Eagle Creek Observatory
"Teaching young minds about the heavens"

Carbon Stars


Why are Carbon Stars so interesting? What exactly is a "Carbon Star" and why are they so red?

I have always like to look at strange objects in the night sky and something like a "Carbon Star" seemed pretty strange to me. I like to see colors and stars are the only object that show any hint of color in a modestly sized telescope anyway.

Stars show different colors because they are different temperatures. How do scientists take a star's temperature? You can't tell a star to "open wide and say ahhhhh" so how do we take the temperature? It's the colo rs of the stars that tells the temperature. The colors range from bright blue to red as they vary from hottest to coolest.

So what does this tell us about Carbon Stars? It tells us that they must be very cool. Very cool usually means very old or very large, sometimes it means both. Also, very old sometimes means that the stars are variable stars. Most Carbon Stars are variable stars and some vary greatly in brightness over just a few days but most vary over months or years. Some Carbon Stars are so red that they are difficult to see. They shine most of the energy in their infra-red.

So where can I see these Carbon Stars? I'm glad you asked. Here is a table of the "reddest" Carbon Stars that I have observed. If there is a camera icon click on it for my photograph if this star.

Where the spectral type is followed by "..." the star is a variable and the spectral characteristics change.
Name Const SAO Magnitude RA Dec Spect
VX And And   7.5 00 19.9 +44 43 N7...
V Aql Aql 142985 6.5 19 04.4 -05 41 C5...
UU Aur Aur 59280 5.5 06 36.5 +38 27 C5...
V466 Cas Cas 22188 10.7 01 19.9 +58 18 M2
Mu Cep Cep 33693 2.5-5.5 21 43.5 +58 47 M2...
V Crb Crb 64929 4.4 15 49.5 +39 34 N2...
V Cvn Cvn 44564 8.8 13 19.5 +45 32 M4-M6
RS Cyg Cyg 69636 7.7 20 13.4 +38 44 C5...
LW Cyg Cyg   4.2 21 55.2 +50 30 R3...
R Dor Dor 249066 7.0 04 36.8 -62 05 M8
UX Dra Dra 9404 6.0 19 21.6 +76 34 C5...
U Hya Hya 156110 6.8 10 37.6 -13 23 C2...
V Hya Hya 179278 5.5 10 51.6 -21 15 C9...
R Lep Lep 150058 5.5 04 59.6 -14 48 C7-C6
T Lyr Lyr 67087 7.3 18 32.3 +37 00 C8...
V Pav Pav 244964 10.0 17 43.3 -57 43 C+
RS Per Per 23257 11.0 02 22.4 +57 07 M4
19 Psc (TX) Psc 128374 5.0 23 46.4 +03 29 C5...
R Scl Scl 193122 9.6 01 27.0 -32 33 C...
SS Vir Vir   4.2 12 25.2 +00 46 C2...

So, why are Carbon Stars so Red? Well, there are two reasons why they are so red.
First, they are very cool when compared with other stars. Most Carbon Stars are cooler than 3500 degrees celsius. That's cool? Yes, when you are comparing stars that is very cool. Our sun, a G-type star, shines at about 6,000 degrees celsius (11,000F). The hottest stars shine at hotter than 30,000 degrees celsius.

Second, their atmospheres are full of "dust." Dust? What kind of dust? Carbon Dust, what else? Actually most of the "dust" is in the form of Carbon Monoxide and Silicon Carbide, yes the sand-paper stuff. This is why they are called "Carbon Stars".

So where does all of this carbon come from?
Good question!
Here's the answer.

Carbon Stars are very old. They are stars that have used up most of their Hydrogen fuel by converting it to Helium. Once a star converts most if its fuel to Helium it begins to colapse under the force of gravity. The core of the star begins to heat up even more under the pressure of gravity. As the Helium in the star's core heats up it begins to "burn" or fuse just like the Hydrogen fused to produce the Helium. What does Helium fusion create? Carbon! Currents in the star's structure bring some of the Carbon to the star's outer layers producing a "dust" in the star's atmosphere. What does dust do in Earth's atmosphere? It makes the sunsets turn red by scattering the shorter wavelengths (blue, green, yellow) allowing the redder wavelengths to pass through. This is what happens in a Carbon Star's upper layers too. The shorter, more blue, wavelengths are scattered and reflected back and the longer, redder, wavelengths are allowed to pass right through. So the Carbon Stars LOOK redder. Not only are they red because they are cooler than other stars but because the Carbon dust in their atmospheres scatter the blue light to make the look even redder.
 
For more on star colors click here.



Back

2001-2005 Kevin Muenzler, Eagle Creek Observatory.