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What Are Hypergiant Stars?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

Blue variables and hypergiant stars

Blue variables and hypergiant stars

What Is a Hypergiant Star?

The cosmos have plenty of high-factory stars out there, but few match the wild times of the hypergiants. They are powerhouses, the “most energetic stars in the Universe” with energy outputs 100,000 times that of our Sun, and yet they don’t get as much coverage as other classes of stars, likely because of their rarity. Most of the stars in our galaxy are red dwarfs, about 10% are like the Sun, and about 0.0001% are supergiants. Hypergiants are even rarer than that, and to make matters worse, their appearance isn’t that different from that of a normal star. But their spectrums give them away, and that boils down to molecular motion (Kaler 33-4).

Temperature can be thought of as the amount of energy released per square meter, and luminosity tells us their size, so by comparing these two quantities, we can get a true feel for our object in question. Just take care to include all aspects of the spectrum besides visible light. Remember, we want to get the total energy. Hypergiants primarily release their energy in the UV portion and not the visible portion, hence why their nature was hidden from us. But for now, let’s briefly go over some of the properties of these stars to see what differentiates them—and how they inform us of their behavior (Ibid).

Luminous Blue Variables

These hypergiants are losing mass like crazy, with strong stellar winds blowing away many of the outer layers and causing 1-2 magnitudes of luminosity differences over years, even decades. Eta Carnae proved indirect evidence of these winds with the nebula that surrounds it, but P Cygni offers direct evidence courtesy of its very bizarre spectrum. It is mainly emission, but some absorption portions do exist in the extreme blue section of the spectrum. The emission comes from the shell of material around the star that emits when hit by the solar wind, but the shell of material that obscures the star in our direct line of sight absorbs other portions of light (Kaler 35-6, Vink).

And because the shell is moving away from the star and towards us, the spectrum is blue-shifted. Hence the mixed, crazy spectrum we see. Based on the blue-shift we see, we can estimate the wind speed to be a couple hundred kilometers a second, yielding a mass loss of roughly 0.0001% the mass of the Sun per year. Not a lot, you may think, but that is over 1 billion times the rate of our own Sun. As far as temperatures go, they can widely from 15000 to 30000 Kelvin, depending on the subclass, with some even being confined to around 8000 Kelvin. And a star can evolve to this stage where helium is being fused as well as higher elements like carbon, oxygen, and nitrogen in as little as 2 million years (Ibid).

It should be noted that blue hypergiants are similar in appearance to these stars but do not mimic their behavior, drawing some scientists to question if these variables belong in the hypergiant family or something else entirely (Ibid).

The connection between luminous blue variables and hypergiants of different classes

The connection between luminous blue variables and hypergiants of different classes

Cool Hypergiants

Also known as red or yellow hypergiants, these stars are mainly 4000 to 7000 degrees Kelvin, but some can get as hot as 10,000 K. Most are temporary stages of red supergiants, but occasionally, if the progenitor is large enough, then the transition to a yellow hypergiant can remain permanent. However, they have lots of chemical as well as luminosity variability and are highly active in the infrared portion of the spectrum, all pointing to highly unstable objects. They, too, like luminous blue variables, expel matter but at a much-reduced rate (Stothers, Vink).

Hot Wolf-Rayet Stars

These stars are mysterious and fascinating. Their emission spectrums reveal that their hydrogen is gone, but plenty of helium, nitrogen, carbon, and oxygen are to be found. That indicates a star with a turbulent past that shed its outer hydrogen-based layers, and these stars often end turbulently, too, in a Type Ib/c supernova. They typically are 10-40 solar masses in size, hinting at a larger parent object. Their temperatures can vary depending on the subclass, with surface readings as low as 35,000 Kelvin but nearly as high as 200,000 Kelvin! This makes these stars among some of the hottest objects in the known Universe (Vink, Crowther).

Tracking the Path

In a typical hypergiant, it starts its life much like other stars: fusing hydrogen into helium. Gravity condenses the core during this process and thus raises temperatures. This, in turn, increases the brightness of the star as matter moves faster and forces the outer layers outward. Now further away from the heat source, the surface cools into a red giant and helium at the core is the main fusion element. Once our helium has been fused into higher elements, the process beings again, and now you have a supergiant due to the extreme swelling from the increased interior temperatures (Kaler 36-7, Vink).

Each phase of the star’s life is shorter and shorter as the fusion process makes less and less readily available materials present. In the case of a hypergiant, the outward pressure of light far exceeds the other spectral classes, in fact pushing right up to the Eddington limit (where the star basically pushes itself apart from the light pressure. But how can we tell if a supergiant is on the road to becoming a hypergiant? (Ibid).

How hypergiants age

How hypergiants age

Roberta Humpheys and Kris Davidson (University of Minnesota) found that stars above 40 solar masses will not become cool hypergiants. This line, known as the HD limit, can be plotted on the HR diagram. So if our star is less than 40 solar masses, then we will have a cool hypergiant, but if it is more than 40 solar masses, then a luminous blue variable. Remember that these are blowing their surfaces away because they can barely keep themselves together. Therefore, luminous blue variables will have a gradually decreasing mass as they push out their layers, lasting for probably 10,000 years (Kaler 37, Vink, Stothers).

And once all the stellar winds have blown away the outer layers, you have a Wolf-Rayet star which can last hundreds of thousands of years. Cool hypergiants are also near their Eddington limit but not to the extremes of their blue luminous variable cousins. Instead, as they approach the HD limit, they seem to “bounce” back in what is known as a blue loop. This is a result of a red supergiant suddenly undergoing a large mass loss (about 1% solar mass per year), heading towards the blue portion of the HR diagram (and so a hypergiant phase) but then cooling off and going back to lower temperatures. Stars travelling a 5000 to 7000 Kelvin loop take about 50 years, while the 6000 to 7000 Kelvin loop is about 20 years. This is why cool hypergiants never become luminous blue variables—they approach the limit but cannot overcome it and return to a red supergiant phase (Ibid).

Are Luminous Blue Variables Capable of Supernovas?

We know that luminous blue variables are on the limit of stability, but does that mean they have the capability to explode? While no conclusive evidence has been found, some tantalizing hints do seem to suggest it is possible. SN 2001ig and SN 2003bg both had radio behavior that would match the shells of material from the winds of the variables. SN 2005gj had portions of its absorption spectrum seemingly matching with that of a luminous blue variable star. And SN 2006jc had an outburst just a few years before its explosion took place that was consistent with a binary system of a Wolf-Rayet star and a luminous blue variable (Vink).

Who knew that so much was going on in the upper-left hand portion of the HR diagram? And yet there is so much more in there to talk about…

Works Cited

Crowther, Paul A. (2007). "Physical Properties of Wolf–Rayet Stars". Annual Review of Astronomy and Astrophysics. 45 (1): 177–219.

Kaler, James B. “Hypergiants.” Astronomy. Mar. 1994. Print. 33-7.

Stothers, Richard B. and Chao-Wen Chin. “Yellow Hypergiants as Dynamically Unstable Post-Red Supergiant Stars.” The Astrophysical Journal, 560: 934-936, 2001 October 20.

Vink, Jorick S. “Eta Carinae and the Luminous Blue Variables.” arXiv:0905.3338.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2022 Leonard Kelley