A class of novae and cataclysmic variables that have multiple observed eruptions. Their prototype is U Geminorum star. Optically, dwarf nova eruptions have amplitudes of 2-6 mag in V, a duration of a
few to 20 days and a recurrence time-scale of weeks to years. Dwarf novae are thought to be semidetached binary stars consisting of a white dwarf primary accreting via Roche lobe overflow from a companion which is usually a late-type, generally main-sequence star. DN outbursts are usually attributed to the release of gravitational energy resulting from an instability in the accretion disk or by sudden mass transfers through the disk.
A new category of astronomical objects in the solar system introduced in a resolution by the 26th General Assembly of the International Astronomical Union (IAU) on August 24, 2006. The characterizing
properties are as follows: 1) It is in orbit around the Sun; 2) It has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape; 3) It has not 'cleared the neighbourhood' around its orbit; and 4) It is not a satellite of a planet, or other non-stellar body. The property 3 reclassified Pluto from a planet to a dwarf planet because it has not cleared the neighborhood of its orbit (the Kuiper Belt). The largest known dwarf planets are: Eris, Pluto, Ceres, Makemake, and 2015 RR245.
A subtype of dwarf ellipticals (dwarf elliptical galaxy), which are companion to the Milky Way and other similar galaxies. The first example of such objects was discovered by Harlow Shapley (1938) in
the constellation Sculptor. 22 such galaxies are known currently to orbit the Milky Way and at least 36 exist in the Local Group of galaxies. Nearby galaxy clusters such as the Virgo, Fornax, Centaurus, and Coma clusters contain hundreds to thousands of individual dSph galaxies. These galaxies have very low surface brightnesses, as low as only 1% that of the sky background. They are also among the smallest, least luminous galaxies known. Most of the radiation from dSph galaxies is emitted by stars in the optical portion of the electromagnetic spectrum. The lack of strong emission lines, infrared, or radio emission suggests that these galaxies are generally devoid of interstellar medium. The velocities of stars within dSph galaxies are so high that them must be disrupting. However, the bulk of mass in these galaxies might be undetected. Dynamical models that include dark matter do adequately explain the velocity dispersion of the stars in all dSph systems. In the most extreme cases, only 1% of the mass of the galaxy is visible. Many of the Local Group dSph galaxies show evidence for star formation more recent than 10 Gyr.