Fundamental Astronomy

Fig. 15.15. Interstellar absorption lines in the ultraviolet spectrum
of ζ Ophiuchi. The strongest line is the hydrogen Lyman
α line (equivalent width, more than 1 nm). The observa-
The average gas density within about 1 kpc of
the Sun derived from the Lyman α observations is
0.7 atoms/cm 3 . Because the interstellar Lyman α line
is so strong, it can be observed even in the spectra of
very nearby stars. For example, it has been detected by
the Copernicus satellite in the spectrum of Arcturus,
whose distance is only 11 parsecs. The deduced density
of neutral hydrogen between the Sun and Arcturus is
0.02–0.1 atoms/cm 3 . Thus the Sun is situated in a clearing
in the interstellar medium, where the density is less
than one tenth of the average density.
If a hydrogen atom in its ground state absorbs radiation
with a wavelength smaller than 91.2 nm, it will be
ionized. Knowing the density of neutral hydrogen, one
can calculate the expected distance a 91.2 nm photon
can propagate before being absorbed in the ionization
of a hydrogen atom. Even in the close neighbourhood
of the Sun, where the density is exceptionally low, the
mean free path of a 91.2 nm photon is only about a parsec
and that of a 10 nm photon a few hundred parsecs.
Thus only the closest neighbourhood of the Sun can
be studied in the extreme ultraviolet (XUV) spectral
region.
The Hydrogen 21 cm Line. The spins of the electron
and proton in the neutral hydrogen atom in the ground
state may be either parallel or opposite. The energy difference
between these two states corresponds to the
frequency of 1420.4 MHz. Thus transitions between
these two hyperfine structure energy levels will give
rise to a spectral line at the wavelength of 21.049 cm
(Fig. 5.8). The existence of the line was theoretically
15.2 Interstellar Gas
tions were made with the Copernicus satellite. (Morton, D.C.
(1975): Astrophys. J. 197, 85)
predicted by Hendrick van de Hulst in 1944, and was
first observed by Harold Ewen and Edward Purcell in
1951. Studies of this line have revealed more about
the properties of the interstellar medium than any other
Fig. 15.16. Hydrogen 21 cm emission line profiles in the galactic
plane at longitude 180 ◦ ,90 ◦ and 1 ◦ (in the direction l = 0 ◦
there is strong absorption). The horizontal axis gives the radial
velocity according to the Doppler formula, the vertical axis
gives the brightness temperature. (Burton, W. B. (1974): “The
Large Scale Distribution of Neutral Hydrogen in the Galaxy”,
in Galactic and Extra-Galactic Radio Astronomy, ed. by Verschuur,
G.L., Kellermann, K.I. (Springer, Berlin, Heidelberg,
New York) p. 91)
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