The star Rho Cassiopeiae belongs to the rare class of yellow hypergiants. These giant luminaries occasionally spew streams of matter from their surfaces. Scientists have investigated the causes of these outbursts.
The study of erupting stars
A recent five-year study has opened up new insights into the properties of yellow hypergiants, a class of heavy stars known for their dramatic outbursts. Scientists focused on Rho Cassiopeiae, HR 8752 and HR 5171A, and found that this luminary exhibits cyclic outbursts every 10-40 years with large variations in surface temperature.
Alex Lobel of the Royal Observatory of Belgium (ROB) was involved in this international study that combined historical data over 138 years. The results, published in the journal Astronomy and Astrophysics, show that strong pulsations cause these massive eruptions. They also offer a clearer understanding of the rapid evolution of yellow hypergiants and their potential transformation into luminous blue variable stars or explosive supernovae.
Yellow hypergiants
Hypergiants are some of the most massive and brightest stars in our galaxy. They show repeated and dramatic outbursts that have puzzled astronomers for decades. Important members are Rho Cassiopeiae, HR 8752 and HR 5171A. They are in the final stages of their rapid evolution, offering unique insights into the life cycle of very heavy stars, which have surface temperatures comparable to the Sun’s but half a million times brighter.
Studying hypergiants gives astronomers a rare chance to peer into the late-life of massive stars. In particular, we see the stage before they turn into supernovae with core collapse, or, alternatively, into another class of hot hypergiants called luminous blue variables.
This transition occurs when yellow hypergiants evolve rapidly because of the so-called yellow evolutionary void at the top of the Hertzsprung-Russell diagram (the dependence of stellar temperature on luminosity). The insights into the periodic outbursts and pulsations of yellow hypergiants help astronomers improve theoretical models of advanced stages of stellar evolution and enhance understanding of cyclic stellar eruption phenomena.
Rho Cassiopeiae
The new study, conducted over the past five years by an international team of scientists from the Netherlands (Leiden University), Belgium (ROB) and the UK (Durham University), also includes valuable data from amateur astronomers from around the world.
The team focused on Rho Cassiopeiae, one of the most studied and observable naked-eye hypergiants, by analyzing the long-term variability in brightness from 1885 to 2023. This large dataset allowed them to investigate its exceptional physical properties and the onset and evolution of three large atmospheric outbursts in 1986, 2000, and 2013.
These long-term observations reveal an interesting pattern: Rho Cassiopeiae demonstrates cyclic atmospheric eruptions about every 10-40 years, each accompanied by significant surface temperature variations ranging from about 4500 to 7500 °C. The new findings offer a unique opportunity to trace a massive star against a backdrop of accelerated evolutionary change.
Giant starbursts
For the first time, the team calculated accurate temperature calibration relations from reliable spectroscopic data combined with photometric observations between 1962 and 2020. This new methodology allows a more precise analysis of these extreme stars, in particular the dynamical behavior of their huge atmospheres (yellow hypergiants have diameters 400-700 times larger than the solar diameter).
The study shows that the pulsations of Rho Cassiopeiae become more intense as the outburst approaches. In particular, the pulsation periods observed in the brightness curve of hypergiants in the V-band (or visible band) lengthen and the pulsation amplitudes increase in the years preceding the outburst. This suggests that strong radial pulsations play a crucial role in triggering recurrent outbursts that have occurred six times in the last 138 years with time intervals of 10, 41, 40, 14, and 13 years.
The study also examined two other known yellow hypergiants, HR 8752 and HR 5171A. It turned out that HR 8752 evolved in a blue direction after 1996, and its visual brightness remained almost constant between 2017 and 2023. HR 5171A resumed its pulsation in early 2018 after a period of gradual low luster.
The new study and observation are significant because they provide important insights into the accelerated evolution of yellow hypergiants. Overall, it not only improves our understanding of extreme stars such as Rho Cassiopeiae and its periodic outbursts, but also contributes to our knowledge of yellow hypergiants, their variability, and their importance for stellar evolution.
According to phys.org
