Our own Sun is a solitary star, whose sparkling stellar siblings wandered off to other regions of our Milky Way Galaxy long ago. However, our lovely, but lonely, Star is an exception to the general rule. This is because most stars dwell in the company of others of their kind. Indeed, observations conducted over the past two centuries indicate that 50% or more of all visible stars are members of multiple stellar systems. The most common multiple star systems are called binary systems, and they contain two sister stars that orbit around their common center of gravity (barycenter). In May 2019, a team of astronomers using the Chandra X-ray Telescope demonstrated what can be achieved when space telescopes like Chandra are used to study galaxy clusters for long periods of time. By combining 15 days worth of Chandra observations of the Fornax galaxy cluster, spread out between 1999 and 2015, the astronomers discovered the existence of unfortunate stellar duos that had been tragically evicted from their host galaxies inhabiting the cluster.
Stars are born when an extremely dense blob of material, embedded within the billowing, swirling folds of one of the many giant, frigid, and dark molecular clouds that haunt our Milky Way Galaxy, collapses under the merciless squeeze of its own gravity. These beautiful, undulating star-birthing clouds serve as the cradles of sparkling baby stars (protostars), that form as the dense blob collapses. However, most blobs fragment, forming multiple sister stars, rather than a single solitary star like our Sun.
Stellar binaries have proven themselves, time and time again, to be valuable tools for astronomers to use in their observations–and they are especially useful for x-ray studies. For a large number of binary systems, the stellar duo circle around their common center of mass as a result of their mutual gravitational attraction. But, complications arise when the two sibling stars evolve independently (wide biaries). Wide binaries can be compared to siblings that live in different cities but nevertheless keep in touch by talking on their phones. Wide binaries play a particularly important role in astronomy because they are the best tool available when astronomers seek to measure stellar masses. This is because astronomers, by observing both the period and size of the orbit, can then go on to apply the theory of gravity to their observations.
However, not all binary stars are so distant, and these more closely-knit stars can be compared to siblings who live on the same street. These kindred stellar duos are appropriately dubbed close binaries, and the two stars involved are so close to each other that they are able to transfer matter to one another. This exchange greatly influences the appearance of both stars, as well as the way that they evolve.
Frequently, stellar binaries are spotted by astronomers using optical wavelengths in order to detect them. For this reason, these double stars are appropriately referred to as visual binaries. A large number of visual binaries possess long orbital periods that can amount to several centuries or millennia. This means that their orbits are poorly determined by astronomers, because they are uncertain or completely unknown. These stellar duos can also be spotted by astronomers using indirect techniques, such as spectroscopy (spectroscopic binaries) or astrometry (astrometric binaries). If a double star system chances to orbit in a plane along astronomers‘ line of sight, its two stellar siblings will eclipse and transit one another. These stellar pairs are referred to as eclipsing binaries. Alternatively, if the two stars are discovered because of alterations in brightness during eclipses and transits, they are termed photometric binaries.
If the two binary sibling stars are sufficiently close to one another they can gravitationally distort their mutual outer stellar atmospheres. Sometimes these close binary systems are able to exchange mass, which can cause them to evolve in a way that is not possible for solitary stars like our Sun. Binary stars are also common as the nuclei of many planetary nebulae, and these stars can serve as the progenitors of both novae and Type Ia supernovae. Both novae and Type Ia supernovae involve a type of stellar corpse called a white dwarf star, which is the lingering core of a small hydrogen-burning „normal“ star like our Sun that has used up its necessary supply of nuclear-fusing fuel and has puffed its outer gaseous layers into space. A nova is a non-fatal explosion that occurs on the surface of a white dwarf in a binary system. It occurs when the white dwarf star has managed to sip up enough gas from its nearby sister star to have it build up on its surface to the point that it triggers a brilliant explosion–that does not destroy the vampire-like white dwarf. However, sometimes the white dwarf pays for its crime and destroys itself completely in a Type Ia supernova explosion. If the thirsty white dwarf dwells in a binary system with a very unfortunate sister star, and it manages to „steal“ enough gas from its victim, the gas piles up on the white dwarf’s surface to the tragic point that it triggers a runaway thermonuclear blast that fatally blows this stellar vampire to smithereens. Nothing of the white dwarf remains to tell the story of how once there was a star that is a star no more.
