peak.
"Currently we know of some 200 dwarf novae and of several hundred nova-like stars and novae."
"Joy pointed out (Joy 1954b) that the spectra of the dwarf novae SS Cyg and RU Peg were rather similar to those of AE Agr and that a physical relationship seemed possible. [In] intervals of about one year AE Aqr underwent outburst-like brightness increases, by one to two magnitudes (Zinner, 1938), that resemble dwarf nova outbursts [...] the explosive U Geminorum requires [...] two stars in a short-period orbit as a necessary, though not sufficient, condition."
EY Cyg is a dwarf nova.
"The most spectacular events in the lives of dwarf novae are the outbursts."
"Instabilities on the surface of the white dwarf lead to nova eruptions."
Dwarf novae are distinct from classical novae in other ways; their luminosity is lower, and they are typically recurrent on a scale from days to decades.
The luminosity of the outburst increases with the recurrence interval as well as the orbital period.

## GQ Muscae

The main plot shows the decline from the 1983 eruption, plotted with data from the AAVSO, Whitelock et al. and Liller. The inset plot (adapted from Narloch et al.) shows the post-eruption variability seen in 1992.
11$^h$ 52$^m$ 02.4285$^s$, -67° 12′ 20.9911″
Apparent visual magnitude = 7.2 Max. 21 Min.
Proper motion RA = −5.056 ± 0.451
Proper motion Dec = 1.193±0.324
Parallax = 0.4702 ± 0.2193
Distance in parsecs = 2480$^{+3780}$ $_{-300}$
Names: American Association of Variable Star Observers (AAVSO) 1147-66, Nova Mus 1983, Gaia DR2=5236081560713688448.
GQ Muscae, also known as Nova Muscae 1983 is a nova in the constellation Musca, which was discovered by William Liller at 03:20 UT on 18 January 1983. At the time of its discovery it was a magnitude ≈7.2 object, and it subsequently faded.
GQ Muscae is a binary star system composed of a white dwarf and small star, the donor star, that is about 10% as massive as the Sun. The two orbit each other every 1.4 hours. The white dwarf accumulates material from the donor star until a runaway nuclear thermonuclear reaction erupts, as it did in 1983. GQ Muscae was the first nova from which X-rays were detected.

## V1974 Cygni

Constellation: Cygnus
Epoch=J2000.0, 20$^h$ 30$^m$ 31.61$^s$, +52° 37′ 51.3″
Apparent magnitude visual = 4.3 (max) >21 (min)
Distance (parsec) = 1631$^{261}$ $_{131}$.
Variable Neon nova.
Names: Nova Cyg 1975, AAVSO 2027+52, V1974 Cyg, 2MASS J20303161+5237513
V1974 Cygni or Nova Cygni 1992 was a nova, visible to the naked eye, in the constellation Cygnus. It was discovered visually with 10×50 binoculars on February 19, 1992, by Peter Collins, an amateur astronomer living in Boulder, Colorado. At that time he first noticed it, it had an apparent magnitude of 7.2. Nine hours later he saw it again, and it had brightened by a full magnitude. For this discovery Collins was awarded the AAVSO Nova Award in 1993. The nova reached magnitude 4.4 at 22:00 UT on 22 February 1992. Images from the Palomar Sky Survey taken before the nova event showed identified a possible precursor which had photographic magnitudes of 18 (blue light) and 17 (red light), but the identification of the precursor is not firm.
V1974 Cygni declined from peak brightness by three magnitudes in 43 days, making it a "fast" nova. Its light curve is classified as type P (Plateau), and it may be a recurrent nova.
In 1995, V1974 Cygni was observed with the Very Large Array at 1.49, 4.9, 8.4, 14.9 and 22.5 GHz. It was also studied with the Hubble Space Telescope instrument the High Speed Photometer. The instrument recorded a short amount of ultraviolet photometry. The nova was also observed in the far-ultraviolet by Voyager 2. It was observed, but not detected, with the Compton Gamma Ray Observatory. It was the first nova to be observed throughout the electromagnetic spectrum, from radio waves to γ-rays.
All novae are binary stars, with a "donor" star orbiting a white dwarf. The two stars are so close to each other that matter is transferred from the donor to the white dwarf. In the case of V1974 Cygni, the binary's orbital period is 1 hour, 57 minutes. The nova has an oxygen-neon-magnesium white dwarf primary, making it a neon nova. Estimates of the mass of the white dwarf range from 0.98 M$_⦿$ to 1.12 M$_⦿$, and it is estimated to be acquiring 3.2 × 10$^{−10}$ M$_⦿$ yr$^{−1}$ of material from the donor star.
V1974 Cygni has a nova remnant shell which has been observed several times with the Hubble Space Telescope, as well as with the Infrared Space Observatory. The shell is nearly circular, and its radius as of 10 February 1998 was 0.983 arc seconds. It is expanding at a rate of about 0.26 milli arc seconds per day. The remnant was also imaged in 6 cm radio waves with the MERLIN interferometer.

