Chromosphere

The Sun has a special layer.

It sits above the brightest part. This layer is a sphere of color. It can look pink or red.

You can only see it during an eclipse. This is because the bright part is too strong. Small jets of gas rise up like hair.

These jets sink back down after ten minutes. Other stars have these layers too. It is a very busy place.

The chromosphere is a layer in a star's atmosphere.

It sits above the photosphere. The photosphere is the bright part we see. The name means "sphere of color." This layer looks pink or red. It looks red because of H-alpha light. This light comes from hydrogen atoms.

You cannot see it easily. The photosphere is too bright. You can see the red color during a total eclipse.

The layer has many moving parts. You can see spicules. These are narrow jets of plasma. Plasma is hot gas. Spicules rise up and then sink down. This takes about ten minutes. You can also see plasma loops. These look like arches. They can appear or vanish in less than an hour. Other stars have these layers too. On a huge star called Antares, the layer is very thick. It is 2.5 times larger than the star's radius.

The chromosphere is a special layer in a star's atmosphere.

Its name comes from Greek words meaning "sphere of color." It sits above the photosphere, which is the bright surface we see. It also sits below the solar corona. This layer is not just found on our Sun. Almost all bright stars have a chromosphere. On some huge stars, this layer is very big. For example, the supergiant star Antares has a chromosphere 2.5 times thicker than the star's radius.

This layer has a very distinct reddish color. It looks like shades of pink or deep red. This happens because of a thing called the H-alpha spectral line. This line is made when a hydrogen atom moves an electron between two energy levels. Specifically, the electron moves from the n=3 level to the n=2 level. This process releases light at a wavelength of 656.3 nanometers. This specific wavelength is in the red part of the light spectrum. You can also see the layer using ionized calcium. This shows up in the violet part of the spectrum at 393.4 nanometers.

An English astronomer named Norman Lockyer first suggested the name. He used systematic observations of the Sun to name it. He wanted to tell this layer apart from the white-light photosphere. Most of the time, the chromosphere is invisible to us. The photosphere is just too bright to look past. You can only see the red color during a total solar eclipse. One famous time people saw this was during the eclipse on August 11, 1999. Scientists also use special tools to study it. They look at the electromagnetic radiation the layer sends out.

Many interesting things happen inside this layer. One common feature is called a spicule. These are narrow jets of plasma that look like hair. Spicules rise up and then sink back down in about 10 minutes. You can also find plasma loops at the edge of the Sun. These look like arches and can change very quickly. Some loops expand in only 10 to 20 minutes. There are also bright regions called plage. These areas are often linked to magnetic activity. Scientists have even found periodic oscillations in the plasma. These waves can happen every three minutes.

The chromosphere has very different physical properties than Earth. The density of the gas drops very fast as you move away from the Sun's center. It is much thinner than the air we breathe at sea level. In fact, it is $10^{-8}$ times the density of Earth's atmosphere. The temperature in the layer changes in a strange way. It starts at about 4,400 Kelvin at the bottom. Then the temperature drops to a minimum of about 2,700 Kelvin. After that, the heat rises again to over 20,000 Kelvin. This heat helps the layer connect to the corona above.

The chromosphere is the second layer of a star's atmosphere. It is located above the photosphere and below the transition region and the corona. The name comes from Ancient Greek words meaning "sphere of color." While the term usually refers to our Sun, it applies to the corresponding layers in other stars. This layer is vital for understanding how stellar atmospheres work. It acts as a bridge between the bright surface and the outer reaches of space.

Scientists study the chromosphere by analyzing electromagnetic radiation. The layer has a very distinct reddish color. This hue ranges between pink and red. This color comes from the H$α spectral line. This line occurs when a hydrogen atom's electron moves from the n=3 energy level to the n=2 level. This specific movement releases light at a wavelength of 656.3 nanometers. Because that wavelength is in the red part of the spectrum, the layer looks red. You can also see it through ionized calcium. This appears in the violet part of the spectrum at 393.4 nanometers.

Physical properties in this layer change rapidly with distance. The density of the gas decreases exponentially from the center of the Sun. It drops by a factor of roughly 10 million from the inner boundary to the outer boundary. The density is $10^{-4}$ times that of the photosphere. It is also $10^{-8}$ times the density of Earth's atmosphere at sea level. Temperature also shifts significantly. It starts at about 4,400 Kelvin at the inner boundary. It drops to a minimum of approximately 2,700 Kelvin. Then, the temperature rises to over 20,000 Kelvin at the outer boundary.

Many dynamic phenomena occur within this layer. One common feature is the spicule. These are narrow jets of plasma that look like hair. Spicules rise through the chromosphere and can extend into the corona. They rise and then sink back down over about 10 minutes. You may also see plasma loops at the border of the solar disk. These are concentric arches with temperatures around 10,000 Kelvin. These loops are very variable. They can appear or disappear in less than an hour. Some can expand rapidly in only 10 to 20 minutes.

Other features include bright regions called plage. These areas are often associated with magnetic activity. The chromosphere also shows a "network" pattern. This consists of bright cells surrounded by darker regions called the internetwork. These patterns look similar to the granules seen on the photosphere. Scientists have also detected periodic oscillations in the plasma. Using the SUMER instrument on SOHO, they found frequencies between 2 and 6 millihertz. This corresponds to a period of about three minutes. These oscillations are typical in the high chromosphere.

History shows how our understanding of this layer grew. The English astronomer Norman Lockyer suggested the name. He conducted systematic solar observations to name it. He wanted to distinguish this layer from the white-light emitting photosphere. For a long time, the chromosphere remained hidden. The photosphere is so bright that it overwhelms the layer. Most people can only see the reddish color during a total solar eclipse. A famous example was the solar eclipse on August 11, 1999.

Chromospheres are not unique to our Sun. They exist on almost all luminous stars except for white dwarfs. They are most prominent on lower-main sequence stars and brown dwarfs. They are also found on giant and subgiant stars. On very large stars, the chromosphere can be a huge part of the star. For instance, the supergiant star Antares has a chromosphere about 2.5 times thicker than the star's radius. Astronomers use the Mount Wilson S-index to measure chromospheric activity on other stars. This helps them understand the magnetic life of different stellar systems.