The Riddle Surrounding Red Supergiant Stars

Red supergiant stars such as Betelgeuse and Antares are the astrobiological fertilizers for our galaxy at large. These massive, evolved stars are largely responsible for the chemical enrichment of newly forming sunlike stars.

Most of these short-lived stars cool and expand and eventually turn into so-called Type II Core Collapse supernovae. In the process, they throw off mountains of dust as well as massive amounts of carbon, nitrogen, oxygen and iron, all of which make up the building blocks of life as we know it.

These massive stars are so large that they could be centered around the Sun and their radii would reach out to the distance of Jupiter, Sarah Healy, a PhD candidate in astrophysics at Virginia Tech in Blacksburg, told me via email. Yet we still fail to understand the complex mechanisms that drive their evolution, she said.

Stellar theory predicts that all red supergiant stars can produce core-collapse supernovae; however, we have never observed a luminous red supergiant and its resulting explosion, said Healy.

This missing population of supernova progenitors has led to what is known as the two-decade old ‘red supergiant problem.’

The problem describes the apparent lack of high-luminosity progenitors detected in supernova pre-images, write the authors of a paper just submitted to The Astrophysical Journal. But in the paper, Healy and colleagues argue that the RSG problem is largely the result of observational bias.

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Massive Clouds Of Dust

We compared a population of pre-explosion imaged progenitor stars; a new RSG population within the Milky Way, and recent statistically complete RSG samples from other galaxies within the Local Group of Galaxies, said Healy, the paper’s lead author.

And after careful comparison and analysis, Healy and colleagues now assert that they found no missing RSG progenitors.

We now know how much light has been blocked by the dust in our previous observations and, therefore, how much light we had been missing, said Healy. So, what we previously had thought were less luminous RSGs, were actually luminous RSGs surrounded by excess dust, she said. This gives us a more correct measurement of luminosity, so we can now properly understand the characteristics of these evolved massive stars, said Healy.

Stars That Live Fast And Die Young

Red Supergiants evolve from high-mass O and B spectral type stars which rapidly exhaust their hydrogen fuel within 30 million years, Edward Guinan, an astronomer at Villanova University in Pennsylvania who was not involved with the study, told me via email.

After they’ve exhausted their hydrogen, they start burning helium in their cores.

During the helium core fusion stage, the star rapidly increases in size, expands, and cools becoming an RSG star, said Guinan. During this time, the stars shed copious amounts of nuclear-processed gas via massive stellar winds which enriches the interstellar medium with nuclear-processed nitrogen, carbon and nitrogen, he said.

As one of the largest, most luminous stars in our Milky Way Galaxy, the variable star VY Canis Majoris is a likely candidates for the next naked-eye core-collapse supernova, said Guinan.

VY Cma lies some 3800 light years away in the southern constellation of Canis Major and is already well above the theoretically required limit of at least 8 solar masses to collapse into a supernova.

These Type II supernovae typically expel several solar masses of ionized gas into the interstellar medium, said Guinan. This results in an expanding shock wave which leads to enhanced star formation, he said.

Why should we care?

It all goes back to the birth and death of stars within our Milky Way Galaxy and in galaxies that lie well beyond our own. The more astrophysicists can glean from the evolution of RSGs, the better they can understand our own solar system and other solar systems like it.

Healy and colleagues took their data mostly from observations made by NASA’s Webb Space Telescope, NASA’s Hubble Space Telescope, and the European Space Agency’s Gaia spacecraft. They also used data from ground-based surveys made by the U.K. Infrared Telescope in Hawaii and the Two Micron All-Sky Survey in the U.S. and Chile.

The Bottom Line?

RSGs are important for astrobiology.

Stars formed from chemically enriched environments are more likely to host earth-like habitable planets, said Healy. Observational evidence has shown that planet populations tend to be more diverse for more metal-rich stars, she said.

Lessons In Observational Astronomy

It is likely that until the number of pre-explosion imaged progenitors gets closer to eighty, the RSG problem will not entirely disappear from discussion, said Healy.

RSGs offer a hard-won lesson to astronomers; that is, the cosmos isn’t always how it appears. Observations can often be biased due to the limitations of telescopic technology in the era in which the data was taken.

What will be needed to finally solve this conundrum?

We need to determine the main driver of red supergiant dust production and increase the sample size of pre-explosion imaging, said Healy.

Future infrared telescopes will help.

So, it now appears that these RSGs will soon give up more of their secrets. In turn, stellar theorists will be rewarded with a much better understanding of these massive stars’ end games and the role they play in cosmic evolution.