Be star

Some stars have a bright glow.

This glow comes from a ring of gas. The star spins very fast. This fast spin helps gas fly out.

The gas forms a flat disk. This disk is around the star. The gas catches light from the star. This makes the star look special.

One star is called Achernar. It is the brightest of these stars. People found it in 1976.

Some stars look like they have a shell. This happens when we see the disk from the side. It is a neat sight to see.

Some stars have a special glow. We call these Be stars.

These stars spin very fast. This fast spin helps throw gas out from the star. The gas forms a flat ring around it. We call this a gaseous disk.

The disk changes the star's light. The disk catches light from the star. This makes the star look different to us. Some stars look like they have a shell. This happens when we see the disk from the side.

Be stars can change over time. This is called variability. Some stars have a disk that comes and goes. Others might pulse.

Angelo Secchi saw the first one in 1866. That star was Gamma Cassiopeiae. Achernar is the brightest Be star. People did not know it was a Be star until 1976.

Some stars have a very special glow. We call these Be stars. These stars are part of a group called B-type stars. They are special because they show Balmer emission lines. These lines are bright marks in the star's light. Most Be stars are main sequence stars. Some are also subgiants or giant stars.

Scientists often call the most common ones classical Be stars. These are stars that are not supergiants. They show these bright light lines at certain times. This makes them very interesting to study.

How do these stars make their special glow? It all starts with how the star moves. Be stars rotate very rapidly. This fast spinning helps throw gas away from the star. This gas forms a gaseous disk around the star. This disk is made of material ejected from the star. The star's ultraviolet light hits this gas disk. Then the disk re-processes that light into new lines. This creates the emission lines we see. The light also scatters in the disk. This causes something called infrared excess and polarization.

People have been studying these stars for a long time. Angelo Secchi was the first to see one. He observed the star Gamma Cassiopeiae in 1866. This was the first star ever seen with emission lines. Later, many other bright stars were found to be similar. However, some of those stars are not classical Be stars now. The brightest Be star is named Achernar.

People did not know Achernar was a Be star until 1976. It took a long time to recognize it.

Sometimes these stars look even more unusual. Some Be stars look like they have a shell. These are called shell stars. This happens when the gas disk is aligned edge on to us. We see the disk as a ring or a detached shell. This creates very narrow absorption lines in the light. Be stars also show variability. This means they change over time. Some are called Gamma Cassiopeiae variables. This happens when the disk is variable or transient. Others might be pulsating stars called Lambda Eridani variables.

Be stars help us understand how stars work. They show us how fast spinning can change a star. You can think of the gas disk like a ring of dust. This ring sits around the star and catches its light. Just like a ring around a planet, it changes how we see it. These stars are active and bright. They show us that stars are not always still. They can spin, throw out gas, and change their glow. This makes the night sky a very busy place.

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.