by Ann » Tue Apr 10, 2012 6:21 am
This is a wonderful portrait of the complex nebulosity surrounding a young cluster of OB stars. All the red emission nebulosity is caused by hot blue stars, whose ultraviolet photons ionize hydrogen atoms, making them glow red.
As a "blue star freak", I always check out the star colors in any portrait of the night sky. The most important star here is, of course, S Monocerotis. S Mon is the chief ionizer of the nebulosity here. S Mon is also a very blue star, having one of the bluest B-V indexes of all naked-eye stars. Yet in this picture S Mon doesn't look particularly blue at all. Why is that?
The reason, as always, is the filters used. The image was made by combining 25 x 1200 sec Ha / 22 x 1200 sec OIII / 6 x 1200 sec SII / 1 x 300 sec RGB. Ha is of course red hydrogen alpha, OIII is blue-green oxygen emission and SII is red ionized sulphur. Most of the exposure time was spent on these "nebulosity filters". It is likely that there is not very much OIII emission in NGC 2264, for the simple reason that there is just too much gas and dust here, and OIII emission demands rarefied conditions. It is likely, on the other hand, that much or possibly most of the red nebulosity that we can see in today's APOD is red sulphur emission. SII emission is all but identical in color to Ha, but represents a lower degree of ionization, and is fainter. Without an SII filter suphur emission is rarely seen at all in RGB images.
Stars produce red sulphur emission, too, and it is a known fact that star colors get changed when an SII filter is used. That's why S Mon doesn't look blue in this image.
But the nebular "landscape" - or "skyscape" - is amazing.
Ann
This is a wonderful portrait of the complex nebulosity surrounding a young cluster of OB stars. All the red emission nebulosity is caused by hot blue stars, whose ultraviolet photons ionize hydrogen atoms, making them glow red.
As a "blue star freak", I always check out the star colors in any portrait of the night sky. The most important star here is, of course, S Monocerotis. S Mon is the chief ionizer of the nebulosity here. S Mon is also a very blue star, having one of the bluest B-V indexes of all naked-eye stars. Yet in this picture S Mon doesn't look particularly blue at all. Why is that?
The reason, as always, is the filters used. The image was made by combining 25 x 1200 sec Ha / 22 x 1200 sec OIII / 6 x 1200 sec SII / 1 x 300 sec RGB. Ha is of course red hydrogen alpha, OIII is blue-green oxygen emission and SII is red ionized sulphur. Most of the exposure time was spent on these "nebulosity filters". It is likely that there is not very much OIII emission in NGC 2264, for the simple reason that there is just too much gas and dust here, and OIII emission demands rarefied conditions. It is likely, on the other hand, that much or possibly most of the red nebulosity that we can see in today's APOD is red sulphur emission. SII emission is all but identical in color to Ha, but represents a lower degree of ionization, and is fainter. Without an SII filter suphur emission is rarely seen at all in RGB images.
Stars produce red sulphur emission, too, and it is a known fact that star colors get changed when an SII filter is used. That's why S Mon doesn't look blue in this image.
But the nebular "landscape" - or "skyscape" - is amazing.
Ann