Mira variable

Some stars change their light.

These stars are very red. They grow big and then small. This happens over many days.

As they change, they look brighter or dimmer. This is because their heat changes too. They can be much brighter than our Sun.

These stars are very old. They lose much of their weight. They may soon leave a cloud of dust behind. It is a big change for a star.

Mira variables are a special kind of star. They are named after a star called Mira. These stars are very red in color. They are also very large. They can be much brighter than our Sun.

These stars change their brightness over time. This happens because the whole star expands and contracts. It grows and shrinks in a set of steps. This way of changing is called pulsation. As the star changes size, its heat changes too. This change in heat makes the star look brighter or dimmer.

Mira variables are in a late stage of life. They are called red giants. They have already lost about half of their mass. These stars change their light over long periods. This period is longer than 100 days. Some stars change their period over many years. This can happen because of thermal pulses. These pulses happen when the star's inner layers reignite. Soon, these stars will lose their outer layers. They will become white dwarfs.

Mira variables are a special group of stars that change their brightness. They are named after a prototype star called Mira. These stars are red giants in the very late stages of their lives. They are on a path called the asymptotic giant branch. These stars are quite large and can be thousands of times brighter than our Sun. They are massive enough to have used helium in their cores. However, they are less than two solar masses. Most have already lost about half of their starting mass.

These stars work through a process called pulsation. This means the entire star expands and then contracts. This movement causes the star's radius and temperature to change. These two changes cause the star's luminosity, or brightness, to vary. The visual brightness changes a lot because energy shifts between different wavelengths. Some energy moves into infrared light while other energy moves into visual light. This shifting makes the star look much brighter or dimmer to us.

Scientists have studied these stars for a long time. Some Mira variables have reliable observations going back over a century. Amateur astronomers love to watch them because their brightness changes are so dramatic. Early computer models assumed these stars were perfectly round. However, a survey using the IOTA telescope found something different. About 75% of the stars they could see were not perfectly round. This means scientists now use supercomputers to make better 3D models.

There are many different Mira variables in our sky. Mira itself has a period of 332 days. Another star, Chi Cygni, has a period of 408 days. R Hydrae is another example that we can study. Some stars like R Leporis are carbon-rich. Most stars usually have more oxygen than carbon on their surfaces. This is because of a process called dredge-ups. These dredge-ups bring carbon from the helium burning shell to the surface.

These stars are constantly changing as they age. They lose mass quickly and create dust shrouds around themselves. Some stars even change how long their pulsation period lasts. This can happen over a few decades or a few centuries. It is caused by thermal pulses when the helium shell reignites. This reigniting changes the structure of the star. Eventually, these stars will expel their outer layers. They will become planetary nebulae and then turn into white dwarfs.

Mira variables are a specific class of pulsating stars. They are named after the prototype star, Mira. These stars are red giants in the very late stages of stellar evolution. Specifically, they reside on the asymptotic giant branch, or AGB. These stars are quite massive, having already undergone helium fusion in their cores. However, they are less than two solar masses. Most have already lost about half of their initial mass. Mira variables are important because they show us how stars change before they die.

The mechanism behind their name is a process called pulsation. This occurs when the entire star expands and then contracts. As the star changes size, its radius and temperature also change. These two factors together cause a variation in the star's luminosity, or brightness. The stars have pulsation periods longer than 100 days. Their visual brightness changes by more than 2.5 magnitudes. Interestingly, the large changes in visual light are not just from luminosity changes. Instead, energy shifts between infrared and visual wavelengths as the temperature changes.

Scientists categorize these stars based on their chemical makeup. Mira variables can be either oxygen-rich or carbon-rich. Most AGB stars tend to have more oxygen than carbon at their surfaces. This is due to a process called dredge-ups. These are periodic deep convection events caused by fusion in alternating hydrogen and helium shells. These dredge-ups bring carbon from the helium burning shell up to the surface. In some stars, like R Leporis, these conditions override the normal oxygen surplus. However, in stars more massive than a certain limit, hot bottom burning occurs. This is a process where CNO cycle fusion destroys carbon in the lower convective regions. This prevents more massive AGB stars from becoming carbon-rich.

These stars are also known for their changing periods. A small subset of Mira variables can change their pulsation period significantly. This change can be as much as a factor of three. Such changes happen over several decades or a few centuries. This is believed to be caused by thermal pulses. During a thermal pulse, the helium shell reignites the outer hydrogen shell. This reigniting changes the physical structure of the star. This process is predicted to happen to all Mira variables. However, thermal pulses only last a few thousand years. Since the AGB lifetime is less than a million years, we only see this in a few stars, such as R Hydrae.

History and observation have shaped our understanding of these objects. Many Mira variables have reliable observations stretching back over a century. This makes them popular targets for amateur astronomers. They are easy to find because their brightness changes are so dramatic. Early computer models assumed Mira stars were spherically symmetric. Scientists used this assumption to keep computer modeling simple. However, a survey using the IOTA telescope changed this view. It found that 75% of resolved Mira stars are not spherically symmetric. Now, astronomers use supercomputers to create realistic three-dimensional models.

There are many notable examples of Mira variables in our galaxy. Mira itself has a period of 332 days. It has a brightness ranging from 2.0 to 10.1 magnitude. Chi Cygni is another example with a 408-day period. R Hydrae is also a known variable. Other stars include R Carinae, R Leonis, and S Carinae. Each star has a unique distance and period. For instance, S Carinae is 149 parsecs away. R Leonis is 310 parsecs away. These specific measurements help astronomers map the stars.

Finally, Mira variables represent a rapid transition in a star's life. They are rapidly losing mass during this stage. This lost material often forms dust shrouds around the star. In some specific cases, the conditions allow for natural masers to form. Eventually, these stars will reach the end of their life cycle. They will expel their outer envelopes to create planetary nebulae. Within a few million years, the remaining core will become a white dwarf. This connects the study of Mira variables to the broader lifecycle of all stars in the universe.