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Solar and Stellar Atmospheres

The atmospheres of stars are very different from those of planets. The Sun’s atmosphere, for instance, is an extremely hot shroud of plasma extending from the surface and stretching many times farther than the Sun’s diameter. “Weather” in that atmosphere can result in plumes of particles that sweep across the Solar System. Astronomers study the atmospheric chemistry and stellar weather on distant stars by studying the spectrum and fluctuations in the light they produce. That helps us understand the structure and evolution of stars, as well as how they interact with planets in orbit.

Fluctuations in the Sun's atmosphere are clearly seen in this image from NASA's Solar Dynamics Observatory. The bright colors represent three different wavelengths of light seen by the CfA-built Atmospheric Imaging Array.

Credit: NASA/SDO

A Star’s Crown

Astronomers have been studying the chemical composition of stars for more than a century, thanks to the way atoms in their atmospheres absorb and emit light. Starting with the work of early 20th-century Harvard College astronomer Annie Jump Cannon and her colleagues, researchers classify stars by their spectrum. Each type of atom has a unique pattern of absorption. The light from different types of stars excite atoms in their atmospheres in different ways, producing an identifiable spectral signature.

Stellar atmospheres are extreme environments. While the surface of the Sun is about 5500º C (9900º F), parts of the atmosphere can reach millions of degrees. That’s enough to strip electrons from atoms, turning the gas into a plasma. The outermost part of the atmosphere is the corona — “crown” in Latin —  which extends many times farther than the Sun’s diameter. Particles in the corona are only loosely held in by the Sun’s gravity, and many escape into space, where they are called the solar wind.

A lot of research on the solar atmosphere involves understanding why it’s that hot, and how it connects with the Sun’s magnetic activity. Magnetic variations produce sunspots and flares, which can affect communications and power grids on Earth. For that reason, astronomers monitor the Sun with a number of different observatories on the ground and in space.

Astronomers also observe flares on other stars, as well as starspot activity. These observations help identify similarities and differences between stars, which are important for understanding the conditions for life on other planets.