Astronomy dictionary – M

Browse through our astronomy dictionary to find definitions for some of the most common terms used in practical astronomy and space science.

Click on one of the letters below to search for a term.



The region dominated by the magnetic field of an object, such as a moon, planet or star.


The brightness of an astronomical body. The lower the number the brighter it is. Magnitudes brighter than 0 are represented with a negative number.

Main Belt

The large asteroid belt between Mars and Jupiter.

Main sequence

The curve where most stars are found on a Hertzsprung-Russell diagram – a graph of stars’ absolute magnitudes versus their surface temperatures. Stars that are still fusing hydrogen nuclei are main sequence stars.

Main sequence star

A main sequence star is one that is in a stable part of its life converting hydrogen and helium through nuclear fusion. Most stars we see in the night sky are in the main sequence stage and our Sun is currently also a main sequence star.


The area in-between the Earth’s crust and its core (roughly 3,000km thick) that consists of hot solid rock and magma (molten rock).


The maria on the Moon are regions where viscous lava has flowed over the surface. They are generally much smoother than surrounding regions and are characterised by large basalt plains enriched with titanium and iron.

Mare (plural maria)

A large, iron-rich basalt plain on the lunar surface. Formed by cooling lava, they are generally much smoother than surrounding regions and appear dark compared with the lunar highlands. Mare is Latin for ‘sea’; early astronomers believed these features were, in fact, bodies of water.


An imaginary line that circles the Earth from north to south that marks the point at which the Sun is at its highest. Anti- and post-meridian (am and pm), mark the times before and after the Sun crosses the Meridian at midday, respectively.

Meridian flip

This is the process that equatorial mount owners have to go through to make sure that their equipment doesn’t collide with a pier or other obstruction as it tracks across the night sky.

Messier catalogue

The famous catalogue of deep sky objects such as nebulae, galaxies and clusters compiled by Frenchman Charles Messier and his colleague Pierre Méchain in the 18th Century.


In astronomy, this term is used to describe any element that is not hydrogen or helium.


Astronomers call anything that isn’t hydrogen and helium a ‘metal’. Therefore, metallicity is the fraction of a star’s mass that is not made up of these two elements.


A small piece of space debris entering the Earth’s atmosphere. It travels so fast it compresses the air ahead of it. This causes it to glow and creates a streak across the night sky.

Meteor trail

Sometimes a meteor leaves a trail of material as it passes through the Earth’s atmosphere. This ionised material glows for a brief moment before fading and is known as a meteor trail, or train.


A rocky piece of debris in space that is smaller than an asteroid. If a meteoroid enters Earth’s atmosphere it is called a meteor. Most meteors burn up in the atmosphere, but those that don’t and survive an impact on the ground are called meteorites.


Another word for one micrometre, which is written scientifically as 10^–6m or µm.

Miller-Urey experiment

An experiment conducted by Stanley Miller and Harold Urey at the University of Chicago in 1953 that simulated the conditions on the early Earth. It was designed to test whether organic molecules could be created from inorganic molecules.

Minor planet

The official name for asteroids, as classified by the International Astronomical Union – the recognised authority responsible for naming celestial bodies.

Mira variable

A red giant star that decreases and increases in brightness over a period of at least 100 days. It’s been shown that Mira variables can change brightness by as much as eight magnitudes.

Molecular cloud

A cold, dense, interstellar cloud in which molecules, mostly molecular hydrogen (H2), form.

Moon illusion

An optical illusion in which the Moon appears larger when it is near to the horizon than when it is high in the sky.


A satellite of a small celestial object, such as a dwarf planet or an asteroid.

Moving Cluster

A physical grouping of stars moving through space that usually shares a common origin. The most famous moving cluster is the Ursa Major Moving Group.

Multi-coated lens

Several clear coatings of a dedicated material that reduce unwanted reflections when applied to a lens or a corrector plate.

Muon collider

A type of particle accelerator capable of smashing particles called muons and anti-muons together to study high-energy collisions.