Stars. II: Binaries and Clusters

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); 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!); [Mira (a strange star, and its companion is a cool white dwarf surrounded by a hot blue accretion disk)].

Spectroscopic Binaries

  • How to spot them: We may only see one "star", but spectral lines of one or both stars may move back and forth periodically because of the Doppler effect.
  • 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].

Other Examples

  • Astrometric binaries: We only see one star, but it wobbles due to the presence of the other one.
  • Systems with more stars: Some triple systems, and double binaries (like Epsilon Lyrae, near Vega) are known; and then there are clusters...
  • How common are binaries? Most stars are formed in multiple systems.
  • Do stars in a binary affect each other? Usually not, because most separations are at least 30 AU, although low-mass stars tend to be closer to their companions; But in very crowded places like globular clusters or galactic centers, they can swap partners, collide, merge... and in close binaries, if one or both stars swells up...
  • Do binary stars stay together? Not always; Many stars retain one or more companions, but couples can break up; Some stars are speeding away from the Orion nebula after breaking up from their partners.
 

  Masses of Stars

  • Why are binaries useful? 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.
  • 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.

Star Clusters
  • Are clusters common? Most stars form in clusters from fragments of large clouds. (Many wander away from their siblings later in life.)
  • Why are clusters interesting? All stars have (approximately) the same age.

Pleiades
Open Clusters
The Quintuplet open cluster
  • Examples: The Pleiades (7 Sisters), 440 ly away in Taurus; M44, the Beehive; NGC 4755, the Jewel Box; The Double Cluster in Perseus.

  Open Clusters

  • What are they? Small systems (a few pc across), with hundreds or thousands of stars; Most crowded groups of blue stars in the galactic disk are open clusters, although some star patterns (asterisms) fool us into thinking they are clusters [like NGC 6997].
  • Surrounding nebulae: Many open clusters have them, because they are young (like NGC 6188, or NGC 2244).
  • How do we know their age? Look at their HR diagrams, and remember that more massive stars burn out faster; For a rough estimate, or for very distant clusters, just look at the color.
  • What ages do we find? For the Pleiades, around 60 Myr; besides, they have heavy elements...

Globular Clusters

  • What are they? Spherical groups of hundreds of thousands of stars, at most about 50 pc across!
  • Where are they? Distributed in a sphere around the disk of our galaxy; Almost 200 are known in our galaxy (there may be more near the galactic center); We also see them in other galaxies (of all types; a famous one is G1 in M31; the LMC has 13 known ones, the Sombrero Galaxy M104 about 2000 of them), and a few are intergalactic.
  • Examples: The largest ones are Omega Centauri (M10 or NGC5139) and 47 Tucanae; M22, spanning an area of the sky as large as the Moon.
  • How do we know their age? From their color or HR diagram.
  • Main Sequence cutoff/turnoff: Compare different ages; More massive stars burn out faster (for example, B stars: 10 Myr; G stars: 10 billion yr).
  • And what ages do we find? In our galaxy at least 10 billion yr, they were formed very early on; In the LMC they are either about 13 Gyr old or about 3Gyr old! Some galaxies (such as Stephan's Quintet) have young, blue globular clusters.
  • What do they tell us? Their age gives constraints on the age of the universe; Their age and distribution in a galaxy may show what happened to a galaxy since the time it formed (galaxy collisions, mergers, etc).

Globular Clusters

The Omega Centauri
globular cluster, the brightest in our sky and the largest known one in our galaxy [so big it may actually be a stripped-down dwarf galaxy!].

page by luca bombelli <bombelli at olemiss.edu>, modified 31 oct 2012