What is an F-star?

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The recent Spitzer observations of epsilon Aurigae support an ongoing view that the central object is an F-class star. What does this mean?

One way to look at it is to compare it with our Sun, a star we all know and love (except for maybe a Texan road crew in August). Astronomers like to use the symbol to represent the Sun. For example, R  is the equivalent to the radius of the Sun. Here is one recent set of measurements we can go on (since it is coauthored by the CS chief scientist, I'm obliged to believe it):

Mass Luminosity Radius  Temperature
2.2-3.3 M* 48,000+ L 130-140 R*  1.3 T*

*From Hoard, Howell & Stencel (2010)
+My estimate, the Sky & Tel article (Naeye, 2010) quotes 130,000 if it is a supergiant.

So the table above tells us that the F-star is about 2-3 times more massive than our Sun. It is also ~135 times larger in radius, meaning that it has to be much less dense than the Sun. The temperature is more-or-less similar to our own Sun, yet it is quite a bit brighter! Putting all it together we get something like this:

These are two artistic renderings of the system. The left is from NASA/JPL the right is an illustration by CS participant Brian Thieme based on early ideas of the system. Notice something similar about the F-star in both images? It's yellow! What else is yellow? Our Sun! (well some argue it's green, but our point remains...) That's because the stars have similar temperatures and temperatures drive the color.

How did it get so large? The star has probably exhausted much of its supply of hydrogen and has begun fusing denser elements (helium, then carbon, oxygen, etc.). This extra energy creates a wind that puffs out the outer layers. If epsilon Aurigae were placed in our Solar System it would engulf Mercury and Venus (one day our Sun will do this too - except it will be red at the time, not yellow, thanks to having less mass). Yellow supergiants aren't incredibly common. Yet, because there are so many stars in the sky - there are still a handful currently known. One of them is our friend Polaris - the north star. But not all yellow supergiants are the same. Polaris, for example, pulsates to a very steady beat. And we know that our F-star likes to also change in brightness - but we've yet to find any beat (period) to its rhythm. That's one of the many things CS participants can help us with as we move into the data analysis stage of the project in 2010.

One of the big unknowns is whether it is a supergiant or a main sequence (a.k.a. stellar "middle age") star. Main sequence stars have a stage near the end of their lifespan where they take on the properties of supergiants. See the problem? We don't know if the star is a main sequence star just going through "a phase" or whether it is inherently a supergiant. Problems like this keep food on the table of astronomers.

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Then again, are F-type stars really yellow? Going with the website What color are the stars?, spectral type F seems to range from very slightly yellow at F8 ranging to pale bluish for F0 types, and the Epsilon Aurigae primary usually seems to be described as F0, though the list of measurements on SIMBAD gives a range A8-F5. So should the primary star be rendered with a bluish colour?

That is a very interesting web page. Thanks for posting it! Maybe the difference between the F0-F8 is close enough that it falls within the subjective judgements Mitchell made when establishing his colors. For example, the white zeropoint and the gamma correction both have big influences on the color (according to his "details" page). So the colors are best viewed as a relative approximation more than an absolute one. Still, I'd take his colors over anyone else at this point. I'd be interested in hear what Brian thinks about the "yellowness" in the illustrations of eps aur. Here is a cartoon on the subject someone recently sent us:

Dr. Bob and I both think the star looks green, but then again everyone's eyes see things differently.

You're not lying about our sun here in Texas in August, been over 100 degrees the past few days and today was 106! So you think the star is green? Then riddle me this... lol Very cool link, thanks for sharing!

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