A star's gravity can measure its age without peeking at its surroundings.

Telling the age of a young star is hard. Most methods rely on the surrounding gas and dust rather than the star itself. In this paper, researchers tested using a star's surface gravity to tell its age, looking at low-mass stars from their class I to class III phases. As a young star forms and shrinks along its developmental path, known as a Hayashi track, its surface gravity steadily rises. This means gravity is an inherent property of the star alone, not its dusty surroundings. To test this, researchers checked young binary star pairs and a cluster called the TW Hya group. Binary stars that formed together showed matching gravities. The TW Hya stars also shared similar gravities, pointing to a median age of 12.8 +/- 1.2 Myr. Finally, interfering light from surrounding disks did not distort the gravity-based age measurements.

Why it matters

Measuring the age of young stars is notoriously difficult, yet it's crucial for understanding how stars and planets form. Current methods rely on the star's dusty surroundings, which can be unreliable. This work shows that a star's surface gravity—a property of the star itself—changes predictably with age, offering a simpler, more direct way to clock young stars.

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