Subdwarf

Some stars are special. They are called subdwarfs. These stars are very old. They do not have much metal in them. This makes them look different.

Cool subdwarfs make food from gas. They have very little metal. This happens because the early universe had little metal. These stars are often found in the halo. The halo is a part of our galaxy.

Some subdwarfs are hot and blue. These are a different kind of star. They can come from stars that lose their outer layers. This can happen if stars are close together.

Scientists find them by how they move. These stars move fast through space. They are very interesting to study.

Some stars are special. We call them subdwarfs. They look different from most stars. They are dimmer than regular stars of the same type. A scientist named Gerard Kuiper gave them this name in 1939.

There are two main kinds of subdwarfs. The first kind is cool subdwarfs. These are often red. They make power from hydrogen. They have very low metallicity. In space, metallicity means having elements heavier than helium. These stars are very old. They formed when the universe had very little metal. Because they lack metal, they are often found in the Milky Way's halo. The halo is a part of our galaxy. These stars also move very fast through space.

The second kind is hot subdwarfs. These are blue and very hot. They are a different type of object. They form when a red giant star loses its outer layers. This might happen if two stars are close together. Sometimes, two white dwarfs might merge to make one. Some hot subdwarfs even have lots of heavy metals. These include metals like lead and strontium. Scientists study these stars to learn about the history of our galaxy.

Some stars are different from the ones we see most often. Scientists call these special stars subdwarfs. They are dimmer than regular main-sequence stars of the same type. On a special chart called an H-R diagram, they sit below the main line. This happens because they have very low metallicity. In space, metallicity means having elements heavier than helium. Because they lack these heavy elements, they look and act in unique ways.

Cool subdwarfs work in a very specific way. Like regular stars, they make energy through hydrogen fusion. However, their low metallicity makes their outer layers less opaque. This means the layers are more clear to light. Because of this, there is less radiation pressure pushing out. This makes the star smaller and hotter for its mass. These stars also emit more ultraviolet light than regular stars. This special trait is called ultraviolet excess.

We have known about these stars for a long time. A scientist named Gerard Kuiper named them in 1939. He used the term to describe stars with strange spectra. At first, people thought they were intermediate white dwarfs. Since then, we have found even smaller stars that fit this group. We also found hot subdwarfs, which are a different class. These hot stars form when a red giant loses its outer layers.

There are many different types of these stars to study. Cool subdwarfs include Kapteyn's Star and GJ 1062. We also see L-type and T-type subdwarfs. The first L-type subdwarf, 2MASS J0532+8246, was found in 2003. In 2020, volunteers found the first extreme T-type subdwarfs. These were named WISEA 0414−5854 and WISEA 1810−1010. Some stars, like Wolf 1130C, are very old. They are thought to be over 10 billion years old.

Subdwarfs help us understand how the galaxy grew. Most cool subdwarfs live in the Milky Way's halo. This is a part of our galaxy that moves fast. These stars are very old because they formed early in the universe. Back then, there were very few heavy elements around. Only later did things like supernovae add metals to space. By studying subdwarfs, we can see what the early universe was like.

A subdwarf is a specific type of star defined by its low luminosity. In the Yerkes spectral classification system, these stars are assigned the luminosity class VI. They are significantly dimmer than standard main-sequence stars of the same spectral type. Specifically, a subdwarf is 1.5 to 2 magnitudes lower in luminosity than a regular star of the same type. On a Hertzsprung–Russell diagram, which maps star brightness against temperature, subdwarfs appear below the main sequence line. This distinction is important because it helps astronomers identify stars with unique chemical histories and physical structures.

Cool subdwarfs, which include spectral types G through M, produce energy through hydrogen fusion. Their unique brightness is caused by low metallicity. In astronomy, metallicity refers to the amount of elements heavier than helium present in a star. Because cool subdwarfs lack these heavy elements, their outer layers have lower opacity. Opacity is a measure of how much a material blocks light. Lower opacity means the layers are more transparent, which decreases the radiation pressure pushing outward. This results in a star that is smaller and hotter for its given mass. This process also creates an ultraviolet excess, where the star emits a higher percentage of ultraviolet light than a Population I star.

There are two distinct categories of subdwarfs: cool subdwarfs and hot subdwarfs. Cool subdwarfs are often found in the Milky Way's halo or the thick disk. These regions contain older stars that move at high space velocities relative to the Sun. Hot subdwarfs, also called extreme horizontal-branch stars, are an entirely different class. They have bluish spectral types, such as O and B. These stars represent a late stage in stellar evolution. They form when a red giant star loses its outer hydrogen layers before the core begins fusing helium. This loss might happen due to interactions in a binary star system or the merger of two white dwarfs.

Scientists have a long history of studying these objects. The term "subdwarf" was coined by Gerard Kuiper in 1939. He used it to describe stars with anomalous spectra that were previously called intermediate white dwarfs. Since Kuiper's discovery, the definition has expanded to include lower-mass stars. Astronomers have also identified heavy metal subdwarfs. These are a type of hot subdwarf with high concentrations of elements like germanium, strontium, yttrium, zirconium, and lead. Notable examples include HE 2359-2844 and LS IV-14 116.

Subdwarfs are categorized into several subtypes based on their decreasing metallicity. For L-type subdwarfs, the categories are subdwarf (sd), extreme subdwarf (esd), and ultra subdwarf (usd). These are measured on a logarithmic scale relative to solar metallicity. For example, the first L-type subdwarf, 2MASS J0532+8246, was later re-classified as an extreme subdwarf. T-type and Y-type subdwarfs also exist, though they are harder to find. T-type subdwarfs have less methane in their atmospheres because they have less carbon. This gives them a bluer color in certain infrared observations compared to objects with solar metallicity.

Specific examples help us understand the age and mass of these objects. Kapteyn's Star is a well-known cool subdwarf of type sdM1. The subdwarf VVV 1256−62B was discovered as a companion to a halo white dwarf. By studying this binary system, scientists measured its age between 8.4 and 13.8 billion years. With a mass between 84 and 87 Jupiter masses, it is likely a red dwarf star. Another example is Wolf 1130C, a companion to an old binary system. It is estimated to be older than 10 billion years and has a mass of 44.9 Jupiter masses, making it a brown dwarf.

Studying subdwarfs provides a window into the history of the universe. The low metallicity of these stars is linked to their extreme age. The early universe had very few elements heavier than helium. These elements were only added later by supernovae, planetary nebulae, and neutron star mergers. Because subdwarfs formed when the universe was young, they belong to the oldest structures in the Milky Way. By analyzing their spectra and movements, astronomers can reconstruct the chemical and structural evolution of our galaxy.