Starship Asterisk*

The Energetic Sun

Our reliable Sun, the benevolent provider of light and warmth as we're used to thinking of it, is in fact a cauldron of violently churning hot plasma, and a rather unstable one at that. Violent solar activity is driven by the interplay between lines of magnetic force entangled with the spinning hot ionized gas that makes up the Sun. This interplay powers incredible outbursts of energy in an activity cycle that waxes and wanes every eleven years. At peak activity the sun's surface is marked by large numbers of magnetized sunspots, and the solar atmosphere erupts in dangerous outbursts that can harm spacefarers and bring an entire civilization to its knees. We are currently nearing the peak of the next solar activity cycle, predicted to occur in mid-2025. But this time around the Sun seems even more active than predicted by solar scientists, showing more than twice the expected number of sunspots, and producing exceptionally powerful solar flares and ejections of mass from its outer atmosphere, or corona. This behavior is certainly something that warrants close inspection. The image above is a recent multi-wavelength study of the Sun from June 2022. At left is a composite image of the Sun spanning energies from the ultraviolet (in red. from NASA's Solar Dynamics Observatory) through the low-energy X-ray band (from Japan's Hinode solar observatory, shown in green) up to the high-energy X-ray band from NASA's NuSTAR observatory (in blue). Although NuSTAR is designed to observe high-energy X-rays from accreting black holes, X-ray binary systems, galaxies and stars beyond our solar system, NuSTAR shows that even the Sun can generate these extreme forms of radiation. The image on the right is the NuSTAR view of the Sun by itself, which shows that extreme high-energy emission is produced in only a very few regions.

Some of the hottest spots in the Sun’s atmosphere appear in the telescope’s X-ray view.

Wavelengths of light from three space observatories are overlapped to provide a unique view of the Sun. Credit: NASA/JPL-Caltech/JAXA

---

Even on a sunny day, human eyes can’t see all the light our nearest star gives off. A new image displays some of this hidden light, including the high-energy X-rays emitted by the hottest material in the Sun’s atmosphere, as observed by NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR). While the observatory typically studies objects outside our solar system – like massive black holes and collapsed stars – it has also provided astronomers with insights about our Sun.

In the composite image ... NuSTAR data is represented as blue and is overlaid with observations by the X-ray Telescope (XRT) on the Japanese Aerospace Exploration Agency’s Hinode mission, represented as green, and the Atmospheric Imaging Assembly (AIA) on NASA’s Solar Dynamics Observatory (SDO), represented as red. NuSTAR’s relatively small field of view means it can’t see the entire Sun from its position in Earth orbit, so the observatory’s view of the Sun is actually a mosaic of 25 images, taken in June 2022.

The high-energy X-rays observed by NuSTAR appear at only a few locations in the Sun’s atmosphere. By contrast, Hinode’s XRT detects low-energy X-rays, and SDO’s AIA detects ultraviolet light – wavelengths that are emitted across the entire face of the Sun.

NuSTAR’s view could help scientists solve one of the biggest mysteries about our nearest star: why the Sun’s outer atmosphere, called the corona, reaches more than a million degrees – at least 100 times hotter than its surface. This has puzzled scientists because the Sun’s heat originates in its core and travels outward. It’s as if the air around a fire were 100 times hotter than the flames. ...