The Coal Sack

Designation:    Dark Nebula Caldwell 99
Constellation:Crux
Apparent size: 375 x 250 arc-min (that’s big! the Moon is 30 arc-min).
Diameter: 53 light years.
Distance: 490 light years.
Exposure:Canon 60D, 50mm fixed lens, 1 x 15 seconds, f/2.5, ISO 3200.
Date: 2016-05-03
Field of View:26.6° x 17.7°
Orientation:Up is 87.1° E of N.
# of stars:Count them yourself! 🙂

Looking like a hole in the Milky Way, the Coalsack is actually a huge dark nebula, which is neither a reflection nebula nor an emission nebula. It can be seen as a silhouette against the Milky Way star fields without optical aid in dark skies.

Probably the most prominent dark nebula in our skies, it is somewhat puzzling that – until the creation of the Caldwell Catalogue in 1995 – it had no identification number, not even an NGC listing – and is not even included in the Barnard List of 366 Dark Nebulae.

For goodness sake, the least they could do is add it as Barnard 367!

The Coalsack is too big to fit in the field of view of most telescopes. I took this single frame image four years ago with a DSLR on a tripod mount, probably with my Star Adventurer which has a motorised RA axis.

Prominent stars in the image include:

Top: Alpha Crucis (Acrux) and Beta Crucis (Mimosa) in Crux.

Bottom: the two Pointers, Alpha Centauri (Rigil Kentaurus) and Beta Centauri (Hadar) in Centaurus.

In retrospect, I probably should have made this object no 8 in my list of the Seven Best Southern Sky objects!

9 Comments

    1. Good question. I’ll answer that by first mentioning what causes nebulae to be bright. There are two types of bright nebulae: emission nebulae and reflection nebulae.
      An emission nebula consists of a cloud of ionised gas which is excited by ultraviolet light from an embedded massive star. This causes the excited atoms in the cloud to emit light of a very specific wavelength, depending on its composition (e.g. hydrogen).
      A reflection nebula is visible because it simply reflects the full spectrum of light from a nearby bright star.
      So those are the two methods which enable us to directly see nebulae.
      When a cloud of gas is neither emitting nor reflecting light, it remains dark and unseen. The only opportunity to visually detect them is when they are located in front of the Milky Way star fields, when they are seen as a silhouette, blocking the starlight beyond.

      Liked by 1 person

  1. Hi, a couple of random thoughts on this.
    Wonderful photo!!!!
    I’d love to see this for myself some day, but it’s so far from NYC.
    I like to tell our guests at star parties that when they notice the dark lanes in Cygnus, they are looking AT something! The ‘hole’ in the sky is dust that blocks the stars. I love that ‘seeing’ something that you can’t ‘see’.
    Question: I heard different answers to this – who dense is the dust? What would it look like flying through it. I should check this out myself (well, not directly, it’s an even longer trip!). all the best, be safe, bob k ardsley, ny

    Like

    1. It is my understanding that nebulae in general are not very dense, except in the regions where gravitational collapse is occurring and star formation processes are under way – which does not seem to be occurring in the Coal Sack.

      You can see some stars inside many nebulae and it is my suspicion that the Coal Sack is probably not as dense at it may look. It blocks starlight from the far side because it is big not because it is dense. So your flight through it might notice a dimming effect once inside but you would probably still be able to navigate through it. Be sure to let me know when you get back!

      Liked by 1 person

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