Binary Star Systems
Are binaries common? Most stars are formed in multiple systems, and many retain one or more companions; In very crowded places like globular clusters or galactic centers, they can swap partners, and collide and merge too...
How do we recognize a binary? We either see both stars, or we see a "single" star with an unusual, variable behavior...

Visual Binaries

  • How to spot them: Both stars are visible, often bright and separated.
  • Examples: Polaris (actually a triple); Mizar in the Big Dipper (and close to Alcor); Castor in Gemini; Albireo in Cygnus, the head of the swan (two colors, perhaps the finest binary for a small telescope); Sirius (Sirius B is a white dwarf, and it had been predicted!); [Mira (its companion is a cool white dwarf surrounded by a hot blue accretion disk)].
  • Remarkable cases: Epsilon Lyrae, near Vega, is a double double!

Spectroscopic Binaries

  • How to spot them: From the Doppler shifts in their spectrum; lines from one or both stars may be seen to move back and forth.
  • Examples: Again Mizar (each of the two visible stars!), and Castor.

Eclipsing Binaries

  • How to spot them: From the changing amount of light we receive when one passes in front of the other.
  • Examples: Algol in Perseus (every 3 days becomes 3 times dimmer for a few hours); [Delta Velorum, a quadruple star system, whose brightest member is an eclipsing binary; Theta-1 Orionis in the Trapezium].

Astrometric Binaries

  • How to spot them: We only see one star, but it wobbles due to the presence of the other one.

  Masses of Stars

  • Usefulness of binary systems: We can determine the stars' masses.
  • How do we find masses? From the orbit's size and period, using Kepler's 3rd law; The separation between the stars is easiest to find for eclipsing binaries. Now we can also use gravitational redshift.
  • Periods: Most are measured at least in years [some in thousands or millions of years], but they can be much shorter in close binaries. [There is a binary white dwarf with a 5-minute period!]
  • What do we find out about the HR diagram? For example, how massive the stars at each location on the main sequence are, and how mass is related to luminosity.
Do binary stars always stay together? No, couples can break up; Examples are known of stars speeding away from the Orion nebula after breaking up from their partners.
Do stars in binaries affect each other?
Usually not, because separations tend to be around 30 AU at least, although low-mass stars tend to be closer to their companions; but they may have to do with planetary nebulas, and in close binary systems, when one or both of the stars swells up...

Close Binary Systems

  • Important concepts: All objects in binary systems have Roche lobes and pull on each other more or less strongly, producing tides; in close star binaries, this can lead to mass transfer and accretion disks.
  • The case of Algol, Beta Persei: A paradoxical variable star - An eclipsing binary with a main sequence massive star and a less massive subgiant!
  • What happens to the star losing mass? Nothing dramatic, but it may become an old brown dwarf.
  • What happens to the star gaining mass? While adjusting, it may become a variable star, emit flashes of radiation and matter jets; Sometimes, much more dramatic things happen.

  Close Binaries with Compact Stars

  • With white dwarfs: Matter in the accretion disk heats up so much that it emits radiation, including UV or X-rays, sometimes producing what is observed as a cataclysmic variable star; In the most violent cases, the disk flares up in a burst of nuclear reactions (nova), or the star can blow up entirely (type I supernova)...
  • With neutron stars: They can become strong X-ray sources, X-ray binaries like SS 433; In interesting cases, a larger but less massive companion can totally eclipse the neutron star; The two stars can also spiral inward until they merge...
Systems with more stars: Some triple systems, ans double binaries are known; and then there are clusters.

page by luca bombelli <bombelli at>, modified 8 oct 2013