By Bill Pellerin
Houston Astronomical Society
There are a variety of amateur observers – those who go to the darkest of sites with the largest of telescopes to see the dimmest of objects, and there are those who (like me), believe that the universe reveals itself in every object we see, including the bright ones. The poet William Blake talked about seeing the world in a grain of sand. If Blake can see the world in a grain of sand surely we can see the universe in a single star.
I write a monthly article for our club’s newsletter in which I provide the information needed to find a star or other bright object in the sky. Then I attempt to tell the reader why that object is interesting. The newsletters are available at: www.astronomyhouston.org.
So, let’s look at a bright star that’s easy to see with any telescope and which comes with a good story.
La Superba (Y CVn) is at RA: 12h, 45m, 08s / DEC: 45 deg, 26 m, 25 s in the constellation Canes Venatici. It is especially well placed for viewing in the spring and it’s easy to see. As this is written, in April, 2013, Y CVn is shining at about magnitude 5.0 (you can see a plot of magnitude versus time at www.aavso.org). The designation Y CVn tells you that it’s a variable star; its brightness range is from 4.8 to 6.3 with a period of about 160 days. It’s also a carbon star (see the Astronomical League Carbon Star observing program on this web site). Other parameters:
Distance: 710 light years
Variable type: SRb (Semi-regular)
Temperature: One of the coolest stars you can see at 2200 Kelvin
Mass: 3 solar masses
Size: Radius of about 2 AU or about 430 times the radius of the Sun
Composition: Significant concentration of Carbon-13
So, what’s a carbon star and why is it red? The answer to this question is what makes this star interesting. Mid-life stars are said to be on the main sequence, and main sequence stars fuse hydrogen (the most abundant element in the universe) to helium. The Sun is doing this now.
When the hydrogen supply runs low, and the star gets hotter it begins fusing helium to carbon at the core of the star. So there’s carbon at the core (the byproduct of helium fusion), with shells of helium and hydrogen surrounding the core. The energy of the star is generated at the core and that energy moves to the outside of the star mostly by convection, a process very similar to what happens in a boiling pot of water. You may have seen images of the Sun that show a granular pattern, most visible in the chromosphere; these granules are the result of convection in the Sun.
In the case of a carbon star, the core carbon is ‘dredged up’ from the center of the star and carried to the outside of the star. For this reason, when we look at the star we’re seeing some of this carbon.
The carbon-based compounds in the outer layers of the star absorb much of the radiation at shorter, bluer wavelengths enhancing the visual redness of the star. The star radiates much of its energy in the infrared (heat) that is invisible to our eyes.
This carbon at the ‘surface’ of the star acts as a blanket around the star causing it to get hotter. As it does, some of the carbon is burned off, the star gets brighter, and a new exterior carbon layer starts to form. This accounts for the variability in brightness that we see.
Carbon Stars are literally off the chart in color. The traditional color designations (OBAFGKM) for stars do not apply to carbon stars, so a special color category for carbon stars was created — ‘C’. This star falls into that category.
This is a low mass star and its final fate will be to become a white dwarf, cool off to a black dwarf, and disappear from view forever. On its way it may produce a planetary nebula, visible for only about 60,000 years.
The name 'La Superba' was attached to this star by Father Angelo Secchi, an Italian astronomer at the Pontifical Gregorian University in the mid 1800's.
Take a look at this star and check out the Carbon Star Observing Program here on the Astronomical League web site. My Observing Stellar Evolution program goes into more detail about how stars are formed and evolve, and ultimately die.