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Surprising dissimilarities in a newly formed pair of ‘identical twin’ stars


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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Figure 1: Light curve of Par 1802 at I band (0.8 μm).
Figure 2: Spectral energy distribution of Par 1802.
Figure 3: Comparison of physical properties of Par 1802 with theoretical predictions.


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We thank A. Prsa for software used in our analyses. This work is supported by grants to K.G.S. and R.D.M. from the National Science Foundation, and a Cottrell Scholar award to K.G.S. from the Research Corporation. K.G.S. acknowledges the hospitality of the Space Telescope Science Institute’s Caroline Herschel Distinguished Visitor programme.

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Correspondence to Keivan G. Stassun.

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The file contains Supplementary Discussion, Supplementary Figure 1 with Legend and Supplementary Table 1. A detailed discussion of the methods is presented by which the flux ratio of the components of Par 1802 is determined from analysis of spectra obtained at multiple orbital phases. The Supplementary Figure 1 illustrates this spectroscopic analysis and the measurement of the flux ratio. The Supplementary Table 1 lists the flux measurements of Par 1802 over the wavelength range 0.36 to 8.0 micrometres, which are used in the paper to determine the distance and interstellar extinction to the system (Figure 2). (PDF 580 kb)

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Stassun, K., Mathieu, R., Cargile, P. et al. Surprising dissimilarities in a newly formed pair of ‘identical twin’ stars. Nature 453, 1079–1082 (2008).

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