by NoelC » Sat Mar 24, 2007 1:15 am
A small point, but worth mentioning...
Note the caption text: "The red glow in the background results from IC 1396, a large emission nebula..."
What they're saying is that the light reddish-brown colored area shown in the APOD is mostly light from glowing gas (e.g., a mix of ionized hydrogen, oxygen, sulfur, etc.), not dust. The dust is showing up as dark areas in front of the molecular gas. Energetic starlight ionizes the gas and makes it glow. This is called an emission nebula. It's shown reddish-brown instead of deep red in the image simply because that's how the person doing the image processing chose to represent it. This sometimes happens with images created from only red and blue filtered exposures.
Here's another rendition showing more the true visual deep red color of the area.
There are nebulae in the sky where the dust lights up due to reflected starlight - this is known as a reflection nebula. Most of the nebulosity around the
Pleiades cluster is one such example. Dust tends to disperse blue light best, so dust reflection nebulae generally look blue.
-Noel
A small point, but worth mentioning...
Note the caption text: "The red glow in the background results from IC 1396, a large emission nebula..."
What they're saying is that the light reddish-brown colored area shown in the APOD is mostly light from glowing gas (e.g., a mix of ionized hydrogen, oxygen, sulfur, etc.), not dust. The dust is showing up as dark areas in front of the molecular gas. Energetic starlight ionizes the gas and makes it glow. This is called an emission nebula. It's shown reddish-brown instead of deep red in the image simply because that's how the person doing the image processing chose to represent it. This sometimes happens with images created from only red and blue filtered exposures. [url=http://starmatt.com/gallery/astro/ic1396.html][u]Here's another rendition[/u][/url] showing more the true visual deep red color of the area.
There are nebulae in the sky where the dust lights up due to reflected starlight - this is known as a reflection nebula. Most of the nebulosity around the [url=http://www.ourdarkskies.com/gallery2/d/762-2/M45_The_Pleiades_Deep.jpg][u]Pleiades cluster[/u][/url] is one such example. Dust tends to disperse blue light best, so dust reflection nebulae generally look blue.
-Noel