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Why Did Betelgeuse Dim?

Why Did Betelgeuse Dim?

The Orion constellation, with Betelgeuse visible as a dull red star on the hunter’s right shoulder. Photo: alchemist_x/Flickr, CC BY 2.0.

Betelgeuse is an incredibly big star located about 700 light years away from Earth. It is about 800-times wider than the Sun and nearly 15-times as massive.1. It is also the second brightest star (fig. 1) after Rigel in the Orion constellation, in the northern hemisphere.

Recently, Betelgeuse’s brightness began to dim in unprecedented  ways, attracting the attention of astronomers and amateur stargazers around the world. The brightness of most stars, including our Sun, vary over time. But these variations are usually small, nothing beyond a few percentage points of the star’s total light output. Betelgeuse, however, has been up to something else.

It began fading sometime in October 2019 and had lost fully two-thirds of its shine by mid-February 2020. Normally ranked the tenth brightest star in the night sky, Betelgeuse has suddenly slipped to 25th. This is a stunning drop the likes of which we haven’t noticed with any other star before, and many astronomers and astrophysicists have been scrambling to make sense of the stellar drama.

Some astronomers suspect the star is nearing its death. They argue that Betelgeuse’s freakish decline could soon culminate in a sudden end triggered by a violent explosion known as a supernova. However, given how old and close we know Betelgeuse to be, a supernova event in our lifetime seems quite improbable.

Figure 1: (a) Image of  Betelgeuse taken with Hubble Space Telescope’s faint object camera in 1996. (b) In the Orion constellation, Betelgeuse at the top left. Photo: NASA/ESA

Betelgeuse belongs to a category of massive stars that are extremely rare. There are more low-mass stars in the Milky Way galaxy than there are high-mass stars. Astronomical surveys of the night sky have found that the star-count drops significantly as the mass increases. On average, for every 200 stars, there is only one Betelgeuse-type star.

So calling Betelgeuse a supergiant wouldn’t be an exaggeration. It is so big that 800 million Suns could fit inside it — and each Sun can pack in 1.3 million Earths.

The luminosity of a star is the amount of energy it releases from its surface every second. Betelgeuse’s luminosity is 100,000-times that of the Sun. However, its surface is also cooler – 3,600 K versus the Sun’s 5,800 K – so only about 13% of its radiant energy is emitted as visible light.

Traditionally, Betelgeuse is classified as a pulsating variable star. This means the star’s brightness changes as the star expands and contracts. In the past, Betelgeuse has displayed striking and unequivocal phases of pulsation. The English astronomer John Herschel first noticed the corresponding changes in brightness in 1836. In the last two centuries, the star is reported to have undergone several intermittent phases of brightening and dimming.

In 1920, Betelgeuse became the first star to have its angular diameter measured with a technique called interferometry, by Albert A. Michelson2 and Francis Pease.

Figure 2: The light curve of Betelgeuse for the past six months. The star’s magnitude had dropped to below 1.6 by early February 2020, amounting to a brightness reduction factor of 2.5. This is the largest observed change for any bright star. Since March 2020, the star has started recovering its brightness. Data source: AAVSO

What happens inside a massive star?

Every star has to constantly grapple with two competing sets of forces throughout its life: the force of gravity that holds the star together and the forces driving the nuclear reactions that are the star’s source of energy. Stars are principally cosmic factories that fuse lighter elements into heavier ones. The star’s gravity pulls everything inwards while the heat and radiation from the reactions exert an outward pressure. The balance of these two opposing forces keeps the star together. Think of how a pressure cooker works. The hot steam inside the cooker is like energy from nuclear reactions. More heat creates more pressure and the steam tries to escape by forcing the lid open. The weight of the lid, or whistle, is like gravity: it keeps the pressure under control.

A star’s mass typically ranges from 0.1- to 150-times the solar mass. A ‘normal’ Sun-like star burns its fuel slowly and lives for several billion years while massive stars like Betelgeuse are short-lived – in the order of millions of years – because they consume their nuclear fuel faster.


Also read: WTF: The Story of the Strangest Star We’ve Known


Supergiant stars produce heavier elements like iron in their interiors in a series of nuclear burning cycles. The time scale of different burning stages is determined by the star’s initial mass. Every star spends about 90% of its lifetime fusing hydrogen into helium inside the core. Subsequently, helium fused into carbon, carbon into neon, neon into oxygen and so on. In high-mass stars, iron is the final product of this series of fusion reactions. And since iron’s atomic nuclei are very stable and tightly bound, they cannot be fused further. So nuclear reactions stop when the star’s core is full of iron.

Figure 3: The different evolutionary stages of a star. A massive star meets its end in a violent explosion called a supernova. Image: NASA

Without nuclear fuel, the core begins to cool even as there’s nothing pushing back against the force of gravity, so gravity takes the upper hand. In less than a second, the iron core collapses catastrophically, forcing the material in the star’s outer parts to fall freely towards the shrinking core. The infalling matter strikes the heated core with tremendous force and rebounds violently in the form of a shockwave that travels outwards into space. This process produces heavier metals such as gold and platinum, as well as gravitational waves and fast neutrinos. The amount of energy released from such powerful explosions can momentarily exceed the combined energy of all stars in the host galaxy.

