AstroGlossary - M
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Magellanic clouds
Two companion galaxies of the Milky Way, called the Large Magellanic Cloud and the Small Magellanic Cloud. They are visible only from the southern hemisphere. They may be gravitationally bound to the Milky Way, so that they orbit it, or they may just be making a close pass by the Milky Way. magnetic field
A force field generated by moving electrical charges. An electrical current running through a loop of wire generates a magnetic field. The strength of the field depends on the current and area of the wire loop. Plasma churning through the atmosphere of the Sun drives powerful magnetic fields that sometimes produce cool magnetic storms called sunspots. A magnetic dynamo underneath Earth's crust generates a magnetic field around Earth. magnification
In a telescope, an increase in the apparent size of an object. The process of magnification expands the apparent size of an object by spreading the image, or light, across a large area. A large primary mirror or objective lens of a telescope focuses incoming light toward an eyepiece lens that actually magnifies what the telescope sees. magnitude
A measure of brightness, or faintness, as perceived by the human eye. In the system used by astronomers, the higher the magnitude, the fainter the object. The magnitude and apparent brightness of a star are related in a logarithmic fashion. For every five steps in magnitude, the apparent brightness of a star, galaxy, or nebula changes by a factor of 100. For example, we receive 100 times more light energy from Vega -- a zero-magnitude star -- than from Eta Ursa Minor -- a fifth-magnitude star in the Little Dipper. Under the clearest, darkest skies, your eye cannot see stars fainter than sixth magnitude. With the aid of binoculars, the human eye can detect 10th-magnitude stars. main sequence
A classification of stars that all shine via hydrogen thermonuclear fusion, and are all in a state of hydrodynamic equilibrium. Stars spend the greatest portion of their luminous (nuclear fusion) lives on the main sequence. mare
Latin for "sea," it is an area of volcanic rock on the surface of the Moon. The plural form of mare is maria. It is thought that most maria formed as a result of impacts on the Moon; lava bled from cracks made in the Moon's surface and filled craters to form the dark "seas" visible from Earth. Maria are the youngest and most crater-free regions on the Moon's surface. Mars
The fourth planet from the Sun in the solar system. Mars orbits the Sun at an average distance of 1.52 AU. The planet has a mass 0.1 times the mass of Earth and a radius 0.5 times the radius of Earth. It is made of solid, rocky material, and has a thin atmosphere of carbon dioxide. Iron oxide, or rust, on the surface of Mars gives it its characteristic red color. The planet has bright polar ice caps made of frozen water and carbon dioxide. Mars also has two small moons, Phobos and Deimos. mass
The total matter content of an object. Also a physical measure of inertia. Newton's law states that mass is related to force and acceleration: m = F/a. Einstein says that mass and space are somehow related, because mass warps space and space directs the motion of mass. matter
The material out of which all ordinary material is made. Protons, neutrons, and electrons are examples of matter, as is a piece of wood, a puddle of water, or an animal or plant. All matter has the properties that it occupies a volume and has inertia. The measure of the amount of matter in an object is that object's mass. See also: mean Sun
An imaginary average solar motion that yields a 24-hour day. Clocks and wristwatches measure time with respect to the mean Sun, not actual solar motion. Mercury
The first planet from the Sun in the solar sSystem. Mercury orbits the Sun at an average distance of 0.4 AU. The planet has a mass 0.055 times the mass of Earth and a radius 0.4 times the radius of Earth. It is made of solid, rocky material that is heavily cratered, and it has no significant atmosphere. Mercury has no moons. Messier catalog
A list of about 110 astronomical objects compiled by French astronomer Charles Messier at the end of the 18th century. The catalog includes objects such as nebulae, star clusters, and galaxies, and the objects are denoted by an M followed by a number from 1 to 110. meteor
A shooting star; a meteoroid that vaporizes in Earth's upper atmosphere as a burst of incandescent gas. meteor shower
A spectacular display of meteors streaking through the sky at rates between several to hundreds per hour. Whenever Earth intercepts a stream of comet debris in orbit around the Sun, some of the debris falls into Earth's atmosphere, producing the meteor shower. Meteor velocities usually fall within a range of about 10 km/s and 70 km/s. meteorite
A space rock that strikes the surface of the Earth. Most meteorites are classified as stony because of their mineral composition, which may include olivine, pyroxene, serpentine, sulfates, organic compounds, iron, and nickel. meteoroid
A tiny grain of silicate or metal between 1 and 10 millimeters across that orbits the Sun. Sometimes meteoroids fall into Earth's atmosphere and become meteors. microwave radiation
A very unenergetic wavelength, or frequency, of light. Microwaves are a type of radio radiation, meaning that they are less energetic than infrared radiation. They are not harmful to life because they are not strong enough to ionize atoms or destroy cells. While the Earth's atmosphere shields us from some microwave radiation, it does allow some frequencies to pass through. Astronomers study these microwaves with large radio telescopes or antennas, which resemble giant satellite dishes. Milky Way
Our home galaxy. The Milky Way is a spiral galaxy with four major arms containing young bright stars, gas, and dust. The mass of the Milky Way is estimated to be between 400 billion and one trillion times the mass of the Sun. The Milky Way's diameter is about 100,000 light-years, with the Sun orbiting about 27,000 light-years from the galactic center. However, most of the galaxy's mass consists of "dark matter" which encircles the Milky Way's disk in a vast halo. Dark matter may account for 90 percent of the Milky Way's mass. minute of arc
One-sixtieth of a degree, also known as an arc minute. See also:
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