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3 years ago, a star in Orion inexplicably dimmed. Astronomers finally know why

Earth’s sun is really a predictable star, and that is a very important thing: if the sun fluctuated in its brightness significantly from year-to-year, the solar system will be a significantly less hospitable place for life. But some stars in the universe are as regular as our sun, a small % aren’t so when a star suddenly dims or brightens, it usually signifies something odd is going on with it, as well as that maybe it’s on the verge of exploding.

When astronomers seen in 2019 thatBetelgeuse had dimmed, some speculated that the massive star would expand right into a supernova so large it might be visible from Earth even through the daytime hours. Considering that Betelgeuse may be the tenth-brightest star in the night time sky, citizens of Earth paid attention. Novas or supernovas which are visible with the naked eye are rare, so when they do happen, they are usually generation-defining events: the final time a nearby star went supernova, in 1604, it had been so bright that it had been visible through the daytime.

Betelgeuse’s mysterious behavior made headlines and it got more mysterious in February 2020. Then,reports surfaced that Betelgeuse was regaining a few of its waning brightness. In a short time, the so-called “Great Dimming” had captured the public’s imagination. Scientists and amateur astronomers alike obsessed over Betelgeuse’s odd behavior, attempting to derive this is; one independent scientist even setup a Twitter bot, “BetelBot,” which issued regular updates on Betelgeuse’s varying brightness.

However now, thanks to several scientists utilizing the Hubble Space Telescope, we have now know the reason for Betelgeuse’s Great Dimming: A coronal mass ejection (CME), or perhaps a phenomenon when a star’s corona (or crown) erupts with an enormous cloud of highly magnetized and energetic plasma.

“It has large convective cells on its surface, this means there’s hot material moving upward from within, much like chocolate sauce boiling in a pot,” explained study author Dr. Andrea Dupree, associate director of the Harvard & Smithsonian Center for Astrophysics, within an email to Salon.

The paper itself wasposted to the preprint database arXiv and accepted for publication by The Astrophysical Journal.

“It would appear that in 2019, [Betelgeuse’s] outward expansion appeared to last an exceedingly very long time and coincided with the current presence of an exceedingly large convective cell,” Dupree said. The dimming that observers noted in those days was due to “an ejection of a considerable portion of the star’s surface accompanied by the current presence of a cooler spot, presumably because of gas expanding to fill the void.”

“Stars live like these live for an incredible number of years, however the end comes relatively quickly,” Murphy wrote to Salon.

Since Betelgeuse is really a massive star (it really is 1000 times bigger than our sun), roughly per year passed before people started to note the results of the event. Yet astronomers “could see material moving out through the star’s atmosphere (in the southern portion of the star) utilizing the Hubble Space Telescope,” Dupree explained. “And that southern part became very dim, as though a dark cloud were covering it. So we believe the dimming is related to the material that has been ejected and cooled, along with the cool spot from the gas that expanded in to the void left by the ejected material.”


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Dr. Avi Loeb, an astronomer at Harvard University, told Salon by email that section of the reason the discovery about Betelgeuse is indeed significant is that it’s a unique star specifically, a red supergiant (that have the biggest radii of most known stars). Moreover, Betelgeuse is indeed large that “if it were at the biggest market of our Solar System, its envelope would engulf the asteroid belt and the orbits of Mercury, Venus, Earth, and Mars.”

Talking about red supergiants, Loeb said that “understanding their properties and evolution is essential for understanding their fate if they consume their nuclear fuel and finally explode. Each of them continue to burn heavier elements and undergo core-collapse producing a supernova.”

Loeb also offered some illumination on the star’s amount of re-brightening in February 2020.

“By 22 February 2020, Betelgeuse began to brighten again,” Loeb explained. “Infrared observations found no significant change in brightness during the last 50 years, suggesting that the dimming was because of change in extinction by large dust grains. Data from the Hubble Space Telescope in 2022 suggested that occluding dust was made by way of a surface mass ejection and caused the dimming.”

“It has large convective cells on its surface, this means there’s hot material moving upward from within, much like chocolate sauce boiling in a pot,” explained study author Dr. Andrea Dupree.

Dr. Phil Massey, an astronomer at the Lowell Observatory in Flagstaff, Arizona, told Salon by email he didn’t believe most astronomers seriously believed that Betelgeuse was ever likely to explode as a supernova. Some believed that the fantastic Dimming have been the effect of a giant starspot somewhere on Betelgeuse’s surface a starspot being the interstellar equal to our sun’s sunspots, blemishes that appear periodically at first glance of stars. Others theorized that Betelgeuse was undergoing a dust formation event when a star will “losemass within an episodic way” that results in a cloud of dust in its vicinity.

Massey noted he andDr. Emily Levesque of the University of Washington, with whom he’s got been studying red supergiants since 2003, had discovered that the star’s temperature have been essentially unchanged over a long time. That was a significant clue in regards to what was going on with the second-brightest star in the Orion constellation.

“To us, that completely eliminated the ‘star spot’ explanation and meant that there have been some kind of large mass ejection, resulting in the forming of dust.” (They later wrote a paper about them for the journal Astrophysical Journal Letters.)

Regarding Dupree’s observations, Massey said they vindicated the sooner research he previously finished with Levesque: “Their results confirm what we’ve long suspected that red supergiants ‘burp’ out massive amount mass every once in awhile, and that mass-loss is more episodic in nature than constant.”

Dr. Alex Murphy, an astronomer at the University of Edinburgh, also praised Dupree’s work with the addition of that it allows visitors to better understand a development which technologically speaking they might have been struggling to appreciate just a few decades ago.

“Stars live like these live for an incredible number of years, however the end comes relatively quickly,” Murphy wrote to Salon. “So we’re really lucky to possess one so near us and in this phase now, when mankind has ‘just’ (astronomically speaking) developed the technology in order to see what’s happening. We’ve pretty good knowledge of what’s happening, but there is nothing like seeing it first hand.”

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