Betelgeuse Is Spinning As Fast As It Can
By Judith E Braffman-Miller
Poised brilliantly on the shoulder of Orion The Hunter, the red supergiant, superstar Betelgeuse sparkles ominously, mysteriously in the sky–silently keeping to itself a well-cloaked secret about its own impending, explosive demise. The ninth brightest star in the night sky, as well as the second-brightest star in the constellation Orion, Betelgeuse is one of the largest and most luminous stars visible to the naked eye. Indeed, if Betelgeuse were situated at the center of our Solar System, its surface would reach past the Main Asteroid Belt–located between the orbits of Mars and Jupiter–completely engulfing with its fierce stellar fires the orbits of Mercury, Venus, Earth, and Mars. Calculations of Betelgeuse’s mass range from slightly under ten to a little over twenty times that of our Sun, and even though it is a famous, well-studied star, it may have had a secret past that is more interesting than what meets the eye today. In December 2016, Dr. J. Craig Wheeler, an astronomer at the University of Texas at Austin, announced that he had–with the help of an international group of undergraduate students–discovered compelling evidence that Betelgeuse may have been born with a companion star, only to ultimately engulf its stellar companion.
For such a famous star, Betelgeuse has managed to keep some important secrets of its past well-hidden from the prying eyes of curious astronomers. Indeed, while astronomers know that Betelgeuse is a red supergiant, that sometime in the future will blast itself to smithereens in a supernova explosion, no one knows exactly when that final blaze of glory will occur!
“It might be ten thousand years from now, or it might be tomorrow night, Dr. Wheeler commented in a December 19, 2016 McDonald Observatory Press Release. The McDonald Observatory is run by the University of Texas.
Red supergiants, like Betelgeuse, are massive stars that are reaching the end of that long stellar road. These enormous, bloated old stars swell up to many times their original size, and are the largest stars known to be dwelling in the Universe–at least in respect to their volume. Red supergiants are not the most massive stars known–although they certainly do display impressively gigantic radii. They are also relatively cool by star-standards, with temperatures of between 3500 and 4500 Kelvin.
Astronomers have calculated that Betelgeuse is about 640 light-years away, and is thought to be less than 6 million years old. Our own Sun, by comparison, is about 4.56 billion years old. However, Betelgeuse evolved much more rapidly than our Star because of its great mass. Having been unceremoniously evicted from its stellar birthplace in the Orion OB 1 Association–which also includes the stars that form Orion’s Belt–this bloated scarlet vagabond has been seen traveling through interstellar space at a supersonic speed of about 30 kilometers per second. Because of this, the speedy old star creates a bow shock that is more than 4 light-years wide. Betelgeuse is in the late stage of stellar evolution, and even though the precise date of its inevitable demise has not been pinpointed exactly, it will probably go supernova sometime within the next million years–or so.
In 1920, Betelgeuse became the second star (after our Sun) to have the angular size of its photosphere measured. More recent studies have suggested an apparent size ranging from about 0.042 to 0.056 arcseconds. Betelgeuse is also encircled by an asymmetric, and rather complex, envelope that is about 250 times the size of the star, resulting from mass loss from Betelgeuse itself.
The bloated star’s original name of Betelgeuse is derived from the Arabic, meaning “the axilla of Orion”. In 2016, the International Astronomical Union (IAU) established a Working Group on Star Names (WGSN) in order to catalog and standardize proper names for stars. The WGSN’s first bulletin released in July 2016 included a table of the first two batches of stellar names that had been approved by the WGSN, which included Betelgeuse for this star. It is now also entered in the IAU Catalog of Star Names.
Betelgeuse and its crimson hue have been observed since antiquity. Indeed, the classical astronomer Ptolemy noted its color–that was later described in the Latin by a translator as “ruddiness”. Three centuries before Ptolemy, Chinese astronomers observed Betelgeuse as sporting a yellow coloration. This ancient observation by Chinese observers suggests that this “ruddy” giant star may have spent some time as a yellow supergiant approximately at the start of the common era. This possibility has stimulated current research into the complex circumstellar environment of these enormous stars.
