# Glossary of Terms

The following is a glossary of terms that may be useful when using the Appreciating Hubble at Hyper-speed application and associated exercises.

Blackbody radiation - the light emitted by a blackbody, which is a perfect absorber and emitter of electromagnetic radiation. The light from stars is well approximated by blackbody radiation, dependent on the temperature of the star. Cepheid variables - a type of star that exhibits a tight correlation between the period of the variation in its brightness and its absolute luminosity. Often used as distance estimators to galaxies. Cosmology - the study of the past, present, and future of the Universe Critical density - the amount of mass per unit volume required to close the universe. If the universe has more than the critical density, the universe will collapse upon itself; if the universe has a density less than or equal to the critical density, the universe will expand forever. Cull size - the smallest size (in pixels) for which a galaxy's image will be displayed. The default cull size is 4. (See "Help" page) Doppler shift - the change in frequency (or wavelength) from a source due to the relative motion of the source and observer Galaxy - a gravitationally bound grouping of stars, gas, and dust that is physically isolated in space General relativity - a theory which argues that everything, including light, is affected by gravity. Introduced by Albert Einstein in 1915. Hubble constant - the proportionality constant that arises from the Hubble Law, the relationship between a galaxy's expansion velocity and the distance to that galaxy. An accurate value of the Hubble constant is required to know the critical density of the universe, as well as the age of the universe. Hubble's Law - a relationship between the distance of a galaxy and its observed redshift, first noted by Edwin P. Hubble in the early 1920's. Light-year - the distance that light travels in one year. Equivalent to 9.48 x 1015 m. Morphology - the study of structure Parallax - the apparent change in the position of an object due to a change in the observer's position Parsec - the distance from the Sun at which an object exhibits one arcsecond of parallax as seen from the Earth. Equivalent to 3.26 light-years. Redshift - The light emitted or reflected from an object is shifted to longer wavelengths (to the "red" end of the spectrum) as seen by an observer with a relative motion away from the object. If two objects have a relative motion towards each other, the light shows a "blueshift". Special relativity - a theory that explored how measurements of time and space are affected by motions at high constant speeds. Developed by Albert Einstein in 1905. Stamp size - within the AHaH application, the size of the image containing a given galaxy. That is, the width (at the galaxy's redshift) of the black box surrounding the galaxy within the Info Box. The stamp size can be used to estimate a galaxy's proper transverse diameter. Type 1a supernovae - an incredibly bright explosion resulting from the collapse of a white dwarf star. Can be used to estimate distances to galaxies. Wave - a traveling disturbance demonstrating periodicity.