Salutations celestial sight seers!
I'm David Fuller, welcome to Eyes on the Sky; what's up this week?
Let's continue through the constellation of Ophiuchus, adding to last week's targets of
IC4665, 70 Ophiuchi and Barnard's star.
But instead of going "outside the house shape," this week we'll stay inside that asterism.
As with any house, let's begin at the foundation.
Starting from the east, looking across the bottom of Ophiuchus finds the 2.5 magnitude
stars Sabik and Zeta Ophiuchi, and then 3.2 and 2.7 magnitude Yed Posterior and Yed Prior
over on the western side.
Capping off the top of the house are Kappa Ophiuchi, Rasalhague at the peak, and then
Cebalrai which is the star we started from last week.
Let's look for a pair of globular clusters, starting with Messier 12.
Magnified finderscope users may wish to start at the cool, red giant star Yed Prior, a 3,700
degrees Kelvin star 171 light years from Earth.
This red giant that has undergone significant expansion, and is nearly 60 times the radius
of our Sun.
While physically large, it only contains 1.5 times the mass of our Sun.
So it's a bit of a 'fluffy" star, as it's density is quite low compared to our host
star.
From here, a 7 degree hop in the direction of Kappa Ophiuchi finds Lambda Ophiuchi first,
a 4-th magnitude binary star having just one arc second of separation.
Try using an 8 inch scope or larger for splitting this tight pair.
From there, it can be a bit of a challenge to hop the five and a half degrees to the
cluster, as there aren't many helpful stars pointing the way.
The best way I've found is to use this 6.4 magnitude star, draw a line through Lambda,
and continue on to the globular.
But there's actually an easier way for red dot or Telrad finder owners.
More on that in a moment.
Dark Sky Fact: In the United States alone, 2.2 billion dollars goes to light up... absolutely
nothing, as misdirected and unshielded light escapes into the atmosphere and doesn't even
light up the ground where the light would be needed.
This is where drawing straight lines and finding intersections, just like in high school geometry,
finally comes in handy!
First, look at a line from Yed Posterior up to Cebalrai.
That's quite a distance, a good 23 degrees of sky.
Next, look from Zeta Ophiuchi, to 3-rd magnitude Kappa.
Then, imagine where those lines intersect, and point your finder there.
In a long focal length eyepiece, Messier 12 should be visible.
Good binoculars can also help track down this object, though it merely appears as a nebulous
spot in them.
Then increase the magnification for the best image size.
With the brightest stars at magnitude 12, smaller telescopes may have difficulty resolving
individual members, while larger telescopes of 6 to 10 inches or larger will begin resolving
the cluster.
Separated by just 3 degrees of sky is an apparent twin of Messier 12, although the two clusters
do have differences.
With a magnified finder, locating Messier 10 is a simple matter of making a small hop
southeast.
Look for these two stars, a 6-th and 7-th magnitude pair, and moving a nearly similar
distance that M12 is from them.
In a 7 degrees field of view finderscope, you can see these two stars and 5-th magnitude
30 Ophiuchi.
Messier 10 sits between these three, but a bit closer to 30.
For zero power finder locating, draw two more lines in the sky, and look for the intersection
of Yed Posterior to Eta Serpentis, both 3-rd magnitude, and Zeta Ophiuchi and Rasalhague.
Visually, the Stellarium depictions are a bit deceiving, as I find Messier 10 to appear
a bit brighter than Messier 12; the core appears brighter and more compact, thus seeming to
be more visually appealing at the eyepiece.
Messier 12 is slightly larger, but also looks a bit dimmer, and is not as perfectly spherical
as M10 is.
What differences do you see between these globular clusters?
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Thanks for watching...
Keep your eyes on the sky and your outdoor lights aimed down, so we can all see, what's
up.
I'm David Fuller, wishing you clear and dark skies.