Why is this nebula red ?

[skysurfer]

Many nebulas are red (Halpha) like Eta Carinae, M42, NGC 7000, Tarantula, California nebula, etc.
I always thought the nebulas emit light from themselves (from what energy source ?).
In Wikipedia (https://en.wikipedia.org/wiki/Xi_Persei) I read that Xi Persei a hot O star lights up the California Nebula NGC1499, but how can it be red when lit by a star of 37000 K (bluish) ? Another example, the Witchhead just east of Rigel is whitish and it lit by the bluish Rigel which indeed is the right color.

Does somebody know this why NGC1499 is red and not bluish white ?

[mental4astro]

There are essentially two types of nebulae: reflection and emission. The mechanisms that cause the each type are different. But what they both share is the source of energy - either a single, but usually a cluster of stars that were created out of the parent gas and dust cloud that surrounds them. Without these stars, the gas and dust clouds would be invisible, resulting in the dark nebulae we see.

Reflection nebulae are the result of the starlight being scattered and reflected off the remnant dust and molecular carbon cloud - something like the scattering effect of our atmosphere giving us a bright blue sky. Much of the hydrogen that would be within these clouds has being blown further away behind the dust and heavier molecular cloud, so we see the distinct blue appearance of the dust and molecular carbon (molecular carbon refers to carbon compounds that had been formed out of earlier generations of stellar formation). The Pleiades is a good example. While the cluster is still young, it is old enough to have dissipated nearly all of the parent gas and dust cloud. Only the heavier dust and molecular carbon cloud remains close to these stars.

Emission nebula are caused by the ionisation of the hydrogen cloud by the intense energy of the young, massive stars that reside within the cloud. The effect if essentially the same one that gives us neon signs: The energy given off by the young hot stars rips off electrons from the hydrogen, and when these electrons recombine with the H atoms, a photon of energy is given off that we see as red light. With neon signs we can control the colour of the sign, but the same mechanism happens here. But with a nebula like NGC 1499, Hydrogen is the predominant gas, so the x-rays that ionise the cloud will only result in light in the red end of the spectrum being visible.

Why wouldn't it be a reflection nebula or reflection & emission? I would say it has to do with the relative density of hydrogen within the parent cloud being enough to overcome the scattering effect of the dust and molecular carbon within it. There are nebulae that have both emission and reflection components, such as M20. This would suggest that there are older and newer clusters within the cloud that would be responsible for the variation in hydrogen density within it, resulting in both reflection and emission areas.

The gas cloud that Rigel formed out of has long stopped being dense enough to produce stars. Now all that remains visibly is the remnant dust and molecular detritus that is being pushed away from Rigel by its stellar wind. Hydrogen being so light weight has been pushed away faster than the heavier dust and molecular carbon, so we get to see only the scattering effect of this dust and molecular carbon.

I hope this helps.

Mental (I feel a little that way now... :lol:)

PS, planetary nebulae are also emission nebulae, but I haven't mentioned them above as it would have complicated the explanation. But these glow at other wavelengths to red emission nebulae due to the different gases within the parent star's atmosphere that is being ionised.