All the colours of sky (continued)
|
| |
Fig. 1: The
spiral galaxy Messier
83: image of the neutral hydrogen (the 21 cm line). |
- Line emission:
a heated low-density gas emits energy in frequency bands
(spectral lines) which depend on the temperature and chemical nature of the
gas. The sequence of lines is so closely linked to the particular gas that
it constitutes an actual fingerprint of it.
Not only stars
Gas (mainly hydrogen), with a percentage of helium and traces of molecules such as carbon dioxide (CO), methane (CH4), ammonia (NH3) and water vapour (H2O), forms a very important component of the galaxies
Neutral hydrogen (HI) in particular, found almost everywhere, can be seen
from great distances in the spiral and irregular galaxies. In fact, at
temperatures of around 100-300
Kelvin degree
and at
densities of around 100 particles per cm3, neutral hydrogen emits a spectral
line with a wavelength of 21 cm, in the radio wave band (figures 1 and
2 show this emission for two spiral galaxies).
Neutral hydrogen is common in the galaxy and is fuel for the stars. It is much more extended than the stellar component of the galaxy.
Fig. 2: Optical image of
NGC 6946
. Thee
same galaxy at the wavelength of 21cm (blue).
|
|
( All the colours of sky- page 5 of 5) < Back | |
