Variable star

Some stars change their light.

They can get bright. Then they get dim.

One star might swell and shrink. This makes the light change. This is called a pulsating star.

Other stars change too. A second star might pass in front. This blocks the light from the first star.

These changes can be fast. They can take less than one hour. Some changes take many years.

People have watched these stars for a long time.

It is fun to watch the sky.

Most stars shine with a steady light. But some stars change their brightness over time. We call these variable stars.

There are two main ways this happens. Some stars change because of their own bodies. These are called intrinsic variables. One type is a pulsating variable. This star swells and shrinks in a cycle.

As the star grows and shrinks, its light changes.

Other stars change for a different reason. These are called extrinsic variables. This happens when something blocks the light. For example, a second star might pass in front. This is called an eclipsing variable.

These changes can happen at different speeds. Some take less than an hour. Others take many years. Scientists use a tool called a light curve to study them. A light curve is a graph that shows brightness over time.

By looking at these graphs, we can learn how stars live and die. We can even use some stars to measure how far away they are in space.

Most stars seem to shine with a steady light. However, some stars change their brightness over time. These are called variable stars.

Scientists group these stars into two main types. The first type is called intrinsic variables. These stars change because of something happening inside their own bodies. For example, a star might swell and shrink in size.

The second type is called extrinsic variables. These stars look like they are changing, but something else is blocking the light. An orbiting companion star might pass in front of the first star. This is known as an eclipsing variable.

How do these stars actually work? For pulsating stars, the change is a cycle of expansion and compression. It often involves a process called ionization. This happens in layers of gas like hydrogen or helium. In a Cepheid variable, helium gas changes how it holds energy. When the star swells, the gas cools down. This makes the gas more transparent, so light can escape easily. This loss of energy causes the star to contract again. As the gas is squeezed, it heats up. This makes the gas more opaque, which traps the radiation inside. This trapped heat causes the star to expand once more.

People have watched variable stars for a very long time. Some ancient Egyptian calendars from 3,200 years ago might record them. Aboriginal Australians also shared stories about the changing brightness of stars like Betelgeuse. In the 1600s, astronomers began to identify specific stars. Johannes Holwarda noticed the star Mira changed in an 11-month cycle in 1638. Later, Geminiano Montanari described the eclipsing star Algol in 1669. John Goodricke found the right explanation for Algol in 1784. By the year 1786, humans had documented ten different variable stars.

As technology improved, we learned much more about these stars. In 1885, the Harvard College Observatory began photographing the whole sky. This helped find many more stars. In 1912, Henrietta Swan Leavitt found a special link for Cepheid variables. She saw a relationship between how bright they were and their timing. In 1924, Edwin Hubble used this to find a galaxy far away. This proved that spiral nebulae were actually separate galaxies. Today, there are 58,200 catalogued variable stars. About 30,000 of them are pulsating variables. Over 10,000 are eclipsing variables.

Astronomers use these stars as tools to map the universe. They use a method called the cosmic distance ladder. This helps them figure out the scale of everything we see. They also study the mass and size of stars using eclipsing binaries. To track the changes, they create a light curve.

A light curve is a graph showing brightness over time. It shows when a star reaches its maximum brightness or its minimum brightness. Amateur astronomers around the world still help by sharing their own observations. They work with groups like the American Association of Variable Star Observers. This teamwork helps us understand how stars live and die.

Most stars appear to shine with a constant intensity. However, some stars undergo systematic changes in their brightness as seen from Earth. These are known as variable stars. This change in brightness is called apparent magnitude.

Astronomers categorize these stars into two primary groups. The first group is called intrinsic variables. These stars change because of physical changes within the star itself. The second group is called extrinsic variables. These stars change because something else affects the light reaching us. For example, an orbiting companion star might block the light. This is known as an eclipsing variable.

Intrinsic variables are further divided into several distinct subgroups. The most common type is the pulsating variable. These stars change brightness because their radius expands and contracts. About two-thirds of all variable stars are pulsating variables. These stars follow a cycle of expansion and compression. Another subgroup includes stars that change due to other internal physical properties. The mechanism for these pulsations is often explained by the Kappa-mechanism. This was previously known as the Eddington valve. It involves instabilities in the interior of the star.

To understand how a pulsating star works, we must look at ionization. Ionization is a process where atoms gain or lose electrons. In stars like Cepheid variables, this happens in layers of helium. These are called partial ionization zones. The process follows a specific sequence of cause and effect. First, the star enters a swelling phase. The partial ionization zone expands and the temperature drops. This decrease in temperature causes the degree of ionization to fall. As a result, the plasma becomes more transparent. This allows the star to radiate its energy more easily.

This loss of energy causes the star to contract. As the gas is compressed, the temperature rises again. The higher temperature increases the degree of ionization. This makes the gas more opaque, or less transparent. The gas then captures radiation, which heats the gas further. This trapped heat causes the star to expand once more. This creates a continuous cycle of swelling and shrinking.

Astronomers use these cycles to study stellar evolution. They can also use eclipsing binaries to determine the mass and radii of stars. This data helps scientists model how stars change over time.

Humans have observed variable stars since ancient times. Some Egyptian calendars from 3,200 years ago may record the eclipsing binary Algol. Aboriginal Australians also incorporated the changes in Betelgeuse into oral traditions. In the modern telescope era, Johannes Holwarda identified the first periodic variable, Mira, in 1638. This discovery helped prove that the sky is not unchanging. It challenged the ancient ideas of philosophers like Aristotle. Later, John Goodricke provided the correct explanation for Algol in 1784. By 1786, scientists had documented ten different variable stars.

Technology has greatly increased our knowledge of these objects. In 1885, the Harvard College Observatory began photographing the entire sky. This led to many new discoveries. In 1912, Henrietta Swan Leavitt discovered the period-luminosity relationship in Cepheid variables. This means a star's brightness is linked to its pulsation period. In 1924, Edwin Hubble used this relationship to measure distances. He found that the Andromeda Nebula was actually a separate galaxy. This helped define the scale of the visible universe.

Today, there are 58,200 catalogued variable stars. Of these, just under 30,000 are pulsating variables. There are also over 10,000 eclipsing variables. Astronomers study these stars using a tool called a light curve. A light curve is a graph of brightness over time. It shows the maxima, or peaks, and the minima, or troughs.

Scientists also use spectroscopy to see changes in temperature and movement. By combining light curves with spectral data, they can explain why a star varies. Amateur astronomers still play a huge role in this work. They share data with professional organizations to help map the cosmos.