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Topic Name: Ohio University Astronomers have Discovered a Faraway Binary Star System May Progenitor of a Rare Type of Supernova

Category: STAR (Space, Telecommunications & Radioscience)

Research persons: Jose Prieto

Location: Ohio State University, United States

Details

Researchers funded by the National Science Foundation (NSF) announced today in Astrophysical Journal Letters that they have discovered a faraway binary star system that could be the progenitor of a rare type of supernova.

The two yellow stars, which orbit each other and even share a large amount of stellar material, resemble a peanut. The Ohio State University astronomers and their colleagues believe the two stars in the system, 13 million light years away and tucked inside a small galaxy known as Holmberg IX, appear to be nearly identical, each 15 to 20 times the mass of our Sun.

This work was funded through an NSF continuing grant to support a systematic study of the most massive stars in the local universe. The study is expected to yield masses and radii for dozens of massive stars discovered in a variety of environments. The data produced can be used to test models of massive star atmospheres, winds, and how they evolve both as single stars and in binaries.

"To have discovered a pair of massive interacting stars in this configuration is truly exceptional--sort of like rare squared," said NSF Program Manager Michael Briley. "There is a lot these stars can tell us about how they work and how they influence their environment. But the really exciting part is they may also hold the key to finally understanding why some massive yellow stars explode."

Lead author Jose Prieto, an Ohio State graduate student who analyzed the new system as part of his doctoral dissertation, searched the historical record to see whether his group had found the first such binary. In a surprising twist, his search uncovered another similar system less than 230,000 light years away in the Small Magellanic Cloud, a small galaxy that orbits the Milky Way. The second binary star system was discovered in the 1980s but misidentified at the time. Prieto reassessed the data and realized the system was another yellow super-giant eclipsing binary. Prieto and his colleague suspect the yellow binary systems could be the progenitors of rare supernova linked to yellow supergiants.

Most stars end their life in a supernova at the cooler red end of the temperature scale and a few end in the hotter blue end, Pietro said. Astronomers didn't believe stars would end during the short transitional phase in between--until now.

"When two stars orbit each other very closely, they share material, and the evolution of one affects the other," Prieto said. "It's possible two supergiants in such a system would evolve more slowly and spend more time in the yellow phase--long enough that one of them could explode as a yellow supergiant."

Note for Binary Star
A binary star is a stellar system consisting of two stars orbiting around their center of mass. For each star, the other is its companion star. Recent research suggests that a large percentage of stars are part of systems with at least two stars. Binary star systems are very important in astrophysics, because observing their mutual orbits allows their mass to be determined. The masses of many single stars can then be determined by extrapolations made from the observation of binaries.

Binary stars are not the same as optical double stars, which appear to be close together as seen from Earth, but may not be bound noticeably by gravity. Binary stars can either be distinguished optically (visual binaries) or by indirect techniques, such as spectroscopy. If binaries happen to orbit in a plane containing our line of sight, they will eclipse each other; these are called eclipsing binaries.

Systems consisting of more than two components, known as multiple stars, are also not uncommon and are generally classified under the same name. The components of binary star systems can exchange mass, bringing their evolution to stages that single stars cannot attain. Examples of binaries are Algol (an eclipsing binary), Sirius, and Cygnus X-1 (of which one member is probably a black hole).

The term binary star was coined by Sir William Herschel in 1802 to designate, in his definition, "a real double star - the union of two stars that are formed together in one system by the laws of attraction". Any two closely-spaced stars might appear to be a double star, the most famous case being Mizar and Alcor in the Big Dipper (Ursa Major). It is however possible that a double star is merely a star pair that only looks like a binary system: the two stars can in reality be widely separated in space, but just happen to lie in roughly the same direction as seen from Earth. Such false binaries are termed optical binaries, or optical pairs. With the invention of the telescope, many such pairs were found. Herschel, in 1780, 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.

A true binary is a pair of stars bound together by gravity. When they can be resolved (distinguished) with a powerful enough telescope (if necessary with the aid of interferometric methods) they are known as visual binaries. In other cases, the only indication is the Doppler shift of the emitted light. Systems in which this is the case, known as spectroscopic binaries, consist of relatively close pairs of stars where the spectral lines in the light from each one shifts first toward the blue, then toward the red, as each 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. If the orbital plane is very nearly along our line of sight, the two stars partially or fully occult each other regularly, and the system is called an eclipsing binary, of which Algol is the best-known example.

