It turns out that our universe is far from a static place.
Thanks to observations from very powerful ground-based telescopes and space telescopes
like the Hubble, we know our universe is seething with activity.
Galaxies are swirling in a cosmic dance breathtaking in its scope.
With so many galaxies hurtling through the heavens, over 100 billion of them - all whirling
around each other, pulled by their gravity - inside this cosmic maelstrom, many galaxies collide.
Some of them come at each other with such force, that both are forever altered and morphed by the encounter
Throughout it's majestic lifetime, the Hubble Space Telescope has been providing us with
unparalleled glimpses of galactic collisions.
In, 2008 the Space Telescope Science Institute released this image containing some of these
moments, 59 individual images, each a snapshot of an instant in a galactic collision.
But how do we know these are glimpses of various points in a several billion year long event?
Here is a simulation that shows a complete galaxy collision over a timescale of one and a half billion years
The two spiral galaxies in the simulation distort, twist, and merge together, and by
rotating the images at different times and viewing angles throughout the simulation,
we can match each epoch with a corresponding Hubble image.
As they approach over a few hundred million years, galaxy trajectories become dictated
by their shape and mass - guided by the laws of gravity.
Soon over the course of another 200 million years, one galaxy buzz saws through the other
slicing it through the middle. Amazingly, because of the distances between the stars
in each galaxy (about 1 light year for every star) very few of the stars in these galaxies
will hit each other.
The individual stars in each galaxy will simply pass each other by, suffering no effects other
than perhaps an altered galactic orbit.
After 300 million years, tidal tails develop - streaming stars into the intergalactic void,
some of these stars will be flung out entirely - left to wander the universe without a galactic home.
At this stage, the centers of the two galaxies are now bound. Held together by gravity and pulling each other closer
As the centers coalesce, each galaxy is no longer distinct - and one and a half billion
years since their dance began they have become one galaxy.
It will take another few billion years for the tidal tails to return.
Galaxy collisions are very common in the universe. Every large galaxy we see has collided with
another galaxy at least once in the past 9 billion years.
Smaller galaxies collide three times more often than large galaxies and anywhere from
five to 25 percent of all galaxies seen in the various Hubble Deep Fields were in the
process of merging at the time they were taken.
Collisions are an essential element of the lifecycle of all galaxies and our own home
- the Milky Way - is not immune.
We are on a collision course with a monster spiral, called the Andromeda Galaxy, which contains some 1 trillion stars
What will that look like?
Our galaxies will begin to crash together in a head-on collision about 4 billion years
from now. The thin disk shapes of these spiral galaxies are strongly distorted and irrevocably
transformed by the encounter.
In around 6 billion years, the two galaxies will merge to form a single elliptical galaxy
and the tiny Triangulum galaxy continues to orbit the merged pair perched high above the chaos.
We on Earth, if we're still around, will be treated to a galactic show unequalled in human
history. Here is the view 25,000 light years from the center of the Milky Way.
This is our sky tonight, the bright belt of our Milky Way clearly visible. The Andromeda
galaxy still lies 2.5 million light-years away and looks like a faint spindle, several
times the diameter of the full Moon.
In 2 billion years, The disk of the approaching Andromeda galaxy is noticeably larger.
Andromeda fills the field of view. The Milky Way begins to show
distortion due to the tidal pull from Andromeda.
During the first close approach, the sky is ablaze with new
star formation, which is evident in a plethora of emission nebulae and open young star clusters.
After its first close pass, Andromeda is tidally stretched out.
The Milky Way also becomes warped.
During the second close passage, the cores of the Milky Way and Andromeda
appear as a pair of bright lobes. Star-forming nebulae are much less prominent now because
the interstellar gas and dust has been significantly decreased by previous bursts of star formation.
Finally - 7 Billion Years hence — The merged galaxies form a huge elliptical galaxy, its
bright core dominating the nighttime sky. Scoured of dust and gas, the newly merged
elliptical galaxy no longer makes stars and no nebulae appear in our sky. The aging star
population is no longer concentrated along a plane, but instead fills an ellipsoidal volume.
The Milky Way and Andromeda galaxies are now one.