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By Abhisek Swain, BS-MS 2019
A bright red star embedded in the shoulder of the Orion Constellation started to dwindle in its brilliance back in October 2019. Edward Guinan and Richard Wasatonic, two astrophysicists at the Villanova University, Pennsylvania and their team first noticed this abnormal dip in its brightness. Within a span of 4 months, the star had dimmed to one-fourth of its original brightness. By mid-February 2020, it had become the 24th brightest star in the night sky, falling down 14 places from being the 10th brightest star 4 months ago. Betelgeuse had never fluctuated by this magnitude in its astronomical records. Betelgeuse is a red supergiant, a stage in the life cycle of high mass stars between 10 and 40 solar masses. Dwarf, Giant and Supergiant are classes that astronomers assign to stars based on the total amount of light they emit in a fixed time interval.
Dwarves are fainter than giants, and supergiants emit more light than giants. Betelgeuse is colossal. It has a radius 900 times that of our Sun. Imagine that Betelgeuse and our Sun have switched places, with its centre located where the Sun's centre once lied. Then, the radius of Betelgeuse would extend beyond the orbit of Jupiter, thus engulfing Mercury, Venus, Earth, Mars, Jupiter, all their moons and roasting Saturn and it's icy moons to a crisp. Betelgeuse is extremely luminous. Luminosity is the intrinsic brightness of an object independent of the distance from which it is measured. To put into perspective, imagine two identical candles, one closer to you and the other at a distance; the former will look brighter while the latter will be dimmer, but they still have some property that is equal, which is their luminosity. They produce the same amount of light. Stars appear dimmer than a torch because they are very far away even though a star emits unfathomably more light. Thus we could say that a star is more luminous than a torch, but a torch is brighter.
In that respect, Betelgeuse outshines the Sun between 90,000 and 150,000 times. Massive stars live aggressively and die violently. They quickly tread through various stages of their stellar life cycles, rapidly burning off their hydrogen fuel and fabricating heavier and heavier elements till their mass permits. They end their lives with a cataclysmic burst called a supernova, dispersing their chemically enriched interiors across the neighbourhood. The star held between 15 and 25 times the Sun's heft when it was a main-sequence star about 40,000 years ago hence falling into the category of high mass stars. Betelgeuse is a toddler amongst stars in cosmic terms. It is only 8 million years old and is already on the verge of ending its brief life in a supernova in about 100,000 years. The sudden plunge in its brightness sent a wriggle of excitement through the astronomy community worldwide, hinting at the likelihood of an early supernova, possibly within our puny lifetimes. But in late February, the brightness began to increase and has been increasing since then, killing the hopes of witnessing an incredibly rare spectacle.
Astronomers measure brightness in units called apparent magnitude, which is reverse logarithmic. It means an increase in apparent magnitude corresponds to a decrease in brightness. It is analogous to the pH scale, in which a higher pH indicates a lower acidity. A difference of 5 apparent magnitudes is parallel to a 100 fold difference in brightness. Doing the math, a one magnitude increment says that the brightness escalates by a factor of 2.51.The dimming of the star had all the astrophysicists puzzled. The brightness had dropped to a magnitude of +1.12 on December 7 from +0.65 in September 2019. It fell to +1.50 on January 18, setting a new record in the last 100 years. It registered a historical minimum of +1.61 on January 30, 2020 nearly equalling the star Bellatrix's brightness in the same constellation. There are two possible explanations so far. One reason for the dimming could be internal changes in the star that could lead to decreased luminosity. For the simultaneous conservation of gravitational and thermodynamic energies inside the star, a decrease in size also reduces the luminosity. To physically interpret this, a star with a large surface area allows a substantial amount of light to escape it's surface, while a small star with a small surface area has little room for light to escape copiously.
Thus a decrease in luminosity corresponds to an increase in star size. Plugging the numbers in, the star should have decreased by 8% in size to account for the plunge in luminosity. But observations do not support this. Stars have convection cells that appear as dark patches on their surface forged in a process called "dredge up", wherein hot material rises from the core and cools at the surface. On the Sun these are numerous and small, about the size of France but on a red supergiant like Betelgeuse, these cells are few and titanic in size, as big as the orbit of Mars. These cells, along with the rotation of the star, cause semi-regular variability in its apparent brightness. But no convection cell could explain the sheer drop that happened to Betelgeuse. The surface temperature of the star has been observed to have decreased by 100° C from 3650 to 3550 degrees. The star has cooled a bit, just not enough to be for evidence of convection cells causing this. No cell could explain the sheer drop that happened to Betelgeuse. A more promising explanation is circumstellar dust. There could be immense blobs of gas and dust surrounding the star that could simply absorb and block a significant part of the light from reaching us. These clouds of dust form when bursts of radiative pressure from the core as it contracts. Material from inside the star bubbles up to the surface, smearing out in the vicinity of the star. Red giants lose mass by this process.
This material then envelopes the star blurring the border between the surface and outer space. Betelgeuse hence doesn't have a well-defined photosphere; it's the body that just gradually transitions into the Corona ( ahem ahem!). Researchers Emily Levesque and Phillip Massey have looked up titanium oxide that is ejected by red giants. Spectroscopy has shown an abundance of absorption lines of titanium oxide in the spectrum, which is directly correlated with the surface temperature. By this method, the temperature was calculated, which didn't show as low a drop to support the former idea. This has led to their research paper being accepted worldwide, and the latter explanation has been agreed upon. Space weather forecasts have once again pushed the dates of the inevitable Betelgeuse supernova explosion to about 100,000 years into the future. Once it does, our descendants will be able to see it as bright as a full moon, visible even in broad daylight. A time bomb is ticking 700 light-years away to set our night sky ablaze.
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