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Key Terms

AstronomyKey Terms

elliptical galaxy
a galaxy whose shape is an ellipse and that contains no conspicuous interstellar material
Hubble constant
a constant of proportionality in the law relating the velocities of remote galaxies to their distances
Hubble’s law
a rule that the radial velocities of remote galaxies are proportional to their distances from us
irregular galaxy
a galaxy without any clear symmetry or pattern; neither a spiral nor an elliptical galaxy
mass-to-light ratio
the ratio of the total mass of a galaxy to its total luminosity, usually expressed in units of solar mass and solar luminosity; the mass-to-light ratio gives a rough indication of the types of stars contained within a galaxy and whether or not substantial quantities of dark matter are present
when lines in the spectra are displaced toward longer wavelengths (toward the red end of the visible spectrum)
spiral galaxy
a flattened, rotating galaxy with pinwheel-like arms of interstellar material and young stars, winding out from its central bulge
type Ia supernova
a supernova formed by the explosion of a white dwarf in a binary system and reach a luminosity of about 4.5 × 109 LSun; can be used to determine distances to galaxies on a large scale
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