Darkness and Light in the Milky Way – Late Summer in Sagittarius

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by Pat Browne

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Early September View of the Milky Way Star Clouds in Sagittarius – courtesy Stellarium Software

 

 



For Canadian observers, the constellation Sagittarius (nicknamed- the teapot), is visible along the southern horizon in September. It is a good time to catch last glimpses of the star clouds and clusters embedded in the galactic band of Milky Way rising at  an angle to our horizon like the steam from our teapot, as shown above. This area contains the most abundant and wide-ranging collection of star clusters, stellar nurseries and inter-stellar dust and gas. So it is not only the pinpoints of light that you see in the Milky Way (which  resolve into  individual suns with binoculars and telescopes), but also the obscuration of these features – seen as voids which are the measureless stretches of intervening dust within the plane of the Milky Way.

To begin our tour of the shapes and shadows, the lights and the darks in the Milky Way … we have to be quick and agile !

The Lagoon Nebula  (M8) and the Trifid Nebula (M20) are the first objects to observe and  clear nights in September offer the last occasions to see them.

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Later in the eveining The teapot starts to dunk below the horizon- taking some of the nebulae with it…

 

 

 

 

 

 

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CCD Image of M8 (Lagoon Nebula – left) and M20 (Trifid Nebula) right. Colours indicate energetic emission and reflection of starlight.
Image taken at FLO courtesy Oscar Echiverri

 

For Charles Messier, his  Catalogue of Celestial Objects helped him ignore these silhouetted shapes of starlight so that he could concentrate on his primary mission: to identify comets. M8 and M20 were simply markers to remind him that the these objects were not to be confused with the special and temporary appearance of a comet traversing inward towards our Sun. When comets do show up in our night sky, their surface brightness makes them appear like star clusters or nebulous patches of sky.

Star Clouds 

As we rise higher up with the ‘steam of the teapot’ above the giant star  star μ Sagitarii, we observe the Sagittarius Star cloud, containing dark nebulae and the open clusters M18, and Nebulae M17 and M16.

 

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Star Map courtesy Sue French, from the most excellent book Celestial Sampler, Chapter Floating on Cloud 24

 

 

 

 

 

 

 

 

In the knots and star clouds of the Sagittarius region, amongst  the swirls of unresolved stars, there are still patches that are overlooked… Why? Because there appears to be nothing there! These are the Dark Nebulae. .. not to be confused with Black Holes. Dark Nebula appear as voids through the telescope or through the Milky Way, but, they really contain interstellar dust, and are by no means empty!

 

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This shows  the ‘cloudlet’ M24 in the star background of the  Milky Way
 – courtesy SEDS database

 

Messier catalog entry M24 is not a “true” seperate , but a huge star cloud embedded in the Milky Way. It is described as a ‘pseudo-cluster’ of stars spread thousands of light years along the line of sight, perceived through a chance tunnel in the interstellar dust. They form a portion of a spiral arm of our galaxy. This cloud is the bright Milky Way patch slightly above the center of the image,  among many other Deep Sky objects (clusters and nebulae)

  • The interstellar dust generally dims the light of stars behind it. But the dust is patchy.
  • For some unknown reason it clumps in clouds typically 25 light years across: many such clouds can be clearly distinguished, projected against the star cloud.
  • There are typically two such clouds in a line of sight 1,000 light years long in the Milky Way. But even over the 30,000 light-years to the central regions of the Galaxy there could be, and by chance are, clearer windows [ more like a light at the end of a tunnel] than normal in the interstellar medium. M24 is in effect one of these windows.
  • These clear windows through the Galaxy have great significance in the study of galactic structure, since they make it possible to study otherwise hidden, distant regions (after Murdin/Allen/Malin’s Catalogue of the Universe, 1979).

This “dim cloudlet (for the naked eye) near Mu Sagittarii” was named “Delle Caustiche” by Fr. Secchi, “from the peculiar arrangement of its stars in rays, arches, caustic curves, and intertwined spirals.”Alternatively, M24 is often referred to as “Sagittarius Star Cloud”, or “Little” or “Small Sagittarius Star Cloud” (in contrast to the “Big” or “Large Sagittarius Star Cloud” which lies more to the south and consists of that portion of our Galaxy’s central bulge which happens to be not obscured by foreground dust). – courtesy http://messier.seds.org/m/m024.html

This wonderful description is provided courtesy of SEDS – Students for the Exploration and Development of Space. You can look up every entry in the Messier Catalog here: http://messier.seds.org/

Just what are Nebulae?

These are ghostly clouds of gas and dust residing in our Milky Way Galaxy.

  • Emission Nebulae (as seen in  Oscar’s Lagoon Nebula M8) shine on their own, as intense ultraviolet radiation from nearby stars excite hydrogen gas, causing it to flouresce
  • Reflection Nebulae (as seen in the right portion of Oscar’s Trifid Nebula M20 do not glow; tiny dust particles merely reflect the (blue) light. The red emission nebula with its young star cluster near its centre is surrounded by a blue reflection nebula which is particularly conspicuous to the northern end.
  • Then there are Dark Nebula… the cold dust and gas that absorb or scatter starlight. We infer their presence by the absence of light visible around them.
DarkNebulaM16
CCD image of theThe Star Queen Nebula, M16 – courtesy the author

 

 

 

 

 

 

 

 

 
Dark Nebulae around M16, the Star Queen Nebulae exhibit this ‘absence’ – the void around the cluster. The open cluster is embedded in a nebula of stellar dust and gas. This image taken last night from my observatory shows faint light patches – reflected light from hot young stars as well as the dark shapes, the dark nebula. As Sue French says in her chapter Floating on Cloud 24: “The most prominent backward ‘L’ of darkness, the Star Queen’s throne” … Can you see this in the upper right? … Just because it’s black doesn’t mean it’s a black hole!

Messier Object M16, like the Star Cloud M24,  displays voids of  ‘nothingness’   –  not the nothingness of empty space, but a space chock full of dust – material that obscures the light …Darkness and Light, Shapes and Void, ‘window’ and ‘blind’ are abundant when we look in the direction of Sagittarius.  To visualize the 3-dimensional view of our spiral galaxy, and appreciate the Milky Way with the NightSky Conservation program at the Mill of Kintail. see   Introduction to our Galaxy the ‘Milky Way ‘