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The "Coal Sack"

Date:9.6.1994 Time:23:16 LT Exposure:61 min
Field of View:10.7o x 8.0o Emulsion:Scotchchrome 400 Filter:none
Instrument:f=100mm 1/4.0 Place: Cerro Tololo Observer:S. Kohle

© Copyright by the observers

In the southern constellation Crux is a large and nearly starless dark patch just lying in very rich star fields of the milky way. It is called the "Coal Sack" and can be seen already by naked eye. It is not really a direction without stars, there is a dark interstellar dust cloud obscuring the light of the stars behind and so appearing as a dark nebulae. Another well known dark nebulae is the Horsehead nebula.

Another astrophysical effect can be seen in the color image above. There are some extremely red stars in the northern and darkest part of the Coal Sack. The absorption of red light in interstellar dust is much smaller than that of blue light. So the starlight is reddened by such dust clouds. In the infrared part of the spectrum such dust clouds get nearly transparent. (see also the M 50 page)

In the optical spectrum such clouds can be detected often just because of this reddening effect. Most of this clouds are not placed so easy visible in front of rich star fields.