9.4 Dust
185
Fig. 9.10 Photograph of the constellation Orion. The three stars on a row in the middle
form the so-called ‘belt’. The structure beneath it is the ‘sword’; its end is nebulous; this
is the Orion nebula of Fig. 9.11. From www.AlltheSky.com [81]
Electrons and protons attract each other. Protons (or atomic nuclei in general)
and electrons can therefore combine again to form an atom, releasing energy
in the form of light (this is called recombination). This then happens at very
specific wavelengths and can be seen as definite emission lines in the spectrum.
We see such light in the Orion Nebula. Because such nebulae are mostly ionized
hydrogen they are called HII-regions, HII being (singly) ionized hydrogen (HI
is neutral, unionized hydrogen).
However, not all structure we see in the Orion Nebula reflects the spatial
distribution of gas. There is dust, also between us and the nebula. The separation between the large Orion Nebula and the small one on the upper left is an
example of this.
Not all luminous nebulae are areas of hot gas like the Orion Nebula. In
Fig. 9.12 we see the Pleiades, which we saw also in Fig. 7.11, in which the
Hyades and the constellation Taurus were illustrated. The stars in this Pleiades
cluster have quite a lot of dust around them and the small particles of dust
185
Fig. 9.10 Photograph of the constellation Orion. The three stars on a row in the middle
form the so-called ‘belt’. The structure beneath it is the ‘sword’; its end is nebulous; this
is the Orion nebula of Fig. 9.11. From www.AlltheSky.com [81]
Electrons and protons attract each other. Protons (or atomic nuclei in general)
and electrons can therefore combine again to form an atom, releasing energy
in the form of light (this is called recombination). This then happens at very
specific wavelengths and can be seen as definite emission lines in the spectrum.
We see such light in the Orion Nebula. Because such nebulae are mostly ionized
hydrogen they are called HII-regions, HII being (singly) ionized hydrogen (HI
is neutral, unionized hydrogen).
However, not all structure we see in the Orion Nebula reflects the spatial
distribution of gas. There is dust, also between us and the nebula. The separation between the large Orion Nebula and the small one on the upper left is an
example of this.
Not all luminous nebulae are areas of hot gas like the Orion Nebula. In
Fig. 9.12 we see the Pleiades, which we saw also in Fig. 7.11, in which the
Hyades and the constellation Taurus were illustrated. The stars in this Pleiades
cluster have quite a lot of dust around them and the small particles of dust