Horizontal branch

Some stars change as they get old.

They grow big and red. Then they change again. They get hot and bright. This makes them look like a line.

Inside the star, a new kind of power starts. It burns helium to make energy. This happens in the center of the star.

This new power makes the star change shape. The star gets smaller. It also gets much hotter.

These stars stay this way for a long time. They stay this way for 100 million years.

On a special chart, they look like a flat bar. This is why they have a special name.

Some stars go through a special stage. This is called the horizontal branch. It happens after a star becomes a red giant.

Inside the star, a new way to make power starts. The star begins to fuse helium in its core. This is the center of the star. It also fuses hydrogen in a shell around that core. This change makes the star look different. The star gets smaller and hotter. Its brightness also goes down.

Scientists use a special chart to study stars. This is the Hertzsprung–Russell diagram. On this chart, these stars form a flat line. This is why we call them the horizontal branch. They stay in this stage for about 100 million years.

Some stars in this group are special. They are called RR Lyrae stars. These stars pulse and change in brightness. This can happen in less than one day.

In old star groups, the branch can look very long. It can even have a "blue tail." This part has very hot stars. These stars can reach 30,000 K. That is a very high temperature.

Stars go through many changes during their lives. One special stage is called the horizontal branch. This stage happens right after a star becomes a red giant.

It is a very important time for stars like our Sun. During this phase, the star finds a new way to stay bright. It changes how it looks and how it works. This stage helps astronomers understand how stars age over time.

To understand how it works, we must look inside the star. The star begins to fuse helium in its core. It uses a process called the triple-alpha process to do this. At the same time, it fuses hydrogen in a shell around that core. This happens through the CNO cycle.

This new energy makes the star change its shape. The star's outer layers contract and get smaller. This makes the surface reach much higher temperatures. The star also becomes less bright than it was as a red giant.

Scientists discovered these stars by studying globular clusters. These are huge groups of very old stars.

Astronomers noticed these stars were missing from open clusters. They found them by using deep photographic studies. The name comes from a special chart called the Hertzsprung–Russell diagram. On this chart, these stars sit along a roughly horizontal line. This happens because stars in a cluster are all about the same distance away. This makes their brightness easy to see on the chart.

There are many interesting facts about these stars. They stay on the horizontal branch for about 100 million years.

Some stars in this group are called RR Lyrae stars. These stars pulse and change their brightness. This change can happen in a period of up to 1.2 days. Some very hot stars are called extreme horizontal branch stars. Their temperatures can reach between 20,000 and 30,000 K. These stars can even form a "blue tail" on the chart.

We can link these stars to things we already know. Just like a campfire might change when more wood is added, a star changes when it starts a new fuel. The helium is like a new kind of fuel for the star's engine.

You might also know about other stars that change brightness. The RR Lyrae stars are a great example of this. Even though they look different, they are all part of the same big story of how stars live and die. Studying them helps us see the life story of the universe.

The horizontal branch (HB) is a specific stage in stellar evolution. It occurs immediately after a star passes through the red-giant branch stage. This phase is most common in stars with masses similar to our Sun. It is a critical period because the star changes its fundamental energy source.

During this time, the star achieves a new state of balance. Understanding this stage helps astronomers map how stars age and change over billions of years.

To understand the mechanism, we must look at the star's internal layers. In the core, the star begins fusing helium into carbon. This specific process is called the triple-alpha process. At the same time, a shell of hydrogen surrounding the core undergoes fusion. This hydrogen fusion happens via the CNO cycle.

The onset of helium fusion causes the star's structure to shift significantly. The star's luminosity, or total brightness, actually decreases compared to the red-giant stage. Meanwhile, the stellar envelope contracts, and the surface temperature rises.

There are different ways a star reaches this stage depending on its mass. For stars up to 2.3 times the mass of the Sun, the helium core becomes degenerate matter. Degenerate matter is a dense state that does not generate its own energy. As hydrogen fusion adds more helium to the core, the temperature rises. Eventually, the core reaches a point where helium fusion ignites. This causes a rapid increase in fusion rates known as a helium flash.

In this event, the core becomes non-degenerate and expands quickly. However, the energy from this flash is absorbed by the plasma layers above. This means the flash is not visible from the outside of the star.

Other stars follow different paths to the horizontal branch. Stars with masses between 0.5 and 2.3 solar masses undergo the helium flash described above. However, stars with slightly different masses might reach the Schönberg–Chandrasekhar mass. At this mass, the core is no longer in thermal or hydrostatic equilibrium. These stars contract and heat up to trigger helium fusion without a flash. Even more massive stars, those above 2.3 solar masses, ignite their helium smoothly. These massive stars eventually become red supergiants instead of following the standard HB path.

Astronomers discovered horizontal branch stars through deep photographic photometric studies of globular clusters. They noticed these stars were absent from all open clusters studied at that time. The name "horizontal branch" comes from the Hertzsprung–Russell (H-R) diagram. On this chart, these stars lie along a roughly horizontal line.

This is because stars in a single globular cluster are all at nearly the same distance from Earth. This allows their absolute magnitudes to be clearly visible without the interference of distance uncertainties.

Horizontal branch stars stay in this stage for about 100 million years. Their appearance on the H-R diagram depends on the mass of the hydrogen envelope remaining around the core. Stars with larger envelopes are cooler, while those with smaller envelopes are hotter. This creates a spread of temperatures along the branch. Some stars are so hot they form a "blue tail" or a "blue hook" on the diagram. The hottest, called extreme horizontal branch stars, reach temperatures of 20,000 to 30,000 K.

These extreme stars may be caused by binary interactions or "late thermal pulses."

One notable feature is the RR Lyrae gap found in many globular cluster diagrams. This gap occurs at the instability strip, where pulsating RR Lyrae variable stars are located. These stars change in brightness with periods of up to 1.2 days.

Because these stars vary so much, they are often omitted from standard color-magnitude diagrams. This omission creates the appearance of a gap where stars are actually present. The different shapes of these branches in various clusters is known as the "Second Parameter Problem." Scientists believe factors like age, rotation, or helium content might cause these differences.

The horizontal branch is also related to a different group called the red clump. Clump giants are the younger, more massive, and metal-rich counterparts to HB stars. While both groups fuse helium into carbon in their cores, their outer layers differ. These differences in structure result in different radii and temperatures. Consequently, they appear in different parts of the H-R diagram despite having the same energy source.

Studying these connections helps scientists understand the complex life cycles of all stars.