Eagle Creek Observatory
“Teaching young minds about the heavens”
Why are Carbon Stars so interesting? What exactly is a Carbon Star and why are they so red?
I have always liked 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" or tell it to lie on it's belly and ...uh... you know... 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 so much so that they can fade completely from the view in an average backyard telescope. Some Carbon Stars are so red that they are difficult to see. They shine most of their energy in the 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.
|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° celsius (11,000°F). The hottest stars shine at hotter than 30,000° 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?
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? These stars fuse Helium mostly int Carbon, Silicon and Oxygen. Currents in the star's structure bring some of the Carbon, Silicon and Oxygen to the star's outer layers producing a "dust" in the star's atmosphere. Much of the dust in a Carbon Star's outer layers are Silicon Carbide (yes, the same thing that is used to make sand paper and some faux diamonds). There is also some Carbon Monoxide and Carbon Dioxide and just plain Carbon dust. 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 containing dust in their atmospheres scatter the blue light to make the look even redder.
For more on star colors click here.