Singing the Blues for Supergiant Stars

James Maynard
May 6, 2019 · 3 min read

Blue supergiants are among the least-understood of any class of star, as these massive bodies burn hot and die young. Now, thanks to the advent of powerful space telescopes, astronomers are understanding, for the first time, how sound waves inside these stars affect their structure and life cycles. This discovery could radically change what we know of supernovas, powerful explosions which produce elements crucial to the development of life.

The lifespans of stars is entirely dependent on their mass, with the largest stars having the shortest lifetimes. Prior to the development of space telescopes, astronomers knew of few blue supergiants, as their lifespans were so short. A team of researchers from KU Leuven in Belgium and Newcastle University in the UK have now confirmed computer models which suggested that nearly all blue supergiant stars shimmer due to waves on their surface.

An artist’s conception of a blue supergiant star in the spiral galaxy NGC 3938. The star imagined here was once 50 times as massive as the Sun, and erupted in a supernova, seen in 2017. Image credit: NASA, ESA, and J. Olmsted (STScI)

“The flicker in these stars had been there all along, we only had to wait for modern space telescopes to be able to observe them… From the frequencies of the waves at the surface, we can derive the physics and chemistry of their deep interior, including the stellar core. These frequencies probe how efficiently metal is produced and how it moves around in the factory,” explained Dominic Bowman, a postdoc researcher at Ku Leuven.

A hydrodynamical simulation of seismic waves within a massive star, three times the size of our own Sun. Different colors represent varying pressure levels within the stellar body. Image credit: © Tamara Rogers (Newcastle University)

Shake, Rattle, and Roll

Astroseismology, the study of tremors on stars, is similar to the seismology on Earth that helps detect earthquakes. However, it was not until the advent of advanced space telescopes that astronomers were able to detect quakes happening within alien stars.

Two different types of waves on the surface of these stars were predicted by computer models. The first were gravity waves, acting like waves on the ocean, breaking the surface of the star. The second were seismic waves, emanating from the interior of these bodies, like those produced beneath the crust of the Earth or Mars.

“Those which break at the surface, similar to the waves breaking on the beach, and the standing wave that just keeps on going and is similar to the seismic waves on Earth. From these we can start to understand how the star is moving and rotating and the physics and chemistry of what is going on inside the deep interior, including the stellar core,” Tamara Rogers, astrophysicist at Newcastle University, explains.

It’s the End of the Star as We Know it, and I Feel Fine

Observations from telescopes in space confirm both types of waves predicted by computer models. These studies allow astronomers to study mechanisms taking place within stars, even down to the stellar core.

“The discovery of pulsation modes or an entire spectrum of low-frequency gravity waves in these stars allow us to map the evolution of hot massive stars towards the ends of their lives,” researchers explain in an article describing their discovery, published in Nature Astronomy.

In 2017, astronomers spotted a supernova in NGC 3938, a spiral galaxy located 65 million light years from Earth. This blue supergiant star was once 50 times as massive as our own Sun. Poor in hydrogen and helium, stars this size erupt in what astronomers term Type 1c supernovas, due to their unique chemistry. This class of eruptions account for roughly 20 percent of all supernova events.

There are four methods by which nature produces supernovae. Here is a look at each of the processes that can lead to these massive eruptions. Credit: NASA Jet Propulsion Laboratory

I Hope I Die Before I Get Old

While the smallest stars have lifetimes stretching for tens of billions of years, the most massive stars live for much shorter periods before exploding into a supernova. The colors of stars is a function of their surface temperature — red stars are the coolest, while blue stars have the highest surface temperatures.

Blue supergiants, the largest and hottest stars, are the metal factories of the galaxy. These stellar bodies produce the majority of all elements heavier than helium before they explode, spreading their material to surrounding space.

Like a rock star burning bright and dying young, these supergiants really know how to sing the blues.

The Cosmic Companion

Exploring the wonders of the Cosmos, one mystery at a time

James Maynard

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Writing about space since I was 10, still not Carl Sagan. Mailing List/Podcast:

The Cosmic Companion

Exploring the wonders of the Cosmos, one mystery at a time

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