Variations in Betelgeuse’s brightness was first noted in 1836 by the English-German astronomer Sir John Herschel, when he published his observations in Outlines of Astronomy. From 1836 to 1840, Herschel observed significant alterations in magnitude when Betelgeuse actually outshone Rigel–the most brilliant star in the Orion constellation–in October 1837 and again in November 1839. A decade-long period of peacefulness then followed. However, again, in 1849, Herschel observed yet another brief cycle of variability, which peaked in 1852. Later observations noted unusually high maxima with an interval of years, but only small variations from 1957 to 1967. The records of the American Association of Variable Star Observers (AAVSO) display a maximum brightness of 0.2 in 1933 and 1942, and a minimum of 1.2, seen in 1927 and 1941.
In 1995, the Hubble Space Telescope’s Faint Object Camera obtained an ultraviolet image of Betelgeuse with an improved resolution that was better than that obtained by ground-based interferometers–the very first “direct-image” obtained of the disk surrounding another star beyond our Sun. Interferometers got there name because they work by merging two or more light sources in order to create an interference pattern, which can then be measured and analyzed. The interference patterns generated by interferometers carry information about the object or phenomenon being studied. They are frequently used to make very small measurements that could otherwise not be obtained.
In Earth’s sky at night, Betelgeuse is an easy star to detect with the naked eye. This is largely due to its distinctive red-orange hue. In the Northern Hemisphere, beginning in January each year, it can be observed to rise in the east soon after sunset. By mid-September to mid-March–with the best in mid-December–, Betelgeuse is visible to literally every inhabited region of our planet, except for a few research stations located in Antarctica at latitudes south of 82 degrees. In May or June, the gigantic red star can be observed briefly on the western horizon after sunset, reappearing again a few months later on the eastern horizon before sunrise. In the intermediate period–June and July–it is invisible to the naked eye, with the exception of around midday on Antarctic regions between 70 degrees and 80 degrees latitude.
Betelgeuse is a variable star whose brightness can vary dramatically. If the human eye could see radiation being emitted at all wavelengths, Betelgeuse would appear as the brightest star in Earth’s sky.
Betelgeuse, like all stars–large and small–is a roiling, seething-hot immense ball of mostly hydrogen gas. The gas has been squeezed very tightly together into a ball by the merciless pull of gravity. This is the reason why the core of a star grows extremely dense–as well as searing-hot! Indeed, stars are so extremely hot that they ignite by way of the process of nuclear fusion which causes the atoms of lighter atomic elements (such as hydrogen and helium) to be fused together–thus creating increasingly heavier and heavier atomic elements. The continual manufacture of increasingly heavier atomic elements out of lighter ones in the searing-hot heart of a star is termed stellar nucleosynthesis. Nuclear fusion begins with hydrogen atoms. Hydrogen is both the lightest and most abundant of all atomic elements in the Universe, and the multitude of stars that do their fiery, glaring dance throughout the Universe fuse their supply of hydrogen atoms into helium atoms. Atomic elements that are heavier than helium are termed metals by astronomers. Therefore, the definition of the term metal by astronomers is not the same as the definition of a metal that is used by chemists. All of the metals–as astronomers define the term–were cooked up in the hot nuclear-fusing hearts of the Universe’s billions and billions of stars or, alternatively, in the explosive death throes of massive stars when they went supernova, and hurled these newly forged heavy metals out into the space between stars.
The process of nuclear fusion manufactures energy, and the liberation of energy is responsible for making fiery, glaring stars shine brightly. The energy that is churned out by stars creates radiation pressure. Radiation pressure causes a very precious and extremely delicate balance to occur between the persistently warring forces of radiation pressure and the star’s own gravity. Radiation pressure pushes all of the stellar material outward, while gravity seeks to pull everything inward. This very necessary balance between gravity and pressure continues for as long as the star “lives” on the hydrogen-burning main-sequence of the Hertzsprung-Russell Diagram of Stellar Evolution. When the star finally grows old, and has managed to burn its necessary supply of hydrogen fuel, the cruel war between the two forces ends tragically–and gravity wins the war against pressure. The doomed old star collapses and “dies”. The precise balance between gravity and pressure, that keeps the main-sequence star bouncy, is determined by its mass–and the most massive stars in the Universe are squeezed the tightest of all by the relentless pull of their own gravity. This powerful, merciless squeezing speeds up the rate of the nuclear reactions occurring within the massive star’s hot heart. This is why massive stars “live” fast–and “die” young.
