Glossary -- M

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magnetic dynamo

a mechanism thought to produce magnetic fields in a planet by the swirling, or circulation, of liquid conducting material in or near the planet's core.

magnifying power

the ability of telescope to enlarge images. Can be increased by using an eyepiece with a shorter focal length.

magnitude

used to quantify brightness. Based on the ancient system of Hipparchus but refined and quantified for measurements today such that a ratio of 100 in brightness corresponds to a magnitude difference of 5. Fainter objects have larger, positive magnitudes (closer to positive infinity), while brighter objects have lower magnitudes (closer to negative infinity).

main sequence

the narrow diagonal band in the Hertzsprung-Russell diagram going from upper left to lower right describing the characteristics of 90% of the stars. Stars spend about 90% of their lives in this stage and are fusing hydrogen to create helium.

main sequence turnoff

the mass of the most massive main sequence star remaining in a star cluster. Stars more massive than the turnoff have already evolved beyond the main sequence stage. The turnoff mass can be used to determine the age of the star cluster (it equals the lifetime of the most massive star still in the main sequence stage).

mass

an intrinsic property of an object that measures its resistance to an acceleration. Mass is measured in units of kilograms.

mathematical models

a set of equations describing the structure and interaction of material in an object or group of objects.

mean Sun

imaginary object that moves uniformly eastward along the celestial equator such that it completes one 360° circuit of the sky in one year. The average solar day is the time between successive meridian crossings of the mean Sun.

meridian

great circle on the sky that goes through the celestial poles and the zenith point. It separates the daytime motions of the Sun into ``a.m.'' and ``p.m.''. The azimuth of an object on the meridian in the northern sky = 0° and the azimuth of an object on the meridian in the southern sky = 180°.

mesophere

layer of a planet's atmosphere above a stratosphere where the temperature decreases with increasing altitude.

metals

what astronomers call all of the elements heavier than helium (like carbon, nitrogen, oxygen, sodium, aluminum, chlorine, calcium, iron, etc.).

meteor shower

what happens when the Earth passes through the dust trail left by a comet in its orbit. The dust grains are the size of a grain of sand or smaller and produce a large number of meteors in a short time that appear to come from a particular point in the sky.

meteorite

a small rock from space that makes it to the surface of a planet without burning up in the planet's atmosphere. This distinguishes it from when it is passing through the atmosphere, glowing hot from the friction with the atmosphere and is called a meteor.

microlens technique (planet detection)

a method of finding exoplanets by looking for the gravitational lensing effect from a planet orbiting a foreground star added to the gravitational lensing effect of the foreground star on the light from a more distant star.

Milky Way Galaxy

the large spiral galaxy in which our Sun and planets reside. Our Sun is one star of several hundred billion in the Milky Way.

model

an abstract construct or idea that is a simplified view of reality. It must enable you to make testable predictions of what will happen under new circumstances.

Glossary links (select a letter for definitions of astronomy terms beginning with that letter):
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last updated: June 7, 2019