Stars begin as large clouds of mostly hydrogen gas and dust. These interstellar gas clouds can be many light years across. There are many examples of these gas clouds such as the North American Nebula and the Eagle Nebula's pillars of creation.
Just like in a chemical reaction where you need to overcome the activation energy for a chemical reaction to occur, gravity needs a boost to get these large gas clouds to begins contracting into a star. As the gas contracts and becomes more dense, the force of gravity increases, speeding up the contraction.
The "activation energy" to begin the cloud contractions usually comes from one of two different events.
1. The shock wave created when the arms of a spiral galaxy pass through the gas cloud. This is why we see so many young stars being born on the leading edge of a galaxies spiral arm.
2. The shock wave from an exploding supernova star travels through space. When this shock wave from a dying star hits a gas cloud it can begin the birth and formation of new stars.
Star forming regions along the galaxies arms Shock wave from the Veil Nebula Super Nova
Young stars begin to shine and produce their own light when the core become hot enough for nuclear fusion to occur. We can see this in the Orion Nebula M42.
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