U Geminorum Stars

Type of variable starStar whose luminosity varies over time. Broadly speaking, variable stars are of two types: (1) stars that are intrinsically variable, that is, their luminosity actually changes, for example because the star periodically swells and shrinks; (2) eclipsing and rotating variables, where the apparent changes in brightness are a perspective effect. displaying sudden and unpredictable increases in brightness, sometimes referred to as dwarf novae. Brightness may increase by as much as 2–5 magnitudes in hours, followed by a slower fading over days to weeks. U Geminorum stars are a binary systemDefinable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with consisting of a white dwarfRemnant of a star with mass <8 Msun. White dwarfs have masses <1.4 Msun (the Chandrasekhar mass) and are supported by electron degeneracy pressure. White dwarfs have radii ~Rearth (<0.02 Rsun) and densities ~105-6 g/cm3. No nuclear fusion or gravitational contraction occurs in white dwarfs, they shine by residual heat. and a red giantGiant and highly luminous red star in the later stages of stellar evolution after it has left the main sequence. These red stars have a relatively cool surface whose core has burned most of its hydrogen. Red giants lose parts of their atmospheres and thus provide new elements into interstellar Click on Term to Read More that has filled its Roche-lobe. Roche-lobe overflow causes matter to fallMeteorite seen to fall. Such meteorites are usually collected soon after falling and are not affected by terrestrial weathering (Weathering = 0). Beginning in 2014 (date needs confirmation), the NomComm adopted the use of the terms "probable fall" and "confirmed fall" to provide better insight into the meteorite's history. If Click on Term to Read More onto an accretion diskDisk-shaped cloud of gas and solids in orbital motion around a central protostar or some other massive central body. Click on Term to Read More around the white dwarf creating a hot spot at the point of impact. Thus, the light from a U Geminorum starSelf-luminous object held together by its own self-gravity. Often refers to those objects which generate energy from nuclear reactions occurring at their cores, but may also be applied to stellar remnants such as neutron stars. has at least four sources: the two stars, the accretionAccumulation of smaller objects into progressively larger bodies in the solar nebula leading to the eventual formation of asteroids, planetesimals and planets. The earliest accretion of the smallest particles was due to Van der Waals and electromagnetic forces. Further accretion continued by relatively low-velocity collisions of smaller bodies in the Click on Term to Read More disk and hot spots on the accretion disk. Sudden increases in brightness probably reflect increases in the amount of material falling onto the white dwarf from the accretion disk.

Image source: http://astronomy.swin.edu.au/cms/astro/cosmos/U/U+Geminorum+Stars.

The causes of sudden increases in accretion rate are not fully understood. One possibility is that sudden increase in the transfer of material from the companion star to the accretion disk causes the disk to collapse onto the white dwarf. Another, preferred, possibility is that the accretion disk can only accumulate a finite amount of gas before becoming unstable and instability causes an abrupt increase in the amount of matter falling onto the white dwarf.