Universe Glossary H-M

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Hawking radiation

A theory first proposed by British physicist Stephen Hawking (1942-2018) that, due to a combination of the properties of quantum mechanics and gravity, the curved space-time surrounding a black hole emits a small amount of heat. For black holes that are not accreting mass to make up for this energy loss, Hawking radiation causes the event horizon to shrink over time, and such a black hole will eventually evaporate in a flash of energetic particles and gamma rays. Stellar-mass black holes would take dozens of times the present age of the universe to evaporate, and supermassive black holes would take even longer.


The second lightest and second most abundant element. The typical helium atom consists of a nucleus of two protons and two neutrons surrounded by two electrons. Helium was first discovered in our Sun, whose mass is about 25% helium.

hertz (Hz)

The derived SI unit of frequency, defined as a frequency of 1 cycle per second. Named after German physicist Heinrich Hertz (1857–1894).

Hubble constant (Ho)

A constant that determines the relationship between the distance to a galaxy and its velocity due to the expansion of the universe. Its value in the present universe has been measured at about 70 km per second per megaparsec. Named for American astronomer Edwin Hubble (1889–1953).

Hubble's law

A relationship between a galaxy's distance from us and its velocity through space, which is interpreted as evidence of the expanding universe. The farther away a galaxy lies from us, the faster it's receding away fromm us. The Hubble constant (Ho) describes the rate of change. Named for American astronomer Edwin Hubble (1889–1953).


The lightest and most abundant element. A hydrogen atom consists of one proton and one electron. Hydrogen makes up about 75% of the Sun's mass, but only a tiny fraction of Earth's.


A violent inward collapse.


The angle between the orbital plane of a planet, comet, or other body in the solar system and the ecliptic, or between the orbital plane of a satellite (such as the Moon) and the equatorial plane of its primary (Earth).


Electromagnetic radiation at wavelengths longer than the red end of visible light and shorter than microwaves (roughly between 1 and 100 microns). Almost no infrared light can reach Earth's surface, although some portions can be observed by high-altitude aircraft or telescopes atop high mountains.

interstellar medium

Gas, dust and other material that exists in the space between stars. 


An atom with one or more electrons stripped off, giving it a net positive charge.


A region of Earth's upper atmosphere containing abundant ions and free electrons mainly formed by ultraviolet light and X-rays from the Sun. The term is also used to describe corresponding regions above the surfaces of other planets. The ionosphere is important because it reflects and modifies radio waves used for communication and navigation. More precisely, the ionosphere consists of three distinct regions, known as the D, E, and F layers, embedded within standard atmospheric layers called the mesosphere and the thermosphere. The D layer ranges from about 60 km (37 miles) to 90 km (56 miles) altitude, the E region extends from around 90 km (56 miles) up to 150 km (93 miles) high, and the F layer ranges from altitudes of about 150 km to up to 600 km (373 miles). The height, fraction of ionized particles, and even the existence of these regions varies over time, with significant differences between day and night, as well as over the course of the Sun's 11-year sunspot cycle.

Japan Aerospace Exploration Agency (JAXA)

The Japanese national aerospace and space agency, formed in 2003 through the merger of three previously independent organizations. JAXA operates the Tanegashima Space Center, the nation's largest spaceport.


Oppositely directed beams of particles moving near the speed of light, usually emitted by supermassive black holes in active galaxies, pulsars, or gamma-ray bursts.

Kelvin temperature scale (K)

A kelvin is the fundamental SI unit of temperature. Water freezes at 273 kelvins (not degrees), which equals 0° Celsius and 32° Fahrenheit. The scale is frequently used in science. A change of 1 kelvin (1 K) is the same magnitude as a temperature change of 1° C, but the Kelvin scale measures from absolute zero, the theoretical coldest possible temperature. The boiling point of water is 373 K (100° C or 212° F), and the Sun's effective temperature is 5,772 K (5,499° C or 9,930° F). Named for Lord Kelvin, the Scottish physicist Sir William Thompson (1824–1907).

Kepler's laws

A set of three scientific laws describing the motion of planets around the Sun published by Johannes Kepler (1571–1630), a German astronomer and mathematician. They define the shape of planetary orbits, explain how a planet's velocity changes during an orbit, and relate a planet's orbital period to its distance from the Sun.1. All planets move along elliptical orbits with the Sun located at one focus. 2. During each orbit, a line connecting a planet with the Sun sweeps out equal areas in equal time intervals.3. The square of the planet's orbital period (P) is proportional to the cube of its semimajor axis (a, or half the longest width of its elliptical orbit): P∝ a3.

kilogram (kg)

The fundamental SI unit of mass. The kilogram is the only SI unit still maintained by a physical artifact (a platinum-iridium bar) kept in the International Bureau of Weights and Measures at Sevres, France. One kilogram is equivalent to 1,000 grams or about 2.2 pounds; the mass of a liter of water.


