How stars can cause comets to strike Earth

As many 24 stars are headed towards our solar system and may deflect comets, causing them to collide into the Earth in the next million years, scientists say.
Researchers at Max Planck Institute for Astronomy in Germany calculated how often stars stray into the Oort cloud, a vast, spherical shell of billions of icy objects that is thought to envelop our solar system.
Such close encounters can dislodge these loosely orbiting comets, sending them hurtling into the solar system, risking a collision course with the Earth, researchers said.
The team predicted that within the next million years between 19 and 24 stars will come within 3.26 light years of the Sun - close enough to deflect comets from their original paths.

"Certainly anything coming within that distance you should worry about," Coryn Bailer-Jones, of the Max Planck Institute was quoted as saying by 'The Guardian'.
Not all close encounters would lead to comets hitting the Earth. The collision would depend on where the Earth is in its orbit relative to the passing star - but the chances of a collision would peak at these time points.

A further 490 to 600 stars will pass the sun within a distance of 16.3 light years within the next million years, the paper estimates, researchers said.
This is far beyond the predicted outer reaches of the Oort cloud, but in the case of a very large star, still potentially close enough to cause comets to swerve in their tracks, they added.

The study was published in the journal Astronomy and Astrophysics.