joule: The SI unit of energy, 1 kg · m2/s2.
Jovian planet: Same as giant planet.
JPL: The Jet Propulsion Laboratory in Pasadena, California, funded by NASA and administered by Caltech; a major space contractor.
Julian calendar: The calendar with 365-day years and leap years every fourth year without exception; the predecessor to the Gregorian calendar.
Julian day: The number of days since noon on January 1, 713 B.C. Used for keeping track of variable stars on other astronomical events. January 1, 2000, noon, begins Julian day 2,451,545.
K line: The spectral line of ionized calcium at 3933 Å.
Keplerian: Following Kepler’s law.
Kuiper belt: A reservoir of perhaps hundreds of thousands of Solar-System objects, each hundreds of kilometers in diameter, orbiting the Sun outside the orbit of Neptune. It is the source of short-period comets.
Kuiper belt object: An object in the Kuiper belt; many comets and perhaps even the planet Pluto are examples.
Large-scale structure: The network of filaments and voids or other shapes distinguished when studying the Universe on the largest scales of distance.
laser: An acronym for “light amplification by stimulated emission of radiation,” a device by which certain energy levels are populated by more electrons than normal, resulting in an especially intense emission of light at a certain frequency when the electrons drop to a lower energy level.
latitude: Number of degrees north of south of the equator measured from the center of a coordinate system.
law of equal areas: Kepler’s second law.
leap year: A year in which a 366th day is added.
lens: A device that focuses waves by refraction.
lenticular: A galaxy of type S0.
light: Electromagnetic radiation between about 4000 and 7000 Å.
light curve: The graph of the brightness of an object vs. time.
light pollution: Excess light in the sky.
light-year: The distance that light travels in a year.
lighthouse model: The explanation of a pulsar as a spinning neutron star whose beam we see as it comes around.
lightweight stars: Stars between about 0.07 and 4 solar masses.
limb: The edge of a star or planet.
line profile: The graph of the intensity of radiation vs. wavelength for a spectral line.
lithosphere: The crust and upper mantel of a planet.
lobes: Of a radio source, the regions to the sides of the center from which high-energy particles are radiating.
local: In our region of the Universe.
Local Group: The two dozen or so galaxies, including the Milky Way Galaxy, that form a small cluster.
Local Supercluster: The supercluster of galaxies in which the Virgo Cluster, the Local Group, and other clusters reside.
Logarithmic: A scale in which equal intervals stand for multiplying by ten or some other base, as opposed to linear, in which increases are additive.
longitude: The angular distance around a body measured along the equator from some particular point; for a point not on the equator, it is the angular distance along the equator to a great circle that passes through the poles and through the point.
Longperiod variables: Mira variables.
look-back time: The duration over which light from an object has been traveling to reach us.
luminosity: The total amount of energy given off by an object per unit time; its power.
luminosity class: Different regions of the H-R diagram separating objects of the same spectral type: supergiants (I), bright giants (II), giants (III), subgiants (IV), dwarfs (V).
lunar eclipse: The passage of the Moon into the Earth’s shadow.
lunar occultation: An occultation by the Moon.
lunar soils: Dust and other small fragments on the lunar surface.
Lyman alpha: The spectral line (1216 Å) that corresponds to a transition between the two lowest major energy levels of a hydrogen atom.
Lyman-alpha forest: The many Lyman-alpha lines, each differently Doppler-shifted, visible in the spectra of some quasars.
Lyman lines: The spectral lines that correspond to transitions to or from the lowest major energy level of a hydrogen atom.