M

Magnifying power

The apparent increase in the size of a body when seen through a telescope over its size when seen with the unaided eye.

 

Magnitude

A number that designatess the brightness of a body. It is a measure of the light received from a body.

 

Main sequence

A narrow band on the H-R diagram on which the majority of the stars lie. The band runs from the upper left to the lower right of the diagram. Stars on the main sequence are burning hydrogen.

 

Mentle

A layer that lies between the crust and the core in the earth's interior.

 

Mare

A large, smooth, circular basin such as those found on the moon. The name comes from the latin word for sea.

 

Mass-luminosity relation

An empirical relationship that states that luminosity of a star, primarily a main sequence or a giant, depends on its mass. The more massive stars are the more luminous.

 

Mean solar day

The time between successive crossing of the observer's meridian by the mean sun.

 

Mean sun

An imaginary body that moves eastward along the celestial equator at a uniform rate and completes its circuit in the sky in the same period as the apparent sun.

 

Meridian

The great circle on the terrestrial sphere which passes through the observer's position and the earth's north and south poles. The great circle on the celestial sphere which passes through the observer's zenith and the celestial north ans south poles.

 

Messier catalog

A catalog of nebulae, star clusters, and galaxies compiled by Charles Messier in 1787. The bodies are designated by M and a number, e.g., M31 for the Andromeda galaxy.

 

Meteor

The bright streak of light that is visible when a meteorid passes through the earth's atmosphere and is heated by friction between it and the air molecules.

 

Meteor shower

Many meteors that appear to radiate from a point in the sky, due to the earth's passing through a swarm or a stream of meteoroid particles.

 

Meteorite

A meteoroid that has survived its flight through the earth's atmosphere and has struck the earth surface.

 

Meteoroid

The stony or metallic particle that produces a meteor when it passes through the earth's atnosphere.

 

Micrometeorite

An extremely small meteoroid whose size causes it to move very slowly through the earth's atmosphere so that it does not burn.

 

Milky Way

A faint, diffuse band of light that completely encircles the sky and consist of a vast number of stars and interstellar material.