Multiple Systems

Abstract

Notes about multiple star systems.

Keywords: multiple system, multiple star, double star, binary star, star cluster

Introduction

Although we are used to thinking of stars coming as individuals because our own Sun appears to be a lone star, this is not the norm. The evidence is that most stars that we see in the sky are parts of multiple star systems revolving around a common center of mass. About 85% of all stars are in double or multiple systems (Smith 1995, p. 208). If there are two stars in the system, it is called a binary star system.

Types of Multiple Systems

Binary Systems

Visual vs. Spectroscopic

Classification of Binary Systems

Higher Multiples

Close Binary Systems

The life history of close binary systems is quite complicated; variations arise from the initial masses and separation of the components, and from the transfer of mass from one component to the other.

Cataclysmic Variables

Comprising white dwarf primaries with M-type companions in semi-detached systems. Characterised by short periods. Accretion onto the white dwarf component ...

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Others

Some of the other classes of close binary are:

• Algols - Algol systems comprise two 'normal' main sequence or subgiant stars in a semi-detached system.
• W Serpens Stars - These are active Algol systems.
• RS Canum Venaticorum Stars - Chromospherically active binaries of F and later spectral type components.
• BY Draconis Stars - Similar to RS Canum Venaticorum systems.
• W Ursae Majoris Contact Systems - Short period (0.2 to 0.8 day) contact binaries exhibiting very high levels of magnetic activity.
• X-Ray Binaries - Binaries with a neutron star or, more rarely, a black hole primary. Accretion onto the primary produces strong x-ray emissions.
• Zeta Aurigae Systems - Long period binaries comprising interacting type G or K supergiant and hot (~type B) companion. Although not originally interacting systems, they become so when the more massive star evolves to become a supergiant.
• VV Cephei Systems - Similar to Zeta Aurigae binaries except a type M supergiant replaces the G or K type.
• Symbiotic Binaries - Long period (~200 to 1500 days) interacting binaries comprising a cool type M giant (sometimes a pulsating Mira-type variable) and a hot accreting companion such as a white dwarf, sub-dwarf, or low-mass main sequence star. Where the cool star fills its Roche lobe, the system becomes a symbiotic Algol.
• Barium and S-Star Binaries - Long period binaries in which the originally more massive component evolved into a white dwarf, transferring some of its nuclear-processed gas to a giant type K or M companion.
• Post-Common Envelope Binaries - Usually comprising a hot white dwarf or subdwarf, and a cooler secondary, which have passed through the common envelope phase. The binary nuclei of planetary nebulae are examples.

Example Evolutionary Model

C&O pp. 706-707

References

Smith, Robert C. 1995: Observational Astrophysics. Cambridge. 443 pp.