Sloan Digital Sky Survey | 2019 Jan 08
Wondering about that New Year’s Resolution to get more exercise? Good news from the Milky Way’s nearest neighbors, the Magellanic Clouds: it’s never too late to get active. After a “lazy” start of star formation for the first few billion years of their lives, both the Large and Small Magellanic Clouds are now forming new stars at a rapid rate.An astrophotographic portrait of the Large and Small Magellanic Clouds (lower
left and upper right, respectively) above Las Campanas Observatory in Chile.
(Credit: Ryan Trainor (Franklin and Marshall College))
Astronomers from the Sloan Digital Sky Survey (SDSS) reached that conclusion by making, and then analyzing, the first-ever detailed chemical maps of galaxies beyond our own. ...
The key to making these new maps is to collect spectra of as many stars as possible. Spectra are observations of the amount of light that stars give off at various wavelengths, or colors. Spectra of stars reveal many things about them, such as how they are moving, their temperature, the chemical elements they contain, and their stage in the stellar life cycle. The APOGEE-2 South spectrograph works primarily at infrared wavelengths, longer than the human eye can see. The maps of the Magellanic Clouds that Nidever’s team made relied on the ability of spectra to reveal the chemical compositions of stars.
Measuring the detailed chemical abundances found in stars can provide both a clock and a speedometer for star formation, because of how the elements in our universe are created. Hydrogen and helium came from the Big Bang, but most heavier elements were formed deep inside stars, and only get released at the end of a star’s life, often by supernova explosions. That means that if you add up all the heavier-than-helium elements you can see in all stars in a galaxy, the total provides a measure of how many stars that galaxy has formed during it life. What’s more, the composition of stars reveals the composition of the gas cloud they were made from, serving as a clock for recording the star’s age. ...
The results show that both the Large and Small Magellanic Clouds have had histories very different from that of the Milky Way. Their stars’ relative abundances of alpha elements came into balance at much lower value of heavy elements (earlier times) than the Milky Way’s, indicating a “lazy” first few billion years in which few new stars formed. But very recently, alpha elements have become more plentiful, indicating a burst of recent star formation. ...