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Moment of truth
The Sun, with a diameter of 1,392,000 km, is made of 94% hydrogen atoms.

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megaflops Comp.
A million FLOPS, or one million floating point arithmatic operations per second.

Mersenne prime Math.
The prime number of the form 2n - 1 where n is also a prime number. It is named after Marin Mersenne (1588-1648) who stated that the numbers 2n - 1 were prime for n = 2, 3, 5, 7, 13, 17, 19, 31, 67, 127 and 257. However, he missed 61, 89 and 107 which were later found to be prime by others. As at June 2004, the largest number discovered is n = 24036583, producing a 7235733 decimal digits prime number.

meson Phys.
Greek word for middle, it is a strongly interacting particle produced in reactions by the strong interaction. It has an integral spin (0, 1, etc) consists of a quark and an antiquark. Mesons are classified as one of the two subgroups of hadron.

Mesons usually decay through the strong, electromagnetic or weak interaction. For example, pion minus, a meson of mass 139.6 MeV/c2, can be produced via proton (p) and neutron (n) collision reaction and the subsequent pion produced then decays via the weak interaction with a half life of 2.6 x 10-8 s.

meson decay

Mesozoic Geol.
The era with a time span from 250 million to 65 million years ago. This includes the Triassic, Jurassic and Cretaceous periods. The Earth was believed to be warmer with higher sea level and no polar ice during this era. The largest global-scale mass extinction (the Permian Extinction) was believed to occur toward the beginning of the Mesozoic Era, of which new life forms such as dinosaurs and mammals began to dominate the Earth.

metal fatigue Eng.
An effect that causes a metal to fail after repeated application of stress or strain, none of which exceeds the yield stress of the material. The effect is more severe if the level of stress or strain increases but is also influenced by a number of other factors such as nature of design and operating environment. Failure can occur in a sudden without a warning, such as formation of a small crack that suddenly undergo complete fracture.

metalloid Chem.
A substance, usually an element, having both metallic and non-metallic properties. Most of these substances are usually used in semiconductor applications. For example, arsenic, antimony or other elements belong to Group 3A to 6A of increasing atomic number in the Periodic Table of the Elements.

metamorphic rock Geol.
Rocks that are formed from igneous and sedimentary rocks due to the action of heat and exposure. These rocks are therefore usually formed deep within the Earth. For example, limestone, a sedimentary rock, that under the action of heat and pressure, changes to marble (metamorphised).

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