It may be a big universe, but it's not big enough for those galaxies that bump into one another. Such interactions can trigger the birth of new stars, creating a so-called starburst; M82 (shown) is the best-known starburst galaxy. Now, as astronomers report online before print in The Astrophysical Journal Letters, they have analyzed 1478 nearby galaxies to compile some statistics: Eighteen percent of starburst galaxies are interacting with other galaxies versus only 9% of normal galaxies. The researchers therefore conclude that galactic interactions do cause starbursts, yet most starburst galaxies are not involved in interactions. That may be because starbursts can arise in other ways, such as when a large cloud of gas falls into a galaxy. A starburst is probably in our distant future: Billions of years from now, the Andromeda galaxy will smash into the Milky Way, stimulating a rash of new star birth as gas clouds in the two giant galaxies crash together.