Even on a sunny day, the human eye cannot see all the light emitted by the nearest star. The new image shows some of this hidden light, including the high-energy X-ray beam emitted by the hottest substance in the Sun’s atmosphere, as observed by NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR). Although the observatory usually studies objects outside our solar system, such as massive black holes and collapsed stars, it has also provided astronomers with information about our Sun.

In the composite image above, NuSTAR data is shown in blue and overlaid by observations from the Japan Aerospace Exploration Agency’s X-ray Telescope (XRT) on the Hinode mission (shown in green) and NASA’s Atmospheric Imager Assembly (AIA) on solar , to find. Dynamics Observatory (SDO) is shown in red. NuSTAR’s relatively small field of view means it cannot see the entire Sun from its Earth-orbiting position, so the observatory’s view of the Sun is actually a mosaic of 25 images taken in June 2022.

High-energy X-rays observed by NuSTAR appear in only a few places in the Sun’s atmosphere. By contrast, Hinode’s XRT detects low-energy X-rays, while SDO’s AIA detects ultraviolet light — wavelengths emitted across the Sun’s surface.

NuSTAR’s vision could help scientists solve one of the biggest mysteries of our nearest star: Why is the temperature of the Sun’s outer atmosphere, called the corona, more than a million degrees, at least 100 times hotter than its surface? This surprised scientists because the Sun’s heat originates in its core and radiates outward. It was as if the air around the fire was 100 times hotter than the flame.

The source of corona heat may be small explosions called nano-flar in the Sun’s atmosphere. Flares are large bursts of heat, light, and particles visible to many solar observatories. Nanoflashes are much smaller events, but both types produce material that is even hotter than the average coronal temperature. Regular flares don’t occur often enough to keep the corona at the high temperatures scientists have observed, but nanoflames can occur much more often—perhaps often enough to heat the corona together.

Although individual nanoflares are too faint to be seen in the bright light of the Sun, NuSTAR can detect light from high-temperature material, which is thought to occur when multiple nanoflares are formed close together. This ability allows physicists to study how often nanoflares occur and how they release energy.

The observations used in these images coincided with the 12th closest approach to the Sun, or perihelion, of NASA’s Parker Solar Probe, which has flown closer to our star than any other spacecraft in history. NuSTAR observations during one of Parker’s perihelion transits allow scientists to correlate distant activity in the Sun’s atmosphere with direct samples from the solar environment taken by the probe.