## Recurrent novas

A recurrent nova is produced by a white dwarf star and a red giant circling about each other in a close orbit. About every 20 years, enough material from the red giant builds up on the surface of the white dwarf to produce a thermonuclear explosion. The white dwarf orbits close to the red giant, with an accretion disc concentrating the overflowing atmosphere of the red giant onto the white dwarf. If the white dwarf accretes enough mass to reach the Chandrasekhar limit, about 1.4 M$_⦿$, it may explode as a Type Ia supernova.

## RS Ophiuchi

RS Ophiuchi is a system consisting of a white dwarf with a red giant companion in a binary system with an orbital period of around 454 days.
| Date of eruption | Years since last eruption |
| 1898 | - |
| 1907 | 9 |
| 1933 | 26 |
| 1945 | 12 |
| 1958 | 13 |
| 1967 | 9 |
| 1985 | 18 |
| 2006 | 21 |
| 2021 | 15 |
Though the 1907 eruption was not observed during outburst, measurements of a dip in brightness from archival observations suggests that RS Oph underwent an eruption in early 1907 during a time when it was obscured by the sun.
The 1945 eruption was also inferred from archival data after the outburst as a result of obscuration from the sun during the peak brightness, but this eruption is more certain than that in 1907, as the tail of the eruption was also observed.
On 12 February 2006 a new outburst occurred, reaching magnitude 4.5, notably observed with the VLTI discovering an elongated fireball as early as 5.5 days after the explosion.
On 8 August 2021, the Brazilian amateur astronomer Alexandre Amorim, from Florianópolis, Brazil detected a new outburst of RS Oph at 21:55 UT and sent a notification to American Association of Variable Star Observers (AAVSO). The outbust was confirmed by an independent observation of Keith Geary from Ireland at 22:20 UT. The Fermi Gamma-Ray Space Telescope corroborated optical observations made by Amorim and Geary of a new outburst associated with RS Oph, with an estimated visual magnitude of 5.0. It reached a peak visual magnitude of approximately 4.6 the following day.

## U Scorpio

U Sco, a recurrent nova as of 1999 unobserved by ROSAT, is a WD (74-76 eV), L$_{bol}$ ~ (8-60) x 10$^{36}$ erg/s, with an orbital period of 1.2306 d.

## V1017 Sagittarius

V1017 Sgr is a recurrent nova.

## 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.
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.
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.

## Kilonovas

Def. a "type of supernova that is underluminous, caused by the merger of two neutron stars" is called a kilonova.
"The presence of [hydrogen-rich] debris [left behind in the remnants of a Type Ia supernova known as SN 2015cp] means that the companion was either a red giant star or similar star that, prior to making its companion go supernova, had shed large amounts of material.”
"On 17 August 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo Interferometer both detected gravitational waves from the collision between two neutron stars. Within 12 hours observatories had identified the source of the event within the lenticular galaxy NGC 4993, shown in this image [on the right] gathered with the NASA/ESA Hubble Space Telescope. The associated stellar flare, a kilonova, is clearly visible in the Hubble observations. This is the first time the optical counterpart of a gravitational wave event was observed."
"Hubble observed the kilonova gradually fading over the course of six days, as shown in these observations taken in between 22 and 28 August (insets)."
"These images [on the left] taken by the NASA/ESA Hubble Space Telescope reveal a new type of stellar explosion produced from the merger of two compact objects."
"Hubble spotted the outburst while looking at the aftermath of a short-duration gamma-ray burst, a mysterious flash of intense high-energy radiation that appears from random directions in space. Short-duration blasts last at most a few seconds. They sometimes, however, produce faint afterglows in visible and near-infrared light that continue for several hours or days and help astronomers to pinpoint the exact location of the burst."
"In the image on the left, the galaxy in the centre produced the gamma-ray burst, designated GRB 130603B. A probe of the galaxy with Hubble's Wide Field Camera 3 on June 13, 2013, revealed a glow in near-infrared light at the source of the gamma-ray burst, shown in the top right image. When Hubble observed the same location on July 3, the source had faded, shown in the below right image. The fading glow provided key evidence that it was the decaying fireball of a new type of stellar blast called a "kilonova"."
"Kilonovas are about 1000 times brighter than a nova, which is caused by the eruption of a white dwarf. But they are 1/10th to 1/100th the brightness of a typical supernova, the self-detonation of a massive star."

