| A | B |
|---|
| parallax | The most basic way of measuring the distance to an object in space by comparing its shift in apparent position over a period of time. |
| apparent magnitude | The perceived brightness of a star as viewed from Earth. |
| absolute magnitude | The actual brightness of a star based on an imagined distance of exactly 32.6 light years. |
| nebula | A cloud of gas and dust in space from which stars are born. |
| main sequence star | A star that whose inward gravitational force is exactly balanced by its outward force of pressure from nuclear reactions. |
| red giant | An expanding, cool star that is running out of "fuel" in its core. |
| planetary nebula | The outer layers of a red giant are being gently "puffed" off in pulses, creating dusty rings around the star. |
| white dwarf | Dead, hot, dense "core" of a star. |
| black dwarf | The remains of a cooled white dwarf. |
| Red Supergiant | Cool, expanding massive star that is nearing gravitational collapse (supernova). |
| neutron star | Dense remains of a Red Supergiant star. |
| black hole | Small, dense are formed by strong gravitational compression of a very massive star following supernova. |
| astronomical unit | A measurement of distance equal to the mean distance from the earth to our sun. |
| light year | A measure of distance equal to how far light travels in a year. |
| Hertzprung-Russell Diagram | A graphical representation of stars that shows the relationship between its spectral class (color) and its absolute magnitude. |
| open star cluster | Formed by dozens to thousands of young stars found in the spiral arms of a galaxy. |
| galactic star cluster | another name for an open star cluster |
| globular cluster | Formed by millions of old stars clumped closely together. |
| galaxy | A large group of stars found outside our own solar system. |
| spiral galaxy | A galaxy shaped like a flat disk with a central bulge, spiral arms, and a surrounding halo. |
| elliptical galaxy | Formed by mostly old stars with little gas/dust and range from spherical to football-shaped. |
| lenticular galaxy | Have a disk, but no arms and little to no gas/dust. |
| irregular galaxy | A catch-all for odd shaped galaxies that contain lots of gas/dust and young stars. |
| the Local Group | Cluster of about 30 galaxies, including our own. |
| Andromeda Galaxy | The closest galaxy to our own and with which we are on a collision course. |
| the Virgo Cluster | A giant galaxy cluster toward which the Local Group is "falling." |
| globule | Concentration of dust and hydrogen gas in a nebula cloud. |
