Superstar Twins!

Eta Carinae is a binary stellar system inhabiting our Milky Way Galaxy that at the turn of the 19th century was both faint and unremarkable. However, during the first half of that century its appearance began to undergo a sea-change, as it became ever brighter–ominously brighter. By the spring of 1843, this strange object was the second brightest star in Earth’s night sky, and it was out-dazzled only by Sirius, the “dog star”–even though Sirius is almost one thousand times closer to Earth! Clearly, at present, Eta Carinae is the most brilliant and massive star system within 10,000 light-years of our planet, and it is most famous for that gigantic eruption of the mid 19th century that blasted out at least 10 times our Sun’s mass into space. As dazzling, bewildering, and intriguing as Eta Carinae certainly is, it is not unique in the Cosmos. Indeed, in January 2016 astronomers announced that NASA’s Spitzer (SST) and Hubble (HST) Space Telescopes had successfully discovered “twins” of the brilliant superstar Eta Carinae inhabiting galaxies far, far away.

The expanding shroud of swirling, glowing gas and dust that astronomers first observed in the mid 19th century–that still hides Eta Carinae behind a heavy, obscuring veil–for years made it the only object of its kind known in our own Milky Way. However, the study released in January 2016–that used archival data from SST and HST–at last has revealed a quintet of objects with similar properties beyond our own Galaxy.

“The most massive stars are always rare, but they have tremendous impact on the chemical and physical evolution of their host galaxy,” commented study lead scientist, Dr. Rubab Khan, in a January 6, 2015 NASA Press Release.

Situated as it is about 7,500 light years from Earth, in the southern constellation Carina, Eta Carinae out-dazzles our own Star, the Sun, 5 million times! This binary system is composed of a duo of massive stars hugging each other in a tight 5.5-year orbit. Astronomers estimate that the more massive star of this closely dancing duo is about 99 times solar-mass, while the smaller of the pair may exceed 30 times our Sun’s mass. The larger star is the one tottering on the brink of self-destruction.

Eta Carinae is one of the closest stellar laboratories to our planet in space, and it handily enables astronomers to observe and study high-mass stars. Indeed, this stellar system has provided a unique astronomical touchstone since its spectacular eruption back in the 1840s. In order to understand why this brilliant eruption occurred and how it relates to the evolution of massive stars, astronomers needed additional examples. Alas, catching such rare massive stars during the very brief aftermath of such a spectacular outburst has been compared to finding a “needle in a haystack.” Nothing matching Eta Carinae had been detected before Dr. Khan’s study–the quest was that extraordinarily difficult.

“We knew others were out there. It was really a matter of figuring out what to look for and of being persistent,” commented study co-investigator Dr. Krzysztof Stanek in the January 6, 2016 NASA Press Release. Dr. Stanek is a professor of astronomy at Ohio State University in Columbus.

Supernova Imposter

During the years following the magnificent eruption of the mid 19th century, Eta Carinae again grew progressively dimmer and dimmer, until by the end of the 20th century it had again become invisible to the naked eye. However, it has continued to vary in brightness ever since. Even though this very strange binary star system is once again visible to observers on Earth without the aid of a telescope, it still has not reached its peak of glaring incandescence observed during the spring of 1843!

The larger of the binary stars, composing the dancing duo of the Eta Carinae system, is unstable as well as enormous. Approaching the end of its “life”, it can blast itself into oblivion at any time. It is the most luminous star in our Galaxy, and it radiates energy at the amazing rate of 5 million times that of our Sun. Surrounded by gigantic clouds composed of matter hurled violently out into space during the 19th century, when this giant star finally goes supernova, it will certainly not go gentle into that good night. The brilliant episode that occurred in back in 1843, that lit Eta Carinae’s brilliant fires, was merely a “near death experience”. In fact, astronomers term such dazzling outbursts “supernova imposter events.” This is because, even though they appear similar to real, and completely fatal, supernovae, they stop their death throes just in the nick of time–before their final stellar farewell performance can occur, blasting them to smithereens.

Only the most massive stars in the entire Cosmos collapse under the pull of their own gravity to create black holes. Usually, their magnificent and brilliant farewell performances are marked by a supernova explosion. Any star containing a core whose weighty mass exceeds at least 10 to 15 times that of our Sun, cannot hold its own against the crush of its own gravity, and this very massive, ill-fated, doomed star must collapse to form the greatest of all gravitational beasts, a stellar mass black hole. All of the matter that originally composed the very massive star before it met its fiery doom is quite literally crushed out of existence. According to Albert Einstein’s Theory of General Relativity (1915), the horizon of a black hole of any size is the surface that keeps the interior of the black hole (where gravity is so strong that nothing, nothing, nothing at all–not even light–can escape to freedom) separate from everything that exists outside of it. The horizon of a black hole is not really a material boundary at all; unfortunate travelers falling into the hole would not even experience anything odd on their journey past that fatal boundary. However, once having passed this bizarre boundary, they would no longer be able to communicate in any way with anyone on the outside. Neither would they ever again be able to return to the outside. An observer on the outside would only be able to receive messages sent out by the doomed travelers before they crossed over the horizon. According to the Theory of General Relativity, a gravitational singularity will form–occupying only an unimaginably tiny, tiny point–when the erstwhile massive star finally explodes to leave in its wake only a stellar-mass black hole.

