One of the long-standing difficulties in astrophysics has been a way to accurately determine the age of a star. Brand new stars are obvious from their location in or near "star nurseries" of interstellar gas and dust, and "adult" stars can be roughly characterized through various methods, including a calculation based on their mass and luminosity. Unfortunately, these methods are approximations at best. Researchers at KU Leuven's Institute for Astronomy have now discovered a way to distinguish young stars from older ones by measuring the acoustic waves that they emit using ultrasound technology.

The result of the accretion of shrinking clouds of gas and dust particles, a star evolves from "newborn" to "adolescent" as growing gravitational forces cause it to contract. As these forces continue, the star becomes smaller, denser, and hotter until the core temperature is great enough to trigger thermonuclear fusion. Once this stage has been reached, and the star has stabilized in its size and fusion energy production over a very long period of time, it is classed as an "adult" and generally remains in this state for many billions of years.


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As a general rule, as an adult star ages it becomes brighter. This means it is possible to approximate the ages of these stars using a calculation based on their mass and luminosity. This method works particularly well for stellar objects in their main sequence, where they are stable, adult stars falling within a particular range of mass, color, and luminosity.

Other methods of calculating a star's age include the amount of dust and gas it may be emitting, aberrant patterns in its light outputs, or inordinate stellar flare or X-ray activity. However most of these methods apply only to older stars or ones nearing the end of their life. Newly-born stars are much more difficult to characterize; the most accurate method is to look in nebula where dense molecular clouds are collapsing and condensing to form nascent stars.

Researchers led by Dr. Konstanze Zwintz at KU Leuven looked in one such nebula – NGC 2264, commonly known as the Christmas Tree Cluster – to study 34 stars known to be under 10 million years of age. Sized between one and four times the mass of our sun, each of the stars was ultrasonically studied to test the theory that the age of a star may be determined by its vibrational frequency.

"Think of it as ultrasound of stellar embryos," explains study co-author Professor Jaymie Matthews from the University of British Colombia. "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."

The notion that immature stars vibrate at different frequencies to more mature ones has been previously contended by theoretical physicists, but Dr. Zwintz's study is the first to confirm these theories using solid data gleaned from the stars themselves.

"Our data shows that the youngest stars vibrate slower while the stars nearer to adulthood vibrate faster," says Dr. Zwintz. "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. We now have a model that more precisely measures the age of young stars, and we are now also able to subdivide young stars according to their various life phases."

As a result of their studies, the researchers claim to show that there is a relationship between a young star's detected pulsation properties and its evolutionary status, thereby showing the potential of ultrasound to determine the ages of other new stars with much greater accuracy.

The scientists obtained their data using the Canadian MOST satellite, the European CoRoT satellite, and from Earth-based facilities including the European Southern Observatory (ESO) in Chile.

The research was published this month in the journal Science

The short video below details the pre-life story of a star, spanning about 10 million years from conception to birth.

Source: KU Lueven