| laser | An acronym for light amplification by stimulated emission of radiation; a device for amplifying a light signal at a particular wavelength into a coherent beam. |
| latitude | A north-south coordinate on the surface of the Earth; the angular distance north or south of the equator measured along a meridian passing through a place. |
| law of areas | Kepler's second law: the radius vector from the Sun to any planet sweeps out equal areas in the planet's orbital plane in equal intervals of time. |
| law of the redshifts | See Hubble law. |
| leap year | A calendar year with 366 days, inserted approximately every 4 years to make the average length of the calendar year as nearly equal as possible to the tropical year. |
| light year | The distance light travels in a vacuum in one year; 1 LY = 9.46 x 10^12 km, or about 6 x 10^12 Mi. |
| line broadening | The phenomenon by which spectral lines are not precisely sharp but have finite widths. |
| line profile | A plot of the intensity of light versus wavelength across a spectral line. |
| Local Group | The cluster of galaxies to which our Galaxy belongs. |
| local standard of rest | A coordinate system that shares the average motion of the Sun and its neighboring stars about the galactic center. |
| Local Supercluster | The supercluster of galaxies to which the Local Group belongs. |
| longitude | An east-west coordinate on the Eartb's surface; the angular distance, measured east or west along the equator, from the Greenwich meridian to the meridian passing through a place. |
| luminosity | The rate at which a star or other object emits electromagnetic energy into space. |
| luminosity class | A classification of a star according to its luminosity within a given spectral class. Our Sun, a G2V star, has luminosity class V |
| luminosity function | The relative numbers of stars (or other objects) of various luminosities. |
| lunar eclipse | An eclipse of the Moon. |
| Lyman lines | A series of absorption or emission lines in the spectrum of hydrogen that arise from transitions to and from the lowest energy states of the hydrogen atoms. |