bipolar flows (392)

the narrow columns of high-speed gas ejected by a protostar in two opposite directions.

Crab Nebula (404)

a supernova remnant in the constellation Taurus. Astronomers in ancient China and the Far East saw the supernova explode in A.D. 1054. A pulsar lies in the middle of the nebula.

Degenerate (397)

the process of which a low-mass star compresses its gas atoms so closely that the gas atoms no longer behave like an ordinary gas.

helium flash (397)

the beginning of helium fusion in a low-mass star. The fusion begins explosively and causes a major readjustment of the star’s structure.

instability strip (399)

a region in the H-R diagram in which stars pulsate.

interstellar cloud (387)

a cloud of gas and dust in between the stars. Such clouds may be many light years in diameter.

main-sequence lifetime (395)

the time a star remains a main sequence star, fusing hydrogen into helium in its core.

nucleosynthesis (402)

the formation of elements, generally by the fusion of lighter elements into heavier ones. For example, the formation of carbon by the fusion of three helium nuclei.

period-luminosity relation (400)

a relation stating that the longer the period of a pulsating variable star, the more luminous it is.

planetary nebulas (401)

a shell of gas ejected by a low-mass star late in its evolutionary lifetime. Planetary nebulas appear as a glowing gas ring around a central star.

protostar (390)

a star still in its formation stage.

shell source (396)

a region in a star where the nuclear energy generation occurs around the core rather than in it.

supernova (404)

an explosion marking the end of some star’s evolution. Astronomers identify two main kinds of supernova: Type I and II. Type I occurs in a binary system in which one star is a white dwarf. The explosion is triggered when mass from a companion star falls onto the white dwarf raising its mass above the Chandrasekhar limit and causing the star to collapse. Collapse heats the white dwarf so that its carbon and oxygen fuse explosively, destroying the star and leaving no remnant. Type II probably occurs when a massive star’s iron core collapses. A Type II supernova leaves either a neutron star or a black hole, depending on the mass of the collapsing core.

supernova remnant (404)

the debris ejected from a star when it explodes as a supernova. Typically this material is hot gas, expanding away from the explosion at thousands of kilometers or more per second.

T Tauri stars (393)

a type of extremely young star that varies erratically in its light output.

triple alpha process (397)

the fusion of three helium nuclei (alpha particles) into a carbon nucleus. This process is sometimes called helium burning, and it occurs in many old stars.

turnoff point (408)

the location on the main sequence where a star’s evolution causes it to move away from the main sequence toward the red giant region. The location of the turnoff point can be used to deduce the age of a star cluster.

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