Binary star

Some stars live in pairs.

They pull on each other. This pull makes them spin in a circle. They are like dancers in space.

Sometimes you see one star.

A telescope can show two stars instead. These are called visual binaries.

Other stars are very close. They move very fast. You cannot see them as two stars.

Sometimes one star moves in front of the other. This makes the star look dimmer.

This is called an eclipse.

Scientists use these stars to learn. They help us learn how stars grow. It is fun to study the sky.

Most stars do not live alone. Some live in pairs called binary stars.

These two stars pull on each other. This pull is called gravity. Gravity keeps them in orbit around each other. They move in circles or ovals.

Sometimes we see only one star. A telescope can show the two stars separately. We call these visual binaries.

Some stars are too close to see with a telescope. We find these using a tool called a spectrometer. This tool looks at the light from the stars. These are called spectroscopic binaries.

Other stars cross paths in front of us. One star moves in front of the other. This makes the light change. We call these eclipsing binaries.

Binary stars help scientists a lot. They help us measure how heavy a star is. They also help us see how stars grow and change. Studying these pairs tells us much about space.

A binary star is a system of two stars. These stars are gravitationally bound to each other. This means their gravity pulls them together. They move in an orbit around each other. These systems are very important to scientists. They help us measure the mass of a star. They also help us test ideas about how stars change over time. Some binary stars can even lead to huge events like novae or supernovae.

There are different ways to find these stars. A visual binary is a pair we can see separately with a telescope. The brighter star is called the primary star. The dimmer star is the secondary star. Some stars are too close to see with our eyes. We find these using spectroscopy, which is looking at light. In these spectroscopic binaries, the light shifts color as the stars move. This happens because of the Doppler effect. The light shifts toward blue when a star moves toward us. It shifts toward red when it moves away.

Sometimes stars cross paths in front of us. This happens in an eclipsing binary. The stars orbit in a way that they block each other's light. This causes the brightness to change as they move. We also use astrometry to find stars. This means measuring a tiny change in a star's position. This change is caused by an unseen companion star pulling on it.

People have studied double stars for a long time. We have seen them since the telescope was invented. Giovanni Battista Riccioli saw that Mizar was a double star in 1650. In 1767, John Michell used math to show stars were in pairs. He looked at the Pleiades cluster to prove this. Later, William Herschel began observing them in 1779. He made lists of about 700 double stars. He even used the term "binary" in 1802.

Binary stars are everywhere in our sky. The Washington Double Star Catalog has over 100,000 pairs. Some stars, like Sirius, are famous examples. Another example is Cygnus X-1, which is a black hole. These stars can even swap material. Close stars can pull gas from each other's outer layers. This can change how the stars grow. Binary stars are a key part of how the universe works.

A binary star system consists of two stars that are gravitationally bound to one another. This gravitational pull forces the two stars to orbit around a common center of mass. These systems are essential to the field of astrophysics. They provide a way for scientists to directly measure stellar masses. They also allow researchers to test complex theories regarding stellar evolution. Furthermore, binary systems act as progenitors, or precursors, for massive cosmic events. These include novae, type Ia supernovae, and the mergers of compact objects.

Astronomers classify binary stars based on how they are detected. Visual binaries are pairs that have enough angular separation to be seen as two distinct objects through a telescope. In these systems, the brighter star is called the primary star. The dimmer star is known as the secondary star, or sometimes the comites. Other systems are identified through spectroscopy, which involves analyzing light. In spectroscopic binaries, the stars are often too close to be seen separately. Instead, scientists detect them by observing periodic changes in spectral lines. This occurs because of the Doppler effect. As a star moves toward Earth, its light shifts toward the blue end of the spectrum. As it moves away, the light shifts toward the red end.

Another method of detection is through photometry. This is used to identify eclipsing binaries. An eclipsing binary occurs when the orbital plane lies nearly along our line of sight. As the stars orbit, they periodically pass in front of one another. This causes the observed brightness of the system to change as one star eclipses the other.

Finally, astrometric binaries are found by measuring tiny deviations in a star's position. These deviations are caused by the gravitational tug of an unseen companion star. A single system can belong to multiple categories. For example, some stars are both spectroscopic and eclipsing binaries. There are only about 40 known examples of such rare systems.

History shows that our understanding of these stars has grown alongside our technology. Humans have observed double stars since the invention of the telescope. In 1650, Giovanni Battista Riccioli observed that the star Mizar was a double star. Later, in 1767, John Michell provided the first mathematical evidence for binary systems. He studied the Pleiades cluster and used statistics to show these groupings were not random. He calculated the chance of such a grouping occurring by accident was one in half a million. He concluded that gravity must be pulling the stars together.

In 1779, William Herschel began a massive study of double stars. He hoped to use them to measure stellar distances through parallax. He eventually published catalogs containing roughly 700 double stars. By 1803, Herschel realized these stars were actually orbiting each other. He introduced the term "binary" in 1802 to describe stars revolving around a common center of mass. Following his work, Félix Savary computed the first known orbit of a binary star in 1827. He focused his calculations on the star Xi Ursae Majoris.

The scale of binary star research is enormous. As of June 2017, the Washington Double Star Catalog contained over 100,000 pairs of stars. This database is compiled by the United States Naval Observatory. While many pairs are listed, orbits are only known for a few thousand of them. This is because many visual binaries have orbital periods lasting centuries or even millennia. Observing a full orbit can take much longer than a human lifetime. This makes the study of long-period binaries a slow and careful process.

Binary stars also influence how stars live and die. When components are very close, they can gravitationally distort each other's outer atmospheres. In some cases, these systems can even exchange mass. This mass transfer can lead to evolutionary stages that a single star could never reach. Some binaries are even the nuclei of planetary nebulae. Notable examples include the star Sirius and the black hole Cygnus X-1. These interactions can eventually lead to the spectacular explosions known as supernovae.