Many stars start their lives in open star clusters. This makes open star clusters valuable to astronomers investigating the evolution of stars and planets, as they enable the study of many stars of similar age that formed in the same environment.
Now, the most thorough research into how magnetically active young stars are has been done by astronomers. This gives scientists a glimpse of how some or all of the atmospheres of planets can evaporate as a result of X-rays from stars similar to the sun, but billions of years younger.
A team of astronomers from Penn State University, led by Konstantin Getman, examined a sample of more than 6,000 stars in ten different open star clusters ranging from 7 million to 25 million years old. Astronomers tried 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 team used NASA’s Chandra X-ray Observatory for the study because stars with more activity coupled 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-metre telescope from ESO’s La Silla Observatory in Chile, shown in red, white and blue.
The Chandra data was then combined with ESA’s Gaia satellite data to determine which stars are in the open star clusters and which are in the foreground or background. Scientists identified nearly a thousand members of the cluster.
Their findings for the open star clusters were combined with previous Chandra observations of stars as young as 500,000. The scientists found that for the first few million years, the X-ray luminosity of young Sun-like stars generally remains constant; between 7 and 25 million years old, however, this brightness decreases. The rate of reduction is faster with heavier stars.
To understand this decline, scientists first gain detailed insights into the interior of the sun and sun-like stars. The dynamo in such stars generates a magnetic field. This process involves the rotation and convection of the star, the rise and fall of hot gas in the star’s interior.
Because their convection zones shrink with age, Sun-like stars around the age of NGC 3293 have much less effective dynamos. This is a fairly slow process for stars with masses less than the Sun. A dynamo shuts down for increasingly massive stars as their convection zone disappears.
In the disk of gas and dust that surrounds all young stars, the rate at which planets form is closely related to the star’s activity. The fastest clearing, most magnetically active newborn stars quickly clear their disks, halting the formation of planets.
In the disk of gas and dust that surrounds all young stars, the rate at which planets form is closely related to the star’s activity. The fastest clearing, most magnetically active newborn stars quickly clear their disks, halting the formation of planets.
The potential habitability of the planets that form after the disk disappears is also affected by this activity, measured in X-rays. Scientists estimate that if a star is particularly active, like many of the NGC 3293 stars in the Chandra data, it will blast the planets in its system with intense X-rays and ultraviolet light.
In some situations, this high-energy attack could vaporize much of the initial, hydrogen-rich atmosphere of an Earth-sized rocky planet within a few million years. Unless a magnetic field shields it, it could also sweep away a later-formed carbon dioxide-rich atmosphere. The magnetic field of our planet itself protected the Earth from such a scenario.
Magazine reference:
- Konstantin V. Getman et al. Evolution of X-ray activity in <25 Myr old pre-main sequence stars. The Astrophysical Journal. DOI: 10.3847/1538-4357/ac7c69
