Dwarf Nova

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UV image of the dwarf nova Z camelopardalis
Artist's impression of a dwarf nova

Dwarf novae ( U-Geminorum stars ) belong to the class of cataclysmic binary star systems and thus to the variable . They are characterized by multiple eruptions in which the apparent brightness of the star changes briefly by around 2 to 8  mag . The term dwarf nova is used for both the astronomical event of the increase in brightness and the stellar class in which these events take place.

Dwarf novae occur like classical novae in binary star systems in which a white dwarf accretes matter from a companion star . The difference lies in the breakout mechanism:

  • In classical novae, a thermonuclear reaction , ie the explosive onset of hydrogen burning on the surface of the white dwarf, leads to an increase in brightness.
  • In the case of dwarf novae, on the other hand, the outbreaks result from increases in brightness in the accretion disk around the white dwarf.

The interval between two outbreaks in dwarf novae is between a few days and a few years, the duration of an outbreak is between two and twenty days; it correlates with the interval length.



A dwarf nova consists of a white dwarf around whom a companion circles on a narrow path , usually a red dwarf . Since this has exceeded its Roche limit volume , it loses mass , which flows over the inner Lagrange point in the direction of the white dwarf. Due to the conservation of angular momentum , it forms an accretion disk around the white dwarf, which dominates the radiation of the dwarf nova in the optical spectral range . The matter orbiting the white dwarf and loses because of the viscosity in the disk slow their momentum . As a result, after a while it falls on the surface of the white dwarf.

Breakout Mechanism

The viscosity of the matter in the accretion disk can have two values:

  • a high one, at which the friction increases and as a result the disk emits more radiation (eruption) and more matter falls on the white dwarf,
  • a low one, in which more matter is stored in the accretion disk than reaches the white dwarf (resting phase).

Magnetorotation instability is assumed to be the cause of the bistable state of the accretion disk (also called accretion disk instability ) .

The development of the accretion disk can be observed in dwarf novae with variable coverage:

  • During a breakout, the radius of the disk increases by up to 30%. This is a consequence of the higher viscosity of the plasma in the accretion disk, which leads to an increase in temperature and thus to an expansion. This widens the minimum brightness that occurs when the companion covers the accretion disk.
  • In the dormant phase, the width of the minimum decreases continuously until a new outbreak begins.

The bright spot at the point where the flow of matter from the companion hits the accretion disk becomes lighter during the eruptions. This is probably a feedback , according to which the more intensely radiating accretion disc heats the front of the companion, which then expands a little and releases more matter.

Whether the mass of white dwarfs in dwarf novae increases due to accretion is controversial, since matter is expelled again during nova outbreaks. If the mass increases, the white dwarfs could cross the Chandrasekharsche limit mass and explode as a type Ia supernova .

Relation to nova eruptions

Although novae and dwarf novae should occur on the same binary stars, studies of historical light curves of novae before and after their eruptions have never shown dwarf nova outbreaks. Instead, they always show a nova-like light change .

This apparent contradiction is explained by the hibernation scenario:

  • During the millennia before a nova outbreak, the rate of mass transfer to the white dwarf is so high that the accretion disk is permanently in its high status and, as a nova-like variable, resembles a dwarf nova in constant outbreak (“hibernation”).
  • If the accumulated hydrogen ignites on the white dwarf, this heats up the companion star, and the mass transfer rate remains high enough even after the eruption to make the binary star system appear as a nova-like variable.
  • Only a few centuries after the nova eruption does the mass transfer rate drop so much that the accretion disk can at least temporarily fall back into its resting state, which corresponds to the Z- Cam subgroup of dwarf novae. This star class should therefore be the best candidate for a search for nova remnants around dwarf novae; Such remains arise when part of the accreted material is thrown off in classical novae. In fact, so far only faint nova remnants have been found around two Z-Cam stars, namely around Z Cam and AT  Cnc . Their rates of expansion each suggest an eruption more than 1000 years ago.

The same cataclysmic variables can show both novae and dwarf nova outbreaks, e.g. B. GK Persei .


