I have been asked several times if the colors presented at astroimages are real or natural, or they are just an effect of processing and author imagination. Well, it depends. They can be natural and only enhanced, but they can also be totally artificial. So what are the deep sky objects colors? How do they look in real?
There are several ways the color can be born in space. One light source are stars. Stars are colourful. Visible star color depends on its surface temperature. Coldest stars are red (when surface temperature is about 3500K or less), then when temperature increases stars become orange (3500-4500K), yellow (4500-5500K), yellowish white (5500-7000K), white (7000-9000K), blueish white (9000-25000K) and blue (over 25000K). Stars actually radiates continuous spectrum, so all colors are present there. But depending on star surface temperature the maximum amount of energy is radiated in different part of spectrum. For cold stars it is somewhere in the red or infrared part, and for hot stars maximum amount of energy is radiated in blue spectrum part, or even ultraviolet. Star color is well approximated with black body radiation, so it is well described by so called black body locus, or Planckian locus.
Using unaided eye you can spot the colors only for brightest stars like Antares, Vega, Betelgeuse and similar. When you use binoculars or telescope, then more and more stars become colorful. When you have a chance try to point telescope to one of the carbon stars (like La Superba in Canes Venatici). These stars have really low surface temperature and are obscured by clouds of carbon molecules and they have deep red hue. Another interesting gems are double or multiple stars with components of different temperature, hence different color. Using small binocular you can view Albireo star as example, but there are many others:
So these are stars, what about other deep sky color objects? We have nebulae, three main types: emission, reflection and dark. Emission nebulae are a clouds of gases that are located nearby very hot stars. These stars need to be so hot, that they radiates much of ultraviolet light. And that high energy radiation excites gas molecules to shine in the very specific colors. This color is determined by gas composition, and most often in emission nebulae we can see (in the visible part of spectrum) radiation that comes from hydrogen, oxygen, nitrogen, sulphur and helium:
All these color lines are available for deep sky imaging, however for visual observing they are not. When we observe at low light level we are using rod cells (scotopic vision), and rod cells are insensitive to red light. That is why we cannot see red hue that comes from H alpha line. But in the few brightest emission nebulae (like M42 or bright planetary nebulae) we can see cyan / blue hue when viewing them with telescope. That is also why there is no H alpha filter for visual observing – we have only O III and H beta.
Imaging deeps sky objects in these narrow emission lines are pretty popular among astronomy amateurs – it is called narrowband imaging. However the light collected from emission lines are often mapped to completely different colors. Probably you all see beautiful astroimages of emission nebulae from Hubble Space Telescope. Many of them have been mapped using so called HST palette, where H alpha signal is mapped to green, O III is mapped to blue and S II is mapped to red, so it is far from natural hues. This kind of mapping enhances contrast in the image and reveals local differences in nebula composition, but they are far from natural look.
Another nebulae type is reflection nebula. This is a cloud of dust that happened to be close to a star. The light from the star is reflected from dust cloud particles and some of this reflected light travels to us, so we can see it. Pure reflective nebulae have the color of the illuminating star. When dust cloud is not illuminated by any nearby star, then it may be a dark nebula. It is dark, but can be observed if there are any light sources behind the nebula. There are many dark clouds in the space, but most of them are observed at the Milky Way. Actually reflection and dark nebulae are pretty the same in terms of how they are build. The only difference is the distance between nebula and nearby stars. If they are close – we have bright reflection nebula. If they are more far away, then nebula becomes fainter. And in extreme case, when closest star is very far away, then nebula is not illuminated at all and becomes invisible, unless there is something bright behind it. In these cases when we observe stars “through” dark nebula the stars are more red. This phenomena is called interstellar reddening. Dark nebula (and interstellar matter in general) absorbs and scatters blue light more than red making stars appear redder than they are.
There are rare cases where we can observe pure emission or pure reflection nebulae. Most planetary nebulae are pretty close to be pure emission. However in many other cases we view nebulae as complex objects that consists of emission, reflection and dark regions as well. That is why they are so magnificent and colorful objects at astroimages. And the colors are real, however these objects can be faint, very faint, so we cannot observe its hue visually, because at low light levels we cannot recognize colors. But they still are there.
And what about other deep sky objects like galaxies or star clusters? These objects actually are built of stars, so its color is an average of all components. Unless you can separate stars, then you will see each star color.