Contact binary
Binary stars that touch or share gaseous envelopes.
In a contact binary, two stars are so close together that they physically touch or have merged their outer gas layers. This arrangement is also called an overcontact binary. The term "contact binary" was coined by astronomer Gerard Kuiper in 1941. Nearly all known contact binaries are eclipsing binaries; those that eclipse are classified as W Ursae Majoris variables, named after the prototype star W Ursae Majoris.
In such a system, both stars have filled their Roche lobes. This allows the more massive primary star to transfer both mass and energy to its less massive companion. As a result, the two stars often have similar surface temperatures and brightness, even if their masses differ. How much energy flows between them depends on their mass ratio and luminosity ratio. If the stars are in physical contact but poor thermal contact, their temperatures can be very different.
Contact binaries should not be confused with common envelopes. A contact binary’s touching configuration can last for millions to billions of years. In contrast, a common envelope is a short-lived, unstable phase that either ejects the star’s outer layers or merges the two stars within months to years.
- Introduced by
- Gerard Kuiper
- Year introduced
- 1941
- Archetype star
- W Ursae Majoris
- Typical lifetime
- millions to billions of years
Lore & Background
The term 'contact binary' was introduced by astronomer Gerard Kuiper in 1941. In a contact binary, both stars have filled their Roche lobes, allowing the more massive primary component to transfer both mass and luminosity to the secondary member. As a result, the components often have similar effective temperatures and luminosities, regardless of their respective masses. The rate of energy transfer between the components depends on their mass ratio and luminosity ratio. In cases where the stars are in geometric contact but thermal contact is poor, wide differences between their temperatures can exist.
Reader's Guide
Contact binaries are notable for their long-lived stable configuration, with typical lifetimes of millions to billions of years. They are distinct from common envelopes, which are dynamically unstable phases that expel the stellar envelope or merge the binary in months to years. Almost all known contact binaries are eclipsing binaries, classified as W Ursae Majoris variables. The energy transfer between components, dependent on mass and luminosity ratios, often equalizes their temperatures and luminosities. Examples include MY Camelopardalis, which shares an atmosphere, and VFTS 352, a massive contact binary in the Tarantula Nebula. KIC 9832227 is a contact binary that was a candidate for stellar merger, and luminous red novae like V1309 Scorpii may result from such mergers. HR 5171 was previously thought to be a contact binary but is now considered a yellow hypergiant.
Did You Know?
- Almost all known contact binary systems are eclipsing binaries, known as W Ursae Majoris variables.
- Contact binaries have typical lifetimes of millions to billions of years, unlike common envelopes which last months to years.
- In a contact binary, the more massive primary transfers both mass and luminosity to the secondary.
More in Binary and Multiple Stars 1-24
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