Astronomers generally refer to massive stars as the chemical factories of the Universe. They usually end their lives in spectacular supernovae, events that forge many elements of the periodic table. How the elemental nuclei blend together within these enormous stars has a major impact on our understanding of how they evolved before they exploded. It also represents the greatest uncertainty for scientists studying their structure and evolution.
A team of astronomers led by May Gade Pedersen, a postdoctoral researcher at the Kavli Institute for Theoretical Physics at UC Santa Barbara, has now measured the internal mixing within a set of these stars using observations waves coming from their deep interiors. Although scientists have used this technique before, this article marks the first time that this has been accomplished for such a large group of stars at a time. The results, published in Nature astronomy, show that the internal mixture is very diverse, with no clear dependence on the mass or the age of a star.
Stars spend the majority of their lives fusing hydrogen into helium deep within their hearts. However, the fusion in particularly massive stars is so concentrated in the center that it leads to a turbulent convective core similar to a pot of boiling water. Convection, along with other processes like rotation, effectively removes helium ash from the nucleus and replaces it with hydrogen from the shell. This allows the stars to live much longer than expected.
Astronomers believe this mixing results from various physical phenomena, such as internal rotation and internal seismic waves in the plasma excited by the convective core. However, the theory has remained largely open to observation because it occurs so deep within the star. That said, there is an indirect method of star scanning: asteroseismology, the study and interpretation of stellar oscillations. The technique has parallels with the way seismologists use earthquakes to probe the interior of the Earth.
“The study of stellar oscillations challenges our understanding of stellar structure and evolution,” Pedersen said. “They allow us to directly probe stellar interiors and make comparisons with the predictions of our stellar models.”
Pedersen and his collaborators at KU Leuven, University of Hasselt and University of Newcastle were able to obtain the internal mixture of a set of these stars using asteroseismology. This is the first time such a feat has been achieved, and was only possible thanks to a new sample of 26 slowly pulsing B-type stars with stellar oscillations identified from NASA’s Kepler mission.
Slow-pulsing B-type stars are three to eight times more massive than the Sun. They expand and contract over time scales in the range of 12 hours to 5 days, and can change in brightness by up to 5%. Their modes of oscillation are particularly sensitive to conditions near the core, Pedersen explained.
“The internal mixing inside the stars has now been measured by observation and turns out to be diverse in our sample, with some stars having almost no mixing while others reveal levels a million times higher.” , said Pedersen. The diversity turns out to be unrelated to the mass or age of the star. On the contrary, it is mainly influenced by internal rotation, although that is not the only factor at play.
“These asteroseismic results finally allow astronomers to improve the internal mixing theory of massive stars, which has so far remained uncalibrated by observations directly from their deep interiors,” she added.
The accuracy with which astronomers can measure stellar oscillations depends directly on how long a star is observed. Increasing the duration from one night to one year results in a thousandfold increase in the measured accuracy of the oscillation frequencies.
“May and his collaborators have really shown the value of asteroseismic observations as probes of the deep interiors of stars in a new and deep way,” said KITP Director Lars Bildsten, Gluck Professor of Theoretical Physics. “I can’t wait to see what she finds next.”
The best data currently available for this comes from the Kepler space mission, which observed the same area of the sky for four consecutive years. Slowly pulsing B-type stars were the highest mass pulsing stars the telescope observed. Although most of them are slightly too small to become supernovae, they share the same internal structure as the more massive stellar chemical factories. Pedersen hopes the information from the study of B-type stars will shed light on the inner workings of their higher-mass O-type counterparts.
She plans to use data from NASA’s Transiting Exoplanet Survey Satellite (TESS) to study clusters of high-mass oscillating stars in OB associations. These groups include 10 to more than 100 massive stars between 3 and 120 solar masses. Stars in OB associations are born from the same molecular cloud and share similar ages, she explained. The large sample of stars and the constraint of their common age offer interesting new opportunities to study the internal mixing properties of high mass stars.
In addition to unveiling the processes hidden in stellar interiors, research on stellar oscillations may also provide information on other properties of stars.
“Stellar oscillations allow us not only to study the internal mixing and rotation of stars, but also to determine other stellar properties such as mass and age,” Pedersen explained. “While these are two of the most basic stellar parameters, they are also among the most difficult to measure.”