Rho Cas and its kin: Study provides new insights into the mysterious outbursts of yellow hypergiants - UNIVERSE

Artist's impression of the Yellow Hypergiant star Rho Cassiopeiae around which gas shells form due to recurring outbursts observed in the last 130 years. Credit: Alex Lobel

A recent five-year study has uncovered new insights into the properties of yellow hypergiants, a heavy star class known for their dramatic outbursts. Scientists focused on Rho Cassiopeiae (Rho Cas), HR 8752, and HR 5171A, revealing that Rho Cas exhibits cyclical outbursts every 10 to 40 years, with large fluctuations of its surface temperature.

Alex Lobel from the Royal Observatory of Belgium (ROB) was part of this international study that combined historical data covering 138 years. The results, published in Astronomy and Astrophysics, show that vigorous pulsations trigger these massive eruptions. They also offer a clearer understanding of the yellow hypergiants' fast evolution and their potential transformation into luminous blue variable stars or explosive supernovae.

Yellow hypergiants

Hypergiants are among the most massive and luminous stars in our galaxy. They show recurring and dramatic outburst events that have puzzled astronomers for decades. Important members are Rho Cassiopeiae, HR 8752, and HR 5171A. They are in the final stages of their fast evolution, offering unique insights into the life cycle of very heavy stars, having surface temperatures comparable to the sun, but being up to half a million times more luminous.

The study of hypergiants offers astronomers a rare glimpse into the late life of massive stars. In particular, we get a look at the stage just before they evolve into core-collapse supernovae, or alternatively, into a different class of hotter hypergiants called luminous blue variables.

This transition occurs when yellow hypergiants quickly evolve through the so-called yellow evolutionary void in the upper Hertzsprung-Russell diagram of stellar temperature vs. luminosity. Understanding the recurring outbursts and pulsations of the yellow hypergiants helps astronomers to refine theoretical models of advanced stellar evolutionary stages and to improve understanding of cyclic stellar eruption phenomena.

The new study, conducted over the last five years by an international team of scientists in The Netherlands (Leiden University), Belgium (ROB), and the UK (Durham University) also incorporates valuable data contributions from amateur astronomers worldwide.

The team focused on Rho Cas, one of the most well-studied and naked-eye hypergiants, analyzing the long-term brightness variability from 1885 to 2023. This extensive dataset allowed them to investigate its exceptional physical properties, as well as the onset and progression of three major atmospheric outburst events in 1986, 2000, and 2013. 

A one minute video clip (animation) about the Yellow Hypergiant Rho Cassiopeiae. Credit: Alex Lobel (shared under a CC BY—NC-SA 4.0 International license)

These long-term observations reveal a fascinating pattern: Rho Cas exhibits cyclical atmospheric eruptions about every 10 to 40 years, each involving a significant fluctuation of the surface temperature, ranging from approximately 4,500 to 7,500 °C. The new findings provide a unique opportunity to follow a very massive star amid accelerated evolutionary changes.

Hypergiant outbursts

For the first time, the team calculated precise temperature calibration relations based on reliable spectroscopic data, combined with photometric observations between 1962 and 2020. This new methodology allows for a more accurate analysis of these extreme stars, in particular the dynamic behavior of their huge atmospheres (yellow hypergiants have diameters of 400 to 700 times that of the sun).

The study reveals that the pulsations of Rho Cas become more intense when approaching an outburst event. Specifically, the pulsation periods observed in the hypergiants' V-band (or visible) brightness curve lengthen, and the pulsation amplitudes increase in the years leading up to an eruption event. This signals that strong radial pulsations play a crucial role in triggering recurring outbursts, which have occurred six times in the last 138 years, with time intervals of 10, 41, 40, 14, and 13 years.

According to Dr. Lobel, co-author of this study, "It is the first time a thorough investigation was done with pretty much all of the available historical data of Rho Cas we collected from the literature, reaching far back to the 19th century. Moreover, we could combine it with new observations, including valuable contributions by amateur astronomers."

The research also investigated two other notorious yellow hypergiants, HR 8752 and HR 5171A. HR 8752 was found to evolve on a blue-ward evolutionary track after 1996, with its visual brightness staying nearly constant between 2017 and 2023. HR 5171A resumed its pulsation pattern in early 2018 after a period of gradual brightness decline.

The new study and observations are significant because they provide important insights into the sped-up evolution of yellow hypergiants. Overall, it not only improves our understanding of extreme stars, such as Rho Cas and its recurring eruptions, but also contributes to the broader knowledge of yellow hypergiants, their variability and importance for stellar evolution. 

by Royal Observatory of Belgium

Source: Rho Cas and its kin: Study provides new insights into the mysterious outbursts of yellow hypergiants