Refers to the calculation or description of the underlying mechanics in the motion of an astronomical object. For example, in radioastronomy, spectral line graphs are used to determine the relative motions of material at the center of a galaxy or surrounding a newborn star.

Kirchhoff's laws

Three laws that explain the formation of spectra, named for German physicist Guastav Kirchhoff (1824–87).

  1. An incandescent solid or gas under high pressure (such as a star) will produce a continuous spectrum.
  2. A low-density gas (such as a neon sign) will radiate an emission (bright line) spectrum.
  3. Continuous radiation viewed through a low-density gas will produce an absorption (dark line) spectrum.

Lagrange points

Locations in the vicinity of two massive bodies (such as Earth and the Moon) where each their respective gravities balance. There are five, labeled L1 through L5. L1, L2, and L3 lie along the centerline between the centers of the two masses. L1 is on the inner side of the secondary, L2 is on the outer side of the secondary, and L3 is on the outer side of the primary. L4 and L5, the so-called Trojan points, lie along the orbit of the secondary around the primary, 60 degrees ahead of and behind the secondary. 


A device that produces a coherent beam of light by stimulating electronic, ionic, or molecular transitions to higher levels so that when they return to lower energies they emit photons.


Electromagnetic radiation of all types, from microwaves to gamma rays. Visible light refers to the wavelengths detectable by the human eye.

light curve

A graph showing how the radiation from an object varies over time.


A unit of length that equals the distance light travels in a year. At the rate of 300,000 kilometers per second (671 million miles per hour), 1 light-year is equivalent to 9.46053 x 1012 km, 5,880,000,000,000 miles or 63,240 AU. See also: scientific notation.


The outer edge of the apparent disk of a celestial body.


The rate at which a star or other object emits electromagnetic radiation.


A type of neutron star possessing an unusually intense magnetic field between 100 and 1,000 times stronger than that of a typical pulsar. Astronomers have identified some 30 magnetars.

magnetic field

A description of the strength of the force exerted by a magnetic object.

magnetic pole

Either of two regions where a magnet's field is most intense. The two regions have opposing polarities, labeled "north" and "south" after Earth's magnetic poles.


The volume of space surrounding an object (such as a star or planet) in which charged particles are subject more to the object's magnetic field than any other. Constant bombardment by the outflow of particles and fields from the Sun (called the solar wind) compresses the Sun-facing side of our magnetosphere. In the opposite direction, it stretches into an immense magnetotail that fluctuates in length and can extend well past the Moon's orbit.


The portion of a planetary magnetosphere that is swept back away from the Sun by the solar wind.


The degree of brightness of a celestial body designated on a numerical scale, on which the brightest nighttime star is measured at -1.4 and the faintest star visible without a telescope is magnitude 6. A decrease of one magnitude represents an increase in apparent brightness by a factor of 2.512; also called apparent magnitude.


A measure of the total amount of material in a body, defined either by the inertial properties of the body or by its gravitational influence on other bodies.


Any substance that occupies space and possesses mass.


A unit of energy used to describe nuclear warheads. The same amount energy as 1 million tons of TNT.1 megaton = 4 x 1022 ergs = 4 x 1015 joules.

meson [MEH-zon]

An unstable subatomic particle in the hadron family composed of a quark and an antiquark. More than 200 types are known, and all decay in less than a ten-millionth (10–7) of a second. Mesons were predicted in 1935 by the Japanese physicist Yukawa Hideki (1907–1981). The first to be discovered, the charged pions, were found through cosmic ray interactions in 1947.

meter (m)

The fundamental SI unit of length, defined as the length traveled by light in vacuum during a period of 1/299,792,458th of a second, and equal to 39.37 inches. A kilometer (km) is equal to 1,000 meters (0.6214 miles).

metric system


Microquasars are stellar-mass black holes that display characteristics of the supermassive black holes found at the centers of some galaxies. For instance, they have jets detectable at radio wavelengths — something not every black hole has.


Electromagnetic radiation with wavelengths between 1 mm and 30 cm. Microwaves can be used to study the universe, communicate with satellites in Earth orbit, and cook popcorn.


A quantity equal to an object’s mass multiplied by its velocity that is always conserved in closed physical systems. The speed of two objects colliding head-on, for example, will change as a result of the impact, but their total momentum before and after the collision will remain the same. Even light and other electromagnetic radiation has momentum, defined as Planck's constant divided by its wavelength. Angular momentum is a related concept applying to rotating systems.