1. Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, USA
2. Department of Astronomy, University of Wisconsin—Madison, Madison, Wisconsin 53706, USA
3. School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK

Correspondence to: Keivan G. Stassun¹ Correspondence and requests for materials should be addressed to K.G.S. (Email: keivan.stassun@vanderbilt.edu).

Abstract

The mass and chemical composition of a star are the primary determinants of its basic physical properties—radius, temperature and luminosity—and how those properties evolve with time¹. Accordingly, two stars born at the same time, from the same natal material and with the same mass, are 'identical twins,' and as such might be expected to possess identical physical attributes. We have discovered in the Orion nebula a pair of stellar twins in a newborn binary star system². Each star in the binary has a mass of 0.41  0.01 solar masses, identical to within 2 per cent. Here we report that these twin stars have surface temperatures differing by 300 K (10 per cent) and luminosities differing by 50 per cent, both at high confidence level. Preliminary results indicate that the stars' radii also differ, by 5–10 per cent. These surprising dissimilarities suggest that one of the twins may have been delayed by several hundred thousand years in its formation relative to its sibling. Such a delay could only have been detected in a very young, definitively equal-mass binary system³. Our findings reveal cosmic limits on the age synchronization of young binary stars, often used as tests for the age calibrations of star-formation models⁴.

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