Roberta Humphreys on Luminous Blue Variables

Professor Roberta Humphreys of the School of Physics and Astronomy recently gave a colloquium that served as a primer on  “some of the most interesting stars I know: luminous blue variables (LBVs). These are among the most massive, most luminous stars known and in their short lifetimes experience high mass loss episodes that are not understood. 

Early in her career Humphreys recognized an empirical upper luminosity boundary in the luminosity –temperature diagrams (the HR-Diagram) for the most luminous stars in the Milky Way and the Large Magellanic Cloud. “We discovered that there was a lack of red supergiant counterparts to the most luminous, hot stars as was expected at that time.  This had important implications for the evolution of the most massive stars, but more important was why.  Humphreys and her collaborator Kris Davidson realized that a few stars known for their instabilities and high mass loss such as eta Carina (add figure) and S Dor in the Large Magellanic Cloud, were near this upper luminosity. They suggested then that the empirical limit was due to mas loss –episodic  high mass loss events. 

One of the stars that Humphreys has observed for decades is the cool hypergiant VY Canis Majoris. It is one of the most luminous red supergiants and one of the largest stars known. It image reveals a complex ejecta with giant arcs and clumps of knots. (add figure). Using imaging and spectra from the Hubble Space Telescope, she and her collaborators we able to reconstruct  its mass loss history. They  calculates the age of the ejecta from these outflows.  The most recent ejection were found to correspond with deep minima in the stars observed light curves earlier in the 19th and 20th centuries “The star has a massive outflow, 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 outflows from VY CMa is large, with masses about the mass of Jupiter and kinetic energy equivalent to the energy our sun radiates in 10,000 years. Humphreys and colleagues suggest that these outflows are due to surface activity, large scale convection on the star.

 In 2019 and 2020, Betelgeuse (alpha Orionis) the bright red star in the winter sky, was prominent in the science news because it had unexpectedly dimmed by more than a magnitude.  The bottom half of the star appeared dark. “Spectra obtained with Hubble confirmed a gaseous outflow prior to the dimming”, and a group of astronomers in France confirmed that the dimming was due to dust. This similarity between Betelgeuse, a typical red supergiant and VY CMa, but on a larger scale, suggests that these massive outflows or ejections are common and a major source of mass loss for red supergiants.

Many evolved luminous hot or blue supergiants experience enhanced mass loss episodes in which the expanded wind becomes dense and the star appears to make a transit to cooler temperatures on the HR Diagram. These stars are known as S Dor variables or Luminous Blue Variables (LBVs).  Their instability may be due to the star’s proximity to the “Eddington Limit” which is the balance between the star’s outward radiation pressure and inward gravity. Near the Eddington limit, surface instabilities may lead to irregular enhanced mass loss. Some of the stars she studies may go through many cycles of eruption in their lifespan. She says that the Eddington Limit also explains why stars with different evolutionary histories and initial masses become LBVs.

A small group of very evolved massive stars experiencing giant eruptions are also known as “supernova imposters.” These stars suddenly increase their luminosity by ten times or more and then fade. They resemble supernovae, but survive, hence “impostors”.  She studied four historical ones, including eta Car. “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: “Why study massive stars? They’re interesting, they’re fun, surprising, and they can change within a human lifespan.” Several stars have appeared to change dramatically due to mass loss events since she started observing them more than forty years ago. Massive stars are short lived, she opined. “Many of the stars I’ve studied in the spiral galaxies M31 and M33, 2 million light years away, 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 discovered constantly by the supernova surveys. “In most cases, we don’t have enough information to say why some of them survive their eruptions and others apparently don’t. They aren’t all alike. It is now time to sort them, according to their observed properties to better understand what they are doing.”