In our own solar system, we have only one sun. Not every star in the night sky shines there on its own: quite a lot of them are double, a few are triple, and some are even more complicated. (It would be quite tough for us, if we had two or more suns all rising and setting at different times, and what would the tides at the seaside be like?) It is fairly easy to find one double star on a clear night in the Northern hemisphere. For many people, the first constellation they learn to recognise is the Great Bear, part of which is called either the Plough or the Big Dipper.
At the bend in its “handle” is a star, at witch
We can’t assume that a double, as we see it from earth, is really two stars close to each other – they might just be accidentally lined up one behind the other but a long way apart. In that case they are called a visual double or optical double. With a telescope, it is possible to measure the distances and angles between double stars to determine the relative motion of the two stars. All stars show some movement over time (known as “proper motion”) relative to distant stars, and if the relative motion of the two stars is large compared to their proper motion, then the pair are probably a visual double. In contrast, if the relative motion of a pair determines a curved arc of an orbit, or is small compared to the common proper motion of both stars, then the pair is a genuine pair, connected in some way. A better word to label this type of double star is binary, i.e. as a binary star with the two stars in mutual orbit around their common centre of gravity. There is a binary star in the Swan constellation which was one of the first that astronomers were able to prove definitely to be a pair. In the story about the Tick Tick pulses, you may remember that the Cygnus (Latin: “Swan”) constellation is in the northern sky. The star 61 Cygni (11.4 light-years from Earth, the 15th nearest star) is a binary star, and was the first to have its distance measured. . William Herschel began systematic observations of 61 Cygni as part of a wider study of binary stars. His observations led to the conclusion that binary stars were separated enough that they would show different movements in parallax over the year, and hoped to use this as a way to measure the distance to the stars.
P.S. It may seem puzzling that losing energy makes these stars – or an earth satellite near the end of its life - speed up. Anyone you know studying physics in college, who knows about potential energy as well as kinetic energy, should be able to explain this. Another puzzle for you: how is it that with our earth-moon system, we have the opposite: tidal friction over millions of years causes the moon to rotate round the earth in a wider orbit, with lower orbital velocity? Knowing about angular momentum as well as energy, the college student can explain that too.
In 1792, Giuseppe Piazzi noted that it had high proper motion, suggesting that it could be one of the closest stars, and confirmed that it could be a prime candidate for an attempt to determine its distance through parallax measurements, using the Earth's orbit as a baseline. A star's positioned is accurately measured twice, six months apart: in the picture below, as shown by the diagonal lines we note whereabouts we see the star superimposed on the layout of distant stars. But this line of sight to the star in December is different to that in June, having swept through the angle shown as the Earth moves to the other side of its orbit. Half of this angle is the parallax, p, and knowing the width of the earth’s orbit, from simple trigonometry:
But the first direct and reliable such measurements were not made until 1838, by Bessel. UNCOMPLICATED ORBITING PAIR
If we look again at the double star in the middle of the handle of the plough or Big Dipper, this is a binary star. The Mi’kmaq First Nations peoples named them Mizar & Alcor, as part of the myth of the seven hunters chasing the great bear. In this myth, Mizar is the hunter Chickadee, and Alcor is his pot in which he cooks the bear. The ability to resolve Mizar and Alcor with the naked eye is often quoted as a test of eyesight, although even people with quite poor eyesight can see the two stars.
With a binary double, the two stars are orbiting each other, as do the earth and the moon. The centre of any such orbit – what they are orbiting about – is their centre of mass: so it’s not accurate to say that the moon is orbiting the centre of the earth: from the centre of the earth, the centre of mass of the earth/moon pair is about three-quarters of the way out to the earth’s surface. Image how complicated day and night would be for us, if our sun were a binary star! A high proportion, perhaps one-half, of all stars in the Milky Way Galaxy are binaries or members of more complex multiple systems. Some of the ancient Greek myths told of pairs of twins, most of which they thought of as placed in the constellations of the night sky, although in most of these myths, twins represent separate stars rather than two members of a double. One such example is Castor and Pollux, or Castor and Polydeuces, who share a bond so strong that when Castor dies, Pollux gives up half of his immortality to be with his brother.
Not all binary stars are of the same type. We remember that the two stars of a binary orbit each other, so that each one feels the gravitational pull of the other, but here, an important complication is that the surface of each star is not just like a perfectly smooth football. Think how the sun appears in images of a total eclipse, when the moon blocks off the direct light from the sun itself: surrounding the black disc of the moon, we see wisps of what is called the sun’s corona, meaning that the sun has an atmosphere. This true of all stars (except for any who are completely cold and dead).
We can think of the atmosphere of a star as resembling the hair on a tennis ball or a human skull. Characters mentioned in several Greek myths are described as twins having spectacular hair. Associated with the constellation corvus (the crow) is the god Apollo, twin of Artemis and frequently described as having golden hair. In Egyptian mythology, Isis and her twin and husband Osiris Spell 562 of the Coffin Texts notes the ability of the hair of Isis and Nephthys to unite things, saying that the hair of the Goddesses is knotted together and that the deceased has come to “be joined to the Two Sisters and be merged in the Two Sisters, for they will never die.” The goddess Isis is frequently associated with the constellation Virgo, particularly when she is depicted seated upon a throne and nursing the infant god Horus (this same pattern is found in the figure of Mary in the New Testament, who can also be shown to be associated with the constellation Virgo). However, Isis can also be associated with other constellations — this association with multiple constellations is not at all uncommon for extremely important figures in ancient myth, such as Isis. She can also be shown to be associated with the constellation Sagittarius when, after discovering that the coffin containing Osiris has been found in the land of Byblos, she visits there in disguise and sits down beside a spring, and braids the hair of the queen’s maidservants as part of her plan to be taken to see the king and queen of the land.
