Ken Tapping, April 24, 2018
Imagine a clear night with just a few faint stars. The only bright, starlike objects in the sky are the other planets, and of course the Moon. This is what our night sky might look like if our Sun were a “runaway star”, flying out of the Milky Way at high speed, heading out into intergalactic space.
Runaway stars are stars that are moving unusually quickly. An average star orbits the core of its galaxy up to speeds of 100 kilometres a second. A runaway star may move at twice that speed or more. The ones moving around inside our galaxy, ploughing through the gas and dust clouds, are preceded by bow waves of hot gas. This makes them easy to spot using infrared telescopes. There are probably thousands of them. Some stars, maybe around 20 found so far, are moving fast enough to escape from our galaxy, into intergalactic space. We have even discovered a star that escaped from another galaxy and is hurtling into ours at 500 kilometres a second! How do stars become runaways? How would becoming a runaway affect a star’s planets or any inhabitants they have? What would be the consequences of being ejected into intergalactic space?
We got an important clue when we found that when we tracked back along the paths of some of the runaway stars in our galaxy, we found they started off from the same place, at the same time. They had been members of the same star cluster. Since then we have seen clusters ejecting the odd member, flinging them off at high speed. How does this happen? What exactly is going on?
Stars are formed in galaxies, through the collapse of clouds of gas and dust. Some stars form singly, but many are born in pairs or clusters, containing between several and hundreds of members. The Pleiades, or “Seven Sisters” is a star cluster most of us have seen in the autumn and winter sky. It contains hundreds of members.
Star clusters are held together by gravity. All the member stars are gravitationally tugging at each other. In such an environment, few if any of the stars have stable orbits. They weave around each other in complex, ever-changing paths, and occasionally two of them pass close by one another, in a near-collision. During the encounter, one star picks up some speed compared with the other, and they swing round each other changing their directions by up to 90 degrees. Since the cluster is already moving at tens of kilometres a second as it orbits its galaxy, the speed increase can be enough to throw one star out of the cluster, while the other one is slowed down and falls deeper into the cluster. This is called the “slingshot effect”. A really violent, close encounter can give a star enough speed to escape from its galaxy.
What happens to a collection of stars gravitationally tugging at each other is something we can simulate in a computer. It is a simple calculation but has to be done for each star, over and over again, millions or billions of times. Fortunately this is the sort of thing that computers are really good at. We find our simulated star clusters behave exactly the same way, with close encounters resulting in members being shot out at high speed and others dropping inward. In principle, if the Sun were already a runaway, it would not affect us much, but the process of becoming one certainly would. In a close encounter with another star, it is likely the Solar System would be completely disrupted. The fact that the planets are still in neat, near-circular orbits suggests that the Sun has not had a close encounter with another star since the planets formed. Even a slight change in Earth's orbit could make our world uninhabitable. We know that has not happened... yet.
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