X-rays could be detected from all nearby dwarf novae . The radiation is weak in the resting phases and increases by a factor of 100 during the outbreaks. The increase in X-ray radiation lags behind that of optical radiation by a few hours.

The source of the high-energy X-rays seems to be the boundary layer between the accretion disk and the white dwarf. The radiation arises from the fact that in this boundary layer the matter in the accretion disk has to be decelerated from the Kepler speed to the much slower rotation speed of the white dwarf. According to the accretion disk instability model, the viscosity increases somewhere in the disk and this change propagates across the disk. When the increased viscosity and thus the increased throughput of matter reaches the boundary layer, the X-ray radiation increases.

A small part of the X-rays can arise from thermal radiation from the white dwarf, which is heated up by accretion.

Regardless of the inclination at which the dwarf nova is viewed from Earth, many X-ray spectra show signs of circumstellar absorption .

Parallel to this observation in the area of ​​X-rays, P-Cygni profiles can occur in the optical . This is interpreted as a sign of a disk wind analogous to a stellar wind . An outflow of matter from an accretion disk has also been suspected in other objects such as X-ray binary stars , T-Tauri stars , etc.

With a high accretion rate, permanent hydrogen burning can occur on the surface of the white dwarf. Since there is only a thin atmosphere over the zone with the thermonuclear reactions according to the Bethe-Weizsäcker cycle , extremely soft X-rays emerge. Because of this low-energy X-ray radiation, these systems are also referred to as super-soft X-ray sources . These are classic novae in eruption over a period of at least decades.


In the outbursts of some dwarf novae and nova-like ones, sinusoidal fluctuations in brightness of low amplitude (up to 0.02%) and with cycle times of 5 to 40 seconds were detected. These variations are as a dwarf Nova oscillations (engl. Dwarf nova oscillation ), respectively. Each star has its own characteristic frequency , which, like the amplitude, is subject to large fluctuations during an outbreak and between different eruptions.

The dwarf nova oscillations were detected in the optical and ultraviolet range as well as in the range of soft X-rays. Due to the high energy of the X-rays, the origin of the dwarf nova oscillations is believed to be in the vicinity of the white dwarf and could be caused by a modulation of the accretion by a weak magnetic field of the white dwarf.

A similar phenomenon is represented by the quasi-periodic oscillations which have been observed in some cataclysmic variables parallel to the dwarf nova oscillations . The difference between the two brightness fluctuations lies in the lower period stability of the quasi-periodic oscillations and in the length of the period , which in the case of the quasi-periodic oscillations is of the order of a few 100 seconds. The quasi-periodic oscillations of the dwarf novae may correspond to those of the X-ray binary stars .


The General Catalog of Variable Stars has the following structure:

  • U-Geminorum Stars (UG): these stars form the super-category of dwarf novae named after the variable star U Geminorum . However, the star itself is also included in the subgroup of the SS Cygni stars and together with it forms the prototype of this subgroup.
    • SS-Cygni-Stars (UGSS): This subgroup of dwarf novae shows pronounced phases of rest in the smallest light, which are almost regularly interrupted by outbursts. The increase to the maximum is faster than the decrease back to the brightness at rest.
    • Z-Camelopardalis-Stars (UGZ): The standstills in the smallest light are very short. Periods of time with a change in brightness are temporarily interrupted by intervals with almost constant light. The standstill begins with a decrease in brightness from the maximum and ends at a minimum.
    • SU Ursae Majoris Stars (UGSU): In this subgroup, so-called super outbreaks occur in addition to normal ones. These are around 0.7 mag lighter and take three to five times longer. So-called superhumps also occur. These are small changes in brightness superimposed on the maximum with a period that is a few percent longer than the period of the binary star system . Example: VY Aqr .
      • TOAD (Tremendous Outburst Amplitude Dwarf novae): The difference to the SU-UMa stars is the lack of normal bursts. Only super-eruptions are observed in the dwarf novae, which are also known as " WZ sagittae stars". In the Variable Star Index (VSX), the WZ Sagittae stars (UGWZ) are considered a subgroup of the SU Majoris stars.

