Binary Stars

Binary Stars

Most stars are not single stars, surrounded only by their dependent planets. There are several kinds of binary star systems, solar systems with two stars. There are even triple systems, where three stars orbit around each other!

Stars of roughly equal mass actually orbit around the center of mass for the system, the balance point. Imagine two young stars playing together on a child's see-saw, wanting to balance perfectly. They move closer to or further away from the fulcrum (the support structure on which the see-saw plank rotates), depending on their relative weights. In order to balance, the lighter star needs to move further away from the fulcrum than the heavier star. The center of mass for a binary system is placed similarly to the fulcrum, nearest to the heavier star.

Optical Doubles: These are just chance superpositions of two stars on the sky. They do not lie the same distance away from us, and they are not real binary systems. Like the stars which form a constellation, they are not really neighbors.

Two-part figure showing the stars in the Ursa Major constellation as they appear when viewed from Earth. On the left a flat two-dimensional plane shows the dipper shape (a simple handle and a cup) traced out on the sky by the stars which make up this constellation. On the right this flat two-dimensional plane has been curved to show that it is a subsection of the round celestial sphere that surrounds the Earth, upon which we see the stars appear. A telescope to the left of the curved shape represents a viewer on Earth. Lines drawn from the telescope start together and diverge as they extend rightwards, each one passing through the position of a single star of the constellation projected onto the curved shape and then extending further rightwards to eventually end at the actual location of the star. Note that some stars are quite close to the curved shape while others are much further away from us, indicating the relative distances to these stars.

A view of the Ursa Major constellation, showing its appearance on the sky (left), and a three-dimensional representation (right). Though the stars in a single constellation may appear to be close neighbors on the sky, some may lie near to Earth while others are much further away; they are simply superimposed. [NMSU, N. Vogt]

Visual Doubles (Visual Binaries): This is the case of a true binary star system, where the spatial separation of the stars is great enough that we can see both members, and, over a loooooong period of time, we can actually watch them move about in their orbits. The orbital periods of known visual binaries range from a few tens of years to thousands of years.
Spectroscopic Binaries: This is the case of a true binary star system, where the two stars are so close together that we cannot distinguish between them on an image. In such a case, how do we know that there are two stars?
- Single-lined Spectroscopic Binary: Sometimes you take a spectrum of a star over several different nights and discover that the positions (observed wavelengths) of the absorption lines in the spectrum change with time. This is ascribed to a Doppler Shift that changes periodically with time, because we are seeing one star in orbit around another.
  
  [NMSU, N. Vogt]
- Double-lined Spectroscopic Binary: Sometimes you take a spectrum of what you think is a single star and see two sets of absorption lines, where each set is drifting back and forth in wavelength, over some length of time. This is interpreted as the result of two stars in orbit around one another. Like the Sun/Earth system, the larger mass star has the smaller orbital velocity.
  
  [NMSU, N. Vogt]

Thanks to Mike Bolte (UC Santa Cruz) for the base contents of this slide.