Transcript Astronomy and Survey of Information

Astronomy and Survey of
Information
Binary Stars
Presented by: Joe Giordano
Guest Speaker: Dr. Frank Maloney
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
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
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
“False Binaries”
• A double star is probably a foreground/background star
pair that only looks like a binary system—the 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.
“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
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.
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.
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.
Algol (β Per / Beta Persei)
http://en.wikipedia.org
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 together—usually too close to be detected as
visual binaries.
• Binaries that are both visual and spectroscopic are usually
relatively close to us.
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.
Astrometric Binaries
• In some instances, one can make a strong case that the
missing companion is in fact a black hole—a 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 sun—far exceeding the maximum
theoretical mass of a neutron star, the other likely
candidate for the companion.
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 20–30 solar
masses
– The compact object has a
mass of 7–13 solar masses
• NOTE: the largest possible
mass of a neutron star can
not exceed three solar
masses
http://en.wikipedia.org
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 > 20 keV) in the sky.
• The distance to Cygnus X-1 is about 2500
parsecs.
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
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.
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.
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 (ζ1 Ret and ζ2 Ret),
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.
Recap
• Another three-category classification is
based on the distance of the stars, relative
to their sizes :
– detached binaries
– semi-detached binaries
– contact binaries
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 star’s orbit is tilted from a
direct face-on view; which must be compensated for.
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.
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.
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),
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.
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.
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.
HD 188753
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•
•
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
Binary Star Examples
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Albireo
Algol (triple, eclipsing binary)
Alpha Centauri (triple)
Castor (sextuple)
Procyon
Sirius
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.