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M7 is
located just off the stinger stars of the far southern constellation
of Scorpius, now riding high due south on July evenings. A favorite
object of mine is this cluster because it is buried inside a
very rich star cloud in the Milkyway, and features several dark
nebula, a globular cluster, and several faint planetary nebula
in the one 2.5 degree field seen above, with north up.
Just
to the left of center is the splashy open cluster M7. Spanning
over 80 arc minutes in size, this 3.3 magnitude object is one
of the most brilliant of its type in the entire sky. About 80
cluster members dominate this object, most being blue B super
giants with a few orange K types.
Near
the right edge of this field you will find the diminutive faint
globular cluster NGC6453 highlighted in the center small panel
above. This 10.2 magnitude object is 8 arc minutes in size and
has a distinctly yellow color due to the amount of dust in this
part of the Milkyway.
Even
more interesting are the two tiny planetary nebula in this field.
The first, highlighted in the left sub panel is M1-30, appearing
as an intense orange star to the upper right of the cluster.
It is 14.7 magnitude, and very tiny at 5 arc seconds in size.
Apparently this object has a lower level of ionizing excitation
than Sanduleak 2-253, which is to the lower right of M7 appearing
as a diffuse teal (blue-green) fuzzy star. The blues to create
this color are from Hbeta and OIII lines. It appears as a sizable
disk, spanning 9 arc seconds in width as seen on the right sub
panel.
Processing
Notes:
This
object is in a part of the Milkyway that is so obscured with
dust, everything appears a shade of brown or yellow. To remove
the brown hue and show the field as it would be without dust
obscuration seen here, you have to histogram equalize all three
color channels in Photoshop over the entire image. To do this
you can either stretch all three colors histograms so they start
and stop at the same point. Or using adjustment layers you can
do a really neat trick to equalize the histograms which is what
I did here. To do this, you duplicate the image in a layer over
the main image. Next add an adjustment layer for Levels over
the new layer. you then blur most of or part of the duplicate
image with a Gaussian blur so huge the entire image is one solid
color - brownish. In the levels adjustment layer, click the gray
eyedropper on the blurred image to make it spectrally flat -
gray. Then delete the duplicated blurred layer. The adjustment
layer stays on top of the original image and fully histogram
equalizes it. Flatten and your done.
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Minkowski 1-30
NGC6453
Sanduleak 2-253