The Sun’s Closest Twins

H-alpha Sun
Is this the Sun or a close-up of 18 Scorpii? You couldn't tell by looking. (It's the Sun.)
S&T photo by Rick Fienberg.
No two people are exactly alike, but if you were to sort through millions of faces, you might find someone who is uncannily similar to you today, others who are similar to you when you were younger, and some who look exactly as you will in the future. You might learn a lot from these people. Similarly, several astronomers at Villanova University think they can learn a lot about our Sun by finding near-twins of it at all the different stages expected in its long lifetime.

At the American Astronomical Society meeting, Edward Guinan described an ongoing program called "The Sun In Time" that is identifying and studying solar analogues. Working at every wavelength, from radio to X-rays, the group is researching stars that have sizes, masses, temperatures, and heavy-element contents that are similar to those of the present-day Sun — as well as solar-twin stars at every age from birth onward.

Perhaps the greatest difference between young solar-type stars and old ones is their surface activity — flares, spots, ultraviolet and X-ray outbursts, and other byproducts of magnetic turmoil. Stars are generally born spinning fast, revolving once in a day or two (compared to the Sun's 27-day rotation period). A fast spin results in a powerful dynamo effect inside the star, which produces strong magnetism and high-energy surface explosions. "These stars are on fire," says Guinan. Over time their spins slow down (due to magnetic braking via stellar winds), and their surface activity declines accordingly. The relation between age and spin is very tight. "If you tell me the rotation period of a G star, I'll tell you its age," Guinan says. This has allowed the group to assemble a zoo of solar analogues at different points along the Sun's lifeline.

One result of this work is an appreciation of just how badly our violent young Sun treated its inner planets. Extreme-ultraviolet radiation and particle winds may have eroded most of Mercury away, leaving just its dense inner portions as the "iron planet" we see today. Venus should have at least lost its atmosphere, meaning that the massive atmosphere it has now must have arrived later, perhaps by volcanism. Earth has always had a magnetic field protecting it from the worst of the particle winds, but the intense far-ultraviolet radiation of the young Sun must have had important photochemical effects, perhaps producing or destroying molecules involved with the origin of life. Mars was protected by a magnetic field for a while and then lost it, which may be why it lost most of its atmosphere.

Ryan Hamilton
Ryan Hamilton comparing the Sun and its almost-identical twin, the faint naked-eye star 18 Scorpii.
S&T photo by Alan MacRobert.
Is there a star exactly like the Sun? No, but 5th-magnitude 18 Scorpii, located 46 light-years away in the southern summer sky, comes very close. Ryan T. Hamilton of Villanova listed its many solar characteristics: the same spectral type (G2 V) and color, a temperature only 11 degrees Kelvin (0.2 percent) hotter than the Sun, a mass 1 percent greater, a diameter 2 percent greater, and a luminosity 4 percent greater. It is 4.2 billion years old (compared to the Sun's 4.56 billion years), and it shows an activity cycle that seems rather like our familiar 11-year sunspot cycle.