Starship Asterisk*

Okay, you are on your way home, it's only five thirty in the afternoon but it is already pitch dark. Arrggh. November.

Right.Well, it has been foggy and rainy and what not for days and days on end, and you can't really remember the last time you saw the stars. It was a while ago. Now as you look at the sky, the color of it is, well, iffy. It hasn't got the thick dull dark orangish cast that is so typical of a completely overcast sky. It is comparatively blackish, but it doesn't seem clear, as if there was a high thin mist hiding the stars, aided and abetted by the light pollution, of course. Because there is nothing to see in the sky.

But wait. What is that peeking out from behind that six-storey hotel? Have they put a small needle-bright sort of spotlight on the roof?

Goodness. That thing is in the sky. What is it?

Okay, look at its color. It is whiter than the street lights and lamplights around here. Big deal. Street light and lamplights are so yellow. Check. The color of that thing in the sky is yellow-white. Make Pollux or perhaps Capella many times brighter and they would probably look like that.

So it isn't a supernova type Ia, at least. Those supernova are blue. Well, they are bluer than Sirius and Vega. You know you have seen Sirius peek out above that six-story hotel several times. It has been nowhere near as bright as that thing in the sky, but it has been so blue it made you jump. And those supernovae Ia are blue. You know you have paid attention to the color of supernovae Ia in color pictures. They have often been so strikingly blue. Besides, you read someplace that the color index of an unreddened supernova type Ia is about -0.01.
Typical supernova type Ia.

Okay, but couldn't this supernova, if it is a supernova, be reddened? Sure it could. It would probably be located in the thin disk of the Milky Way. And there could so easily be loads of dust between the supernova and us.

Of course it could also be a supernova type II, a core-collapse supernova. They are yellow. Remember how yellow the famous supernova 1987A looked in photographs.



Yellow supernova 1987A.

But if this thing that I can see in the sky this November afternoon is a supernova type II, isn't it too white? Shouldn't it be yellower? It is not that astoundingly bright after all, so it would be relatively far away and definitely located in the thin disk of the Milky Way - well, almost definitely. And if it is yellow to begin with, how come it does not look even yellower due to dust reddening?

Okay, hold your horses. Look again. That thing in the sky is shining with a rock steady light, see? Rock steady. You know what that means, right? Right.

Planet. That's what it means.

We can't see the disks of Venus, Mars, Jupiter and Saturn, but the light that reaches us from these planets don't originate at a single point. They originate at small disks. Therefore the photons that reach us from these planets follow different paths through the atmosphere. Therefore the photons don't encounter the same movements and instabilities in the atmosphere, unlike the light from bright stars which reach us seemingly from a single point, so that the photons encounter the same kind of "obstacles" through the atmosphere the whole time. Therefore bright stars twinkle. Therefore a supernova would twinkle.

That yellow-white light is in the south-eastern part of the sky, far away from the Sun, and it is as bright as a medium bright Venus. Duh. Good-bye, supernova. Hello, Jupiter.
Ann

Ann wrote:So it isn't a supernova type Ia, at least. Those supernova are blue...

Again, I'd like to point out that there are no really blue stars, and certainly no blue supernovas. Above about 5000K, almost everybody perceives a blackbody as white. It is only by comparison to a truly white source like the Sun that hotter objects are seen with a slight blue cast. At peak intensity (when you would expect to visually observe a SN) all types have a color index not far from zero. Local supernovas observed in the 16th and 17th centuries were described as white, and more recent supernovas that can be observed telescopically are seen as white. SN 1987A, which was bright enough to be seen visually, was also described as white. It is easy to believe that there are blue stars when we see images, but those images almost always misrepresent the saturation compared with actual human vision.

If we are lucky enough to observe a nearby supernova (not too near!) it will almost certainly appear white, or maybe just slightly red-cast from dust extinction or atmospheric extinction.

Chris, you insist on saying that stars are colorless, or white, and that blue stars don't exist. Of course there are no stars whose color is a saturated blue. But to me, some stars are obviously blue, and since their color indexes agree with me, I'll keep on saying that I'm right. And you will keep on saying that I'm wrong... oh well... Let me say, at least, that to me the color of that bright light in the sky was a very big help in identifying it as Jupiter. And if an unreddened supernova type Ia had appeared that was as bright as that, or brighter, its color in itself would have told me that it was a supernova. No kidding!

Ann

Ann wrote:Chris, you insist on saying that stars are colorless, or white, and that blue stars don't exist. Of course there are no stars whose color is a saturated blue. But to me, some stars are obviously blue, and since their color indexes agree with me, I'll keep on saying that I'm right. 8-) And you will keep on saying that I'm wrong... oh well... Let me say, at least, that to me the color of that bright light in the sky was a very big help in identifying it as Jupiter. And if an unreddened supernova type Ia had appeared that was as bright as that, or brighter, its color in itself would have told me that it was a supernova. No kidding! 8-)

I'm not saying you are wrong. I just think it is confusing to identify any stars as blue, when almost nobody with normal human vision would call them that. At most, a "blue" star has a very slight blue cast, and even seeing it as such requires very well trained vision, or a companion white source for comparison.

A local supernova, bright enough to be visible to the unaided eye, will almost certainly not appear blue, or even show a blue cast.

I looked at Jupiter again tonight. Definitely the color of Capella!

Ann