Fundamental Astronomy

ers first increase in the same way, but beyond some
limiting magnitude (10 in the figure) the number of
stars falls below that outside the cloud. The reason for
this is that the fainter stars are predominantly behind the
Fig. 15.8a–c. Bright and dark nebulae in Scorpius and
Ophiuchus. Photograph (a) was taken in the blue colour
region, λ = 350–500 nm, and (b) in the red colour region,
λ = 600–680 nm. (The sharp rings in (b) are reflections
of Antares in the correction lens of the Schmidt camera.)
The nebulae located in the area are identified in drawing
(c). B44 and H4 are dark nebulae. There is a large
reflection nebula around Antares, which is faintly visible
in the blue (a), but bright in the red (b) regions. Antares
is very red (spectral class M1) and therefore the reflec-
15.1 Interstellar Dust
nebula, and their brightness is reduced by some constant
amount ∆m (2 magnitudes in the figure). The brighter
stars are mostly in front of the nebula and suffer no
extinction.
tion nebula is also red. In contrast, the reflection nebulae
around the blue stars ρ Ophiuchi (B2), CD-24 ◦ 12684 (B3),
22 Scorpii (B2) and σ Scorpii (B1) are blue and are visible
only in (a). In(b) there is an elongated nebula to the
right of σ Scorpii, which is invisible in (a). Thisisanemission
nebula, which is very bright in the red hydrogen Hα
line (656 nm). In this way reflection and emission nebulae
can be distinguished by means of pictures taken in different
wavelength regions. (Photograph (a) E. Barnard, and (b)
K. Mattila)
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