Think also of the story of Samson, in the Bible, to whom God gave immense strength to aid him against his enemies and allow him to perform superhuman feats, including slaying a lion with his bare hands and massacring an entire army of Philistines using only the jawbone of a donkey. In the sky, the jawbone is almost undoubtedly represented by the V shape of the brightest stars of the Hyades cluster in the constellation Taurus, just above the well-known figure of Orion.
However, Samson’s strength was given to him in return for his mother vowing that he would not cut his hair or shave. if Samson's long hair were ever cut, then the vow would be violated and he would lose his strength. Samson was betrayed by his lover Delilah, who, sent by the Philistine officials to entice him, ordered a servant to cut his hair while he was sleeping and turned him over to his Philistine enemies. It was not until his hair had grown again that he
recovered his strength and was able to bring down the columns of
the temple, killing himself as well as all of the Philistines.
Well, Samson wasn’t even a twin, so this stuff about hair - what has it to do with double stars?
If they are so far apart that the atmosphere of each one is not affected by the other, then the stars have no effect on each other and evolve separately. This is the case with most binaries. (In mythology, this reminds us of the companionship and collaboration shown by the twins Zethos and Amphion, who build the wall that fortifies Thebes).
But when two stars are pulling on each other, if they are close enough, some of the atmosphere of one star can be dragged into the atmosphere of the other, as if the hair on someone’s skull was being pulled out. This could have a serious effect on how each star is moving. The phenomenon of one star in a double “stealing” some atmosphere from the other can have different consequences, depending on how far apart the pair are.
The simplest such situation is when the binary stars are what is known as semidetached. In this case, gas from the surface of one component, the “donor” star, is transferred to the other, the “accreting” star. In many cases, the inflowing gas forms a detectable disc of material around the accretor. When gas is being shifted from donor to accretor, this transfer of material dominates the evolution of the system. The shape of the donor star will become distorted, and mass can be transferred from the distorted star to the companion star in the binary.
We can think of Samson, or Nisos whose hair was cut off by his daughter Scylla at the request of her lover Minos the king of Crete: in both cases the affect was drastic. Delta1,2 (δ1,2) Tauri is a true double star located just 2° northwest of the Thetas. Here’s another image of the Hyades. Delta1 is easy to pick out at magnitude 3.8, but its companion is a full magnitude fainter, but with averted vision for a clear view, it is possible to make them out with the naked eye.
A more drastic consequence of the dragging of atmospheres is when we detect what is called a “common envelope binary”. This is a type of binary star in which the uppermost part of both their atmospheres forms a common envelope that surrounds both stars. Remember that the two stars in a double are both orbiting about their centre of mass. But when the stars are sharing an atmosphere, the orbital motion results in friction. Using a kitchen whisk to mix and stir a cake mix can provide an analogy:
when you turn the handle, you feel the friction that results from the rotation of the loops that are immersed in the cake mix, like the gravitational pull of both stars stirring the atmospheric envelope. But with double stars, there’s no-one to turn a handle: thus, as the friction of the envelope brakes the orbital motion and the two objects lose energy, they drift into a closer and closer orbit, while actually increasing their orbital velocities. This is what happened to the European Space Agency's gravity-mapping GOCE satellite which eventually fell to Earth in November 2013.
In this process, the common envelope of gas is heated up and expands, and the stars may eventually merge, in a timescale of months to years. A star known as KIC 9832227 in the constellation Cygnus is an example, predicted to become a luminous red nova - an exploding object whose light lasts for weeks or months, becoming dimmer and redder over time. It’s as if the larger of the two stars is eating the other – you might think of the titan in Greek mythology who was called Cronos, who began to devour them all his sons as soon as they were born, in order to prevent a prophecy that he was destined to be overcome by them.
A star in the constellation Aries was named after Cronos when it was reported to have swallowed its planets, but a better analogy is found within African mythology. Ibeji twins are viewed as one soul shared between two bodies. If one of the twins dies, the living twin is almost destined for death because the soul of the dead twin is believed to be missing half of soul of the living one. To prevent this, the parents then create a doll that portrays the body of the deceased child, so that its soul can merge with the soul of the living twin.
Alternatively we can find a contact binary, which is a binary star system whose component stars are so close that they touch each other; they share their gaseous envelopes which rotate with them, giving the system a peanut shape .There is an example in the Great Bear, shown by the red dot in this map: This system consists of a pair of stars in a tight, circular orbit. Once every eight hours and 26 seconds, each star eclipses the other, resulting in a decrease in magnitude. It is too faint to be seen with the naked eye, but it can be viewed with a small telescope.
A particular type of binary system, consisting of a "white dwarf" and its binary companion, can generate a special kind of supernova known as a type 1a. Here the explosion of the white dwarf will produce mainly heavy metals such as iron and nickel, and by neutron capture, copper, silver, iodine and more. Indeed, processes in such supernovae are thought to be responsible for some nucleosynthesis of other heavy elements. Since white dwarfs are the final evolutionary phase for stars with masses less than around five times that of the sun, type Ia supernovae begin to occur soon (about one billion years) after star formation starts, and this can continue indefinitely as new white dwarfs are formed in binary systems.
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