SupernovaThe explosion of a star, which can shine for a few weeks as brightly as a whole galaxy. Many are found every year in distant galaxies, sometimes by amateurs, at magnitude 12 to 16. The last one seen in our own Galaxy was in 1604.
NebulaA cloud of gas and dust in space. Depending on how it shines, or does not, it is called an emission, reflection, dark or planetary nebula. The word once also covered galaxies, whose nature was unknown.
Nebula filterA filter that passes only the few colours emitted by nebulae, notably ionised oxygen (OIII, around 500 nm) and hydrogen. It darkens the sky background and brings out emission and planetary nebulae, such as the Veil Nebula. It does not help with galaxies or star clusters.