Two Stars Too Close For Comfort!

All stars, from the largest to the smallest, are enormous roiling balls of seething hot, mostly hydrogen gas. Hydrogen is the most abundant atomic element in the Cosmos, as well as the lightest. Most of the billions of stars, that have set our large barred-spiral Milky Way Galaxy on fabulous fire, do not live alone, but instead formed in binary or multiple star systems close to one or more sparkling stellar siblings. In December 2014, astronomers announced that they have observed a duo of sister stars that are so close together that they will wind up merging into a single supermassive star. In fact, this study suggests that the most massive stars are born as a result of the merging of smaller stars, as predicted by certain theoretical models.

Some of the binary or multiple star systems in our Galaxy are eclipsing. Such a system is composed of two or more sister stars which, when observed from our own planet, undergo eclipses and mutual transits. This is because their orbital plane faces the Earth. One such system is the eclipsing binary dubbed MY Camelopardalis (MY Cam), the subject of a recent study appearing in the journal Astronomy & Astrophysics. The observations of MY Cam were conducted by astronomers using the Calar Alto Observatory (Almeria). The researchers were from the University of Alicante, the Astrobiology Centre of the Spanish National Research Council (CAB-CSIC) and the Canaries' Astrophysics Institute (IAC)--with additional, and extremely valuable help, coming from amateur astronomers!

The article suggests that MY CAM -- the most massive binary stellar system known -- is composed of an enchanting duo of sparkling sibling stars of spectral type O. Spectral type O stars are very hot, bright, and blue. The two glittering sibling stars are 38 and 32 times our Sun's mass, and are still on the hydrogen-burning main sequence. The massive stars are also extremely close to each other--indeed, they may be too close for comfort, with an orbital period of less than 1.2 days! This is the shortest orbital period known for this particular type of stellar system, indicating that the stars were born as they are seen now. In other words, the stars were almost in contact with each other from the time they were both born!

The predicted upshot of this very close sibling relationship is the merger of both stars into a single stellar object of more than 60 solar-masses. This will likely occur before either one of them has had sufficient time to significantly evolve. Therefore, these results add credibility to certain theoretical models suggesting that most massive stars are born as the result of merging less massive stars.

Stellar Matters

Stars are enormous, glaring, roiling balls of hot nuclear-fusing gas. The billions upon billions of glittering stars dancing around the Cosmos transform their hydrogen into heavier and heavier atomic elements in their seething-hot cores, in a process that is termed stellar nucleosynthesis.

The first stars to light up the Cosmos were unlike the stars we wish upon today: they were born directly from the lightest of all primordial gasses--primarily hydrogen. Both hydrogen and helium (the second-lightest atomic element) were born in the hot Big Bang birth of the Universe about 13.8 billion years ago (Big Bang nucleosynthesis). Indeed, the only atomic elements born in the Big Bang fireball are hydrogen, helium, and small quantities of lithium. All of the rest of the atomic elements listed in the familiar Periodic Table were manufactured deep in the nuclear-fusing hearts of stars, their incredibly hot cores progressively fusing the nuclei of atoms to form increasingly heavier atomic elements. Without these heavier elements fused in the searing-hot hearts of our Universe's stars, there would be no life. The oxygen we breathe, the elements composing the dirt, stones, and sand beneath our feet, the water that we drink, the iron in our blood, the carbon that is the basis for life on our planet, were all manufactured by the stars--or else in the explosive supernovae that marked their deaths. We are star dust. When very massive stars perish, they do not go peacefully--they blast themselves to pieces in the raging, fiery tantrum of a supernova explosion. When these heavy stars go supernova they throw their batch of freshly fused heavy atomic elements out into the space between stars. The first generation of stars were probably enormous--weighing-in at, perhaps, hundreds of times more than our Sun. They lived fast and died young. The heavier the star, the briefer its stellar existence. When the first stars blew themselves up in supernova fireworks, they hurled out the very first newly fused batch of heavy atomic elements into the Cosmos. These heavy atomic elements, or "metals" in the jargon of astronomers, are those that are heavier than hydrogen and helium.

Stars of all masses live out their lives on the hydrogen-burning main-sequence, whereby they manage to keep a necessary, and very delicate, balance between two constantly warring opponents--radiation pressure and gravity. The radiation pressure of a star tries to push everything out and away from the star. Basically, this pushing pressure keeps the immense seething sphere of roiling gas bouncy against the merciless, relentless pull of its own squeezing, crushing gravity--that tries to pull everything in towards the star. The star's radiation pressure is the result of nuclear fusion, which begins with the burning of hydrogen into helium. This process of stellar nucleosynthesis progressively fuses lighter atomic elements into heavier ones.