The unstable, giant, dying star of the Eta Carinae dancing duo, is definitely sufficiently massive to leave behind a stellar-mass black hole when it goes supernova. However, it actually contains so much mass–100 to 150 solar-masses–that it may very well meet its fate in a very special type of blast termed a pair-instability supernova–that leaves behind absolutely nothing. A pair-instability supernova can explode only in the case of stars that possess between 130 and 250 solar-masses. The candidate progenitor stars for such a special, catastrophic form of explosive death, must also possess slow to moderate rates of rotation, and a very low metallicity–meaning that they must be composed almost entirely of hydrogen and helium. Astronomers do not define the term metal in the same way that chemists do. A metal in the jargon of astronomers is any atomic element found in the familiar Periodic Table that is heavier than helium. For example, oxygen, carbon, calcium, and neon, are all metals to astronomers–but not to chemists.

During that special type of catastrophic–and completely fatal–blast, that heralds a pair-instability supernova, the doomed massive star’s core becomes so extremely hyper-energetic that atomic nuclei and gamma-rays brutally blast into each other forming electron-positron pairs (a positron is an electron’s antimatter twin, possessing a positive charge, as opposed to the negative charge of an electron). This process sucks up a large amount of available thermal energy, and triggers a precipitous fall in pressure. The drop in pressure results in the doomed star’s collapse as a result of the merciless pull of its own gravity.

The regions of stellar gravitational collapse are superheated very quickly to extreme temperatures and pressures. This causes the very rapid fusion of atomic nuclei and a magnificent, monumental, and very powerful blast of energy. The resulting amount of thermal energy is so enormous that it blasts the doomed star out of existence–leaving nothing behind to tell the story. Where once there was a star, there is a star no more. All other supernovae leave behind either a tattle-tale stellar-mass black hole or, if the dead star was somewhat less massive, a neutron star.

Pair-instability supernovae are probably uncommon occurrences in today’s Cosmos, where most stars are too petite and richly endowed with metals–as astronomers define the term–to trigger such utterly catastrophic stellar explosions. However, such supernovae were probably more common in the ancient Universe when stars were much more massive than stars are today–and less metal-rich. This is because only hydrogen, helium, and trace amounts of lithium were born in the Big Bang birth of the Universe about 13.8 billion years ago (Big Bang nucleosynthesis). Literally, all of the atomic elements heavier than helium–the metals–were fused in the cores of our Universe’s stars (stellar nucleosynthesis) –or else in supernova blasts, that formed the heaviest atomic elements of all, such as gold and uranium.

One pair-instability supernova candidate, inhabiting today’s Cosmos, has been called the “the brightest stellar explosion ever recorded.” Named SN 2006gy, it was ten times more powerful than the more common form of core collapse supernova, and it was designated a hypernova.

Eta Carinae may very well meet its doom in a pair-instability supernova. Since it is “only” 7,500 light-years from Earth, when it finally does go supernova, it will put on quite a show–lighting up Earth’s entire sky enough to make it possible to read by its brilliant fires at night. The supernova might even be visible during the day, as a gauzy and ghostly object similar to Earth’s Moon after sunrise.

Eta Twins

Working with Dr. Scott Adams and Dr. Christopher Kochanek of Ohio State University and Dr. George Sonneborn of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, Dr. Khan devised a kind of optical and infrared “fingerprint” for detecting potential Eta Carinae twins, or “Eta twins” for short.

Dust forms in the gas that is hurled out by a massive star. This cloak of dust veils the star’s visible and ultraviolet light. However, the dust absorbs and then reradiates the energy as heat at longer mid-infrared wavelengths. “With Spitzer we see a steady increase in brightness starting at around 3 microns and peaking between 8 and 24 microns. By comparing this emission to the dimming we seen in Hubble’s optical images, we could determine how much dust was present and compare it to the amount we see around Eta Carinae,” Dr. Khan explained in the January 6, 2016 NASA Press Release.

The original survey of seven galaxies from 2012 to 2014 did not detect any Eta Twins. This was further evidence of their rarity in the modern Universe. However, this initial survey did identify a class of less luminous and less massive stars of scientific interest, thus demonstrating that the hunt itself was sufficiently sensitive to spot Eta Twins had they been around to detect.

In the updated follow-up survey of 2015, the team of astronomers successfully discovered two Eta Twins dwelling in the galaxy M38, located 15 million light-years from our own Milky Way Galaxy, as well as one each in the galaxies NGC 6946, M101, and M51, all located between 18 and 26 million light-years away. This fascinating quintet of Eta Carinae-type stars match both the optical and infrared properties of Eta Carinae, suggesting that each probably sports a high mass star buried like a scientific treasure in five to 10 solar-masses of gas and dust. Additional studies will enable astronomers to more accurately calculate the stellar quintet’s physical attributes. The findings of this study were published in the December 20, 2015 edition of the Astrophysical Journal Letters.

NASA’s upcoming James Webb Space Telescope (JWST), scheduled to launch in late 2018, carries an instrument well-suited for further studies of these Eta Twin stars. The Mid-Infrared Instrument (MIRI) carries 10 times the angular resolution of instruments aboard the SST and is most sensitive at precisely the same wavelengths where Eta Twins shine most brightly.

Dr. Sonneborn explained in the January 6, 2016 NASA Press Release that “Combined with Webb’s primary mirror, MIRI will enable astronomers to better understand these rare stellar laboratories and to find additional sources in this fascinating phase of stellar evolution.” Dr. Sonneborn is NASA’s project scientist for JWST operations. It will take JWST to confirm the Eta Twins as true identical siblings of Eta Carinae.

Source by Judith E Braffman-Miller

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