The bright red star Betelgeuse, called ‘Thiruvathirai’ or ‘Ardra’ in Indian astronomy, is easily spotted in the constellation Orion.
By examining its pulsation – the periodic contraction and expansion of the star – researchers from Japan and Switzerland recently reported that the star is in its late carbon-burning stage. In massive stars like Betelgeuse, the carbon-burning stage lasts only up to a few hundreds of years, after which the star ‘dies’ and collapses into a supernova within a few months.
In humans, slow or abnormal heartbeats indicate a possible blockage of the heart. Likewise, the researchers say Betelgeuse’s observed pulsation matches theoretical estimates from a late carbon-burning stage, suggesting the red supergiant is in its death throes.
“From the pulsation periods, we can infer the radius of the pulsating object. From the calculated radius, luminosity and mass of Betelgeuse, we determine that it must be a late stage of core carbon-burning,” Devesh Nandal, a PhD student in the Department of Astronomy at the University of Geneva and one of the authors of the study, told The Hindu.
A death foretold
Most stars, including our Sun, fuse the simplest element in the universe, hydrogen, to produce helium and some energy as a byproduct. This energy’s outward push balances gravity’s inward pull, and keeps the star from collapsing.
Massive stars like Betelgeuse run out of hydrogen fuel in only a few crore years, when they switch to using helium to make carbon. The energy released in the fusion of helium is less than that of hydrogen, so the star burns more helium to stay stable and not collapse. The helium runs out in about ten lakh years.
At this time, red giants like Betelgeuse burn carbon, then silicon, and briskly consume one by one the elements of the periodic table, until finally their core brims with iron – whose fusion requires more energy than it releases – and some cobalt and nickel.
Each of these stages is shorter than the predecessor. In a star like Betelgeuse, carbon burns in a few hundred years whereas silicon lasts about a day. So the late-carbon stage is the terminal phase of Betelgeuse.
Once the core is rich in iron, the temperature and pressure within the star drop. With nothing to stop it, gravity compresses the core and turns it into a neutron star or a black hole. The shock wave resulting from the collapse blasts the surrounding layers into interstellar space. The star explodes in a celestial firework display.
Betelgeuse’s heartbeats
Like the lid covering a pot of boiling water, which periodically lifts to release the built-up steam, red giant stars expand and contract due to the periodic heating and cooling of the hydrogen in their outermost layers.
The temperature of hydrogen in the outermost part of the envelope of a red giant star is 3,800-4,000 K. Neutral hydrogen in this cooler region absorbs the heat from the interior. As the temperature soars to more than 7,000K, each hydrogen atom’s lone electron is stripped away and the hydrogen is ionised.
Ionised hydrogen can soak up heat, so the star suddenly expands with great force and the outermost shell of the star is ejected. Once this hot mass is expelled, the region becomes cooler and the hydrogen becomes neutral again by recombining with electrons.
As this process repeats itself, the star appears to a distant observer to dim and brighten at regular intervals.
Checking the pulse
Astronomers detect the expansion and contraction of a distant star by dispersing its starlight into its various colours, and examine the resulting spectrum. This pulsation also corresponds roughly to periodic variations in the observed brightness of the star. For Betelgeuse, astronomers have observed four approximate semi-regular pulsations with periods of 2,190, 417, 230, and 185 days.
The membrane of a drum oscillates at a certain frequency depending on the beat. In addition to this fundamental vibration, the membrane also vibrates at other frequencies, called overtones. Likewise, the heartbeats of the stars also have a fundamental and overtones.
Of the four observed pulsations, which one is Betelgeuse’s fundamental? Earlier, a team that included László Molnár, of Konkoly Observatory, Budapest, and Meridith Joyce of Australian National University, Canberra, took the 417-day period to be fundamental, placing Betelgeuse at its early core helium-burning phase. This means the star has roughly one lakh years before it explodes.
They set aside the 2,190-day pulse as mysterious and of unknown origin. However, Devesh Nandal’s team took the 2,190-day pulse to be the fundamental and the other three to be harmonics, and concluded that Betelgeuse is in the final stage of burning carbon.
In a model developed by Nandal et al., a red giant star that commences its life with 19 solar masses will shed part of its material in each pulsation. By the time it reaches the end of the core carbon burning stage, it will be left with 11 solar masses. According to the model, the star will by this time have expanded to 1,300 times the radius of the Sun and glow at a specific luminosity.
These parameters in the model match that of Betelgeuse, and the model also predicts that such a red supergiant will have a fundamental pulsation period of about 2,200 days and three overtones consistent with the observed pulsation.
“The pulsation itself has nothing to do with core carbon-burning,” Mr. Nandal said. “Nevertheless, the pulsation periods tell us the radius, luminosity, and mass of the pulsator. Knowing these parameters, we can conclude that Betelgeuse is most likely in a late stage of core-carbon burning.”
A death disputed
The work of Nandal et al., uploaded as a preprint paper on June 1, stirred a hornet’s nest. Social media was set aflutter with news of the giant star’s imminent death. However, the rival team led by Dr. Molnár and Dr. Joyce responded with a rebuttal. In their paper, they wrote that Betelgeuse’s assumed radius of 1,300 solar radii is too high and that studies indicate it is 600-1,000 solar radii.
How far away and how large Betelgeuse is is a matter of dispute among astronomers. Estimates of its distance range from 550 to 730 lightyears; its estimated luminosity is also in the range 90,000-150,000 (times that of the Sun).
While the team led by Mr. Nandal estimated the distance of Betelgeuse to be about 730 lightyears, Dr. Joyce and her team pegged it at around 550 light-years. “The distance assumptions affect the luminosity and therefore the predictions of Devesh Nandal and his team,” said Kuntal Misra of the Aryabhatta Research Institute of Observational Sciences.
“It has been long known that Betelgeuse is a good candidate for a core-collapse supernova,” Firoza Sutaria, an associate professor at the Indian Institute of Astrophysics, Bengaluru. “The paper is interesting, but the result is quite model dependent, hence it is a bit of an overstretch to predict a supernova ‘in a few tens of years’.”
Both of them are supernova experts but weren’t associated with either paper.
“There is no dispute that Betelgeuse is fated to explode into a supernova,” Dr. Sutaria added. “The only question is when.”
T.V. Venkateswaran is a scientist with Vigyan Prasar. The views expressed here are personal.
