Astro Glossary

  • Magellanic Clouds

    The Large Magellanic Cloud and Small Magellanic Cloud are companion galaxies of the Milky Way. They are visible only from the southern hemisphere. They appear to be gravitationally bound to the Milky Way, so that they orbit it. Each is less than 200,000 light-years away, making them some of our closest known galactic neighbors.

  • Magnetar

    A magnetar is a rapidly spinning, highly magnetic neutron star, which is the ultra-dense corpse of a supermassive star. A magnetar's magnetic field can be trillions of times the strength of Earth's. Most magnetars are quite young. They form when a supermassive star can no longer produce nuclear reactions in its core. The core collapses to form a ball that's up to twice the mass of the Sun, but no larger than a city. It is so dense that a chunk of its matter the size of a sugarcube weighs millions of tons. As the core collapses, it spins faster and faster. The magnetic field of the entire core is then compressed into the rotating neutron star, so it is extremely powerful. 

  • Magnetism and Magnetic Fields

    Magnetism is 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.

  • Magnitude Scale and Star Brightness

  • Main Sequence

    A classification of stars that shine via hydrogen thermonuclear fusion in their cores. Such stars are stable, with no major pulsations or variations. Most stars spend the greatest portion of their lives on the main sequence.

  • Mapmaking and Navigation

  • Mariner Probes

    A series of American interplanetary missions. In 1962, Mariner 2 staged the first successful encounter with another planet when it flew past Venus. Its instruments recorded Venus’ surface temperature and pressure, and the composition of its atmosphere and the clouds that envelope the planet. The next successful mission in the series, Mariner 4, snapped the first close-range pictures of Mars in 1965. The images revealed a Moon-like landscape. Mariner 9 became the first spacecraft to orbit Mars, and found that the planet has a much more varied and interesting surface than scientists had expected. Mariner 10 flew by Mercury three times in 1974 and 1975, and mapped about half of its battered, Moon-like surface, which is dominated by impact craters. Other successful missions in the series included Mariner 5 to Venus and Mariner 6 and 7 to Mars.

  • Mars

  • Mars

    The fourth planet from the Sun. Mars orbits the Sun at an average distance of 1.52 times Earth’s distance. The planet is about 10 percent as massive as Earth, and a bit more than half as big. 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.

  • Mars Climate Orbiter

    A spacecraft that was designed to provide detailed information about the atmospheric conditions on Mars, including temperature, dust, clouds, water vapor, and carbon dioxide levels. The mission failed when it approached too close to Mars and was destroyed as it plunged into the planet’s atmosphere.

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