There are several possibilities. If the collision speed is higher than a particular threshold speed, say about 300 miles per second, enough kinetic energy would be imparted to the two masses that the stellar material would dissipate into a vast expanding cloud of gas, never to reassemble itself into a new star.
If the speed were very slow, the stars would merge into a new, more massive, star. The evolution of the new star would begin with a rejuvenated core of fresh fuel since the merging of the two stars would have mixed new hydrogen fuel into the core of the new star.
If the speed of the impact is moderate and off center, the stars will go into a very tight orbit around one another, perhaps even sharing a common gaseous envelope. Over time, the two separate cores would spiral into each other, and you would again be left with one new, massive star. Since the escape velocity of the Sun is about 1.3 million miles per hour, this is about equal to the threshold speed of the impact.
If a smaller star, like a white dwarf or neutron star, smashes into a bigger star, like a red giant, most of the giant's outer envelope would be blown off as it absorbs the impact. The results get a little more violent when two smaller stars collide. Neutron stars are very small and dense. If a neutron star reaches a certain mass, it will implode and form a black hole. Therefore, if two neutron stars merge but their combined mass is more than the maximum mass a single neutron star can have, they implode into a black hole. If the circumstances are the same when two white dwarf stars collide, they will implode into a neutron star.
A team of astronomers is making a bold prediction: In 2022, give or take a year, a pair of stars will merge and explode, becoming one of the brightest objects in the sky for a short period. It’s notoriously hard to predict when such stellar catastrophes will occur, but this binary pair is engaged in a well-documented dance of death that will inevitably come to a head in the next few years, they say. The researchers began studying the pair, known as KIC 9832227, in 2013 before they were certain whether it was actually a binary or a pulsating star. They found that the speed of the orbit was gradually getting faster and faster, implying the stars are getting closer together. The pair is so close, in fact, they share an atmosphere (see an artists version of this above:ESO/L. Calçada). KIC 9832227’s behavior reminded the researchers of another binary pair, V1309 Scorpii, which also had a merged atmosphere, was spinning up faster and faster, and exploded unexpectedly in 2008. Now, after 2 years of careful study to confirm the accelerating spin and eliminate alternative explanations, the team predicted in 2017 that the pair will explode as a “red nova”—an explosion caused by a binary merging—in about 5 years’ time.
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