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Using Ultrasound to Determine the Age of Baby Stars

1441 Using Ultrasound to Determine the Age of Baby Stars
A star is born once it becomes optically visible (bottom right). As it evolves, the star contracts and gets hotter until its core starts nuclear burning of hydrogen (bottom left) / ESO

Like an ultrasound used by doctors, astronomers are using sound waves to monitor the heartbeats of baby stars. Young stars born in clusters are often lumped together and assigned a single age. By measuring the acoustic vibrations they emit, the new technique can distinguish infant stars from adolescent ones. 

Stars are born when they become optically visible, and as they age, gravitational pull causes them to contract. They get smaller and hotter until their core temperature is sufficient to start nuclear burning of hydrogen; this is when they stabilize and leave their stellar childhood phase behind. They stay this way for vast tracts of time, until the hydrogen fuel at the core is exhausted, and they enter the last stages of their lives. Until now, asteroseismology -- the study of stars’ oscillations, or pulses -- has only been used on old, “main sequence” (MS) stars that have already started the nuclear fusion of hydrogen in their cores. 


A large international team led by Konstanze Zwintz from KU Leuven in Belgium used satellite data from the Canadian MOST and the European CoRoT, as well as ground-based observations from the European Southern Observatory (ESO) in Chile, to study the pulses of 34 pre-MS stars aged 10 million years or younger. These stars are between one and four times the mass of our sun and belong to the nebula NGC 2264, also known as the Christmas Tree Cluster. 

According to theory, young stars quake and quiver before fusion ignites in their cores. "Think of it as ultrasound of stellar embryos," study coauthor Jaymie Matthews of University of British Columbia explains in a UBC release. "Stars can vibrate due to sound waves bouncing inside. 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."

Sure enough, the team confirmed that the stars in their cohort pulsate faster as they grow older, hotter, and denser. "The youngest stars vibrate slower while the stars nearer to adulthood vibrate faster,” Zwintz explains in a KU Leuven release

Furthermore, a star's mass has a major impact on its development, Zwintz adds. Stars with a smaller mass evolve slower; heavy stars grow faster and age more quickly. A pre-MS star with a solar mass of 3.0 needs about 8 million years after it emerges from its molecular cloud of gas and dust particles before hydrogen fusion ignites in its core. A smaller pre-MS star with a solar mass of 1.5, on the other hand, would take about 33 million years. 


"We now have a model that more precisely measures the age of young stars," Zwintz says. "And we are now also able to subdivide young stars according to their various life phases." The work was published in Science last week. In this video, you can watch the pulsation properties of the star change from its birth to the beginning of hydrogen burning. 



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