The Milky Way galaxy is going to become a cosmic snack for another galaxy, so you might want to mark your calendar -- for 5 billion years from now.

That's when our neighboring Andromeda galaxy is going to start chowing down on us, astronomers say.

Not that our galaxy isn't sometimes hungry itself, they say, and in fact our Milky Way is going consume two nearby small galaxies over the next 4 billion years -- before Andromeda starts in on us.

"All galaxies start off small and grow by collecting gas and quite efficiently turning it into stars," explained lead study author Aaron Robotham of Australia's International Center for Radio Astronomy Research (ICRAR.)

"Then every now and then they get completely cannibalized by some much larger galaxy."

That's going to be the fate of the Large and Small Magellanic Clouds, with our Milky Way as the cosmic predator.

"The Milky Way hasn't merged with another large galaxy for a long time but you can still see remnants of all the old galaxies we've cannibalized," Robotham says.

The study, published in the Monthly Notices of the Royal Astronomical Society, is based on seven years of data gathered as part of the Galaxy And Mass Assembly survey led by ICRAR.

Large galaxies turn cannibalistic because they tend to have slowed down their own rate of star formation, probably because of the extreme conditions present in their center regions, known as active galactic nucleus.

"The topic is much debated, but a popular mechanism is where the active galactic nucleus basically cooks the gas and prevents it from cooling down to form stars," Robotham says.

So size does matter, and when our galaxy encounters Andromeda in around 5 billion years, size will determine who makes a meal of whom, he says.

Gravity is the central factor in the life and death of galaxies, and eventually all of the universe's clusters and groups of galaxies will come together until the entire universe it inhabited by only a few super-giant monster galaxies.

Don't bother with your calendars; Robotham says we've many, many billions of years to mark time before it happens.

"If you waited a really, really, really long time that would eventually happen but by really long I mean many times the age of the Universe so far," he says.