Other groups of stars show dwarf nova outbreaks, but most of them are assigned to the novae:

  • UX-UMa stars: The nova-like ones are dwarf novae in permanent outburst and show absorption lines in the spectrum .
  • RW Tri-Stars: These nova-like double stars are dwarf novae in permanent eruption and they show emission lines in the spectrum .
  • VY-Scl stars: These dwarf novae are similar to the UX-UMa stars. They sometimes show a minimum and return to the maximum after a short time. They are therefore also called "Anti-Novae".


The classification of dwarf novae is not always clear. In 1985 the prototype of the normal dwarf novae, U Geminorum , showed a super maximum with an outbreak duration of 39 instead of 12 days and the occurrence of superhumps.

The super-bursts of the SU-Ursae Maioris stars and TOADs require a different mechanism than that of normal maxima. In doing so, all super-eruptions develop from a failed normal eruption and these systems have an orbital period of less than 2 hours. During a super-eruption, up to 80% of the mass stored in the accretion disk is transferred to the white dwarf compared to a few percent in the case of the U-Gem stars. Three models are discussed in the literature:

  • A normal eruption leads to a warming of the companion, which then loses more mass to the accretion disk and this starts the super eruption.
  • During a normal eruption, the accretion disk grows to such an extent that there is increased friction at the outer edge of the disk under the influence of a 3: 1 resonance with the companion. This leads to an increased flow of matter towards the white dwarf and thus to a super-eruption.
  • According to the third model, a super-eruption is the result of a normal variation in eruptions. The SS-Cyg and U-Gem prototypes also show a change between narrow and wide maxima. The difference between the two types is the course of the warming front, which runs from the inside out in the case of narrow eruptions and from the outside in in the case of wide eruptions. Because the wide outbursts are rarer in SU-UMa stars, they show up as super outbursts. Continuous observations with the Kepler satellite at the SU-UMa stars V1504 Cyg and V344 Lyr support model 2, which is also known as the "thermal tidal instability model".

Occurrence in star catalogs

The General Catalog of Variable Stars currently lists a little over 400 stars (almost 1% of the stars in this catalog), which are divided into a subgroup of dwarf novae. Of these, not quite 200 are assigned the abbreviation UG for U-Geminorum stars, about 120 with UGSU the SU Ursae Majoris stars and about 80 with UGSS the SS Cygni stars. The Z-Camelopardalis stars form the smallest subgroup with about 25 pieces. In addition to this group, there are a little over 100 suspected dwarf novae.

Related outbreaks

The accretion disk instability model is used not only to describe the outbreaks of dwarf novae, but also for the following phenomena:

  • With X-ray novae or soft X-ray transits , matter falls from an accretion disk onto a compact star, which is probably a black hole . Since the compact companion has a smaller radius and a greater gravitational potential than a white dwarf, the matter can circle the black hole in narrower orbits and reach higher temperatures. This is why most of the radiation is observed in the X-ray range during soft X-ray transits . The X-ray novae, like the dwarf novae, receive matter from a companion in a binary star system that has exceeded its Roche limit .
  • The AM Canum Venaticorum stars correspond in many ways to the dwarf novae. Only the orbital period of the erupting binary star systems is shorter with 20 to 40 minutes, since the companion of the white dwarf is a partially degenerate helium star. The dwarf nova-like outbreaks occur in an accretion disk around the white dwarf, which consists mainly of helium. In addition, superhumps have also been observed in short-period AM-CVn systems with periods of rotation between 5 and 20 minutes.
  • In the case of the FU Orionis stars , the accretion disk is fed by a protostellar cloud. With these young single stars, too, the disk can become overloaded, which lights up when there is an increased mass transfer. Since the protostellar accretion disks have a larger diameter than the disks around a white dwarf in a cataclysmic binary star system, the eruptions last for up to several decades.


Web links

Commons : Zwergnova  - collection of images, videos and audio files

Individual evidence

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