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Past Rates and Future Fates: A Galaxy's Tale

Right now, the Milky Way hosts about 100 billion stars and each year that number grows. The growth rate of new stars, however, is a sluggish crawl compared to what it was a few billion years ago, explained Dominik Riechers at this year’s American Association for the Advancement of Science annual meeting in Chicago, Illinois.

Riechers is an assistant professor of astronomy at Cornell University who studies galaxies and how they form and change over time. Galaxies are stellar factories, manufacturing new stars each year. Astrophysicists describe a galaxy’s star formation rate by the total mass of new stars it produces annually. Today, the Milky Way creates between one and two times the mass of our sun in the form of stars each year. But this was not always the case.

NASA's Spitzer Space Telescope took this infrared image of the Sunflower galaxy.  The dusty, red patches mark spots of star formation within the galaxy. Credit: NASA Jet Propulsion Laboratory

If you add the mass from all of the stars in the Milky Way, you could make about 70 billion stars the size of the sun. The Milky Way would take about 35 billion years to produce this amount of mass in the form of stars at its current star formation rate of two solar masses each year. Since the age of the universe is 14 billion years old, astrophysicists know that the star formation rate of our galaxy must have been higher during its former years, Riechers said.

The universe is riddled with galaxies that have larger star formation rates than the Milky Way. Many galaxies between 7.7 and 11.5 billion light years from Earth produce between ten and thirty times more stellar mass each year than the Milky Way today. That's nothing, however, compared to some even more distant galaxies.

For example, a distant, dusty galaxy called AzTEC-3 produces about one thousand more stars each year than the Milky Way, Riechers said. Galaxies churning out this many stars in such a short period of time are called, appropriately, starburst galaxies.

When astrophysicists like Riechers observe AzTEC-3, they’re seeing the galaxy as it was 12.5 billion years ago. At this age, the galaxy is still relatively young, turning out stars faster than you can say “AzTEC-millimeter-wave camera” – the instrument that found AzTEC-3. Ok, maybe not that fast. The galaxy creates about 5 stars the size of our sun per day, which is still more than twice what the Milky Way takes a year to churn out.

The Atacama Large Millimeter/submillimeter Array. Credit: ESO/C. Malin

AzTEC-3 is also remarkably dusty. Dust is the fuel that feeds star formation within galaxies and is also why some types of starburst galaxies shine brightly in the infrared.

Dust absorbs visible starlight and reemits it at longer wavelengths, which correspond to the infrared band of the electromagnetic spectrum. Riechers recently captured a clear image of the dusty AzTEC-3 at these longer wavelengths using the highly sensitive Atacama Large Millimeter Array (ALMA).

So far, astrophysicists have an idea of how galaxies change and evolve over time: At some point in time they undergo a burst of star formation, which gobbles up the resource of gas and dust within the galaxy. As the galaxy depletes that resource, its star formation rate takes a dive eventually reaching zero. Galaxies that no longer produce stars are what astrophysicists refer to as “dead” galaxies.

What determines how quickly galaxies will reach their peak star formation rates, how long it will last or what triggers this burst of stellar manufacturing in the first place is still unclear. But as instruments like ALMA come onto the scene, astrophysicists like Riechers will begin constructing larger samples, clearer images and, with any luck, a more complete understanding of the billions of galaxies throughout our universe as well as the Milky Way's past star formation rate and future fate.


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