When massive stars “die” they go out with a bang. Massive stars end their main-sequence “lives” in catastrophic supernova explosions that rip the old star apart. This powerful, violent explosion hurls off the gaseous outer layers of the doomed, old star, creating a beautiful death-shroud in the form of a supernova relic composed of whirling, swirling multi-colored gases. The core of the once-massive star collapses under the force of gravity, leaving behind either a very dense city-sized neutron star or stellar mass black hole–that tell the tragic tale of the existence of a once brilliant star that is no more.
Betelgeuse Is Spinning As Fast As It Can!
A new method for determining the future of Betelgeuse involves its rotation. When a dying, elderly star inflates to morph into a supergiant, its rate of rotation usually slows down. “It’s like the classic spinning ice skater–not bringing her arms in, but opening her arms up,” Dr. Wheeler explained in the December 19, 2016 McDonald Observatory Press Release. As the ice-skater opens her arms, she slows her spin rate down. In the same way, the rate of Betelguese’s rotation rate should have slowed down as the crimson giant expanded. However, this is not what Dr. Wheeler’s team observed.
“We cannot account for the rotation of Betelgeuse. It’s spinning 150 times faster than any plausible single star just rotating and doing its thing,” Dr. Wheeler added.
Dr. Wheeler directed his team of undergraduate students including Sarafina Nance, Manuel Diaz, and James Sullivan of the University of Texas at Austin, as well as visiting students from China and Greece to study Betelgeuse with a computer modeling program dubbed MESA. The students used MESA to model Betelgeuse’s puzzling rotation rate for the first time.
Dr. Wheeler noted in the December 19, 2016 McDonald Observatory Press Release that while studying the star’s strangely speedy rate of rotation, he began to devise a new theory. “Suppose Betelgeuse had a companion when it was first born? And let’s just suppose it is orbiting around Betelgeuse at an orbit about the size that Betelgeuse is now. And then Betelgeuse turns into a red supergiant and absorbs it–swallows it,” he speculated.
Dr. Wheeler added that the companion star, once swallowed, would transfer the angular momentum of its orbit around Betelgeuse to that star’s outer envelope–thus speeding up Betelgeuse’s rotation.
Dr. Wheeler estimates that the companion star would have had approximately the same mass as our Sun, in order to explain Betelgeuse’s puzzling current spin rate of 15 kilometers per second.
But is there any evidence for this swallowed-stellar-companion theory? That is the question, and the answer is “perhaps”.
If Betelgeuse did indeed swallow its luckless smaller companion star, it is probable that the interaction between the duo would result in the crimson supergiant hurling out some matter into interstellar space, according to Dr. Wheeler.
Having learned how fast matter sheds off of a red giant star–approximately 10 kilometers per second–Dr. Wheeler was able to come up with a rough estimate about how far from Betelgeuse this rudely evicted matter should be today.
“And then I went to the literature, in my naivete, and read about Betelgeuse, and it turns out there’s a shell of matter sitting beyond Betelgeuse only a little closer than what I had guessed,” Dr. Wheeler commented in the December 19, 2016 McDonald Observatory Press Release.
Infrared images taken of Betelgeuse in 2012 by Dr. Leen Decin of the University of Leuven in Belgium, with the orbiting Herschel telescope, revealed two of shells of interacting matter situated on one side of the crimson giant star. This observation can be explained by various differing interpretations; some astronomers propose that this matter is a bow shock that formed as Betelgeuse’s atmosphere rushed through the interstellar medium as it rampaged through our Milky Way Galaxy.
However, no one knows for certain how the two shells of interacting matter formed. Nevertheless, “The fact is there is evidence that Betelgeuse had some kind of commotion on roughly this timescale”–that is, about 100,000 years ago when the star bloated into a red supergiant, according to Dr. Wheeler.
The ill-fated engulfed companion theory could account for both Betelguese’s speedy rotation rate and this nearby matter.
Dr. Wheeler and his team of students are continuing their observations into this puzzling giant red star. Next, Dr. Wheeler added, he and his team hope to probe Betelgeuse using a technique called asteroseismology. Asteroseismology involves searching for sound waves impacting the surface of the star, in order to get precious clues about what is happening deep within its shrouding cocoon. The team also plans to use the MESA code in order to attain a better comprehension about what would occur if Betelgeuse had swallowed an unlucky companion star.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, newspapers, and journals. 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.