HomeScience & TechAstronomers have conducted the most extensive study to date of how magnetically...

Astronomers have conducted the most extensive study to date of how magnetically active stars

Astronomers have conducted the most extensive study to date 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 vaporize the atmospheres of planets orbiting them. Many stars begin life in “open clusters,” loosely assembled groups of stars with up to a few thousand members that all formed at roughly the same time. This makes open clusters valuable to astronomers studying the evolution of stars and planets, as they allow the study of many stars of a similar age formed in the same environment.

6,000 stars in 10 different open star clusters

A team of astronomers led by Konstantin Getman of Penn State University studied a sample of more than 6,000 stars in 10 different open star clusters between 7 million and 25 million years old. One of the goals of this study was to determine how the levels of magnetic activity of stars like our Sun change during the first tens of millions of years after their formation. Getman and his colleagues used NASA’s Chandra X-ray Observatory for this study because stars that have more activity associated with magnetic fields are brighter in X-rays.

This composite image shows one such cluster, NGC 3293, which is 11 million years old and located about 8,300 light-years from Earth in the Milky Way Galaxy. The image includes X-rays from Chandra (purple) as well as infrared data from ESA’s Herschel Space Observatory (red), longer-wavelength infrared data from NASA’s retired Spitzer Space Telescope (blue and white), and optical data from the MPG/ESO 2.2-meter telescope at ESO’s La Silla Observatory in Chile, it appears as red, white and blue.

The researchers combined Chandra data on stellar activity with data from ESA’s Gaia satellite — not shown in the new composite image — to determine which stars are in open clusters and which are foreground or background. The team identified nearly a thousand members of the cluster. They combined their results for open clusters with previously published Chandra studies of stars up to 500,000 years old. The team found that the X-ray luminosity 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 decline occurs more rapidly in more massive stars.

To explain this decline in activity, Getman’s team used astronomers’ understanding of the interior of the Sun and Sun-like stars. The magnetic fields in such stars are generated by the dynamo, a process involving the rotation of the star as well as convection, the rising and falling of hot gas inside the star. Around the age of NGC 3293, the dynamos of Sun-like stars are much less efficient as their convection zones shrink with age. For stars with a mass less than the mass of the Sun, this is a relatively slow process. For more massive stars, the dynamo disappears because the stellar convection zone disappears.

The activity of the star directly affects the processes of planet formation in the disk of gas and dust that surrounds all forming stars. The most turbulent, magnetically active young stars quickly clear their disks and stop the growth of planets. This activity, measured in X-rays, also affects the potential habitability of planets that emerge after the disc collapses. If the star is extremely active, as many of NGC 3293’s stars are in the Chandra data, then scientists predict it will blast the planets in its system with energetic X-rays and ultraviolet light.

In some cases, this high-energy burst could cause a rocky Earth-sized planet to lose most of its original hydrogen-rich atmosphere to vaporization over a few million years. It can also remove the carbon dioxide-rich atmosphere that forms later if not shielded by a magnetic field. Our planet has its own magnetic field, which prevented such an outcome for Earth. A paper describing these results was published in the August issue of The Astrophysical Journal and is available online. The paper was co-authored by Eric D. Feigelson and Patrick S. Broos of Penn State University, Gordon P. Garmire of the Huntingdon Institute for X-ray Astronomy, Michael A.

 Kuhn of the University of Hertsfordshire, Thomas Preibisch of Ludwig-Maximilians-Universitat, and Vladimir S. Airapetian from NASA’s Goddard Space Flight Center. NASA’s Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center manages science operations from Cambridge, Massachusetts and flight operations from Burlington, Massachusetts.

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