Date: January 17, 2009
Title: January’s Garnet Star
Organization: RapidEye Observatory - a private observatory in rural Lee County, NC
Description: A discussion about what are carbon stars, what they look like, and how to find winter’s nicest carbon star: R Leporis.
Bio: I’ve been captivated by astronomy ever since I was a kid, living in NW Colorado where the Milky Way was bright enough to read by. I can be found most clear nights in my pasture with either my 4.5″ Dob, 10″ Dob, or my binoculars.
Since January’s birth stone is the Garnet, it is appropriate that one of the reddest stars in the evening sky rides high in the south this time of the year: R Leporis.
Beyond its intrinsic beauty, R Leporis is interesting because it is a very nice example of a category of uncommon stars called Carbon Stars.
Late in a star’s lifetime its supply of Hydrogen starts to dwindle and it creates elements heavier than Helium such as Carbon, Nitrogen, and Oxygen. As an abundance of Carbon accumulates in the upper layers of the star, where it combines with itself and the other elements to create a sooty layer that blankets the star, obscuring much of its visible light.
Light that does escape the solar soot is primarily red and infrared, with very little blue or yellow light escaping. This leaves a star whose color is perceived as anywhere from a deep blood red to a lighter, salmon or poppy pink color.
Being late in their stellar lifecycle, most carbon stars are also pulsating variables and will raise and lower their brightness by easily observable amounts. According to the American Association of Variable Star Observers, or the AAVSO, R Leporis varies its visual magnitude between 11.7 at its minimum and 5.5 at its maximum: that is an increase in brightness of over 250X!!! An interesting effect of this variability is that our perception of a carbon star’s color will change throughout its light cycle. When near minimum, they will appear a deeper red than when near maximum. During this winter’s pass, R Leporis will probably be somewhere near magnitude 6.0 towards the top of its brightness curve. Having already observed it many times this winter, I can tell you that it is putting on a very nice show. Under modest telescopic power, it looks like a small sparkling red garnet on a black velvet background.
While all Carbon stars are red, not every red star in the sky is a Carbon Star. Some, like Mu Cephei, also known as “Herschel’s Garnet Star”, Alpha Scorpii, known as “Antares”, and Alpha Orionis, known as “Betelgeuse” are famous, dazzling, bright, red stars that are also late in their stellar lives; however, their spectral lines are quite different from carbon stars and their red color comes from a different mechanism. But that would be a topic for another podcast, and not this one.
A more detailed discussion about carbon stars and what distinguishes them from other red stars can be found in B.C. Stephenson’s “General Catalogue of Cool Carbon Stars”. The Warner and Swayse Observatory astronomer released his catalog in 1973, subsequently revised it in 1989, and a third revision was released in 2001 by six Latvian astronomers who’s names I wont even try to pronounce. Links to this paper will be provided in this podcasts’s show notes. The locations and brightness of almost 7000 carbon stars, many of which are within reach of amateur visual observers, can be found as an appendix to the paper.
Now how to find it. R Leporis can be found in the northern end of the border of the fainter and lesser known constellations, Lepus the Hair and Eridanus the River. Although Eridanus typically requires dark country skies to be seen, Lepus’ key stars should be visible south of Orion the Hunter from many suburban locations. When Lepus transits, to me, it appears as a lopsided figure eight laying on its side.
Tonight around 10PM local time Alpha and Mu Leporis form a line along the north west quadrant of the figure eight and serve as pointers to R Leporis. Simply follow the line created by the two brighter stars approximately one finder scope field of view west of Mu. Switch to a low power eyepiece and you should see the dim, dark red glow of R Leporis. Don’t worry, if you don’t see it right away, dark adapted vision doesn’t perceive color very well and the deep red color can easily be overlooked. Keep trying, the hunt is worth the effort! Take your time and study the star field before moving the eyepiece. Still no luck? Links to detailed finder charts can be found in this podcast’s show notes.
In the northern hemisphere, R Leporis is not the only dazzling carbon star easily visible to amateur astronomers. In small telescopes, T Lyra certainly rivals R Leporis in visual impact, even if it doesn’t have as wide of a brightness swing. While binoculars are required to see V Aquilae and S Scuti, both are very nice examples of the more pinkish flavor of carbon stars. From modestly dark skies, no optical aid is needed to see TX Piscium and U Hydrae, however binocular or telescopic magnification will certainly help in pulling out their red color. With 7000 to chose from, certainly at least one is visible from your observing location on any given night.
This podcast is dedicated to my wife who’s support for my astronomy hobby goes to the moon… and back. This week is her birthday and if the weather is clear tonight, I’ll show her January’s biggest Garnet!
January All Sky Map – Chart to find Lepus:
Lepus Detail Map:
http://seds.org/maps/stars_en/fig/lepus.html” R Lep AAVSO Detail Charts:
R Lep data:
Info about Stephenson’s catalog:
365 Days of Astronomy
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