Be star

Be stars are a unique and diverse group of stars. They belong to the B spectral type, which describes their temperature and light. What makes them stand out is the presence of Balmer emission lines. These lines are specific patterns of light that appear in their spectrum. While many stars show hydrogen emission lines, Be stars are a specific category. Scientists often use the term "classical Be stars" to be more precise. These are non-supergiant stars that show one or more Balmer series lines. This distinction helps astronomers separate them from other types of stars.

The way these stars function is quite remarkable. Most Be stars are main sequence stars, which are stars in their stable middle age. However, some are subgiants or giant stars. The most important factor in a Be star is its rapid rotation. This star spins very quickly on its axis. This fast spinning helps the star eject material into space. This ejected material forms a gaseous disk around the star. This structure is known as circumstellar material.

This gaseous disk is responsible for almost everything we see. The disk forms from material that the star throws off. Once the disk is there, it interacts with the star's light. The star emits ultraviolet light, which hits the gaseous disk. The disk then re-processes this ultraviolet light. This process creates the Balmer emission lines we detect. The disk also causes infrared excess and polarization. This happens because stellar light scatters as it moves through the disk.

Some Be stars appear even more unusual due to their orientation. These are known as shell stars. A shell star is actually a Be star where the disk is aligned edge-on to us. Instead of seeing the disk from above, we see it from the side. This makes the disk look like a detached shell or a ring. This specific view creates very narrow absorption lines in the star's spectrum. It is not a different kind of star, just a different way of looking at the disk.

Humans have been studying these stars for over a century. The first Be star ever recognized was Gamma Cassiopeiae. Angelo Secchi observed this star in 1866. This was a major discovery because it was the first star ever seen with emission lines. Since then, many other bright stars have been studied. Some stars once thought to be Be stars are no longer classified as classical Be stars. The brightest star in this group is Achernar. Even though it is very bright, it was not recognized as a Be star until 1976.

Be stars are also known for being variable, meaning they change over time. They can change in how they look or how their spectrum behaves. Some are called Gamma Cassiopeiae variables. This name is used when a transient or variable disk is observed. This means the disk might appear and disappear or change shape. Other stars are listed simply as BE in the General Catalogue of Variable Stars. This is used when they show variability without a clear known mechanism.

Some variability might be caused by the star itself pulsing. These are sometimes called Lambda Eridani variables. These stars are thought to be pulsating stars. Understanding these changes helps scientists learn about the life of a star. Be stars are part of a larger, heterogeneous set of stars. This includes O and A shell stars, which are sometimes included under the Be star banner. By studying these stars, we learn how rotation and gas disks shape the universe.