Evrim Yazgin
Cosmos science journalist
Almost exactly 100 years ago, Edwin Hubble made a claim which changed the way we view the universe – and our place in it – forever.
On December 30, 1924, Hubble announced the discovery of a spiral nebula outside our galaxy. His breakthrough proved for the first time that our Milky Way galaxy didn’t make up the whole universe. Instead, the Milky Way was just one of many galaxies.
It was an astonishing find and is among several which led him to be referred to as one of the fathers of astronomy.
Today, we cosmologists estimate there are somewhere between 100 billion and 2 trillion (2 with 12 zeroes after it!) galaxies in the universe.
Hubble discovered that the object named Andromeda was not a nebula, but a spiral galaxy.
The Andromeda Galaxy is visible with the naked eye as a cigar-shaped object in the night sky. It is just 2.5 million light-years from Earth, making it the nearest large galaxy neighbour to the Milky Way. It is home to an estimated to have about 1 trillion stars.
In roughly 4 billion years, the Milky Way and Andromeda will collide.
Even though we live in a spiral galaxy, it’s very hard to find out much about the structure and history of the Milky Way because we are living in it. It’s like trying to determine the city plan of a major metropolitan area while standing on the corner of a downtown street.
Over the years, Andromeda has helped cosmologists understand the evolution of spiral galaxies like our own more than any other galaxy.
Now, the telescope named after Hubble has completed the most comprehensive survey of Andromeda, revealing millions of stars and clues to our galactic neighbour’s history.
“With Hubble we can get into enormous detail about what’s happening on a holistic scale across the entire disk of the galaxy. You can’t do that with any other large galaxy,” says principal investigator Ben Williams of the University of Washington.
Images from NASA’s Hubble Space Telescope can resolve more than 200 million stars. Even with Hubble’s powerful resolution, only stars brighter than our Sun are visible.
The final product required Hubble to orbit Earth more than 1,000 times, taking pictures over the course of more than a decade. It started in 2012 with the Panchromatic Hubble Andromeda Treasury. Now, the Panchromatic Hubble Andromeda Southern Treasury (PHAST) has added about 100 million stars in the southern half of Andromeda to the Treasury.
The new results are published in the Astrophysical Journal.
One surprising insight from the panorama is that, despite forming at around the same time, Andromeda and the Milky Way appear to have very different histories.
Andromeda has more younger stars and unusual features like coherent streams of stars. It seems to have had more active recent star-formation than the Milky Way.
“Andromeda’s a train wreck,” says senior author Daniel Weisz from the University of California, Berkeley. “It looks like it has been through some kind of event that caused it to form a lot of stars and then just shut down. This was probably due to a collision with another galaxy in the neighbourhood.”
The culprit may have been the compact satellite galaxy Messier 32. This galaxy looks like the stripped-down core of a former spiral galaxy.
“Andromeda looks like a transitional type of galaxy that’s between a star-forming spiral and a sort of elliptical galaxy dominated by aging red stars,” Weisz adds. “We can tell it’s got this big central bulge of older stars and a star-forming disk that’s not as active as you might expect given the galaxy’s mass.”
