Behold: the Horsehead Nebula – the most famous (and beloved) interstellar cloud in the sky. To wit:
It is visible as the dark indentation to the red emission nebula in the centre of the above photograph. The horse-head feature is dark because it is really an opaque dust cloud that lies in front of the bright red emission nebula. Like clouds in Earth’s atmosphere, this cosmic cloud has assumed a recognisable shape by chance. After many thousands of years, the internal motions of the cloud will surely alter its appearance. The emission nebula‘s red colour is caused by electrons recombining with protons to form hydrogen atoms. On the image left is the Flame Nebula, an orange-tinged nebula that also contains filaments of dark dust. Just to the lower left of the Horsehead nebula featured picture is a blueish reflection nebulae that preferentially reflects the blue light from nearby stars.
(Image: José Jiménez Priego)