On a summer night, high above our heads, where the Northern Crown and Herdsman meet, a titanic new galaxy is being born 4.5 billion light years away. You and I can’t see it but astronomers using the Hubble Space Telescope just released photographs today showing how the merger of two enormous elliptical galaxies will one day create one of the heavyweights of the universe.
The two giants, each about 330,000 light years across, are members of a large cluster of galaxies called SDSS J1531+3414. They’ve strayed into each other’s paths and are now helpless against the attractive force of gravity which pulls them ever closer.
Galactic mergers are violent events that strip gas, dust and stars away from the galaxies involved and can alter their appearances dramatically, forming large gaseous tails, glowing rings, and warped galactic disks. Stars on the other hand, like so many pinpoints in relatively empty space, pass by one another and rarely collide.
Elliptical galaxies get their name from their egg-like oval shapes. They lack the spiral arms, rich reserves of dust and gas and pizza-like flatness that give spiral galaxies like Andromeda and the Milky Way their multi-faceted characters. Ellipticals, although incredibly rich in stars and globular clusters, generally appear featureless.
These two appear to be different. Unlike their gas-starved brothers and sisters, they’re rich enough in the stuff to induce star formation during the merger. Take a look at that string of blue blobs stretching across the center – astronomers call it ‘beads on a string’ star formation. The knotted rope of gaseous filaments with bright patches of new star clusters stems from the same physics which causes rain or water from a faucet to fall in droplets instead of streams. In the case of water, surface tension causes water to ‘snap’ into individual droplets; with clouds of galactic gas, gravity is the great congealer.
Nineteen compact clumps of young stars make up the length of this ‘string’, woven together with narrow filaments of hydrogen gas. The star formation spans 100,000 light years, about the size of our galaxy, the Milky Way. Astronomers still aren’t sure if the gas comes directly from the galaxies or condensed like rain from X-ray-hot halos of gas surrounding both giants.
The blue arcs framing the merger have to do with the galaxy cluster’s enormous gravity, which acts like a lens to bend the light of more distant background galaxies into curvy strands of blue light that are highly distorted images of real objects.
Milky Way and Andromeda collide in 4 billion years – a simulation
Four billion years from now, Milky Way residents will experience a merger of our own when the Andromeda Galaxy, which has been heading our direction at 300,000 mph for millions of years, arrives on our doorstep. After a few do-si-dos the two galaxies will swallow one another up to form a much larger whirling dervish that some have already dubbed ‘Milkomeda’. Come that day, a hard stellar rain’s a gonna fall.