The Allure of Carbon Stars

The carbon star R Leporis. Credit: Damian Peach

The carbon star R Leporis. Credit: Damian Peach

For visual observers with small optics, the colors of the deep sky range from subtle to nonexistent. Galaxies and nebulae cast too little light to stimulate the color-sensing cone cells in our retinas, so they appear pale gray-white or, in the case of a bright planetary nebulae, gray-green. Bright stars are a little more colorful. Betelgeuse appears clearly orange, even to the unaided eye, Rigel shines blue-white, and the showpiece double star Albireo in the constellation Cygnus shows off a blue-green primary and red-orange secondary in even the smallest telescope. Otherwise, star colors are quite subtle, especially to new stargazers. But there is one exception– carbon stars. These deep ruby-red stars, which dredge up nuclear soot from their innards, give off a striking glow that’s easy to see in a small telescope. See your first carbon star and you’ll want to see many more.

Nearly all carbon stars are red giants that have run out of hydrogen fuel in their cores. As the hydrogen became sparse, the core of the star compressed and grew much hotter, hot enough to start the process of nuclear fusion of helium into carbon and oxygen. The hotter core causes the star’s outer layers to expand, and within these layers form convective currents that dredge up carbon and oxygen into the star’s outer layers like bubbles churning in a pot of stew. In the cool outer layers of the star, carbon and oxygen atoms combine to form small molecules like C2, CH, and even SiC2.This material scatters blue light from inside the star and passes red light unscathed resulting in an enhancement of the star’s already reddish-color. This is the same process that reddens the sky during a sunrise or sunset in our sky.

Red giant stars are common, but carbon stars are much less so because the mechanism the produces them lasts for a relatively short span in the star’s late life. Carbon stars are also highly variable. Their brightness can change by many magnitudes over the course of hundreds of days. At peak brightness, many such stars are visible even in binoculars. At their lowest, they fade to near invisibility in small optics. The apparent ‘redness’ of the stars also varies with brightness and tends to be most pronounced in the middle to lower range of their brightness.

An image using millimeter-wave telescopes of the carbon star TT Cygni and a surrounding shell of CO molecules.

An image using millimeter-wave telescopes of the carbon star TT Cygni (center) and a surrounding shell of CO molecules. The outer shell has a radius of 1/4 light year.

One of the brightest and best-known carbon stars lies to the south of the feet of Orion in the constellation Lepus, the Hare. The star is cataloged as R Leporis, but it’s more commonly known as Hind’s Crimson Star or the Vampire Star because of its deep blood-red color (see image at top). You can find the star about 3° west of the blue-white star μ (mu) Leporis or Neshmet. When the star is within a few magnitudes of peak brightness, it’s easily discernible by its color relative to other stars in the field of view.

The Vampire Star lies about 1,100 light years away, which makes it one of the closest carbon stars. It’s highly variable, ranging in brightness over a 420-day period from magnitude 5.5, nearly bright enough to see without optics, to magnitude 11.7, faint enough to require a 6″ telescope to see. R Lep is nearing the end of its life as it burns the rest of its fuel in a shell around its core and begins to cast off its dusty and gaseous outer layers in a planetary nebula. In a few tens or hundreds of thousands of years, the core of the star will remain as a dim, hot, bluish-colored white dwarf.

The carbon star R Leporis lies south of Orion and about 3 degrees west of the star Neshmet (mu Leporis)

The carbon star R Leporis (in red circle) lies south of Orion and about 3 degrees west of the star Neshmet (mu Leporis)

The outer layers of the star are cool, just 2300 K, and highly dynamic, with strong winds blowing around molecules made of hydrogen, carbon, and oxygen atoms. In the sooty outer shell of the star, a powerful maser (a microwave laser) has self-assembled and blasts intense microwaves into space. The star is an astrophysicist’s dream.

When observing carbon stars like R Lep, you may experience the rather wonderful Purkinje effect in which the star appears to grow brighter the longer you stare at it. This makes it hard to accurately estimate the true brightness of the star, but it’s great fun for casual observers of carbon stars.

If you want to get an estimate of the current brightness of R Lep, go to the light-curve generator at the American Association of Variable Star Observers (AAVSO), enter the name ‘R Lep’ in the top line, then hit ‘Plot Data’. You’ll get a plot of estimated brightness of the star over the past 200 days.

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