Physics faculty member discusses significance of reports that supergiant star Betelgeuse is dimming

Supergiant Betelgeuse—normally the 10^th-brightest star in the night sky—made headlines earlier this month based on analysis that has revealed the star dimmed significantly in 2020. The red-colored star—part of the constellation of Orion—is well-known to the science world because it has been observed and studied for centuries and is, at 750 times the size of the sun, easily visible to the naked eye. Melissa Halford, a lecturer in Auburn University’s Department of Physics in the College of Sciences and Mathematics, or COSAM, took some time to discuss the popular star, which is roughly 530 light years from Earth.

Media reports have detailed Betelgeuse’s recent dimming, saying it has entered the helium-burning phase that is the first stage of a star going supernova. What does this mean for the star and our ability to observe it in Orion?

The light we see coming from a star is a form of energy, and that energy has to come from somewhere. For most of a star’s life, it generates energy by fusing hydrogen atoms into helium atoms in its core. Eventually, the core has a lot of helium and runs out of hydrogen to fuse. When this happens, the longest part of the star’s lifetime is over. But it’s not the end of the line yet—massive stars then go through several stages burning other types of fuel, including a stage when helium fuses into carbon.

As the energy generation process in the star’s interior changes, its outer layers may change as well—for example, a higher energy generation rate may increase pressure inside the star, pushing outward on the star’s outer layers and causing them to expand and cool. These changes may result in visible effects, like changing brightness. Dimming can also result when the star’s light is blocked, which may explain the fairly dramatic recent dimming of Betelgeuse: the star ejected some material, which then blocked some of its light from reaching us on Earth.

Can you talk about stellar evolution in general and specifically how long Betelgeuse is expected to remain a constant in the night sky?

High-mass stars live shorter lives than low-mass stars. They are much brighter, so they burn though their fuel more quickly. It’s like a small car vs. a large truck—the truck has a larger gas tank, but it uses gas much more quickly so it can’t go as far as the car will go on a single tank.

Betelgeuse is somewhere around 20 times the mass of the sun, which makes its lifetime “only” several million years. Millions of years sounds like a long time, but on astronomical time scales, it’s quite short—the lifetime of a star like the sun is 10 billion years. Each stage a star goes through after hydrogen burning is quicker than the last. The helium-burning phase that Betelgeuse is entering is expected to last around 100,000 years.

Betelgeuse is clearly visible with the naked eye, but what might Auburn students who are taking astronomy classes be able to see by utilizing the Leach Science Center’s new state-of-the-art telescope terrace?

Individual stars like Betelgeuse aren’t the most interesting objects to observe with a telescope—they just look like points of light. Betelgeuse is a red supergiant star that is relatively close to Earth, so its main distinguishing features are that it has a red-orange color, and it is one of the brightest stars in the night sky.

The constellation Orion, however, contains many deep-sky objects that students can observe from the astronomy terrace. The brightest and most well-known is the Great Nebula in Orion, which showcases the opposite side of stellar evolution—young stars that are still in the process of forming along with their surrounding clouds of gas and dust. There are many other star clusters and nebulae in Orion, including the Horsehead Nebula and Flame Nebula, which are near one of the stars in Orion’s belt.

About Melissa Halford:
Melissa Halford is a lecturer in the Department of Physics at Auburn University. She received her Bachelor of Arts degree in astronomy from Cornell University in 2012 before attending the University of Arizona, where she received her Master of Science in astronomy in 2014 and her doctorate in astronomy and astrophysics in 2018. Her research focuses on star clusters and star formation, specifically looking for variations in populations of stars due to the different environments in which those stars form.