Binary stars that are both visual and spectroscopic binaries are rare, and are a precious source of valuable information when found. Visual binary stars often have large true separations, with periods measured in decades to centuries; consequently, they usually have orbital speeds too small to be measured spectroscopically. Conversely, 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 thus must be relatively close to Earth.

Astronomers have discovered some stars that seem to orbit around an empty space. 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 emits little or no electromagnetic radiation, for example a neutron star. In some instances, there is strong evidence that the missing companion is in fact a black hole: a body with such strong gravity that no light is able to escape. Such binaries are known as high-mass X-ray binaries. Probably the best known example at present is Cygnus X-1, where the mass of the unseen companion is believed to be about nine times that of our sun; far exceeding the Tolman-Oppenheimer-Volkoff limit (the maximum theoretical mass of a neutron star, the only other likely candidate for the companion). In this way, Cygnus X-1 became the first object that was widely accepted as being a black hole.

Note for Supernova
A supernova (plural: supernovae or supernovas) is a stellar explosion that creates an extremely luminous object. A supernova causes a burst of radiation that may briefly outshine its entire host galaxy before fading from view over several weeks or months. During this short interval, a supernova can radiate as much energy as the Sun could emit over its life span. The explosion expels much or all of a star's material at a velocity of up to a tenth the speed of light, driving a shock wave into the surrounding interstellar medium. This shock wave sweeps up an expanding shell of gas and dust called a supernova remnant.

Several types of supernovae exist that may be triggered in one of two ways, involving either turning off or suddenly turning on the production of energy through nuclear fusion. After the core of an aging massive star ceases to generate energy from nuclear fusion, it may undergo sudden gravitational collapse into a neutron star or black hole, releasing gravitational potential energy that heats and expels the star's outer layers. Alternatively, a white dwarf star may accumulate sufficient material from a stellar companion (usually through accretion, rarely via a merger) to raise its core temperature enough to ignite carbon fusion, at which point it undergoes runaway nuclear fusion, completely disrupting it. Stellar cores whose furnaces have permanently gone out collapse when their masses exceed the Chandrasekhar limit, while accreting white dwarfs ignite as they approach this limit (roughly 1.38] times the mass of the Sun). White dwarfs are also subject to a different, much smaller type of thermonuclear explosion fueled by hydrogen on their surfaces called a nova. Solitary stars with a mass below approximately nine solar masses, such as the Sun itself, evolve into white dwarfs without ever becoming supernovae.

On average, supernovae occur about once every 50 years in a galaxy the size of the Milky Way and play a significant role in enriching the interstellar medium with heavy elements. Furthermore, the expanding shock waves from supernova explosions can trigger the formation of new stars.

While it is not impossible that some binaries might be created through gravitational capture between two single stars, given the very low likelihood of such an event (three objects are actually required, as conservation of energy rules out a single gravitating body capturing another) and the high number of binaries, this cannot be the primary formation process. Also, the observation of binaries consisting of pre main sequence stars, supports the theory that binaries are already formed during star formation. Fragmentation of the molecular cloud during the formation of protostars is an acceptable explanation for the formation of a binary or multiple star system.

The outcome of the three body problem, where the three stars are of comparable mass, is that eventually one of the three stars will be ejected from the system and, assuming no significant further perturbations, the remaining two will form a stable binary system.

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. From the orbital pattern of a visual binary, or the time variation of the spectrum of a spectroscopic binary, the mass of its stars can be determined. In this way, the relation between a star's appearance (temperature and radius) and its mass can be found, which allows for the determination of the mass of non-binaries.

Because a large proportion of stars exist in binary systems, binaries are particularly important to our understanding of the processes by which stars form. 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.

