| A | B |
|---|
| perihelion | point in a planet's orbit where it is closest to the Sun |
| photosphere | the lowest visible layer of the Sun's atmosphere and the surface of the sun |
| chromosphere | layer of the Sun's atmosphere above the photosphere and below the corona |
| corona | top layer of the Sun's atmosphere that extends from the top of the chromosphere and ranges in temperature from 1 million to 2 million K |
| solar wind | a stream of high-speed, ionized particles flowing outward through the solar system from the Sun's corona |
| sunspot | relatively cool, dark spots appearing on the surface of the photosphere |
| solar flare | a sudden eruption of particles and radiation from the surface of the Sun |
| prominence | an arc of flaming gas rising from the chromosphere |
| fusion | a nuclear reaction in which lightweight nuclei combine into heavier nuclei |
| fission | a nuclear reaction in which an atomic nucleus splits into smaller, lighter nuclei releasing energy |
| spectrum | visible light arranged according to wavelengths |
| astronomical unit | the Earth's average distance to the Sun |
| constellation | group of stars that forms a pattern in the sky |
| binary star | two stars gravitationally bound together and orbiting about a common center of mass |
| parallax | an apparent change in position caused by motion of the observer |
| parsec | the distance at which an object has a parallax of one arcsecond |
| apparent magnitude | ancient Greek classification system based on how bright a star appears to be |
| absolute magnitude | brightness an object would have if it were placed at a distance of 10 pc |
| luminosity | energy output from the surface of a star per second, measured in watts |
| Hertzsprung-Russell diagram | graph that relates stellar properties |
| main sequence | in an H-R diagram, the diagonal band that includes about 90% of all stars |
| isolate | to set apart or cut off from others |
| nebula | a cloud of interstellar dust or gas or both, which collapses on itself as a result of its own gravity |
| protostar | the hot condensed disk-shaped object at the center of a collapsing star |
| neutron star | collapsed, dense core of a star that forms quickly while its outer layers are falling inward |
| supernova | massive explosion that occurs when the outer layers of a star are blown off |
| black hole | small, extremely dense remnant of a star whose gravity is so immense that not even light can escape its gravity field |
| dynamic | relating to or tending toward change |
