A common trigger would be a nearby nova or supernova. Note that it would not need to collapse the entire nebula; just creating a spot of critical density would cause a protostar to form.
Stars form when there is a sufficient concentration of interstellar gas, to begin the process of gravitational collapse into a star.
if they are massive enough and have sufficient gravitational force to overcome pressure forces and begin the process of nuclear fusion, which powers stars. This collapse is triggered by disturbances such as shock waves from supernovae or gravitational interactions with other clouds.
Stars usually begin their lives in stellar nurseries: (relatively) dense clouds of interstellar gases. Critical densities may be achieved when nearby stars go nova, at which time pockets in the nebula collapse under their own weight to form a protostar. If the mass of the protostar is sufficient, further gravitational collapse will cause the temperature and pressure in the core to rise to the point where hydrogen starts to fuse. At that point, a star is born.
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A solar nebula begins to form when a cloud of gas and dust in space collapses under its own gravity. This collapse can be triggered by a nearby supernova explosion, a shockwave from a passing star, or other disturbances in the interstellar medium.
Interstellar clouds collapse due to gravity, which overcomes the outward pressure from internal heat or other forces. As the cloud collapses, it undergoes fragmentation to form denser regions where stars eventually form. This process is triggered by various factors such as the shockwaves from supernovae or interactions with other clouds.
All-stars begin their lives as a part of a molecular cloud in space. These clouds contain gas and dust that eventually collapse and form stars through the process of nuclear fusion. The intense pressure and heat at the core of the collapsing cloud triggers the fusion of hydrogen atoms into helium, leading to the birth of a new star.
it started in 400BC and began to collapse in 900AD
As interstellar clouds approach the spiral arm region it helps them to begin the collapsing process and eventually form a star.
Glowing cloud sections that will eventually become stars are protostars. These protostars are clouds of interstellar gas and dust, which gradually collapse causing a hot dense core to form and evolve into a star.
The collapse of the North ministry and the Whig takeover of the British government
The empires that fell were the roman empire since they fell in 407 Ad