Many experienced stargazers are connoisseurs of carbon stars, deep-red and highly evolved stars that are dredging up carbon and other nuclear reactants from their innards on their way to becoming, briefly, planetary nebulae. These striking stars are the most colorful of all celestial sights and they’ve long intrigued astronomers who are trying to fine tune their theories of how stars come to the end of their lives. Carbon stars are too far away to reveal much detail directly, so astronomers study them indirectly by examining their spectra. But a team of researchers at the remarkable ALMA telescope in northern Chile have captured an amazingly beautiful and revealing image of a carbon star in the constellation Antlia. Let’s have a look at what they saw, then set some time aside to go see a carbon star for ourselves with a small telescope or pair of binoculars.
Astronomers have long known that carbon stars cast shells of gas as they evolve. But they wanted to capture an images of this process. So they turned the ALMA telescope array to the carbon star U Antliae, a relatively bright carbon star that’s easily visible in the southern sky from March through July. In optical telescopes, this star is just a point of light. But using ALMA’s razor sharp resolution, astronomers captured a detail image of a shell of gas ejected by the star some 2,700 years ago (see image at top). This was an early round of mass ejection by the star, which, as its core heats up as it desperately burns the last of its nuclear fuel, will start to eject nearly all its outer layers as a planetary nebula. Future astronomers will see U Antliae as an object that resembles the Helix Nebula or the Dumbbell Nebula, for example. After showing off as a planetary nebula for a hundred thousand years, the core of U Antliae will settle down as a white dwarf star, a dim but blazing-hot crystalline cinder that slowly radiates its heat into space for a hundred billion years.
Astronomers knew that U Ant had a gas shell, but they’ve never been able to see it directly. And while the image of the ejected gas shell is impressive enough, the ALMA scope produced another image, a false-color image that reveals the direction and speed of the gas shell towards and away from Earth. In the image below, the blue and green sections of the nebula are moving towards us, while the yellow, orange, and red sections are moving away from our point of view.
It’s pretty impressive.
But this website is about seeing celestial sights for yourself. So let’s go see a carbon star. U Antliae itself is visible in the little constellation Antlia, the Air Pump, south of Hydra, in the months of March through July, roughly, so we can’t see it this month. The map below shows its location in the event you want to inspect the star next year when it returns to the sky. The deep-red star varies irregularly from magnitude 5.3 to 6.0, so it’s still plenty bright enough to see in a pair of binoculars. The star has swollen to 350 times the radius of the Sun and shines nearly 8,000 times brighter than the Sun. U Antliae lies about 900 light years from Earth.
If you’re keen to see a carbon star now, then TX Piscium is your best bet. This carbon star lies near the distinctive asterism called the Circlet of Pisces near the ecliptic. TX Psc varies in brightness from magnitude 4.8 to 5.2, which makes it easily visible in binoculars or any telescope. It’s not as deep red as some carbon stars such as R Leporis, the Vampire star, but you will still see a remarkable red-orange color in this star. TX Psc has swollen to some 240 times the Sun’s radius. It lies 760 light years from Earth. Go see it!