The Small Sagittarius Star Cloud
Posted by Brian Ventrudo
Today, an excerpt from the new Stargazer University guide “What To See In A Small Telescope”. In this short sample, you get a look at Messier 24, also known as the Small Sagittarius Star Cloud. This scintillating patch of millions of stars is not a star cluster, but a gap in the dust lanes of the Milky Way that gives you a glimpse of a distant arm of the Milky Way. It is an astonishing sight…
Let’s begin the tour of Sagittarius with what’s called the “Small Sagittarius Star Cloud”, M24. M24 is unlike any other entry in Charles Messier’s famous catalog. While it looks like a rich open star cluster, M24 is simply a distant cloud of stars in a spiral arm of the Milky Way.
To be more specific, M24 is a small section of the Sagittarius Arm of our galaxy that’s clearly visible because we’re looking through a gap in the obscuring dust along the plane of the Milky Way. If there was no dust or cold gas, the entire Milky Way Cygnus from Scutum and into Sagittarius (and beyond into Centaurus and Crux in the southern hemisphere) would appear as bright and luminescent as M24.
Dark sky is essential to see M24, and indeed all of the Milky Way. M24 appears as a brightening in the Milky Way just 4 degrees north of mu (μ) Sagittarii, and spans a fairly small 1°x2° rectangular patch of sky. It is splendid in binoculars and in a telescope with a field of view of at least 2°. The individual stars range from magnitude 6 down to invisibility in a small telescope.
The cloud appears to shimmer and take on a 3-dimensional quality in a good scope, and some observers see the color as blue or even green. As you gaze at the star cloud, you may also see a network of dark lanes and channels as your eye and brain try to make sense out of the profusion of patterns formed by thousands of sparkling stars.
M24 is over 330 light years wide and lies a fairly distant 9,400 light years from Earth, nearly as distant as some globular clusters.
M24 has within it several dark nebulae, and the bright background of the cloud make these nebulae easier to observe. Perhaps the most straightforward to see is Barnard 92 (B92), a 1/4-degree oval of darkness along the middle of the northwestern edge of M24. Once you find B92, examine it with a range of magnification to examine the numerous bright stars around it, as well as a single lone foreground star apparently embedded in the nebula.
If you can spot B92, try for B93 just 1/3 of a degree to the northeast. This nebula is narrower than B92 but nearly as long. It’s a little trickier to see because its borders are less defined. The image at the top of the page shows you the star cloud along with the two dark nebulae…