When an extremely heavy main-sequence star, that weighs-in at approximately 8 solar-masses, has finally burned up its necessary supply of hydrogen fuel, it is ready for its grand finale. The massive doomed star, at this unfortunate stage, can no longer keep itself bouncy against the squeeze of its own relentless, crushing gravity. At this point, gravity prevails over its rival, radiation pressure, and the star goes supernova. The supernova explosion blasts the dying heavy star to pieces, ferociously hurling its lovely, brilliant, fiery rainbow of varicolored gaseous layers out into the space between stars. This violent, brilliant event occurs when the iron core possessed by the heavy star attains the impressive weight of about 1.4 solar-masses. This triggers the star's final blaze of glory, and the most massive of all stars collapse and blast themselves into the oblivion of a stellar-mass black hole. Heavy stars--that are not quite that heavy--also blast themselves to smithereens, but they leave behind a sad testimony to the way they were in the form of a neutron star.

Two Stars Too Close For Comfort!

Stars, like our own loner of a Sun, travel through our Galaxy bereft of the comfort of sibling stars. These lonely stellar inhabitants of the Milky Way are trailed only by their planetary systems--however, such solitary stars are in the minority. Most stars spend their lives bound by gravity to a sibling star (binary system) or several siblings (multiple system). Dr. Javier Lorenzo explained to the press on December 5, 2014 that in these systems all stars describe their orbits around a common center of mass. In particular, the stars that are much more massive than our Sun, and contain an equivalent mass to many Suns, have a tendency to be accompanied by a sibling or siblings. Dr. Lorenzo is of the University of Alicante and is first author of the Astronomy & Astrophysics article. He added that recent studies indicate that these very massive stars, that are considerably hotter and larger than our Sun, are born members of systems with at least one other stellar companion of comparable mass.

MY Cam is an especially fascinating example of such a binary system. Located in the constellation known as the Giraffe, this stellar object is the brightest star in the open cluster Alicante 1--which has been identified as a small nursery composed of lovely baby stars by astronomers at the University of Alicante. Even though it has been known for half a century that MY Cam is a very massive star, it was not until a decade ago that it was finally recognized to be an eclipsing binary--a system in which a star floats in front of its companion star every time they complete their orbit, leading to alterations in the brightness of the system as seen from Earth. This property displayed by eclipsing binaries enables astronomers to decipher many of the characteristics of the component sibling stars through a careful analysis of the light that emanates from them--and the simple application of Isaac Newton's law of universal gravitation.

For the study of MY Cam, professional astrophysicists collected a large number of spectra of the system with the FOCES spectrograph, which operated for years in the 2.2 meter telescope of Calar Alto Observatory. Using the Doppler Method, these spectra enable scientists to measure the velocities at which the stars travel in their orbits. In addition, astrophysicists can determine the basic properties possessed by the stars--such as their surface temperature and size by way of a comprehensive analysis of the properties of the spectra. In order to complete their work, the team of professional astronomers had a great deal of help from amateur astronomers who measured the alterations in the quantity of light emanating from the system along the orbit--what astrophysicists term the light curve of the system. The analysis of MY Cam revealed that it is truly a remarkable system!

Dr. Sergio Simon explained to the press on December 5, 2014 that the light curve reveals that the orbital period of the system is only 1.2 days! Given the extremely large size of the stars, they have to be very, very close together to be able to do a full turn so speedily. The stars are traveling at a speed of more than one million kilometers per hour, but being so close, the tidal forces between the hefty duo make them rotate around themselves with the same period--that is, each star turns on itself in just over a day, in marked contrast to our Sun, which is a much more petite star, that turns on itself once every 26 days. Dr. Simon is an IAC researcher and one of the study's co-authors.

At 38 and 32 times solar-mass, the sparkling sibling stars are extremely massive. Such enormous, heavy stars do not fit comfortably into such a small orbit. Therefore, the researchers have concluded in their study that the duo are actually touching each other--and the material of their outer layers is in the process of mixing. This close contact forms a common envelope (contact binary). MY Cam is one of the most massive contact binaries known--and by far the most massive whose constituents are so youthful that they have not as yet begun to evolve.

The most interesting aspect of MY Cam may be that its predicted future fate matches some of the current theories of formation, Dr. Ignacio Negueruela told the press on December 5, 2014. Dr. Negueruela, also of the University of Alicante, is another author of the new paper. He went on to explain that the properties of the two stellar siblings composing MY Cam indicate that they were born "only" within about two million years. This extreme youth makes scientists suspect that the system was formed the way it is now--even though the two stars were likely not touching at first. As the stars age, their natural evolution will cause them to grow even larger. Because they have no space existing between them, this process will result in the merger of the two stellar siblings into a single enormous star--a true supermassive sparkler! The details of the predicted merging process are still mysterious, because they have never been observed before. Some theoretical models indicate that the merging process is very fast, releasing an enormous quantity of energy in a kind of explosion.

Many astrophysicists think that the merger of two stellar siblings, composing a close binary, is probably the most effective way to create extremely massive stars. MY Cam is the first observed example of such a system that can ultimately lead to one of these enormous stellar objects.

Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various journals, newspapers, and magazines. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, "Wisps, Ashes, and Smoke," will be published soon.

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