Mysteries are fun to solve, and some stars can lead perplexed scientific detectives on a merry chase to solve their myriad mesmerizing mysteries. Such a puzzling star is KIC 8462852, frequently referred to as Tabby’s Star or Royajian’s Star. This strange stellar inhabitant of our Milky Way Galaxy is an F-type main-sequence star situated in the constellation Cygnus (The Swan), which is approximately 1,280 light-years from Earth. Weird fluctuations in the light streaming out from Tabby’s Star were discovered by citizen astronomers as part of the Planet Hunter’s project, and, in September 2015, an enticing interpretation of these mysterious fluctuations suggested the possible existence of an alien megastructure orbiting this undeniably bewitching star. Alas, the true explanation may be somewhat less dramatic, though certainly fascinating in its own right. In January 2018, a team of more than 100 astronomers came one step closer to solving the mystery behind what has been called “the most mysterious star in the Universe”–and it debunks the luring Sirens’ song of the existence of an alien megastructure circling Tabby’s Star.
If not for its unusual light fluctuations, KIC 8462852 would just be an average star. It is approximately 50 percent larger and 1,000 degrees hotter than our Sun. However, it has been inexplicably dimming and brightening repeatedly like no other star observed to date. Indeed, the mystery of Tabby’s Star is so bewitching that more than 1,700 people donated over $100,000 through a Kickstarter campaign in support of dedicated ground-based telescope time to observe and collect more information about the star. The new data was collected by a network of telescopes around the world. As a result, a treasure trove of information was gathered by the discovery team, led by Louisiana State University in Baton Rouge Assistant Professor Tabetha Boyajian and her colleagues –in partnership with the Las Cumbres Observatory in California. The team’s findings are now available in a paper published in The Astrophysical Journal Letters, under the title The First Post-Kepler Brightness Dips of KIC 8462852.
In September 2015, astronomers and citizen scientists associated with the Planet Hunters project posted a preprint of an article describing the wealth of new data, as well as some possible interpretions. The discovery itself was made from data obtained from NASA’s planet-hunting Kepler space telescope, which looks for alterations in the brightness of faraway stars in order to spot exoplanets.
A number of hypotheses have been proposed to explain Tabby’s Star’s strange behavior–specifically, the large irregular alterations in brightness as measured by its light curve. However, none of the data obtained could adequately explain all aspects of the weird light curve. The most probable explanation, proposed by NASA on October 4, 2017, is that there is an “uneven ring of dust” circling Tabby’s Star. However, there are other interpretations proposed, as well. A second hypothesis, based on a lack of observed infrared light, suggests there is a swirling swarm of dusty, frigid comet fragments existing in a highly eccentric orbit. However, the idea that disturbed comets, inhabiting such a cloud, could exist in numbers great enough to obscure 22% of the strange star’s observed luminosity, has been met with considerable skepticism. A third hypothesis proposes that there is a myriad of small masses existing in “tight formation” in orbit around this strange star. However, a spectroscopic study of the system detected no evidence for coalescing material, hot close-in dust, or circumstellar matter from an evaporating or exploding planet, within a few astronomical units (AU) of the mature central star. One AU is equal to the average distance between Earth and Sun which is about 93,000,000 miles.
The fourth hypothesis–and the one that has captured the most interest–is that the alterations in Tabby’s Star’s brightness could be signs of activity performed by intelligent aliens. These extraterrestrial lifeforms would construct a Dyson swarm. However, the astronomers involved in the new study are very skeptical of this, and others are calling it implausible.
Despite the interest in Tabby’s Star, it is not the only star that has displayed such large irregular dimmings. However, all of the other stars that show this behavior are young stars termed YSO dippers, that have different dimming patterns. Tabby’s Star is classified as an F-type main-sequence star (FV) on the Hertzsprung-Russell Diagram of Stellar Evolution. This means that it is of spectral type F and luminosity class V. Main-sequence stars, regardless of type, are still fusing hydrogen in their searing-hot hearts.
FV stars, like Tabby’s Star, sport from 1.0 to 1.4 times solar-mass, and have surface temperatures of between 6,000 and 7,600 Kelvins. Such extremely toasty temperatures give F-type stars a yellow-white color. Because a main-sequence star is generally called a dwarf star, FV stars are frequently referred to as yellow-white dwarfs. Some of the best-known examples of FV stars are Procyon A, Gamma Virginis A and B–and, of course, KIC 8462852!
More recent light fluctuation events of KIC 8462852 started in mid-May 2017, and totaled, as of September 16, 2017, four prominent dimmings–the final dimming being the largest of 2017. On October 10, 2017, an increase in brightness–lasting approximately two weeks– of the starlight flowing from Tabby’s Star was recorded–and a potential explanation for this event was provided. On November 20, 2017 a fifth prominent dimming event commenced, and then deepened. As of December 16, 2017, the dimming event recovered, and leveled off at dip bottom for 11 days before it faded again. It is currently recovering from this dimming event. Dimming and brightening events of Tabby’s Star continue to be carefully watched, and related light curves are (as of this writing) being updated and released frequently.
