Astronomy and Survey of Information

1
Astronomy and Survey of Information

• Binary Stars
• Presented by Joe Giordano
• Guest Speaker Dr. Frank Maloney

2
Binary Stars

• Definition
• A binary star system consists of two stars both
  orbiting around their barycenter. For each star,
  the other is its "companion star".
  http//en.wikipedia.org
• Binary Stars Two stars, bound by their mutual
  gravity and revolving around a common center of
  mass. -Weighing and Sizing Stars Binary Systems

3
Binary Stars

• Term
• The term "binary star" was apparently coined by
  Sir William Herschell in 1802 to designate "a
  real double star the union of two stars that are
  formed together in one system by the laws of
  attraction".

William Herschel
http//en.wikipedia.org
4
Facts

• Binary Stars can be misleading
• Any two closely-spaced stars might appear to be a
  double star, the most famous case being Mizar and
  Alcor in the Big Dipper.

http//en.wikipedia.org
5
False Binaries

• A double star is probably a foreground/background
  star pair that only looks like a binary
  systemthe two stars are, in reality, widely
  separated in space but just happen to lie in
  roughly the same direction as seen from our
  vantage point.
• "false binaries" are termed optical binaries.
• Many such pairs were found with the invention of
  the telescope.

6
False Binaries

• In 1780 Herschel measured the separation and
  orientations of over 700 pairs that appeared to
  be binary systems and found that about 50 pairs
  changed orientation over two decades of
  observation.

The Pleiades star cluster
http//en.wikipedia.org
7
True Binaries

• A true binary is a pair of stars bound together
  by gravity.
• When they can be resolved (distinguished) with a
  powerful enough telescope (with the aid of
  interferometric methods) they are known as visual
  binaries.
• In other cases, the only indication of binarity
  is the Doppler shift of the emitted light.
• These systems are known as spectroscopic
  binaries.

8
Spectroscopic Binaries

• Consist of relatively close pairs of stars such
  that the spectral lines in the light from each
  one shifts
• It shifts first toward the blue, then toward the
  red, as it moves first toward us, and then away
  from us, during its motion about their common
  center of mass, with the period of their common
  orbit.

9
Eclipsing Binary

• If the orbital plane is very nearly along our
  line of sight, the two stars partially or fully
  occultation each other regularly, and the system
  is called an eclipsing binary.
• Algol is the best-known example.

10
Algol (ß Per / Beta Persei)
http//en.wikipedia.org
11
Binary Stars

• Binary stars that are both visual and
  spectroscopic binaries are rare.
• Are a precious source of valuable information
  when found.
• Unless they are relatively close to Earth, visual
  binary stars have a large true separation, and
  consequently usually have orbital speeds too
  small to be measured spectroscopically.
• Spectroscopic binary stars move fast in their
  orbits because they are close togetherusually
  too close to be detected as visual binaries.
• Binaries that are both visual and spectroscopic
  are usually relatively close to us.

12
Astrometric Binaries

• Scientists have discovered some stars that seem
  to orbit around an empty space called Astrometric
  binaries.
• Astrometric binaries are relatively nearby stars
  which can be seen to wobble around a middle
  point, with no visible companion.
• With some spectroscopic binaries, there is only
  one set of lines shifting back and forth. The
  same mathematics used for ordinary binaries can
  be applied to infer the mass of the missing
  companion.
• The companion could be very dim, so that it is
  currently undetectable or masked by the glare of
  its primary, or it could be an object that does
  not emit visible light, or in fact any
  electromagnetic radiation, like a neutron star.

13
Astrometric Binaries

• In some instances, one can make a strong case
  that the missing companion is in fact a black
  holea body with such strong gravity that no
  light is able to escape.
• Perhaps the best known example at present is
  Cygnus X-1, where the mass of the unseen
  companion is about nine times that of our sunfar
  exceeding the maximum theoretical mass of a
  neutron star, the other likely candidate for the
  companion.

