Astronomy and Survey of Information Binary Stars Presented by: Joe Giordano Presented by: Joe Giordano Guest Speaker: Dr. Frank Maloney.

Astronomy and Survey of Astronomy and Survey of Information Information

Binary StarsBinary Stars Presented by: Joe GiordanoPresented by: Joe Giordano

Guest Speaker: Dr. Frank Maloney Guest Speaker: Dr. Frank Maloney

Binary Stars 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 StarsBinary 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

FactsFacts

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.


““False Binaries”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”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


True Binaries 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 BinariesSpectroscopic 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 BinaryEclipsing 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)Algol (β Per / Beta Persei)


Binary StarsBinary 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 BinariesAstrometric 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 BinariesAstrometric 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-1Cygnus 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


What does all this mean?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 DiskAccretion 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.


Why are Binaries Helpful? 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?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?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 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 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 BinariesFinding 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 FindingsResearch 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.


• 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 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),


• 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.


Warning Science Warning Science Content!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.

MythbusterMythbuster

• 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 188753HD 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

Binary Star ExamplesBinary Star Examples

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

Binary Stars in Fiction 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.

Astronomy and Survey of Information Binary Stars Presented by: Joe Giordano Presented by: Joe Giordano Guest Speaker: Dr. Frank Maloney.

Documents