By Thomas Sumner- Santa Cruz Sentinel
Our galaxy wasn't always the beautiful cosmic ballet it is today. Billions of years ago, the Milky Way was a chaotic jumble of stars and gas moving every which way.
Only over time did the Milky Way morph into its current uniform shape, with flat arms of starlight reaching out from a galactic core.
Now, a new NASA study shows this process happened much more recently than scientists had thought.
Astronomers previously believed that all of the pancake-shaped "disk galaxies," such as our Milky Way, had been stagnant for the last 8 billion years -- more than half of the universe's 13.7-billion-year existence. But as a result of the study, astronomers are reconsidering how our own galaxy transformed over time.
"We found that galaxies 8 billion years ago were different from galaxies today," said lead researcher and NASA astronomer Susan Kassin, who began the study as a postdoctoral researcher at UC Santa Cruz. "This was pretty shocking to a lot of people."
Watching changes
Astronomers hadn't noticed changes in the galaxies over the past 8 billion years ago because the instruments used to examine the motions of the gas, stars and dust in a galaxy -- called spectrographs -- could look only at one galaxy at a time. Astronomers had to be picky about their galactic subjects.
"Back in the day, every shot was precious in terms of both time and money," Kassin said. "So they didn't look at galaxies that had something about them that was a little bit broken, like they were warped or disordered."
This partial sample led astronomers astray, Kassin said. The galaxies that were still changing were out there -- astronomers just weren't looking at them.
"It's as if you were taking a survey of people's heights but decided to only measure people over 6 feet tall," Kassin said. "You just end up thinking that everyone is tall."
Many galaxies
The game changer for Kassin's group was a new spectrograph that can look at one hundred galaxies at once. This meant the group could measure the inner motions of many galaxies, including the ones previous studies had skipped over. In total, Kassin studied 544 galaxies from the DEEP2 Redshift Survey, a UCSC project led by astronomers Puragra Guhathakurta, Sandra Faber and David Koo.
The study looked at the distant galaxies through telescopes. Since the light from each distant galaxy takes time to travel across the universe, astronomers glimpsed into the past. The farther away a galaxy, the more ancient the image astronomers see. A galaxy 8 billion light years away appears as it was 8 billion years ago.
Looking back in time
Using the Hubble Space Telescope and Hawaii's Keck telescope, Kassin peered back in time at hundreds of galaxies dating back to 6 billion years after the Big Bang created the universe. She found many galaxies evolving in the 8-billion-year period previously thought to be devoid of change.
"It was nice to see someone look at this in such detail," said Shelley Wright, an astronomer at the University of Toronto who studies galaxies similar to those in Kassin's research. "She has a great collection of galaxies that proves this trend in a really nice way."
Disk galaxies become more ordered over time for several reasons, Kassin said. Soon after the Big Bang, everything in the universe was much closer together, so galaxies were more apt to collide. These collisions ripped apart galaxies and jumbled their contents. In time, the universe spread out, and there were fewer collisions; galaxies settled down.
Also, earlier in the universe much more gas was around to form large stars. Larger stars tend to burn out faster and explode as supernovae, blasting their evolving host galaxies back into disarray. Since larger galaxies hold their shape better after these explosions than smaller galaxies, a galaxy's size will affect how long it takes to form its ordered shape.
Our own Milky Way likely settled into its current, ordered form about 4.5 billion years ago, around the same time our planet formed.
Celestial snapshots
Looking at similar galaxies through a telescope is like looking at snapshots of our own galaxy's past, said UCSC astronomer David Koo who co-authored the study published in the Astrophysical Journal. "It tells us a little more about our home in the universe and how we fit into the grand scheme of things; it gives us perspective on our own history from the Big Bang to life on Earth."
For her part, Kassin finds the new insight into our cosmic origins reassuring.
"Usually things tend toward more disorder," she said. "In this case, they're forming ordered systems, which is kind of nice. It's like the universe is saying to me, 'Susan, there's more order with time. Just relax; it's going to be OK.'"