14
Cygnus X-1

• Cyg X-1 is a binary star that contains a O9-B0
  supergiant
• Surface temperature of 31,000 kelvins
• It also contains a compact object
• Mass of the supergiant is approximately 2030
  solar masses
• The compact object has a mass of 713 solar
  masses
• NOTE the largest possible mass of a neutron star
  can not exceed three solar masses

http//en.wikipedia.org
15
What does all this mean?

• Cygnus is believed to be a black hole.
• The X-rays are produced in an accretion disk that
  is formed by matter flowing from the supergiant
  into the black hole.
• Cygnus X-1 is the brightest persistent source of
  hard X-rays (E gt 20 keV) in the sky.
• The distance to Cygnus X-1 is about 2500 parsecs.

16
Accretion Disk

• A structure formed by material falling into a
  gravitational source.
• Conservation of angular momentum requires that,
  as a large cloud of material collapses inward,
  any small rotation it may have will increase.
• Centrifugal force causes the rotating cloud to
  collapse into a disc, and tidal effects will tend
  to align this disc's rotation with the rotation
  of the gravitational source in the middle.
• Viscosity within the disc generates heat and saps
  orbital momentum, causing material in the disc to
  spiral inward until it impacts in an accretion
  shock on the central body if the body is a star,
  or slips toward the event horizon if the central
  body is a black hole.

http//en.wikipedia.org
17
Why are Binaries Helpful?

• Binaries provide the best method for astronomers
  to determine the mass of a distant star.
• The gravitational pull between them causes them
  to orbit around their common center of mass.
• The mass of its stars can be determined from the
  orbital pattern of a visual binary or the time
  variation of the spectrum of a spectroscopic
  binary.

18
Why are Binaries Helpful?

• Binaries are particularly important to our
  understanding of the processes by which stars
  form because a majority of stars exist in binary
  systems.
• In particular, the period and masses of the
  binary tell us about the amount of angular
  momentum in the system.
• Because this is a conserved quantity in physics,
  binaries give us important clues about the
  conditions under which the stars were formed.

19
Why are Binaries Helpful?

• In a binary system, the more massive star is
  usually designated "A" and its companion "B."
• Thus the bright main sequence star of the Sirius
  system is Sirius A, while the smaller white dwarf
  member is Sirius B.
• If the pair is very widely separated, they might
  be designated with superscripts as with Zeta
  Reticuli (?1 Ret and ?2 Ret),

20
Recap

• Binary stars are classified into four types,
  according to their observable properties.
• visual binaries
• spectroscopic binaries
• eclipsing binaries
• astrometric binaries
• Any star can belong to several of these classes.
• Several spectroscopic binaries are also eclipsing
  binaries.

21
Recap

• Another three-category classification is based on
  the distance of the stars, relative to their
  sizes 
• detached binaries
• semi-detached binaries
• contact binaries

22
Finding Mass using Binaries

• If both stars are visible
• We can trace out their orbital motion by
  observing them for a long time, giving the
  angular size of the orbit and the orbital period.
• Then we need to find the distance to the binary
  system so that we can convert their angular
  separation into a physical one.
• It is likely that the plane of a stars orbit is
  tilted from a direct face-on view which must be
  compensated for.

23
Research Findings

• During the past 200 years a large amount of
  research has been carried out on binary stars
  leading to some general conclusions.
• It is believed that at least a quarter of all
  stars are at least binary systems, with as many
  as 10 of these systems containing more than two
  stars (ternary etc.).
• There is a direct correlation between the period
  of revolution of a binary star and the
  eccentricity of its orbit, with systems of short
  period having smaller eccentricity.

24
Research Findings

• Binary stars may be found with any conceivable
  separation.
• Pairs orbiting so closely that they are
  practically in contact with each other.
• Pairs so distantly separated that their
  connection is indicated only by their common
  proper motion through space.
• Remarkably, among gravitationally-bound binary
  star systems, there exists a log normal
  distribution of periods, with the majority of
  these systems orbiting with a period of about 100
  years.

