Betelgeuse! Betelgeuse! Betelgeuse! – Amateur Astronomers Association

The reviews are in: Beetlejuice, the movie and sequel – meh.

Betelgeuse the star – spectacular!

With the coming of the winter months, we will have a front-row view of Betelgeuse. This 4th magnitude star (arrow) is easily located in the constellation Orion; the mighty hunter. (The name Betelgeuse is thought to be Arabic for “the shoulder of the hunter”) It is the tenth brightest star in the night sky.

This is the first direct image of a star other than the Sun, made with NASA's Hubble Space Telescope. Called Alpha Orionis, or Betelgeuse, it is a red supergiant star marking the shoulder of the winter constellation Orion the Hunter.
Andrea Dupree (Harvard-Smithsonian CfA), Ronald Gilliland (STScI), NASA and ESA — _{This is the first direct image of a star other than the Sun, made with NASA’s Hubble Space Telescope. Called Alpha Orionis, or Betelgeuse, it is a red supergiant star marking the shoulder of the winter constellation Orion the Hunter. Credit: Andrea Dupree (Harvard-Smithsonian CfA), Ronald Gilliland (STScI), NASA and ESA}

Betelgeuse Facts

Betelgeuse has a diameter that would approximately reach beyond the orbit of Jupiter (700 times our Sun). Yet its density is in the range of 0.012 g/m³ which means it is about 12 billionth the density of our Sun. Given its wispy character, it has been hard to nail down its exact distance from Earth, or much of anything else for that matter. It is approximately 400-650 light years away.

It is classified as a red supergiant of spectral class M2lab (red supergiant) which means that it has a “short” lifespan of about 10 million years. By comparison, the lifespan of our Sun (Type G2) is 10 billion years. This type of star “burns” (a colorful way of describing the fusion of hydrogen into helium) through its mass at an enormous rate: ultimately leading to a supernova. While its core temperature is estimated to be in the billions of degrees, its surface temperature is cooler than our Sun (3300 compared to 5500 Celsius)

It is considered a pulsating semiregular variable star that varies significantly in brightness due to changes in its size and temperature. Its apparent magnitude ranges from 0.0 and 1.6, the largest variation seen in any first-magnitude star. By comparison, Spica located 260 light-years away in Virgo, has a steady apparent magnitude of 1.02.

‘A Troubled Glare’

During the Chinese Han Dynasty (100 BCE), the court scribe astrologer, Sima Quian, noted that Betelgeuse appears yellow. Some 200 years later Ptolemy (Alexandria) described it as ruddy or red. Around the same time, the Roman poet Horace described it as having a “troubled glare”.

In 1836, the European astronomer John Herschel determined that Betelgeuse was a variable star (meaning its brightness changes as it expands and contracts). It is likely that Australian Aboriginals orally recorded its variable brightness and color much earlier.

Because of its prominence in the night sky, it has been studied extensively. Observations have been done at a multitude of wavelengths, visible, infrared, and ultraviolet.

_{This spectral plot is based on Hubble Space Telescope observations from March 2019 to February 2020. Hubble recorded a surprising outburst in the atmosphere of the nearby red supergiant star Betelgeuse. Measurements of emission from magnesium II were used to trace motion in the star’s pulsating atmosphere. Credit: NASA, ESA, A. Dupree (CfA), and E. Wheatley (STScI)}

The indeterminate nature of its status and the variability of its color and brightness have led to a flurry of speculation about its fate. Recent studies have confirmed that it is a volatile star with a rapidly (relative astronomical term) changing landscape. In 2017 a team, Gorman et al, using the ALMA radio telescope array, reported that the surface temperature of the star is not homogeneous. While our Sun shows small variations in surface temperatures, their highly technical study shows significantly larger variations on the surface of Betelgeuse.

These color graphs indicate just how much variation there is. In the words of the authors, these differences are attributed to “the shifting of contributing heights to higher and hotter layers in the atmosphere.” The cause of these differential heights is surmised to come from dynamic magnetic convection currents in the star’s core.

In 2019-2020 astronomers observed a remarkable dimming of the star. Andrea Dupree, associate director of the Center for Astrophysics | Harvard & Smithsonian (CfA), Cambridge, Massachusetts, proposed that the star ejected a massive amount of material which formed a huge dust cloud. This dust cloud wound up covering a significant portion of the star’s surface, thereby dimming it. Several months later, it was reported that the star had returned to its former brightness.

_{This four-panel graphic illustrates how the southern region of the rapidly evolving, bright, red supergiant star Betelgeuse may have suddenly become fainter for several months during late 2019 and early 2020. In the first two panels, as seen in ultraviolet light with the Hubble Space Telescope, a bright, hot blob of plasma is ejected from the emergence of a huge convection cell on the star’s surface. In panel three, the outflowing, expelled gas rapidly expands outward. It cools to form an enormous cloud of obscuring dust grains. The final panel reveals the huge dust cloud blocking the light (as seen from Earth) from a quarter of the star’s surface. Illustration Credit: NASA, ESA, and E. Wheatley (STScI)}

These brightness fluctuations correlated with temperature asymmetry and a mass ejection have led astronomers to believe that there is a real possibility that Betelgeuse will explode as a highly visible supernova – and soon!

More recently, Jared Goldberg et. al at the Flatiron Institute in Manhattan conducted a comprehensive computer-generated simulation of Betelguese’s unusual orbital dynamics. They concluded that the most likely explanation for all these phenomena would be the presence of a companion star; (mass of 1.4M⊙, orbiting at a distance of 1900R⊙). Their conclusion was based on the presence of two different light curve periods. The first shorter one of 417 days is thought to represent the star’s fundamental radiation pulsation. The longer pulsation at about 2200 days would best be explained by the presence of a companion star dubbed “Betel-buddy”.

*_{Infographic describing how the Betelbuddy affects Betelgeuse’s apparent brightness. Credit: Lucy Reading-Ikkanda/Simons Foundation}*

The fluctuations in brightness and mass ejections could be attributed to the physical dynamics between the star and its companion. By wonderful coincidence, on December 6, 2024 (this month!), astronomers will have a chance to test these two hypotheses: supernova prelude or Betel-buddy. Simulations suggest that the star and its hypothesized companion will be in a position for telescopes to visibly detect the presence of this projected companion. Astronomers around the world are getting ready.

Whatever the result, this star is certain to stimulate further observations and analyses. Its volatile character will continue to intrigue observers for a long time.