Tour: Chandra Sees Stellar X-rays Exceeding Safety Limits

Astronomers have made the best study yet of how magnetically active stars are when they are young. This gives scientists a window into how X-rays from stars like the Sun, but billions of years younger, could partially or completely evaporate the atmospheres of planets orbiting them.

Many stars begin their lives in “open clusters”, loosely packed groups of stars with up to a few thousand members, all formed roughly at the same time. This makes open clusters valuable for astronomers investigating the evolution of stars and planets, because they allow the study of many stars of similar ages forged in the same environment.

A team of astronomers studied a sample of over 6,000 stars in 10 different open clusters with ages between 7 million and 25 million years. One of the goals of this study was to learn how the magnetic activity levels of stars like our Sun change during the first tens of millions of years after they form. The researchers used NASA’s Chandra X-ray Observatory for this study because stars that have more activity linked to magnetic fields are brighter in X-rays.

The researchers combined the Chandra data of the stars’ activity with data from ESA’s Gaia satellite, Herschel Space Observatory, and NASA’s Spitzer Space Telescope. They also compared their results for the open clusters with previously published Chandra studies of stars as young as 500,000 years old. The team found that the X-ray brightness of young, Sun-like stars is roughly constant for the first few million years, and then fades from 7 to 25 million years of age. This decrease happens more quickly for heftier stars.

A star’s activity directly influences the formation processes of planets in the disk of gas and dust that surrounds all nascent stars. The most boisterous, magnetically active young stars quickly clear away their disks, halting the growth of planets. The X-rays also affect the potential habitability of the planets that emerge after the disk has disappeared. If a star is extremely active, as with many NGC 3293 stars in the Chandra data, then scientists predict they will blast their planets with energetic X-ray radiation and ultraviolet light. Luckily for us here on Earth, our planet possesses its own magnetic field that protected it and its atmosphere against this high-energy radiation from a very young Sun.