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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Variable star | 8/10 | https://en.wikipedia.org/wiki/Variable_star | reference | science, encyclopedia | 2026-05-05T13:33:52.261350+00:00 | kb-cron |
Supernovae are the most dramatic type of cataclysmic variable, being some of the most energetic events in the universe. A supernova can briefly emit as much energy as an entire galaxy, brightening by more than 20 magnitudes (over one hundred million times brighter). The supernova explosion is caused by a white dwarf or a star core reaching a certain mass/density limit, the Chandrasekhar limit, causing the object to collapse in a fraction of a second. This collapse "bounces" and causes the star to explode and emit this enormous energy quantity. The outer layers of these stars are blown away at speeds of many thousands of kilometers per second. The expelled matter may form nebulae called supernova remnants. A well-known example of such a nebula is the Crab Nebula, left over from a supernova that was observed in China and elsewhere in 1054. The progenitor object may either disintegrate completely in the explosion, or, in the case of a massive star, the core can become a neutron star (generally a pulsar) or a black hole. Supernovae can result from the death of an extremely massive star, many times heavier than the Sun. At the end of the life of this massive star, a non-fusible iron core is formed from fusion ashes. The mass of this iron core is pushed towards the Chandrasekhar limit until it is surpassed and therefore collapses. One of the most studied supernovae of this type is SN 1987A in the Large Magellanic Cloud. A supernova may also result from mass transfer onto a white dwarf from a star companion in a double star system. The Chandrasekhar limit is surpassed from the infalling matter. The absolute luminosity of this latter type is related to properties of its light curve, so that these supernovae can be used to establish the distance to other galaxies.
==== Luminous red nova ====
Luminous red novae are stellar explosions caused by the merger of two stars. They are not related to classical novae. For a brief period prior to the merger event the two components share a common envelope, which is followed by a mass ejection event that expels the envelope. They have a characteristic red appearance and a lengthy plateau phase following the initial outburst. The luminosity of these transient events lies between those of novae and supernovae, and their evolution lasts from several weeks to months. The galactic rate of these events is 0.2 per year.
==== Novae ====
Classical novae are the result of dramatic explosions, but unlike supernovae these events do not result in the destruction of the progenitor star. They form in close binary systems, with one component being a white dwarf accreting matter from the other ordinary star component, and may recur over periods of decades to centuries or millennia. Novae ignite from the sudden onset of runaway thermonuclear fusion at the base of the accreted matter, which under certain high pressure conditions (degenerate matter) accelerates explosively. They are categorised by their speed class, which range from very fast to very slow, depending on the time for the nova to decrease by 2 or 3 visual magnitudes from peak brightness. Several naked eye novae have been recorded, V1500 Cygni being the brightest in the recent history, reaching 2nd magnitude in 1975. Recurrent novae are defined as having undergone more than one such event in recorded history. These tend to occur on higher mass white dwarfs and have smaller ejecta mass. M31N 2008-12a, a recurrent nova in the Andromeda Galaxy, erupts as often as every 12 months. It has an estimated mass close to the Chandrasekhar limit, and thus is a Type Ia supernova progenitor candidate.
==== Dwarf novae ====
Dwarf novae are double stars involving a white dwarf in which matter transfer between the component gives rise to regular outbursts. They are dimmer and repeat more often than "classical" novae. There are three types of dwarf nova:
U Geminorum stars, which have outbursts lasting roughly 5–20 days followed by quiet periods of typically a few hundred days. During an outburst they brighten typically by 2–6 magnitudes. These stars are also known as SS Cygni variables after the variable in Cygnus which produces among the brightest and most frequent displays of this variable type. Z Camelopardalis stars, in which occasional plateaux of brightness called standstills are seen, part way between maximum and minimum brightness. SU Ursae Majoris stars, which undergo both frequent small outbursts, and rarer but larger superoutbursts. These binary systems usually have orbital periods of under 2.5 hours.
==== Z Andromedae variables ====
These symbiotic binary systems are composed of a red giant and a compact star (typically a white dwarf) enveloped in a cloud of gas and dust. They undergo nova-like outbursts with amplitudes of 1–3 magnitudes, and are caused by accretion rates greater than is needed to maintain stable fusion. The prototype for this class is Z Andromedae.
==== AM CVn variables ====
AM CVn variables are symbiotic binaries where a white dwarf is accreting helium-rich material from either another white dwarf, a helium star, or an evolved main-sequence star. They can undergo complex variations, or at times no variations, with ultrashort periods. The orbital periods of these systems are in the range of 5–65 minutes, with those between 22–44 minutes showing outburst behavior that increases the brightness by 3–4 magnitudes. The last is due to instabilities in the accretion disk.
=== Variable X-ray sources === These optically variable binary systems are sources of intense X-ray emission that do not belong to one of the other variable star categories. One of the components is an accreting compact object: either a white dwarf, neutron star, or stellar mass black hole. A notable example of such a variable X-ray source is Cygnus X-1.
==== DQ Herculis variables ====