When the sun was just 1 million years old--1/4600th its current age--it was brighter than it is today. But an elaborate new computer simulation indicates that the young sun was even hotter and brighter than astronomers have thought. If the results hold up, they could change the way scientists date young stars.
In its early years, the sun was still a protostar--a ball of gas in which the nuclear fusion of hydrogen into helium was just about to start. It's no surprise that this protostar shined more brightly than today's sun, because its contracting gases temporarily released more energy than fusion does now.
To arrive at the new estimate, Günther Wuchterl of the Max Planck Institute of Extraterrestrial Physics in Garching, Germany, and colleagues used a special-purpose supercomputer called GRAPE (Science, 13 July, p. 201). They simulated the entire star-forming process, starting with a fragmenting interstellar molecular cloud and continuing through the formation of protostellar "embryos" and the accretion of gas onto the young protostar. Whereas current evolutionary models peg the young sun's luminosity at just twice the present value, the new simulations suggest it was four times as bright as it is now and that its surface was 500 degrees hotter, the team reports in a paper accepted for publication in Astrophysical Journal Letters.
Most earlier simulations treated each sun-spawning step "separately and, hence, inconsistently," says theoretician Adam Burrows of the University of Arizona in Tucson. "For 40 years, the astrophysics community has been seeking a comprehensive and predictive theory of star formation. This new work is a big step toward that goal."
Astronomers deduce the mass and age of a young star from its luminosity and surface temperature, on the assumption that young protostars get fainter with age. But if protostars start out brighter than current models predict, their ages may well be underestimated, Wuchterl says.