The term binary star was used for the first time by the German-born English astronomer Sir William Herschel (1738-1822) in 1802, when he wrote:
If, on the contrary, two stars should really be situated very near each other, and at the same time so far insulated as not to be materially affected by the attractions of neighboring stars, they will then compose a separate system, and remain united by the bond of their own mutual gravitation towards each other. This should be called a real double star; and any two stars that are thus mutually connected form the binary sidereal system which we are now to consider.
The term binary star itself is usually limited to only those stellar duos that revolve around their shared center of mass. The more general term double star designates pairs of stars which are observed to be very near to one another in the sky. This distinction between binary stars and double stars is not usually made in languages other than English. Hence, double stars can be either binary systems or merely two stars that appear to be close to one another in the sky but, nevertheless, actually have very different real distances from our own Sun (optical doubles or optical pairs).
Ever since the invention of the first telescopes, a large number of double stars have been detected. Early examples include the stars dubbed Mizar and Acrux. Mizar is located in the Big Dipper (Ursa Major) constellation, and it was observed to be a double star by the Italian astronomer Giovanni Battista Riccioli (1598-1671) in 1650. However, Mizar may have been detected earlier by another Italian astronomer Benedetto Castelli (1578-1643) and the great Italian astronomer and physicist Galileo Galilei (1564-1642). Acrux is a brilliant southern star, situated in the Southern Cross. It was discovered to be a double star by the French mathematician Father Jean de Fontenay (1643-1710) in 1685.
The English natural philosopher John Michell (1724-1793) was the first to propose that double stars are physically bound to one another gravitationally. In 1767, Michell argued that the likelihood that a double star was merely the result of a chance alignment was improbable. William Herschell began observing double stars in 1779 and shortly thereafter published catalogs listing around 700 double stars. By 1803, Herschel had managed to detect alterations in the relative positions of numerous double stars over the span of two and a half decades. He went on to conclude that the double stars that he had discovered must be binary systems. However, the first orbit of a binary star was not determined until 1827, when the French astronomer Felix Savary (1797-1841) calculated the orbit of Xi Ursa Majoris.
Since those early discoveries, many more double stars have been measured and catalogued. The Washington Double Star Catalog is a database that lists visual double stars compiled by the United States Naval Observatory. This catalog includes more than 100,000 double stars, including binary stars and optical doubles. Orbits have been determined for only a few thousand of these double stars, and most have still not been confirmed as either optical double stars or true binaries. This confirmation can be determined by watching the relative motion of the stellar duos. If the motion is part of an orbit, or if the two stars sport similar radial velocities and the difference in their proper motions is small compared to their common proper motion, the duo is deemed to likely be physically bound. One of the tasks remaining for visual observers of double stars is to collect enough observations to prove or disprove a gravitational connection between stellar pairs.
When Misery Loves Company
Binary pairs can consist of a number of differing combinations of stellar duos.The two sibling stars can be a combination of main-sequence (hydrogen-burning) stars like our own Sun, or they can be a pair of exotic and dense oddballs–such as neutron stars and black holes of stellar mass. Neutron stars are born when a massive star has managed to burn its necessary supply of nuclear-fusing fuel and then blasts itself to smithereens in the fury of a Type II supernova explosion. This explosion occurs when the core of the doomed massive star collapses onto itself. However, under certain conditions, these fiery and enormous blasts are not symmetric. The powerful recoil that results from this lack of symmetry can kick the luckless star screeching out of its host galaxy, where it formerly resided peacefully, as a still-living star on the hydrogen-burning main-sequence of the Hertzsprung-Russell Diagram of Stellar Evolution. However, the new Chandra results reveal that sometimes misery loves company. This is because the newborn neutron star’s unfortunate companion is also booted out of its home galaxy along with the wreckage of its sister star.
In addition to these rudely evicted X-ray binaries, the Chandra astronomers discovered about 150 other sources dwelling outside the boundaries of the galaxies being observed by the space telescope. One possible proposal explaining this observation suggests that these sources reside in the halos, or in the far suburbs, of the Fornax cluster’s central giant galaxy–where they were born. A second proposal suggests that they are X-ray binaries that were torn away from a galaxy by the powerful gravitational force of a nearby galaxy during a flyby. Yet a third explanation suggests that they are X-ray binaries left behind as a part of the remnants of an unfortunate galaxy that had been stripped of most of its stars as the result of a galactic smash-up. Such interactions are thought to be relatively frequent in a crowded region like the one in the Fornax cluster.
A paper describing these new results is published in the May 1, 2019 issue of The Astrophysical Journal.
by Judith E Braffman-Miller