Palomar 4 (April-May 2020)

Palomar 4 is fascinating—a very distant globular cluster of stars orbiting the Milky Way far outside our galaxy, 356,000 light years from us.

At that distance, the stars of Palomar 4 are very dim, from magnitude 17.9 down to magnitude 28.  Please note, however, that the two brightest stars in my image of the cluster stand out.  I noticed that the brighter of the two seemed much brighter than when I imaged the cluster in 2009, https://polarisb.blogspot.com/2009/03/palomar-4-final.html.  The star seemed so much brighter that I believed it must be variable.  I checked with the AAVSO, the world's experts on variable star light curves, and yes, both the two brightest stars in my image are variables.  Thanks to Brian Skiff and Sebastian Otero:
https://www.aavso.org/vsx/index.php?view=detail.top&oid=380883
https://www.aavso.org/vsx/index.php?view=detail.top&oid=380882

The cluster is relatively sparse as globs go.  See this deep image from the Dark Energy Camera Legacy Survey telescope, a 4m scope.

The whole cluster has less than 30,000 solar masses.  See https://arxiv.org/pdf/1205.2693.pdf.  This study also contains a color magnitude diagram that shows the cluster's star magnitudes.  Even the 4m scope attached to the DECam only reaches those as bright as mag 23 or 24.

In addition to Palomar 4, the image contains some deep galaxies.  To the right is PGC 1855809, mag 17.92 and 330 million light years away. Above that, and very dim, is PGC 1856336, mag 17.97, 2.2 billion light years away, and 167,000 light years wide.

To the left of Pal 4 is PGC 1846998, mag 16.18 and 330 million light years away.

Bottom left is PGC 1845035, at mag 17.69, 2.1 billion light years away.  The galaxy must be huge, 250,000 light years across. Another galaxy smudge, just left of PGC 1846998, is at the same distance.  It is a small smudge on my image, almost starlike.  It is called PGC 1846201 and at 2.1 billion light years is probably 150,000 light years across.

There are other galaxies in the image, but I did not identify them.

This is 11x900" with the 203mm Synta-ONTC Newtonian at f/4.95 and the Atik 460EXC.

Palomar 4 Final

Palomar 4 is the Milky Way's second most distant globular cluster (the other is 10% further out, see here). It is 356,000 light years from the Sun and 364,000 light years from the center of the galaxy. For some perspective as to how far out that is, consider that the galaxy itself is only about 100,000 light years across. That means Palomar 4 is 3.5 times further from the Milky Way's center than the galaxy is wide. If the Milky Way is a frisbee ten inches wide, Palomar 4 is three feet away from its center! There are several other galaxies closer to the Milky Way than that.

At this great distance, only very bright stars show up as stars. Compare my results with this image taken from the Sloan Digital Sky Survey.
These are the brightest stars in the cluster. Many more have been detected individually by professional astronomers that are much dimmer, down to magnitude 25 or 26. These stars in my image are further than any individual stars I have imaged before now. They are very dim. I could never see them with my eyes through the telescopes I have, but the camera catches much dimmer objects than my eyes ever could. Here is my image with magnitudes of the brighter stars filled in (mags provided by Sky-Map.org). Please click on the image for a readable view.
Palomar 4 is something someone with a really large telescope (i.e., 20") might try to see (as here). I feel fortunate that I can catch it from my backyard in my little 5.3" scope.

This image is 87 minutes worth of 20" exposures taken with the Atik 16 through the Vixen R135S, unguided, processed in Nebulosity, Registax, and PSE7.

Update: NHAC member and accomplished astrophotographer Paul Downing also recently shot Palomar 4. I'm honored to show his image here at Polaris B:
As you can see, Paul's image is as deep as the survey. And it has greater resolution than mine, and color. This image begins to show Palomar 4 as the globular that it is. Great! The image was taken with a Celestron C14 at f/7 on a Takahashi NJP. It is an L:R:G:B combination using an SBIG ST10-XME CCD camera - 4x5min exposures of each component, dark subtracted and stacked in MaxIm CCD and color combined in Photoshop. It was taken in southern Spain at 36d 51m 41s N; 3d 15m 28s W. More of Paul's work can be seen at http://www.paulandliz.org

Anyone else?