Our Milky Way galaxy is an anomaly in more ways than one. And now, NASA scientists say they know exactly when it will come to an end.

In a universe that is forever spreading apart, the Milky Way has been moving closer to celestial neighbor the Andromeda galaxy. But whether we are in for intergalactic Armageddon or an extraterrestrial fender bender has been a mystery - until now.

“Very interestingly, we find that Andromeda galaxy does appear to be coming straight at us,” said Roeland van der Marel, an astronomer at the Space Telescope Science Institute in Baltimore. He was scheduled to speak at a NASA press conference Thursday.

The discovery was made thanks to images taken over the 22-year lifespan of the Hubble Space Telescope. But the quest to determine the Milky Way galaxy’s expiration date has been undertaken by astronomers for more than 100 years. Now, for the first time, NASA scientists say they know “with certainty” when our beloved galaxy will cease to exist as we know it, what it will look like and how it will happen.

New data collected by the Hubble Space Telescope proves, NASA says, that in 4 billion years the Milky Way and Andromeda will collide or pass each other by so closely that the gravitational force each exerts on the other will cause them to slow down to the point of merging. The merger will be completed 6 billion years from now.

When galaxies collide

At the outset of universal existence, Andromeda and the Milky Way were each going their separate ways, van der Marel said.

“They have been approaching each other for the last 5 billion years," he said. "For the first 8 billion years, they were moving away from each other, and now they are moving closer together - and that will continue, always.”

In our local universe, which consists of around 50 galaxies, according to van der Marel, the Milky Way and Andromeda are goliaths. There are two galaxies that have around one-tenth the mass of two giants, and the rest are less than 1%. Each exists at the center of its own universal microcosm, with smaller galaxies swirling around similar to a solar system.

It is the massive gravitational pull that ultimately drew the Milky Way and Andromeda together, and will ultimately cause them to become one.

“The clear finding is, we are going to merge with Andromeda,” van der Marel said. “In the past, it was just a possibility, but now it is a known fact that this will happen.”

The finding was determined by comparing pictures of the sky taken by the Hubble Space Telescope over a number of years and comparing the movement of the galaxies, allowing scientists to determine the trajectory of the Andromeda for the first time.

There is a 9% chance that M-33, a satellite galaxy of Andromeda, will hit the Milky Way first in what van der Marel called a "one-two punch," causing it to become a satellite of the new galaxy that is formed.

What an intergalactic merger looks like

Van der Marel described it as a “really big cosmic pileup” that will light up the sky.

When the two galaxies hit, each containing its own set of stars and cosmic gases, the result will be the formation of many new stars - all shining bright.

The Milky Way, as it exists now, is a flat disc shape similar to a frisbee. Andromeda is more spherical. When the two combine, they will form an "elliptical galaxy," or what van der Marel described as a football-shaped galaxy. Rather than seeing a band of stars on the cosmic horizon, someone on Earth would theoretically be surrounded by them as shown in a simulation unveiled to show what the night sky will look like in 6 billion years.

The new vision of the stars won't be the only earthly changes.

“Our sun and Earth will go on a new journey through the universe,” van der Marel explained.

When Andromeda gets here, the sun will likely be pushed out much farther into the universe. By that time though, Earth will have become too hot to be inhabited by humans anyway.

Our sun will not be directly hit when the initial collision happens in 4 billion years. But in 6 billion years, when the merger is complete, our sun will die.

Meaning in the cosmos

Life on Earth as we know it will certainly not be possible by the time this great galactic merger is expected to take place. But as van der Marel said, there are “many more uncertainties than the laws of physics,” and the human race may well have figured out how to carry on with existence.

"It's a very long time from now, so people don’t have to lie awake or take out insurance," van der Marel said. "What makes it really special is it is going to happen to us, it's our sun, it's our planet, and humans are really fascinated about what our fate is going to be."