Planetary Nebula are the outer layers of a star that are lost when the star changes from a red giant to a white dwarf. A star is a luminous globe of gas producing its own heat and light by nuclear reactions (nuclear fusion). They are born from nebulae and consist mostly of hydrogen and helium gas.
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20.4 years is 20.4/10.2 = 2 half-life cycles, which means a quarter of the starting mass or 15.2 g will remain after this time.
It's weird but technically correct to say that a radio wave can be considered a low-frequency light wave. Radio and light are both electromagnetic waves. The only difference is that radio waves have much much much longer wavelengths, and much much much lower frequencies, than light waves have. But they're both the same physical phenomenon.
However, a radio wave CAN'T also be considered to be a sound wave. These two things are as different as two waves can be.
-- Radio is an electromagnetic wave. Sound is a mechanical wave.
-- Radio waves travel more than 800 thousand times faster than sound waves do.
-- Radio waves are transverse waves. Sound waves are longitudinal waves.
-- Radio waves can travel through empty space. Sound waves need material stuff to travel through.
-- Radio waves can be detected by radio, TV, and microwave receivers. Sound waves can't.
-- Sound waves can be detected by our ears. Radio waves can't.
-- Sound waves can be generated by talking, or by hitting a frying pan with a spoon. Radio waves can't.
-- Radio waves can be generated by an alternating current flowing through an isolated wire. Sound waves can't.
<span>BSL Physics Glossary - air resistance - definition. Translation: Air resistance is a force that is caused by air. The force acts in the opposite direction to an object moving through the air.</span><span>
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