Roberta Humphreys on Luminous Blue Variables

Professor Roberta Humphreys of the Minnesota Institute for Astrophysics recently gave a colloquium that served as a primer on  “some of the most interesting stars I know: luminous blue variables (LBVs)". Humphreys, College of Science and Engineering Distinguished Professor Emerita, recently gave a plenary lecture on this topic at the 238th meeting of the American Astronomical Society. The LBVs form a class of extremely rare stars (only 12 have been identified in our galaxy) that are very luminous, massive, and radiate blue light. Humphreys coined the term “luminous blue variables” in 1978.

They were originally thought to be primordial stars, at the earliest stage of stellar evolution. The problem with this theory was that it wasn’t borne out by data. Much like what is found for flames from a fire, “red” stars are cooler and “blue” stars are hotter.  “We discovered that there was a lack of red supergiants which had important implications for the evolution of stars. It had been expected that [the red supergiant stage] was a part of evolution.” Humphreys and her collaborators discovered that there was a red luminosity boundary in galaxy surveys. Having spent years observing the supermassive star, Eta Carinae, which was losing mass, she theorized that this upper limit was due to episodic high mass loss such as Eta Carinae exhibited. During these periods, the Red Supergiants were erupting, increasing luminosity, changing from red to blue.

One of the stars that Humphreys has studied for decades is VY Canis Majoris. It is a red supergiant that periodically experiences mass loss. Humphreys and her collaborators were able to calculate the age of the ejecta from these periodic eruptions and compare them to the changes in luminosity of the star and found that they synched up. “The star has a massive outflow, and it cools and forms dust which causes the dimming. The ejecta expands and moves outward, and the star returns to its former brightness.” The scale of these eruptions is so large that one mass of ejecta from VY Canis Majoris was equivalent to the energy our sun radiates in 10,000 years.

The size of these stars poses a problem for astronomers trying to understand their mechanics. “The methods that work well for red giants don’t work for red supergiants.” The reason for this is the “Eddington Limit” which is the maximum luminosity of a star before the gravity of its massive size causes it to collapse inward. Near the Eddington limit, large-scale surface convective cells form that project outflows. These stars erupt, cool, and return to previous brightness. Some of the stars Humphreys studies may go through many cycles of eruption in their lifespan. She says that the Eddington Limit also explains why stars of varying histories and ages can become LBVs, since they are all of a similar massive size.

These changes in commonly observed stars pop up from time to time in the news. In 2019 Betelgeuse, which is among the brightest stars in the night sky, dimmed massively. It appeared that the bottom half of the star just disappeared. “Spectra obtained with Hubble confirmed a gaseous outflow prior to the dimming.” A group of astronomers in France confirmed that the dimming was due to dust.

One group of LBVs are also known as “supernova imposters.” These are a class of large stars that got suddenly brighter and then disappeared entirely. They were thought to have exploded as supernovae, but then suddenly reappeared. In 1994 Humphreys studied four stars which she believed were imposters. “Three of the four are still here. People are still arguing about the 4th one,” she noted.

In her recent colloquium for the School, Roberta Humphreys summed up her career-spanning interest in massive stars thusly: “Why study massive stars? They’re interesting. They can change within a human lifespan.” Several stars such as VY Canis Majoris have changed drastically since she started observing them more than forty years ago. Massive stars are short-lived, she opined. “These stars I’ve studied are likely gone now. It’s very sad, I feel like I know these stars.”

After a career getting to know these massive stars, Humphreys still has a lot of unanswered questions, mainly about the mechanism that drives the eruptions. “We need some clever modeling.” There are new imposters being observed constantly by supernova surveys. “In most cases, we don’t have enough information to say why some of them survive their eruptions and others don’t. “We need to go and sort them out because they aren’t all alike.”