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The Hubble telescope zooms in on the galaxy next door. Explore it like never before.
Cloudy blob or massive galaxy?
For most of human existence, no one knew what they were looking at when they noticed the cloud-like 'nebula' in the constellation of Andromeda. The 18th-century astronomer Charles Messier included it in a catalogue of celestial objects, the 31st entry on his list, and it came to be known as M31.
Many astronomers assumed this and other nebulae were clouds of dust and gas. The influential Harvard astronomer Harlow Shapley believed there was only one galaxy, our Milky Way, and that M31 and other nebulae were within it — and, in the cosmic scheme of things, not so far away.
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Astronomers deployed the Hubble over the course of a decade to conduct 600 separate observations to produce an extraordinary mosaic of the great spiral galaxy. (Brian Monroe and Drea Cornejo/The Washington Post)
But some scientists speculated that the nebulae might be separate galaxies of stars at a great distance.
This led to the 'Great Debate' about the scale of the universe. It was resolved early in the 20th century due to a crucial discovery by Henrietta Swan Leavitt, a 'human computer' at Harvard College Observatory. She realized that stars known as Cepheid variables get brighter and dimmer in a pattern that reveals their absolute luminosity and thus their distance from Earth.
Astronomer Edwin Hubble made the next leap when he identified a Cepheid variable star in Andromeda. 'Var!' he wrote on a photographic plate that, a century later, is kept secure in a fireproof vault at the Carnegie Observatories in Pasadena, California.
The discovery proved Hubble's conjecture that the nebula was a galaxy outside the Milky Way.
The universe kept getting vaster and vaster. Hubble (the astronomer) had observed Andromeda with a 100-inch telescope (that's the diameter of the mirror) on Mount Wilson in Southern California. Two and a half decades later, on Palomar Mountain farther to the south, astronomers began looking at the universe with a 200-inch telescope. And then came Hubble (the telescope).
It was launched in 1990 with an infamous flaw in the mirror, called a 'spherical aberration,' that made stars look like squashed spiders. Astronauts visited and installed a second, smaller mirror that precisely corrected the flaw. The Hubble became the world's most famous telescope, enjoyed four more repair visits and is still a workhorse, in demand by astronomers.
Today we know there are at least 100 billion galaxies.
An illustration of the predicted merger between our Milky Way and Andromeda, as it will unfold over the next several billion years. (ESA/Z. Levay/ R. van der Marel/STScI/T. Hallas/ A. Mellinger/NASA)
Stitching together a gift from the stars
The new Hubble mosaic offers insights about the history of Andromeda, including evidence that it has been disturbed by collisions with galaxies in the past, said Benjamin Williams, an astronomer at the University of Washington and the lead scientist on the project.
The Hubble has a small field of view — like looking into space through a narrow straw — and thus it can't possibly see the whole of Andromeda in a single observation. To accomplish the mosaic, astronomers aimed the Hubble at Andromeda during more than 1,000 of the telescope's orbits of Earth.
The resolution is so sharp that astronomers have been able to catalogue 200 million individual stars in Andromeda.
'Pictures like this remind us that we live in an incredible universe,' said NASA's Wiseman.
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For many years the consensus has been that Andromeda and the Milky Way will someday merge. A recent report in Nature Astronomy says there's only a 50-50 chance over the next 10 billion years. In any case, the stars will mostly just be ships passing in the night.
'Stars don't crash into each other,' Williams said. 'The size of the star compared to the distances between the stars is very, very small.'
The inescapable question for anyone studying Andromeda is whether there's life there, and intelligent life. Anyone staring at images of a galaxy, a cluster of galaxies or one of the Hubble 'deep field' images showing thousands of galaxies, is presented with evidence that the Earth is a minuscule element in the cosmic scheme of things.
'It's just so beautiful, and causes us to keep asking the big questions,' said Amber Straughn, a NASA astrophysicist, referring to the Andromeda mosaic. 'Can't you imagine that there might be another advanced civilization there among the trillion stars, who have also built a telescope and are looking back at us?'
About this story
Editing by Lynh Bui and Christian Font. Additional development by Dylan Moriarty. Video editing by Drea Cornejo. Photo editing by Maya Valentine. Copy editing by Briana R. Ellison.