## Hypernovas

A hypernova is a type of stellar explosion with a luminosity 10 or more times higher than that of standard supernovae.
Gamma ray bursts (GRBs) were initially detected on July 2, 1967 by the Vela satellites which were capable of detecting explosions behind the moon, but the satellites detected a signal unlike that of a nuclear weapon signature, nor could it be correlated to solar flares.
In February 1997, Dutch-Italian satellite BeppoSAX was able to trace GRB 970508 to a faint galaxy roughly 6 billion light years away. From analyzing the spectroscopic data for both the burst and the galaxy, it was concluded that a hypernova was the likely cause and that same year, hypernovae were hypothesized in greater detail.
The first hypernova observed was SN 1998bw, with a luminosity 100 times higher than a standard Type Ib.
The first confirmed superluminous supernova connected to a gamma ray burst was when GRB 030329 illuminated the Leo constellation. SN 2003dh represented the death of a star 25 times more massive than the sun, with material being blasted out at over a tenth the speed of light.
In June 2018, AT2018cow was detected and found to be a very powerful astronomical explosion, 10 – 100 times brighter than a normal supernova.
Today, it is believed that stars with M ≥ 40 M$_☉$ produce superluminous supernovae.
SLSNe events use a separate classification scheme to distinguish them from the conventional type Ia supernova, Type Ib and Ic supernovae, and type II supernovae.
(1) Hydrogen-rich SLSNe are classified as Type SLSN-II, with observed radiation passing through the changing opacity of a thick expanding hydrogen envelope; (2) Most hydrogen-poor events are classified as Type SLSN-I, with its visible radiation produced from a large expanding envelope of material powered by an unknown mechanism; and (3) A third less common group of SLSNe is also hydrogen-poor and abnormally luminous, but clearly powered by radioactivity from $^{56}$ Ni.
Increasing number of discoveries find that some SLSNe do not fit cleanly into these three classes, so further sub-classes or unique events have been described; so many or all SLSN-I show spectra without hydrogen or helium but have lightcurves comparable to conventional type Ic supernovae, and are now classed as SLSN-Ic. PS1-10afx is an unusually red hydrogen-free SLSN with an extremely rapid rise to a near-record peak luminosity and an unusually rapid decline. PS1-11ap is similar to a type Ic SLSN but has an unusually slow rise and decline.
Various light curves including hypernovas are compared to super novas in the diagram on the right.
A good example of a collapsar SLSN is SN 1998bw, which was associated with the gamma-ray burst GRB 980425.
The "origin of 'long gamma ray bursts', the most powerful electromagnetic phenomena in the universe, [which] release as much energy in a second or so as the Sun will release over its entire lifetime [...] come from the visible surface of high-speed jets, emitted as massive stars tear themselves apart. [...] It’s known as 'photospheric' emission, where the rays come from the surface of the jets as they expand."
"To us [supercomputer simulations of how the gamma rays were released] strongly suggests that photospheric emission is the emission mechanism of gamma-ray bursts."

## Symbiotic novas

Symbiotic novae are slow irregular eruptive variable stars with very slow nova-like outbursts with an amplitude of between 9 and 11 magnitudes. The symbiotic nova remains at maximum for one or a few decades, and then declines towards its original luminosity. Variables of this type are double star systems with one red giant, which probably is a mira variable, and one white dwarf, with markedly contrasting spectra and whose proximity and mass characteristics indicate it as a symbiotic star. The red giant fills its Roche lobe so that matter is transferred to the white dwarf and accumulates until a nova-like outburst occurs, caused by ignition of thermonuclear fusion. The temperature at maximum is estimated to rise up to 200,000 K, similar to the energy source of novae, but dissimilar to the dwarf novae. The slow luminosity increase would then be simply due to time needed for growth of the ionization front in the outburst.
It is believed that the white dwarf component of a symbiotic nova remains below the Chandrasekhar limit, so that it remains a white dwarf after its outburst.
One example of a symbiotic nova is V1016 Cygni, whose outburst in 1971–2007 clearly indicated a thermonuclear explosion. Other examples are HM Sagittae and RR Telescopii.
"Though typical symbiotic systems consist of a M giant and a white dwarf companion, systems containing a G or K giant ("yellow symbiotic") are known as well."