A Peek at the Glowing Red Shell Around an Aging Star

A large glowing red star that appears to have a red bubble around it
ALMA / ESO / NAOJ / NRAO / F. Kerschbaum

Most pictures of stars in our galaxy show them as tiny jewels against a sea of darkness. This one’s a little different.

U Antliae, one of the members of the constellation Antlia, is located in the southern hemisphere of the sky. About 2,700 years ago, the star expelled a bunch of gas at high speed, creating a thin shell around itself. Thanks to powerful telescopes, astronomers can now take a close-up picture of it.

U Antliae is a carbon star, which means its atmosphere contains more carbon than oxygen. As the star has aged, carbon has given it and its surrounding bubble a red hue.

Stars in their prime produce light through nuclear reactions in their cores that turn hydrogen into helium. But when stars start to lose their hydrogen reserves, their cores can become so hot that they transform helium into carbon. That carbon is ejected to the star’s outer layers, where it produces a material that scatters blue and green light. The red light persists, and eventually travels many light-years to Earth, where telescopes detect it and astronomers turn it into the mesmerizing image at the top of this story.

The photo, released Wednesday along with research in Astronomy & Astrophysics, comes from the the Atacama Large Millimeter/submillimeter Array, or ALMA. ALMA, a radio telescope stationed in the desert of northern Chile, is one of the most powerful observatories in the world. It’s built to measure the wavelengths of light coming from some of the most distant astronomical objects, including the very first stars and galaxies in the universe.

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Because ALMA can pick up different wavelengths, it can measure gas moving at different speeds and in different directions. The photo below shows gas moving toward Earth’s line of sight in blue, and gas moving away in red:

ESO / NAOJ / NRAO / F. Kerschbaum

Pretty trippy.

Astronomers have known U Antliae has a bubble of ejected mass around it, but they’ve never seen it in such fine detail. “It’s amazing that this has been captured ‘on film,’” said Anna Frebel, a Massachusetts Institute of Technology astrophysicist who studies the early stars of the universe, and who was not involved in the ALMA research. “We know that such mass loss has to happen, but seeing it like this is truly gratifying.”

The study of carbon stars and their shells can help astronomers learn more about stars’ life cycles. It can also contribute to our understanding of how galaxies form. Carbon stars spew a variety of chemical compounds along with carbon that can drift into the dusty space between stars.

“These stars quietly expel material into space, and with it bits of all the new elements made inside the star. This is particularly carbon, the most important element for life, but also heavy elements from the bottom of the periodic table, like strontium and barium,” Frebel explained in an email. “Because there are many, many of these types of stars, all together they are shaping the chemical composition of local regions within their host galaxies.”