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Why Did Betelgeuse Dim?

Updated: May 17, 2022

Betelgeuse is a massive star situated around 700 light-years from Earth. It is over 800 times broader and approximately 15 times as big as the Sun. It is also the second brightest star in the northern hemisphere's Orion constellation, behind Rigel. Betelgeuse's brilliance has recently begun to decline in unusual ways, catching the attention of astronomers and amateur stargazers worldwide. Most stars, including our Sun, have varying brightnesses throughout time. However, these changes are often minor, amounting to only a few percentage points of the star's total light output. Betelgeuse, on the other hand, has been up to something else.


It began to fade in October 2019 and had lost about two-thirds of its luster by mid-February 2020. Betelgeuse, normally the tenth brightest star in the night sky, has dropped to 25th. This is a spectacular plunge that we haven't seen with any other star before, and astronomers and astrophysicists have been rushing to make sense of the celestial drama. Some astronomers believe the star is reaching the end of its life. They believe that Betelgeuse's unusual fall will shortly come to an end, caused by a tremendous explosion known as a supernova. However, considering how ancient and near Betelgeuse is, a supernova occurrence in our lifetime is quite doubtful.


Betelgeuse belongs to an extraordinarily unusual class of big stars. The Milky Way galaxy contains more low-mass stars than high-mass stars. Astronomical studies of the night sky have revealed that when mass increases, the number of stars decreases substantially. There is one Betelgeuse-type star for every 200 stars on average. So it wouldn't be an exaggeration to label Betelgeuse a supergiant. It is so massive that 800 million Suns could fit inside it, with each Sun holding 1.3 million Earths. A star's brightness is the amount of energy it emits from its surface per second. The brightness of Betelgeuse is 100,000 times that of the Sun. However, because its surface temperature is 3,600 K, as opposed to the Sun's 5,800 K, only approximately 13% of its radiant energy is radiated as visible light.


Betelgeuse is traditionally categorized as a pulsing variable star. This means that the brightness of the star fluctuates as it grows and collapses. Betelgeuse has previously demonstrated strong and unmistakable periods of pulsation. In 1836, the English astronomer John Herschel detected comparable fluctuations in brightness. The star is said to have gone through multiple intermittent stages of brightening and fading during the previous two centuries. Albert A. Michelson and Francis Pease used interferometry to estimate the angular diameter of Betelgeuse for the first time in 1920.

What takes place inside a huge star?


Throughout its existence, every star must contend with two opposing sets of forces: the force of gravity that keeps the star together and the forces driving the nuclear processes that provide the star with energy. Stars are mostly cosmic furnaces that convert lighter materials into heavier ones. The gravity of the star pushes everything inside, while the heat and radiation from the reactions push everything outward. The star is held together by the balance of these two opposing forces. Consider how a pressure cooker works. The heated steam within the cooker is analogous to the energy produced by nuclear processes. As the temperature rises, so does the pressure, and the steam attempts to escape by forcing the lid open. The weight of the lid, or whistle, acts similarly to gravity in that it maintains the pressure under control.


The mass of a star normally ranges from 0.1 to 150 times that of the Sun. A regular Sun-like star consumes its fuel slowly and lives for billions of years, but huge stars like Betelgeuse survive for millions of years because their nuclear fuel is used quickly. In a series of nuclear burning cycles, supergiant stars create heavier metals like iron in their innards. The starting mass of the star determines the time scale of the various burning phases. Every star spends around 90% of its existence inside the core fusing hydrogen into helium. Helium then fused with carbon, carbon with neon, neon with oxygen, and so on. Iron is the last result of this chain of fusion events in high-mass stars. Furthermore, because the atomic nuclei of iron are exceedingly stable and strongly linked, they cannot be fused further. When the star's core is full of iron, nuclear processes come to a halt.


