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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Stars are born in nebulae. Huge clouds of dust and gas collapse under gravitational forces, forming protostars. These young stars undergo further collapse, forming main sequence stars.
<span>Stars expand as they grow old. As the core runs out of hydrogen and then helium, the core contacts and the outer layers expand, cool, and become less bright. This is a red giant or a red super giant (depending on the initial mass of the star). It will eventually collapse and explode. Its fate is determined by the original mass of the star; it will become either a black dwarf, neutron star, or black hole </span> [i got this from google because i have stuff to do but if it's against the rules and you somehow get effected by it then my bad]
C. Electromagnetic waves don't always need a medium to travel. Note that they do vary in wavelength and frequency however their speed is fixed. Also, EM waves are always transverse and they consist of vibrating electric and magnetic fields.
After all, "Spring Tides" (peak ocean tides that arise bi-monthly) occur when the Sun, the Moon and the Earth are nearly in a straight line around the times of the New Moon and Full Moon.