Note for Small Magellanic Cloud
The Small Magellanic Cloud (SMC) is a dwarf galaxy. It contains several hundred million stars.
Some speculate that the SMC was once a barred spiral galaxy that was disrupted by the Milky Way to become somewhat irregular. It still contains a central bar structure.
At a distance of about 200,000 light-years, it is one of the Milky Way's nearest neighbors. It is also one of the most distant objects that can be seen with the naked eye.
With a mean declination of approximately -73 degrees, it can only be viewed from the Southern Hemisphere and the lower latitudes of the Northern Hemisphere. It is located in the constellation of Tucana and appears as a hazy, light patch in the night sky about 3 degrees across. It looks like a detached piece of the Milky Way. Since it has a very low surface brightness, it is best viewed from a dark site away from city lights.
It forms a pair with the Large Magellanic Cloud (LMC), which is positioned a further 20 degrees to the east. The Small Magellanic Cloud is a member of the Local Group.

Note for Milky Way
The Milky Way is a barred spiral galaxy that is part of the Local Group of galaxies. Although the Milky Way is one of billions of galaxies in the observable universe, the Galaxy has special significance to humanity as it is the home galaxy of the planet Earth. The plane of the Milky Way galaxy is visible from Earth as a band of light in the night sky, and it is the appearance of this band of light which has inspired the name for our galaxy.

Some sources hold that, strictly speaking, the term Milky Way should refer exclusively to the observation of the band of light, while the full name Milky Way Galaxy, or alternatively the Galaxy should be used to describe our galaxy as an astrophysical whole. It is unclear how widespread the usage of this convention is, however, and the term Milky Way is routinely used in either context.

Visible from Earth as a hazy band of white light that is seen in the night sky, arching across the entire celestial sphere, the visual phenomenon of the Milky Way (as seen in the night sky) originates from stars and other material which lies within the galactic plane.

The Milky Way looks brightest in the direction of the constellation of Sagittarius, toward the galactic center. Relative to the celestial equator, it passes as far north as the constellation of Cassiopeia and as far south as the constellation of Crux, indicating the high inclination of Earth's equatorial plane and the plane of the ecliptic relative to the galactic plane. The fact that the Milky Way divides the night sky into two roughly equal hemispheres indicates that our Solar System lies close to the galactic plane. The Milky Way has a relatively low surface brightness, making it difficult to see from any urban or suburban location suffering from light pollution.

The stellar disk of the Milky Way galaxy is approximately 100,000 light years in diameter, and is believed to be, on average, about 1,000 light years thick. It is estimated to contain at least 200 billion stars and possibly up to 400 billion stars, the exact figure depending on the number of very low-mass stars, which is highly uncertain. Extending beyond the stellar disk is a much thicker disk of gas. Recent observations indicate that the gaseous disk of the Milky Way has a thickness of around 12,000 light years - twice the previously accepted value. As a guide to the relative physical scale of the Milky Way, if it were reduced to 130 km (80 mi) in diameter, the Solar System would be a mere 2 mm (0.08 inches) in width.

The Galactic Halo extends outward, but is limited in size by the orbits of the two Milky Way satellites, the Large and the Small Magellanic Clouds, whose perigalacticon is at ~180,000 light-years.

Observations by the Spitzer Space Telescope in 2005 backed up previously collected evidence that suggested the Milky Way is a barred spiral galaxy. It consists of a bar-shaped core region surrounded by a disk of gas, dust and stars. Within the disk region are several arm structures that spiral outward in a logarithmic spiral shape. The mass distribution within the Galaxy closely resembles the Sbc Hubble classification, which is a spiral galaxy with relatively loosely-wound arms. It was only in the 1980s that astronomers began to suspect that the Milky Way is a barred spiral rather than an ordinary spiral, which observations in 2005 with the Spitzer Space Telescope have since confirmed, showing that the Galaxy's central bar is larger than previously suspected. This argues for a classification of type SBbc (loosely wound barred spiral). In 1970 Gérard de Vaucouleurs predicted that the Milky Way was of type SAB(rs)bc, where the "rs" indicates a broken ring structure around the core region.

In figure 1, Hubble image of the Sirius binary system, in which Sirius B can be clearly distinguished (lower left)

In figure 2, A nearly identical pair of stars is one of two known yellow binary systems

In figure 3, An eclipsing binary, with an indication of the variation in intensity