Our galaxy wasn't always the beautiful cosmic ballet it is today. Billions of years ago, the Milky Way was a chaotic jumble of stars and gas moving every which way.
Only over time did the Milky Way morph into its current uniform shape, with flat arms of starlight reaching out from a galactic core.
Now, a new NASA study shows this process happened much more recently than scientists had thought.
Astronomers previously believed that all of the pancake-shaped "disk galaxies," such as our Milky Way, had been stagnant for the last 8 billion years -- more than half of the universe's 13.7-billion-year existence. But as a result of the study, astronomers are reconsidering how our own galaxy transformed over time.
"We found that galaxies 8 billion years ago were different from galaxies today," said lead researcher and NASA astronomer Susan Kassin, who began the study as a postdoctoral researcher at UC Santa Cruz. "This was pretty shocking to a lot of people."
Watching changes
Astronomers hadn't noticed changes in the galaxies over the past 8 billion years ago because the instruments used to examine the motions of the gas, stars and dust in a galaxy -- called spectrographs -- could look only at one galaxy at a time. Astronomers had to be picky about their galactic subjects.
"Back in the day, every shot was precious in terms of both time and money," Kassin said. "So they didn't look at galaxies that had something about them that was a little bit broken, like they were warped or disordered."
This partial sample led astronomers astray, Kassin said. The galaxies that were still changing were out there -- astronomers just weren't looking at them.
"It's as if you were taking a survey of people's heights but decided to only measure people over 6 feet tall," Kassin said. "You just end up thinking that everyone is tall."
Many galaxies
The game changer for Kassin's group was a new spectrograph that can look at one hundred galaxies at once. This meant the group could measure the inner motions of many galaxies, including the ones previous studies had skipped over. In total, Kassin studied 544 galaxies from the DEEP2 Redshift Survey, a UCSC project led by astronomers Puragra Guhathakurta, Sandra Faber and David Koo.
The study looked at the distant galaxies through telescopes. Since the light from each distant galaxy takes time to travel across the universe, astronomers glimpsed into the past. The farther away a galaxy, the more ancient the image astronomers see. A galaxy 8 billion light years away appears as it was 8 billion years ago.
Looking back in time
Using the Hubble Space Telescope and Hawaii's Keck telescope, Kassin peered back in time at hundreds of galaxies dating back to 6 billion years after the Big Bang created the universe. She found many galaxies evolving in the 8-billion-year period previously thought to be devoid of change.
"It was nice to see someone look at this in such detail," said Shelley Wright, an astronomer at the University of Toronto who studies galaxies similar to those in Kassin's research. "She has a great collection of galaxies that proves this trend in a really nice way."
Disk galaxies become more ordered over time for several reasons, Kassin said. Soon after the Big Bang, everything in the universe was much closer together, so galaxies were more apt to collide. These collisions ripped apart galaxies and jumbled their contents. In time, the universe spread out, and there were fewer collisions; galaxies settled down.
Also, earlier in the universe much more gas was around to form large stars. Larger stars tend to burn out faster and explode as supernovae, blasting their evolving host galaxies back into disarray. Since larger galaxies hold their shape better after these explosions than smaller galaxies, a galaxy's size will affect how long it takes to form its ordered shape.
Our own Milky Way likely settled into its current, ordered form about 4.5 billion years ago, around the same time our planet formed.
Celestial snapshots
Looking at similar galaxies through a telescope is like looking at snapshots of our own galaxy's past, said UCSC astronomer David Koo who co-authored the study published in the Astrophysical Journal. "It tells us a little more about our home in the universe and how we fit into the grand scheme of things; it gives us perspective on our own history from the Big Bang to life on Earth."
For her part, Kassin finds the new insight into our cosmic origins reassuring.
"Usually things tend toward more disorder," she said. "In this case, they're forming ordered systems, which is kind of nice. It's like the universe is saying to me, 'Susan, there's more order with time. Just relax; it's going to be OK.'"