25
Research Findings

• In pairs where the two stars are of equal
  brightness, they are also of the same spectral
  type.
• In systems where the brightnesses are different,
  the fainter star is bluer if the brighter star is
  a giant star, and redder if the brighter star
  belongs to the main sequence.
• Since mass can be determined only from
  gravitational attraction, and the only stars for
  which this can be determined are binary stars,
  these are a uniquely important class of stars.
• (with the exception of the Sun, and
  gravitationally-lensed stars),

26
Research Findings

• The combined mass of two visual binary stars may
  be obtained by a direct application of the
  Keplerian harmonic law if
• The orbit has been determined of a visual binary
  star.
• The stellar parallax of the system has been
  determined.

27
Research Findings

• Warning Science Content!
• It is impossible to obtain the complete orbit of
  a spectroscopic binary unless it is also a visual
  or an eclipsing binary, so from these objects
  only a determination of the joint product of mass
  and the sine of the angle of inclination relative
  to the line of sight is possible.
• Therefore, without additional information
  regarding the angle of inclination, the mass can
  only be inferred in a statistical sense.
• In the case of eclipsing binaries which are also
  spectroscopic binaries, it is possible to find a
  complete solution for the specifications (mass,
  density, size, luminosity, and approximate shape)
  of both members of the system.

28
Mythbuster

• Science fiction has often featured planets of
  binary or ternary stars as a setting.
• In reality, some orbital ranges are impossible
  for dynamical reasons
• The planet would be expelled from its orbit
  relatively quickly, being either ejected from the
  system altogether or transferred to a more inner
  or outer orbital range.
• Other orbits present serious challenges for
  eventual biospheres because of likely extreme
  variations in surface temperature during
  different parts of the orbit.

29
HD 188753

• Detecting planets around multiple star systems
  introduces additional technical difficulties,
  which may be why so far (as of July 2005) only
  one such planet has been found HD 188753 Ab.
• HD 188753 Ab is the first known planet in a
  triple star system. It has been discovered by a
  Polish astronomer working in the United States,
  Dr. Maciej Konacki. The planet, a gas giant
  slightly larger than Jupiter, orbits the main
  star of the HD 188753 system (HD 188753A), in the
  constellation Cygnus.
• The stellar trio (yellow, orange, and red) is
  about 149 light-years from Earth. The planet,
  which is at least 14 larger than Jupiter, orbits
  the main star (HD 188753 A) once every 80 hours
  or so (3.3 days), at a distance of about 8 Gm, a
  twentieth of the distance between Earth and the
  Sun.
• The other two stars whirl tightly around each
  other in 156 days, and circle the main star every
  25.7 years at a distance from the main star that
  would put them between Saturn and Uranus in our
  own Solar System.

Artist concept of a triple sunset on a
moon of HD 188753 Ab
http//en.wikipedia.org
30
Binary Star Examples

• Albireo
• Algol (triple, eclipsing binary)
• Alpha Centauri (triple)
• Castor (sextuple)
• Procyon
• Sirius

31
Binary Stars in Fiction

• Isaac_Asimov's Nightfall is set in a six-sun
  system.
• The 1994 computer game Little Big Adventure was
  set on a planet estabilised between two stars.
• The planet Tatooine in the movie Star Wars orbits
  the binary star system of Tatoo. The names of the
  stars are Tatoo 1 and Tatoo 2.
• Manticore system is a binary star in Honorverse
  with three habitable planets, two of them
  (capital Manticore and Sphinx) orbiting Manticore
  A and one (Gryphon) orbiting Manticore B.
• In the Star Trek Enterprise episode
  "Singularity" the ship visits a three-sun
  (Trinary or Ternary) star system.