S-type star

Some stars are very special.

These are called S stars. They are big and cool. They are not like most stars. Most stars have more oxygen than carbon. S stars have almost the same amount of both.

We can see them in a special way. They have marks from a thing called zirconium oxide. This helps us find them.

S stars come in two kinds. One kind makes its own special parts. The other kind gets parts from a neighbor star. They are rare in the sky. It is fun to learn about them!

S-type stars are special, cool giant stars. Most stars have more oxygen than carbon in their air. S stars are different. They have almost equal amounts of carbon and oxygen.

Scientists find these stars by looking for specific marks. These marks come from zirconium oxide. This is a part made of zirconium and oxygen. These marks are very easy to see. They help us tell S stars apart from other stars.

There are two main kinds of S stars. The first kind is called intrinsic S stars. These stars make their own special parts. They use a process called convection to move things to their surface. These stars are on a stage of life called the asymptotic giant branch. This stage lasts less than a million years.

The second kind is called extrinsic S stars. These stars get their parts from a neighbor star in a binary system. These stars live longer than intrinsic stars. They are also less bright.

S stars are quite rare. Intrinsic S stars make up less than 10% of certain giant stars. Extrinsic S stars are even harder to find in the sky.

S-type stars are a special group of cool giant stars. Most cool stars, like class M giants, have much more oxygen than carbon in their atmosphere. S-type stars are different because they have almost equal amounts of carbon and oxygen. This makes them an intermediate step between normal giants and carbon stars. These stars are relatively rare in our universe.

Scientists identify them by looking for specific marks in their light. The most important mark is the presence of zirconium oxide (ZrO) bands. These bands appear as distinct patterns in the star's spectrum.

There are two main ways these stars form. The first type is called intrinsic S stars. These stars are on the asymptotic giant branch, which is a specific stage of a star's life. During this stage, the star undergoes thermal pulses. These pulses create strong convection, which is a way that material moves within the star. This movement carries fusion products and special elements up to the surface. This stage of life is quite short, lasting less than a million years.

The second type is called extrinsic S stars. These stars form through mass transfer in a binary system. This means they have a close companion star that shares material with them.

Astronomers first defined the S-type class in 1922. A scientist named Paul Merrill originally identified them. He noticed they had unusual absorption lines and molecular bands. At that time, the exact cause of these lines was not known. We now know these lines come from s-process elements. Over many years, the way we classify these stars has changed. New tools and better spectra have allowed for much more detail. Scientists have revised the rules many times as they learned more about how these stars work.

Measuring an S star involves looking at many different elements. Scientists look for zirconium oxide (ZrO) to define the star. They also look at titanium oxide (TiO) bands. In S stars, the TiO bands are often weaker than in normal M class giants. Other elements like strontium (Sr), barium (Ba), and lanthanum (LaO) are also much stronger. Some S stars even show traces of a rare element called technetium (Tc). The presence of technetium is a key way to tell intrinsic stars from extrinsic ones.

Classifying these stars is like using a complex code. In 1954, a system was created using two digits, like S6,6e. The first digit represents the temperature of the star. The second digit represents the abundance of certain elements. Later, scientists like Ake and Keenan created different ways to index these stars. They used the ratio of different chemicals to find where a star fits on the scale. This helps astronomers understand if a star is moving toward becoming a carbon star. It is a way to map the life story of a giant star.

S-type stars are a unique class of cool, giant stars. They occupy a middle ground in the evolution of stars. Most cool giants, known as class M stars, are oxygen-rich. This means their atmospheres contain much more oxygen than carbon. In contrast, carbon stars have more carbon than oxygen. S-type stars are intermediate because they have approximately equal amounts of carbon and oxygen in their atmospheres.

This balance makes them a vital link in understanding how stars change over time.

Astronomers identify these stars by studying their spectra. A spectrum is a pattern of light that reveals chemical signatures. The defining feature of an S-type star is the presence of zirconium oxide (ZrO) bands. These molecular bands appear in different parts of the visible spectrum. There is an alpha series in the blue light. A beta series appears in the yellow light. A gamma series is found in the red light. While normal M-type giants show strong titanium oxide (TiO) bands, S-type stars show much weaker TiO. Instead, they show strong lines from s-process elements. These include strontium (Sr), barium (Ba), and lanthanum oxide (LaO).

There are two distinct ways these stars form. The first group is called intrinsic S stars. These stars are on the asymptotic giant branch (AGB). This is a specific stage of a star's life. During this stage, the star has an inert carbon-oxygen core. It undergoes fusion in both a helium shell and a hydrogen shell. These processes create thermal pulses. These pulses cause strong convection, which is the movement of material within the star. This convection carries fusion products and s-process elements to the surface. This stage is quite short, lasting less than one million years.

The second group is known as extrinsic S stars. These stars do not create their chemical signatures through internal convection. Instead, they form through mass transfer in a binary system. This means the star has a close companion. The companion star transfers material to the S-type star. Extrinsic S stars are generally less luminous than intrinsic ones. They are also longer-lived. They often appear as semiregular or irregular variable stars. Scientists use the presence of technetium (Tc) to tell the two groups apart. Intrinsic S stars show technetium, but extrinsic stars do not.

The history of S-type stars began in 1922. A scientist named Paul Merrill first defined the class. He noticed stars with unusual absorption lines and molecular bands. At that time, the specific elements causing these lines were unknown. We now know they are caused by s-process elements. Since then, classification has changed many times. Advances in spectral resolution have allowed for much better identification. Earlier definitions required ZrO bands to be easily seen on photographic plates. Modern technology allows us to identify stars with much weaker ZrO bands.

Classifying these stars is a complex process. In 1954, a formal scheme used two digits, such as S6,6e. The first digit represents the temperature class. It ranges from 1 to 9, similar to the M-type sequence. The second digit represents the abundance class. This was calculated by multiplying the ratio of ZrO and TiO bands by the temperature class. Later, researchers like Ake introduced an abundance index. This index uses a scale from 1 to 7 to show the transition from MS stars to carbon stars. This helps astronomers track the changing carbon-to-oxygen (C/O) ratio.

S-type stars are relatively rare in the universe. Intrinsic S stars make up less than 10% of asymptotic giant branch stars with similar luminosity. Extrinsic S stars make up an even smaller proportion of all red giants. Studying them helps us understand the chemical enrichment of the universe. As these stars evolve, they move from being oxygen-rich to being carbon-rich. They serve as a laboratory for studying how elements are built through neutron capture. This process is known as the s-process. By watching S stars, we see the transition of matter in the cosmos.