Astronomic Nebulae

The colors in astronomical images provide information about the properties and chemical composition of astronomical objects in space.
Even if astronauts could theoretically get close to deep sky objects in space, their eyes could only see the visible portion of the radiation emitted.

However, telescopes can collect the radiation from astronomical objects and the ionized gases from deep sky objects. Filters in the beam path between the telescope and the camera chip separate the visible parts of the radiation and are recorded by the camera's recording chip.
In One-Shot-Color (OSC) cameras, a color filter is already vapour-deposited on the camera chip (Bayer sensor).
With the Mono cameras (black and white) mostly used for astrophotography, special astro filters are inserted into the beam path using a filter wheel or filter drawer.

The camera saves these visible wavelengths (= colors) as a colored astro image on the camera's memory card.
The colored astro image thus corresponds to the visible wavelengths of the ionized gases and chemical elements in space.

Stars are also usually colored in astronomical images. In addition to white, they are often bluish, yellow or red. These color variations depend on the temperature and the evolutionary stage of the stars.

Bluish stars
are usually very hot and young stars. They have high surface temperatures and emit mainly in the short-wavelength blue and violet regions of the electromagnetic spectrum. These blue stars are often found in regions of active star formation.

Yellow stars
are usually slightly older than the blue stars. They have a slightly lower surface temperature and therefore appear yellowish. Yellow stars are often found in star clusters and in the galactic disk.

Red stars
tend to be the oldest and coolest stars. They have low surface temperatures and emit mainly in the long-wave red part of the spectrum. Red stars can occur at different stages in their evolution, including giant stars and red giants.

It is important to note that the colors in astronomical photos are often enhanced or manipulated to emphasize specific information or to make the image more pleasing to the human eye.
The actual color of an astronomical object can be inconspicuous to the human eye or even outside the visible spectrum.
To make recorded data comparable and easier for us to interpret, coordinated color palettes such as the Hubble Color Palette are used.

Types of astronomical nebulae

A distinction is made between the following types of astronomical nebulae:

1. Emission Nebula
These nebulae are composed of glowing gas excited by energetic sources such as hot young stars. The atoms in these nebulae absorb the energy of the stars and re-emit it as visible light. Each atom or type of molecule has characteristic emission lines that lie at specific wavelengths of the electromagnetic spectrum. When these lines fall within the visible part of the spectrum, we can perceive them as distinct colors. For example, hydrogen in an emission nebula often appears red, while oxygen can appear green or blue.

2. Reflection Nebula
These nebulae are made up of dust particles that reflect light from nearby stars. The color of a reflection nebula depends on the composition and size of the dust particles. Typically, reflection nebulae appear blue because the dust particles tend to scatter blue light more than light of other colors. A well-known example of a reflection nebula is the Pleiades Nebula in the constellation of Taurus.

3. Dark Nebula
These nebulae are made up of dense interstellar dust that blocks light from stars behind. They appear dark or black because they do not transmit visible light. However, these nebulae can be indirectly illuminated by light from surrounding stars or emission nebulae, causing them to appear as high-contrast structures against the bright background.

3. Planetary Nebulae
Planetary Nebulae are astronomical nebulae composed of a shell of gas and plasma expelled from old stars at the end of their evolution, e.g. Owl Nebula Messier 974.

4. Molecular Clouds
Molecular clouds are interstellar gas clouds and are considered the birthplace of stars. More than half the mass of our Milky Way consists of molecular clouds. They consist mainly of molecular hydrogen (H2).