Without nuclear fuel, the core begins to cool even though there is nothing to oppose the force of gravity, therefore gravity wins. The iron core collapses catastrophically in less than a second, causing the material in the star's outer regions to fall freely towards the diminishing core. The infalling matter strikes the hot core with great power and bounces forcefully in the form of a shockwave that flows out into space. Heavy metals such as gold and platinum are produced, as well as gravitational waves and fast neutrinos. The energy unleashed by such tremendous explosions can briefly exceed the combined energy of all-stars in the host galaxy. Following the disaster, whatever remains of the core transforms into a neutron star or, if massive enough, a black hole.


Betelgeuse is roughly 10 million years old and the most likely star in the night sky to go supernova in the future. We can only conjecture on Betelgeuse's destiny and cross our fingers in hope. There is no perfect method to estimate when it will die. However, if it does go supernova, Earth-based detectors will detect gravitational waves and fast neutrinos from the explosion many hours before the visible fireworks begin. This is due to the fact that gravitational waves are formed seconds before the explosion, travel at the speed of light, and are unaffected by intervening matter. Neutrinos travel at almost the speed of light and have little interaction with matter.

Spot hypothesis


The energy created at the star's core must escape and reach the surface. Large blobs of heated and ionized material rising to the surface of high mass stars convey energy, much like bubbles rising from the bottom of a pot of boiling water over a fire. Convective cells are the name given to these superheated blobs of plasma. Convective cells in a Sun-like star are just a few hundred kilometers across. The overall gap between Earth and Mars is around 240 million kilometers wide on Betelgeuse. As it happens, the surface of most stars is periodically lined with intense magnetic fields known as starspots (just like sunspots). The magnetic field in these areas prevents energy from the star's innards from convecting to the surface. As a result, spot regions are colder and produce less energy. And, of course, the greater the star, the larger the dots. It's likely that a massive region on Betelgeuse's surface has momentarily obstructed convection across a vast area, decreasing the supergiant's surface temperature. This would account for the present dimming.


However, subsequent measurements at the Lowell Observatory in Arizona by astronomers Emily Levesque and Philip Massey show that Betelgeuse isn't so cool after all. They reported a measured temperature that was not significantly different from what previous studies had found in a scientific paper published in the March 2020 issue of The Astrophysical Journal, indicating that the star hasn't undergone the kind of substantive cooling that could explain its brightness deficit. As a result, the spot theory is unlikely to be the predominant source of fading.


Dust hypothesis


Every star is known to lose mass at the end of its development. While Betelgeuse is massive, it is 117.5 million times less dense than the Sun, resulting in low surface gravity and a low escape velocity of 60 km/s compared to the Sun's 600 km/s. As a result, gas and dust may escape from Betelgeuse's surface more easily into the circumstellar medium. In this fashion, Betelgeuse has been losing one Earth's mass of material every year, material that condenses to form a nebula-like envelope of gas and dust seen in infrared photos. Part astronomers believe that an unusually formed column of dust and gas created in this manner simply got in the way of our line of sight, preventing some of Betelgeuse's illumination from reaching Earth.


This fortunate alignment appears to have continued until around mid-February 2020. The star looks to be recovering its lost luster these days. The dust idea appears to provide an adequate explanation for the dimming. However, further observations are required to prove this notion beyond a reasonable doubt. All-stars perish. The larger ones just perish in a more dramatic manner. Betelgeuse is too far away from Earth to pose a significant threat when it explodes, but it is close enough to provide astronomers and astrophysicists with a once-in-a-lifetime opportunity to examine the unusual cosmic event in exquisite detail. Its supernova will be visible even during the day and will be brighter than the full moon at night.


Betelgeuse is likewise too young (in stellar terms) for planets to develop around it, let alone sustain life. Betelgeuse and its supergiant companions, on the other hand, are progenitors of life in a different way. Following a supernova, the heavier elements generated in the core of a giant star are blasted into the interstellar medium. This debris combines with gas and dust to form the building blocks for the next generation of Sun-like stars, which eventually sustain planets. In truth, humans are the result of the demise of a gigantic star. Our Solar System was constructed from the remnants of a comparable explosion that occurred before the Sun's birth. Many important components of the human body were generated in a distant supernova. In the larger scheme of things, we are actually the children of stardust, and this is arguably the most profound and sobering realization that contemporary science has assisted us in discovering.

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