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magnetograph
an instrument that maps the strength, distribution, and direction of magnetic fields on the sun's disk
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magnetometer
an instrument used to measure the strength and direction of a magnetic field
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magnetopause
the boundary between Earth's magnetic field and the solar wind
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magnetosphere
the dynamic region around a planet where the magnetic field traps and controls the movement of charged particles from the solar wind
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magnitude
the measurement of an object's brightness; the lower the number, the brighter the object
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main sequence
the band of stars on a Hertzsprung-Russell diagram stretching from the upper left to the lower right; stars spend most of their lives in the main sequence phase, in which they are fusing hydrogen into helium in their cores
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mantle
the portion of a planet’s interior above the core but below the crust
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mare
a dark and relatively smooth area on the surface of the moon or a planet.
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mass
a measure of the total amount of matter within an object
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mass loss
the loss of mass by a star during its evolution; some of the causes of mass loss include stellar winds, bipolar outflows, and the ejection of material in a planetary nebula or supernova
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megaparsec
one million parsecs; equal to 3.26 million light-years
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meridian
an imaginary circle on the celestial sphere that connects the zenith to the north (or south) celestial pole
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meteor
a flash of light that occurs when a meteoroid burns up in Earth’s atmosphere; also popularly known as a shooting star
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meteor shower
a period of enhanced meteor activity that occurs when Earth collides with a swarm of meteoroids; an individual shower happens at the same time each year and has all its meteors appearing to radiate from a common point
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meteor showers
a period of enhanced meteor activity that occurs when Earth collides with a swarm of meteoroids; an individual shower happens at the same time each year and has all its meteors appearing to radiate from a common point
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meteor storm
Meteor storms are rare events that occur when Earth encounters dense regions within a meteor stream. Such encounters can increase normal meteor rates by more than 1,000 meteors per minute.
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meteorite
a rock from space that survives passage through Earth’s atmosphere and falls to the ground
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meteoroid
a small rock that orbits the sun
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microgravity
a condition in which the force of gravity is very low, producing a near-weightless environment
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microlensing
the effect of gravity from a small astronomical body or bodies focusing light rays, similar in manner to lenses
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micron
one-millionth of a meter
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microwaves
the most energetic form of radio waves
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million
1,000,000 (in American usage)
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millisecond pulsar
a neutron star that rotates hundreds of times per second, which typically accretes matter from a stellar companion
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minor planet
a rocky body that orbits the sun; also known as an asteroid
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mirror
a piece of glass coated with a highly reflective material
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molecule
a combination of two or more atoms that represents the smallest part of a compound that has the chemical properties of that compound
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moon
a smaller body orbiting a larger body; often refers to Earth’s moon
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morning star
the planet Venus when it appears in the morning sky
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motor drive
see clock drive
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multicultural astronomy
the variety of ways in which cultures of the past and present have observed, recorded, interperted, and made use of astronomy to structure their lives and/or satisfy their curiosity about the universe
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multiple star system
a gravitationally bound system in which two or more stars orbit a common center of mass.
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