After this cataclysm, whatever is left of the core turns into a neutron star or, if it is dense enough, a black hole.

Figure 4: The inward gravitational pull of the gravity is balanced by outward pressure generated by heat and radiation produced by nuclear fusion in the core.

Betelgeuse is already about 10 million years old, and it is the most promising star in the night sky to go supernova in future. We can only speculate the fate of Betelgeuse, and cross our fingers in hope. There is no exact way to predict the exact time of its demise. This said, when it does go supernova, instruments on Earth will register gravitational waves and fast neutrinos from the explosion several hours before the visual fireworks come on. This is because the gravitational waves are generated moments before the explosion, travel at the speed of light and aren’t disturbed by intervening matter. The neutrinos also travel at nearly the speed of light and don’t interact much with matter.

Figure 5: Images of Betelgeuse taken almost a year apart using ESO’s SPHERE/VLT facility at the Paranal Observatory in northern Chile. Recent dimming seems more prominent in the southern hemisphere of the star.

Plausible explanations

Spot hypothesis

The energy produced at the star’s centre has to come out and reach the surface. In high-mass stars, the energy is transported by large blobs of hot and ionised material rising to the surface – much like bubbles rising from the bottom of a pot of water boiling over a stove. These superheated blobs of plasma are called convective cells. In a Sun-like star, convective cells are only a few hundred kilometres wide. On Betelgeuse, they are about 240 million km wide – the entire distance between Earth and Mars.

As it happens, the surface of most stars is laced occasionally by strong magnetic fields called star spots (just like sunspots). The magnetic field in these spots prevents energy in the star’s interior from being convected to the surface. Spot regions are therefore cooler and emit less energy. And yes, the larger the star, the bigger the spots.

Figure 6: Reconstructed images of Betelgeuse showing large convective cells responsible for transporting energy from the deeper layers of the star to its surface. Source: https://doi.org/10.1051/0004-6361/201936189

It’s possible that a giant spot covering the surface of Betelgeuse has temporarily impeded convection over a large area, thus lowering the supergiant’s surface temperature. This would explain the current dimming.

However, astronomers Emily Levesque and Philip Massey have found in newer observations at the Lowell Observatory, Arizona, that Betelgeuse isn’t so cool after all. In a scientific paper that appeared in the March 2020 issue of the Astrophysical Journal, they reported a measured temperature not very different from what previous studies have found, meaning the star hasn’t undergone the sort of substantive cooling that could explain its brightness deficit.

The spot hypothesis is therefore unlikely to be the primary cause of  dimming.

Figure 7: An image of Betelgeuse taken by ESA’s Herschel Space Observatory at infrared wavelengths, in 2012. The clumpy dust shells around the star suggest episodic and asymmetric mass loss.

Dust hypothesis

In the last stage of its evolution, every star is known to lose mass. While Betelgeuse is huge, it is 117.5-million-times less dense than the Sun, which means it has a low surface gravity and a small escape velocity: 60 km/s versus the Sun’s 600 km/s. This in turn means gas and dust escape more easily from Betelgeuse’s surface into the circumstellar medium3. And this way, Betelgeuse has been losing one Earth’s mass worth of material every year – material that condenses to form a nebula-like envelope of gas and dust that can be seen in images taken at infrared wavelengths.

Some astronomers think an oddly shaped column of dust and gas produced this way has simply come in the way of our line of sight, and obstructed some of Betelgeuse’s starlight from reaching Earth.

This fortuitous conjunction seems to have lasted until about mid-February 2020. These days, the star appears to be regaining its lost shine.

The dust hypothesis seems to offer a satisfactory explanation of the dimming. However, we still need more observations to confirm this possibility beyond any reasonable doubt.

§

All stars die. The bigger ones just die more spectacularly.

Betelgeuse is too far from Earth to pose any major threat when it eventually explodes – but it’s close enough to offer a unique chance for astronomers and astrophysicists to study in great detail the rare cosmic event. Its supernova will be brighter than the full moon at night and will be visible even during the day.

Betelgeuse is also too short-lived (in stellar terms) for planets to form around them, leave alone harbour life. However, Betelgeuse and its supergiant peers are progenitors of life in a different way. The heavier elements formed in the core of a massive star are expelled into the interstellar medium after the supernova. This debris mixes with gas and dust to become the material for the subsequent generation of Sun-like stars, which then support planets.


Also read: The Story of Dust, Through Space and Time


In fact, we owe our existence to the death of a massive star. Our Solar System was formed from the remains of a similar explosion that predated the birth of the Sun. Many essential ingredients of the human body were first created in a faraway supernova. In the grand scheme of things, we are truly the children of stardust – and this is possibly the most profound and humbling thing modern science has helped us find.

Ravinder Banyal is a research scientist at the Indian Institute of Astrophysics, Bengaluru.


  1. If it had been in the Sun’s place, it would have swallowed Jupiter

  2. Of the Michelson-Morley experiment’s fame

  3. The part of space immediately surrounding the star

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