Black dwarfA black dwarf is a theoretical stellar remnant, specifically a white dwarf that has cooled sufficiently to no longer emit significant heat or light. Because the time required for a white dwarf to reach this state is calculated to be longer than the current age of the universe (13.8 billion years), no black dwarfs are expected to exist in the universe at the present time. The temperature of the coolest white dwarfs is one observational limit on the universe's age.
Chronology of the universeThe chronology of the universe describes the history and future of the universe according to Big Bang cosmology. Research published in 2015 estimates the earliest stages of the universe's existence as taking place 13.8 billion years ago, with an uncertainty of around 21 million years at the 68% confidence level. For the purposes of this summary, it is convenient to divide the chronology of the universe since it originated, into five parts.
Fixed starsIn astronomy, the fixed stars (stellae fixae) are the luminary points, mainly stars, that appear not to move relative to one another against the darkness of the night sky in the background. This is in contrast to those lights visible to naked eye, namely planets and comets, that appear to move slowly among those "fixed" stars. The fixed stars includes all the stars visible to the naked eye other than the Sun, as well as the faint band of the Milky Way.
T Tauri starT Tauri stars (TTS) are a class of variable stars that are less than about ten million years old. This class is named after the prototype, T Tauri, a young star in the Taurus star-forming region. They are found near molecular clouds and identified by their optical variability and strong chromospheric lines. T Tauri stars are pre-main-sequence stars in the process of contracting to the main sequence along the Hayashi track, a luminosity–temperature relationship obeyed by infant stars of less than 3 solar masses () in the pre-main-sequence phase of stellar evolution.
Carbon starA carbon star (C-type star) is typically an asymptotic giant branch star, a luminous red giant, whose atmosphere contains more carbon than oxygen. The two elements combine in the upper layers of the star, forming carbon monoxide, which consumes most of the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds, giving the star a "sooty" atmosphere and a strikingly ruby red appearance. There are also some dwarf and supergiant carbon stars, with the more common giant stars sometimes being called classical carbon stars to distinguish them.
Stellar collisionA stellar collision is the coming together of two stars caused by stellar dynamics within a star cluster, or by the orbital decay of a binary star due to stellar mass loss or gravitational radiation, or by other mechanisms not yet well understood. Astronomers predict that events of this type occur in the globular clusters of our galaxy about once every 10,000 years. On 2 September 2008 scientists first observed a stellar merger in Scorpius (named V1309 Scorpii), though it was not known to be the result of a stellar merger at the time.
Convection zoneA convection zone, convective zone or convective region of a star is a layer which is unstable due to convection. Energy is primarily or partially transported by convection in such a region. In a radiation zone, energy is transported by radiation and conduction. Stellar convection consists of mass movement of plasma within the star which usually forms a circular convection current with the heated plasma ascending and the cooled plasma descending. The Schwarzschild criterion expresses the conditions under which a region of a star is unstable to convection.
ChromosphereA chromosphere ("sphere of color") is the second layer of a star's atmosphere, located above the photosphere and below the solar transition region and corona. The term usually refers to the Sun's chromosphere, but not exclusively. In the Sun's atmosphere, the chromosphere is roughly in height, or slightly more than 1% of the Sun's radius at maximum thickness. It possesses a homogeneous layer at the boundary with the photosphere. Hair-like jets of plasma, called spicules, rise from this homogeneous region and through the chromosphere, extending up to into the corona above.
Red clumpThe red clump is a clustering of red giants in the Hertzsprung–Russell diagram at around 5,000 K and absolute magnitude (MV) +0.5, slightly hotter than most red-giant-branch stars of the same luminosity. It is visible as a denser region of the red-giant branch or a bulge towards hotter temperatures. It is prominent in many galactic open clusters, and it is also noticeable in many intermediate-age globular clusters and in nearby field stars (e.g. the Hipparcos stars).
Solar radiusSolar radius is a unit of distance used to express the size of stars in astronomy relative to the Sun. The solar radius is usually defined as the radius to the layer in the Sun's photosphere where the optical depth equals 2/3: is approximately 10 times the average radius of Jupiter, 109 times the radius of the Earth, and 1/215th of an astronomical unit, the approximate distance between Earth and the Sun. The solar radius to either pole and that to the equator differ slightly due to the Sun's rotation, which induces an oblateness in the order of 10 parts per million.
