Nebula are an accumulation of gas particles.
Bearing in mind that there untold billions of atoms in the cosmos.
Any two atoms have a gravitational attraction between them. they come together, thus attracting more gas particles by gravitational attraction.
These gas particles become so crowded that they begin to glow, (the nebula).
The analogy, is a dance hall/disco; maybe two people will statr to dance. Eventually a lot more people arrive on the dance floor, and it becomes hot and crowded.
Under further gravitational attraction these hot crowded gas particles keep accumulating into a hot glowing ball. Because partiucles are now so crowded nuclear fusion starts, and releases energy. Hence a Star/Sun.
Because this process takes billions of years we do not instantaneously see it going on.
NB All stars are nuclear reactors in space.
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Emission nebula glow and reflection nebula reflect the light form other stars
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An absorption nebula is a dark nebula, which absorbs all incident radiation without reemission.
Well, isn't that a happy little thought! If the pressure inside a nebula were greater than the force of gravity, it might cause the nebula to expand and disperse into the surrounding space. Just like a gentle breeze carrying flower petals through the air, the nebula's beautiful gases could drift away and create new wonders in the cosmos. Remember, in the vast universe, there's always room for new beginnings and endless possibilities.
Another popular nebula is the Eagle Nebula, also known as Messier 16. It is located in the constellation Serpens and is famous for the "Pillars of Creation," which are towering columns of interstellar gas and dust where new stars are forming.