Surprising dissimilarities in a newly formed pair of 'identical twin' stars

Nature453, 1079-1082 (19 June 2008) | doi:10.1038/nature07069; Received 7 February 2008; Accepted 2 May 2008
http://www.nature.com.libproxy.ucl.ac.uk/nature/journal/v453/n7198/full/nature07069.html

News Article

In 2007 a group of astronomers from UK and USA discovered an interesting stellar pair in the famous Orion Nebula star forming region – a binary system, with a given name of Par 1802 – a symbiosis of two exceptionally young stars of the age of approximately 1 million years, which, despite their common natal material they were born from, have surprising dissimilarities in the main physical properties such as size, mass and temperature, with one being almost 10 per cent hotter, larger and thus more luminous than its companion.

It is worthy to mention that mass and chemical composition are the key parameters, which determine the future of all stars in the whole Universe. The first factor, mass of the twins, was calculated to be the same to a high degree of precision and the second one, chemical composition, was presumed to be common for both stars, as they were born in the same molecular-dust cloud.

Observing and analysing two stars, which are orbiting the same gravitational centre, trough two big telescopes in America enabled scientists to investigate the discovered system. The team of astronomers lead by Keivan G. Stassun (Vanderbilt University, Nashville, USA) calculated the stellar twins’ masses to be almost equal to each other and compose 40 per cent of that of our own star; also being cooler and almost two times larger than the Sun we are orbiting. For age comparison: since Par 1802 twins are one million years old and 4.5 billion years have passed since a tiny piece of the Milky Way at the periphery was firstly illumined by our newborn Sun, it is 4500 times older.

The surprise is in the fact, that according to the common sense, two stars of the same mass and chemical composition should evolve in time similarly, but the lack of correspondence in predictions and practical observations always pushes theoreticians to find alternative ways of explaining phenomena. Those 10 per cent differences in parameters were finally explained by a theoretical model, which predicts that young stars with the approximate mass of the stars discovered undergo a short period of rapid evolution at the age of one million years.

The practically obtained parameters fit precisely into the theoretical model, thus explaining the difference between those twins of Par 1802. According to the article in “Nature”, where the results of the investigation were published in May 2008, the predicted rapidity of evolution proposes that a cooler and less luminous companion had “a 'lag' of only a few hundred thousand years” in its growth.

Par 1802 is the youngest binary system of equal mass ever discovered. The observed case helps scientists-theoreticians to find proofs for their theories of stellar evolution at early stages. The stars observed also reveal a limit on the age synchronization of young binary stars, which is frequently used by astronomers for age calibrations of star-formation models.

The main techniques the team of Keivan G. Stassun employed included measuring radial velocities by a high-resolution spectrograph, and obtaining light curves of rotating twins by telescope detectors, which recorded the light intensity variations during their mutual motion.