PDA

View Full Version : What color is Alioth?



AlanMintaka
2009-Oct-16, 02:14 AM
Hey Everyone,
Here's a question that's been bugging me for some time: what color is Alioth, Epsilon Ursae Majoris?

The spectral type is listed in various places as A0pCr, where the (p)eculiar aspect is the presence of strong Chromium(Cr) lines in the spectrum. It has a B-V index of -0.02.

That's spectrum color talk, of course. When I look at Alioth in a small telescope, it looks brilliant white in color.

What I'm wondering is if the presence of Chromium in Alioth would affect its visual appearance if it were magnified enough to show a disk. Would it still appear brilliant white, or would there be a subtle effect from the Chromium that would give it some kind of slight coloring other than white?

There must be better ways to ask that question. I can't think of any, but one problem at a time: what color is Alioth if seen as a disk?

Thanks for your time and patience,

StupendousMan
2009-Oct-16, 09:43 AM
The "strong" spectral lines of chromium in this star are much too weak to affect the apparent color of the star in any significant way when a human looks at the star with his eye.

You can see a spectrum of the star at the Elodie archive. For example, go to

http://atlas.obs-hp.fr/elodie/E.cgi?c=i&z=vs&o=elodie:19960204/0022

The spectrum shows a peak at the blue end and declines towards the red end, like an ordinary A star. The very strong absorption lines you see are due to hydrogen, as in an ordinary A star. The chromium lines are much weaker -- for example, there's a chromium line at a wavelength of 4558 Angstroms; does it stand out?

AlanMintaka
2009-Oct-24, 11:34 PM
The "strong" spectral lines of chromium in this star are much too weak to affect the apparent color of the star in any significant way when a human looks at the star with his eye.

You can see a spectrum of the star at the Elodie archive. For example, go to

http://atlas.obs-hp.fr/elodie/E.cgi?c=i&z=vs&o=elodie:19960204/0022

The spectrum shows a peak at the blue end and declines towards the red end, like an ordinary A star. The very strong absorption lines you see are due to hydrogen, as in an ordinary A star. The chromium lines are much weaker -- for example, there's a chromium line at a wavelength of 4558 Angstroms; does it stand out?


Sorry for the delay in this response, StupendousMan.

I want to thank you for that link. I didn't know that this detailed spectra data was available online; or I should say, I wouldn't have known where to look for it.

I see what you mean about that Chromium line. If you hadn't told me where it was, I never could have picked it out.

Good stuff!

Thanks again,

George
2009-Oct-26, 02:19 PM
Hey Everyone,
Here's a question that's been bugging me for some time: what color is Alioth, Epsilon Ursae Majoris?
My guess is that it would look bluish-white, especially in the central zone of the disk, if you could observe its disk at a reduced level conducive to the eye. At ~9400K, a Planck peak intensity would be around 310nm (UV), and its photon flux peak is about 390nm (far violet). The red flux is not quite half that of the blue end of the spectrum, so I suspect blue would be favored somewhat.