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- Machine Code
- the code, readable by computer hardware that is created after source code is compiled.
- Magnetic Pole
- Either of two regions usually near opposite ends of a magnet where the magnetic intensity is greatest.
- Magnetosheath
- the region in space where a planet’s magnetic field interacts with the charged particles of the solar wind.
- Magellanic Clouds
- Two irregular dwarf galaxies in orbit around the Milky Way GalaxyThe Large Magellanic Cloud is about 163,000 light years distant and the Small Magellanic Cloud is about 206, 000 light years from the Milky Way.
- Magnetar
- A neutron star with a particularly powerful magnetic field. The strength of a magnetar’s magnetic field is generally in excess of a billion Tesla.
- Magnitude
- astronomers measure how bright an object is in terms of its apparent magnitude. Counter intuitively, the brighter an object is, the lower its magnitude. A first magnitude star is about 2.5 times as bright as a second magnitude star, and so on. The brightest object in the sky is, of course, the Sun; with a magnitude of -26.73. The full moon is -12.6, and with the naked eye we can see all the way down to about a magnitude of 6.
- Main Sequence
- the name given to a region on a graph, known as the Hertzsprung–Russell (HR) diagram, of stellar color versus brightness. While on the main sequence, which runs from hotter and brighter stars (upper left) to cooler and dimmer ones (lower right), a star generates energy by nuclear fusion in its core. In the Sun these fusion reactions convert hydrogen to helium.
- Mare
- maria is latin for “seas”, mare is singular. The dark regions that you can see on the Moon are the maria.
- MASER
- Microwave Amplification by Stimulated Emission of Radiation. It’s like a laser, but in microwave light instead of higher energy optical light.
- Mass
- a measurement of how much stuff, or “matter,” an object is made from.
- MeV
- Mega Electron Volts. The electron volt (eV) is a measurement of energy; One eV is the energy gained by an electron (or proton) dropping through an electrical potential of one volt. This is a tiny amount of energy, appropriate for describing the energies of atomic and subatomic processes. To give an example, the energy of visible light photons is about 1 eV, as is the typical energy of outer electrons in atoms. Adding the prefix “Mega” means we are talking about one million electron volts (MeV).
- Meson
- mesons are particles made up of a quark and an anti-quark pair. Because mesons are unstable they exist only for a short time before decaying into other particles. Mesons, like exotic baryons, are often created in particle accelerators.
- Meteor
- the flash of light we see when a particle or chunk of solid matter falls into our atmosphere and disintegrates.
- Meteor Shower
- when the Earth passes through a cloud of dust, we see large numbers of meteors in a relatively short time. This is called a meteor shower. During its peak, observers might notice one or two meteors a minute from the Perseids.
- Microorganism
- as the name implies, a microorganism is an organism that is microscopic in size, usually too small to be seen with unaided human eye.
- Microwave
- the energy of microwaves is a bit higher than radio waves. Their wavelengths are therefore shorter, and are typically measured in centimeters (cm or 10-2m).
- Milliamperage Second
- a common X-ray imaging setting; the total X-ray current measured in milli-amps times the total exposure time.
- Milky Way Galaxy
- A barred spiral galaxy containing our Solar System. It is generally considered to be the second largest and second most massive galaxy within the Local Group, after the Andromeda Galaxy.
- Missing Mass
- the phrase used by Fritz Zwicky to refer to the material needed to gravitationally bind galaxy clusters. We now call this material dark matter.
- Molecular Cloud
- also sometimes referred to as a stellar nursery if new stars are being formed within the cloud. Molecular clouds are giant regions of diffuse gases that can be as big as 326 light years `(3.1 x 10^15 km)` across. They are composed mostly of Hydrogen and Helium, with a few other elements dispersed throughout. Internal gravitation in denser regions of the cloud can trigger the collapse of parts of the cloud, and when this happens, new stars can be formed.
- moon
- a celestial body that orbits a planet or smaller body.
- M87
- the 87th object in the Messier Catalogue is a very large elliptical galaxy some 55 million light years away at the center of the Virgo Cluster of galaxies (our local supercluster). M87 is an active galaxy; its strong nuclear radio emission makes it a radio galaxy. It is also one of the brightest radio objects in the sky.
