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.