APOD: Zeta Oph: Runaway Star (2011 Feb 04)

[Beyond]

Not knowing much about dealing with colors and stars, since you're talking about red-blue-green filters, is it possible to make a 3-D picture of a star? And would it help at all, or just be a waste of time?

[Chris Peterson]

Not knowing much about dealing with colors and stars, since you're talking about red-blue-green filters, is it possible to make a 3-D picture of a star? And would it help at all, or just be a waste of time?

Only our own star.

[zloq]

In this Hubble palette image, the orange foreground star and the blue stars inside the nebula all seem to be more or less the same color:

Hi Ann-

That's a good example of what I'm talking about. The key thing to note about the image is that it has a full range of hues - from red to blue to green and many things in between. If the colors truly represented the raw emission lines in the three channels on the same scale, then it would be a huge coincidence that that all had about the same variation range so that no single channel dominated. In fact - the rosette, like many other emission nebulae, will have much stronger Ha than others - and in order to make the picture look this nice and color balanced, you have to scale down the green. When you do that, the stars - regardless of their temperature, will end up looking magenta.

The bandwidth of the filters plays an indirect role because it will alter the power received in each channel from continuum sources like stars. But if there is a gross rescaling of the green channel, then that is what will dominate the cause of the magenta.

"Hubble Palette" just means that Sii, Ha, Oiii have been assigned to R, G, B respectively - but it does not specify how those channels are manipulated before they are combined. The image you show looks like it was arbitrarily stretched to bring out rich colors in the result. Even so, if there is an overall suppression of the green, then you will get magenta stars. So - even though an image is shown in "Hubble Palette" - there could still be huge artistic license exercised in bringing out colors that don't really map to anything directly interpretable.

Whatever color manipulation is done to bring out the nebula, the stars just ride along with whatever chunks of their continuum that got grabbed by the filters. For Ha dominated nebulae and filters that don't have hugely different bandwidths, they will tend to end up magenta.

zloq