Have you ever looked at a star and wondered about its origins? How long ago did it form? Is it a child on the astronomical time scale? Is it ancient?

Scientists just figured out a way to tell. While methods could roughly glean the ages of stars, distinguishing between "infants" and "adolescent" stars was always difficult.

Using astronomical "ultrasounds", or measurements of acoustic (sound) waves, astronomers can now figure out, with precision, the ages of young stars. Previous theories stated that young stars produce different vibrations than old stars. But until this study, no one had yielded concrete evidence of the theory.

Konstanze Zwintz of the Institute for Astronomy in Belgium, lead author of the paper published in Science on July 3, observed vibrations produced by radiation pressure inside stars. She and her colleagues found that the youngest stars vibrate the slowest, and the older stars vibrate faster. She also found that the masses of the stars factor greatly in the rate at which they age.

"A star's mass has a major impact on its development: stars with a smaller mass evolve slower. Heavy stars grow faster and age more quickly," says Zwintz.

When a star is born it is often in a cluster due to the gas and dust of molecular clouds contracting around it. Between the infant and adolescent stages of a star's life, the star itself shrinks a bit, due to gravitational pull. As it gets smaller, the core temperature increases until it is high enough to begin burning hydrogen. This is when a young star becomes an adult star. 

Zwintz and her team of researchers looked at 34 stars that were younger than 10 million years and about 1-4 times the mass of the sun. With the Canadian Space Agency's Microvariability and Oscillations of Stars (MOST) telescope, and the French Convection, Rotation, and Planetary Transits mission (CoRoT), the researchers were able to age multiple baby stars.

Jaymie Matthews, a MOST Mission Scientist and corresponding author of the study, says to "think of it as ultrasound of stellar embryos. We detect the sound vibrations across the vacuum of space by the subtle changes in stellar brightness. Then we translate the frequencies of those vibrations into models of the structures of those stars' hidden interiors."

Scientists hope to harness this "ultrasound" method to understand the evolution of our solar system and the universe as a whole.