The APOD photo for the above date shows galaxy M33 with red emission nebulae. Just a few days earlier, APOD featured the Pleiades cluster (M45), w blue reflection nebula. Am I missing something here? I thought higher-energy emission nebulae showed up in blue, and lower-energy reflection nebulae in red. I'd appreciate it if someone could set me straight on this.
http://apod.nasa.gov/apod/ap091017.html
Galaxy M33 with Emission Nebulae (APOD, Oct. 17, 2009)
Re: Galaxy M33 with Emission Nebulae (APOD, Oct. 17, 2009)
[b][color=#FF0000]Wikipedia: Emission Nebula[/color][/b] wrote:An emission nebula is a cloud of ionized gas (i.e. a plasma) emitting light of various colors. The most common source for ionization are high-energy photons emitted from a nearby hot star.
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The nebula's color depends on its chemical composition and degree of ionization. Due to the prevalence of hydrogen in interstellar gas, and its relatively low energy of ionization, many emission nebulae appear red due to the strong emissions of the Balmer series. If more energy is available, other elements will be ionized and green and blue nebulae become possible.
[b][color=#0000FF]Wikipedia: Reflection Nebula[/color][/b] wrote:In Astronomy, reflection nebulae are clouds of dust which are simply reflecting the light of a nearby star or stars. The energy from the nearby star, or stars, is insufficient to ionize the gas of the nebula to create an emission nebula, but is enough to give sufficient scattering to make the dust visible. Thus, the frequency spectrum shown by reflection nebulae is similar to that of the illuminating stars.
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Reflection nebulae are usually blue because the scattering is more efficient for blue light than red (this is the same scattering process that gives us blue skies and red sunsets).
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NoelC
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Re: Galaxy M33 with Emission Nebulae (APOD, Oct. 17, 2009)
Please don't confuse false coloring with visual coloring, nor star colors with nebula colors... In the cases of the images mentioned, they are represented in visual color (if not a bit enhanced).
The emission nebulae in the M33 image were clearly enhanced to show their presence, but the images showed M33 and the Pleiades reflection nebulae in visual color - i.e., what you'd see if you had really big eyes and essentially the same color sense you have now.
There are a few predominant colors in nebulae we see fairly often... Ionized hydrogen (often referred to as Ha) glows red, ionized oxygen (OIII) glows teal, reflection (dust) nebulae often reflect bluish light the best. What I think is the coolest thing is that the latter also sometimes look brownish and dirty when not so brightly lit. To wit, this image of the Iris Nebula in visual color:
The universe is certainly a spectacular place!!
-Noel
The emission nebulae in the M33 image were clearly enhanced to show their presence, but the images showed M33 and the Pleiades reflection nebulae in visual color - i.e., what you'd see if you had really big eyes and essentially the same color sense you have now.
There are a few predominant colors in nebulae we see fairly often... Ionized hydrogen (often referred to as Ha) glows red, ionized oxygen (OIII) glows teal, reflection (dust) nebulae often reflect bluish light the best. What I think is the coolest thing is that the latter also sometimes look brownish and dirty when not so brightly lit. To wit, this image of the Iris Nebula in visual color:
The universe is certainly a spectacular place!!
-Noel