Binary star

A binary star or binary star system is a pair of stars that are gravitationally connected and spin around each other. These pairs of stars are important for scientists who study space. They help us measure how much stars weigh and test ideas about how stars change over time.

Sometimes, we can see these two stars as one bright dot in the night sky with just our eyes. But when we use a telescope, we can often see that they are really two stars close together. These are called visual binaries. Some of these pairs take hundreds or even thousands of years to go around each other.

We can also find binary stars using other clever methods, like watching how their light changes or measuring tiny shifts in their positions. When a binary star system's orbit is lined up just right with us on Earth, one star can pass in front of the other. This causes the pair to dim like a flashlight being covered—this is called an eclipsing binary.

If the two stars in a binary system are very close, they can pull and shape each other's outer layers. Famous examples of binary stars include Sirius and Cygnus X-1, where Cygnus X-1 is a well-known black hole. Binary stars are also found at the centers of many glowing clouds called planetary nebulae.

Discovery

Double stars, which are pairs of stars that look close together, have been noticed since telescopes were invented. Early examples include Mizar and Acrux. Mizar is in the Big Dipper in the Ursa Major constellation. It was first seen as a double star by Giovanni Battista Riccioli in 1650. Acrux, a bright star in the Southern Cross, was found to be double in 1685.

In 1767, John Michell used math to show that stars often appear in pairs more often than by chance. He studied the Pleiades cluster and found that stars being so close together randomly was very unlikely. This suggested that these stars might be pulled together by gravity, giving the first proof that binary stars exist.

William Herschel began watching double stars in 1779, hoping to measure distances to stars. By 1803, he noticed that some double stars were moving around each other. In 1827, Félix Savary calculated the orbit of Xi Ursae Majoris, the first known orbit of a binary star.

Today, the Washington Double Star Catalog, kept by the United States Naval Observatory, lists over 100,000 pairs of double stars. We only know the orbits of a few thousand.

Etymology

The word "binary" was first used for pairs of stars by Sir William Herschel in 1802. He described stars that are close together and stay connected because of gravity.

Today, a "binary star" means two stars that orbit around a common center. When we can see these stars as separate with a telescope, they are called "visual binaries." Many of these stars have not completed one full orbit around each other yet, but we can see parts of their path.

The term "double star" is used for stars that look close together in the sky. They might be a binary system, or they might just appear close from our view but be far apart in space. The latter are called "optical doubles."

Classifications

Binary stars are grouped into four types based on how we study them: visually, by looking through telescopes; spectroscopically, by watching changes in light patterns; photometrically, by noticing brightness changes during eclipses; or astrometrically, by tracking small shifts in a star’s position caused by an unseen companion. Some binary stars fit into more than one group.

Methods of observation

Visual binaries

A visual binary is a pair of stars close enough together that we can see them separately through a telescope or strong binoculars. The brighter star is called the primary, and the dimmer one is the secondary. By watching how these stars move over time, scientists can learn about their orbits and distances.

Spectroscopic binaries

Sometimes, we only learn about a binary star by studying the way its light changes. As the stars move toward or away from us, their light shifts slightly, showing us their motion. These stars are often very close together, so we can’t usually see them separately even with powerful telescopes.

Eclipsing binaries

An eclipsing binary is a system where the stars line up so that one passes in front of the other from our viewpoint, causing regular drops in brightness. By studying these brightness changes, scientists can figure out the stars’ sizes, orbits, and even distances to faraway galaxies.

Non-eclipsing binaries that can be detected through photometry

Some stars that don’t eclipse can still be found by watching how they affect each other’s light, either by reflecting light, changing shape, or other subtle effects.

Astrometric binaries

Astronomers sometimes find stars that appear to wobble in their path across the sky. This wobble suggests an unseen companion, like a very dim star or a neutron star, pulling on the visible star. By tracking these movements, scientists can estimate the companion’s mass and orbit.

Orbital period

Orbital periods can be very short, less than an hour for some stars, or a few days for others. They can also be very long, lasting hundreds of thousands of years.

The Applegate mechanism helps explain why some binary stars change their orbital periods over time. As a star changes shape during its activity cycles, it affects the orbit of the two stars. This causes small changes in the orbital period.

Some binary stars also show steady increases in their orbital periods. This is often because mass moves from one star to the other.

Main article: Applegate mechanism

Designations

The two stars in a binary star system are given letters. The brighter star is called A and the dimmer one is called B. Sometimes, we call the pair together using both letters, like AB. If there are more than two stars, we can use extra letters like C or D.

Stars can also be named after the person who first discovered them. For example, α Centauri was found to be two stars by Father Richaud in 1689 and is called RHD 1.

In some binary stars, one star is hotter than the other. The hotter star is called the hot companion, and the cooler star is called the cool companion. For instance, Antares has a cooler red supergiant star and a hotter blue star companion.

Evolution

Formation

Binary stars are two stars that orbit each other. They often form when stars are just being born. Sometimes two stars form together from a big cloud of gas and dust. This is why we see many binary stars in the sky.

Mass transfer and accretion

As stars get bigger, sometimes one star can get so large that material from it moves to its partner star. This happens when the big star stretches into an area where the gravity of the smaller star pulls some of its material away. This material can fall onto the smaller star or form a spinning disc around it. This disc can sometimes be the brightest part of the binary system that we can see.

Runaways and novae

Sometimes, binary stars can move apart and evolve on their own. If two binary systems come close to each other, they can break apart, sending some stars flying away at high speeds.

When a small, dense star called a white dwarf has a partner star close by, it can pull material from that star. This material builds up on the white dwarf and can sometimes explode in a bright burst of light called a nova.

Main article: Algol paradox

Astrophysics

Binary star systems are very useful for learning about stars. Because these stars move around each other, scientists can measure their masses very well. This helps us understand how stars are made and change over time.

Binary stars come in many shapes. Some move very close to each other, while others are far apart. By studying these systems, astronomers can learn about how stars are born. Scientists have found that about one-third of the stars in our Milky Way are part of binary or multiple star systems. These systems also help us learn about planets — some binary stars have planets moving around them, though this is less common than around single stars.

Examples

Some stars that look like one bright light in the night sky are actually two stars moving around each other. One easy example is Albireo. It looks like a single star but can be seen as two with a telescope. It is located in the constellation Cygnus. Another famous star, Sirius, the brightest star in the sky, was found to be two stars moving around each other in 1844. Scientists later learned that one of these stars, Sirius B, is a very dense type of star called a white dwarf.

Epsilon Aurigae is another example of a pair of stars where one star moves in front of the other from our viewpoint, making the star look dimmer. These special pairs are called eclipsing binaries. Beta Lyrae is another example of this type of star system. Other interesting pairs of stars include 61 Cygni and Procyon.

Multiple-star examples

Systems with more than two stars are called multiple stars. Algol is a famous example of a three-star system and is found in the constellation Perseus. Another well-known three-star system is Alpha Centauri, located in the southern constellation of Centaurus.

Other examples include Castor, a six-star system that is the second-brightest star in the constellation Gemini. The Alcor–Mizar system in Ursa Majoris is also made of six stars. QZ Carinae is an even more complex system with at least nine stars.