All stars are born surrounded by a whirling, swirling disk composed of gas and dust. This disk, termed a protoplanetary accretion disk, contains all of the necessary ingredients to create a family of planets in orbit around the young star. Astronomers have observed many protoplanetary accretion disks circling faraway, brilliant, fiery baby stars, and these disks form at about the same time that the baby star (protostar) is born, tucked within the dense, obscuring blanket of its natal cloud.
Protoplanetary accretion disks harbor large amounts of nurturing gas and dust that feed the voracious, growing, sparkling protostar. Our own Solar System, as well as other planetary systems, emerge when a relatively small, unusually dense pocket embedded within the secretive billows of one of the numerous dark, frigid, and enormous molecular clouds–which serve as bizarre stellar cradles–collapses under the powerful pull of its own gravity. These cold, enormous star-birthing clouds float through our Milky Way Galaxy in huge numbers, and these beautiful objects are primarily composed of gas–with smaller quantities of dust. Most of the collapsing pocket, squeezed mercilessly by its own gravity, collects at the center, and ultimately ignites as a result of the process of nuclear fusion–thus forming a protostar.
The remaining gas and dust gradually evolves into the protoplanetary accretion disk from which planets, moons, and assorted smaller objects ultimately emerge. In their earliest stages, protoplanetary accretion disks are both seething-hot and very massive, and they can linger around the young star for as long as 10 million years.
Within the disk, tiny motes of dust begin to collide with one another–and then ultimately merge within the dense environment of the protoplanetary accretion disk. As a result, ever larger and larger objects form from the merger of smaller ones. What started out as tiny particles of sticky dust begin to bind together and grow–from pebble size, to boulder size, to mountain size, to moon size, to planet size. These growing objects become planetesimals–the building blocks of planets. Planetesimals can reach sizes of 1 kilometer across, or larger, and these primordial chunks are a very abundant population within the swirling accretion disk. These surrounding, swirling disks can hang around their parent-stars long enough for some of them to still be present billions of years after a mature planetary system has formed around a star. In our own Solar System, the asteroids are similar to the relic rocky and metallic planetesimals that went into the construction of the quartet of relatively small, solid inner planets: Mercury, Venus, Earth, and Mars. In contrast, the frigid comets that swarm in our Solar System’s outer limits, are the left over icy and dusty planetesimals that built up the quartet of giant, gaseous planets inhabiting the outer Solar system: Jupiter, Saturn, Uranus, and Neptune.
The Most Mysterious Star In The Universe May Not Be So Mysterious
The team of scientists led by Dr. Boyajian, closely monitored Tabby’s Star using the Las Cumbres Observatory from March 2016 to December 2017. Then, the supporters from the crowdfunding campaign nominated and voted to name the four distinct episodes of dimming that began in May 2017. The first duo of dips were dubbed Elsie and Celeste. The last two were named after ancient lost cities–Scotland’s Scara Brae and Cambodia’s Angkor. The authors write that in several ways what is occurring with Tabby’s Star is like these lost cities.
“They’re ancient; we are watching things that happened more than 1,000 years ago. They’re almost certainly caused by something ordinary, at least on a cosmic scale. And yet that makes them more interesting, not less. But most of all, they’re mysterious,” wrote the authors.
Dr. Boyajian noted in a January 3, 2018 LSU Press Release that “Dust is most likely the reason why the star’s light appears to dim and brighten. The new data show that different colors of light are being blocked at different intensities. Therefore, whatever is passing between us and the star is not opaque, as would be expected from a planet or alien megastructure.”
The method in which Dr. Boyajian’s team studied Tabby Star’s heralds a new era of astronomy.
“We’re gathering so much data on a single target. This project is reflective of changes in astronomy, with the access to this flood of data,” commented Tyler Ellis in the January 3, 2018 LSU Press Release. Ellis is a doctoral student at LSU, who is studying this mysterious star.
Citizen scientists, the Planet Hunters, while carefully sifting through large amounts of data derived from NASA’s Kepler space mission, were the first to spot Tabby’s Star’s weird behavior.
“If it wasn’t for people with an unbiased look on our Universe, this unusual star would have been overlooked. Again, without the public support for this dedicated observing run, we would not have this large amount of data,” Dr. Boyajian commented in the LSU Press Release.
Yet, there is still much more to be discovered about this mysterious star.
As Dr. Boyajian continued to explain:
“It’s exciting. I am so appreciative of all of the people who have contributed to this in the past year, the citizen scientists and professional astronomers. It’s quite humbling to have all of these people contributing in various ways to help figure it out.”
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various newspapers, journals, 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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