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Cosmic Challenge: Palomar 4
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Cosmic Challenge: Palomar 4
May 2024Phil Harrington
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Target |
Type |
RA |
DEC |
Constellation |
Magnitude |
Size |
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Palomar 4 |
Globular |
11h 29.3m |
+28° 58.4’ |
Ursa Major |
14.2 |
1' |
When you think of deep-sky objects in Ursa Major, you probably don't think of globular clusters. Galaxies, sure! Planetary nebulae? There's the Owl Nebula. But globular clusters? Probably not.
Well,
guess what? There is actually a renegade globular cluster within the Great
Bear. Today, we know it as Palomar 4, the fourth entry in the list of 15
challenging globular clusters discovered on the plates of the Palomar Sky
Survey of sixty years ago. This particular globular was discovered by Edwin
Hubble in 1949 and confirmed a year later by A.G. Wilson. Based on early
observations, Palomar 4 was initially misclassified as a dwarf spherical
galaxy, in part because of its remote distance. Palomar 4 is estimated to be
330,000 light years away from the Sun. That's further away than the two
Magellanic Cloud satellite galaxies. More recent studies, however, prove that
Palomar 4 is, indeed, a member of the Milky Way's family of globular clusters,
albeit the most secluded save for Arp-Madone
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Above: Evening star map. Credit: Map adapted from Star Watch by Phil Harrington |
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Below: Finder chart for
this month's Cosmic Challenge. |
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Palomar 4 lies near the star Alula Australis [Xi (ξ) Ursae Majoris], the Bear's big toe on its hind leg. Alula Australis, together with its twin star just to the north, Alula Borealis [Nu (ν) Ursae Majoris], is located 10° south of Phecda [Gamma (γ) Ursae Majoris]. Be sure to pause at both on your way to Palomar 4, as each is an attractive binary system. Nu's golden primary is accompanied by a 10th-magnitude companion 7" to its south-southeast. Xi is tougher thanks to its 4.3- and 4.8-magnitude suns being separated by only about 1½” at present. Incidentally, Xi was the first binary star whose components were proven to be physically related. William Herschel drew that conclusion in 1802 after he had found each star changed orientation relative to the other after a span of 22 years. A quarter century later, the French astronomer Felix Savary determined that the stars orbited each other in just under 60 years.
Palomar 4 is parked about 3½° southeast of Alula Australis. To find it, move 2½° southeastward from the star to a westward-aimed isosceles triangle of 7th-magnitude stars. Extend the triangle's eastern side southward to 7.8-magnitude SAO 81872, and then turn toward the west, passing a 9.3-magnitude star 12' later and coming to a 9.8-magnitude star in another 24'. Palomar 4 is just 6' further west of that last star. Look for its faint glow just to the west-southwest of a 13th-magnitude field star.
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Above: Palomar 4 as seen through the author's 18-inch (46-cm) reflector.
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Its
far-flung distance coupled with its inherently weak stellar concentration
conspires to dilute the cluster to nothing more than a very faint glow
measuring just an arc-minute or so across. From my suburban backyard
observatory, my 18-inch (46-cm) reflector revealed only the faintest hint of
the cluster at 206x, and then only fleetingly with averted vision. From darker
sites in North Carolina and
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Above: Hubble Space Telescope image of Palomar 4 Credit: Fabian RRRR, CC BY-SA 3.0, via Wikimedia Commons |
If searching for Palomar globulars is your thing, here is a complete listing of the 15 objects in that elite list. I'll profile some in future columns.
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Palomar # |
Constellation |
Right Ascension |
Declination |
Magnitude |
Size (') |
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01 |
Cepheus |
03 33 23.0 |
+79 34 50 |
13.6 |
2.8 |
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02 |
Auriga |
04 46 05.8 |
+31 22 55 |
13.0 |
2.2 |
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03 |
Sextans |
10 05 31.4 |
+00 04 17 |
13.9 |
1.6 |
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04 |
Ursa Major |
11 29 16.8 |
+28 58 25 |
14.2 |
1.3 |
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05 |
Serpens |
15 16 05.3 |
-00 06 41 |
11.8 |
8.0 |
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06 |
Ophiuchus |
17 43 42.2 |
-26 13 21 |
11.6 |
1.2 |
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07 |
Serpens |
18 10 44.2 |
-07 12 27 |
10.3 |
8.0 |
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08 |
Sagittarius |
18 41 29.9 |
-19 49 33 |
10.9 |
5.2 |
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09 |
Sagittarius |
18 55 06.0 |
-22 42 06 |
8.4 |
5.4 |
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10 |
Sagitta |
19 18 02.1 |
+18 34 18 |
13.2 |
4.0 |
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11 |
Aquila |
19 45 14.4 |
-08 00 26 |
9.8 |
10.0 |
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12 |
Capricornus |
21 46 38.8 |
-21 15 03 |
11.7 |
2.9 |
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13 |
Pegasus |
23 06 44.4 |
+12 46 19 |
13.8 |
0.7 |
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14 |
Hercules |
16 11 04.9 |
+14 57 29 |
14.7 |
2.5 |
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15 |
Ophiuchus |
16 59 51 |
-00 32 31 |
14.2 |
3.0 |
Until next month, remember that half of the fun is the thrill of the chase. Game on!
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About
the Author: |
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Phil Harrington's Cosmic Challenge is copyright 2024 by Philip S. Harrington. All rights reserved. No reproduction, in whole or in part, beyond single copies for use by an individual, is permitted without written permission of the copyright holder. |
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- random, John O'Hara, BoldAxis1967 and 2 others like this
10 Comments
In 1996 I excitedly thought I'd seen it, but back in the light looking at my drawing I realised it was just a mag 13.8 field star nearby at the edge of visibility. 8" dob under urban lights.
The Palomars are good fun!
Here's an observation with a 15" f/4.5 under dark and transparent skies:
"Very small and even glow appears next to a field star with averted vision. Quite faint!"
Palomar 4 is well placed for observation in the northern hemisphere, but its central surface brightness is quite low, ~23.5 V mpsas in a 1995 paper and falls off to 25 mpsas by ~1.3 arcmin diameter. This makes it quite challenging in anything other than very dark sky.
While there is a hazy general glow around it, with the 20" there is some subtle lumpiness. I reobserved it last night 5/7/24 and at 357x identified the positions of at least three threshold stellar points spread around with hints of one or two more. The ones seen match the 1985 color magnitude star data for the cluster. It looks like at least two of these in the 17.8+ range were part of the CMD plot at the tip of the red giant branch. Sky conditions were ~21.3 mpsas at the time with mediocre to good variable seeing.
So do we think that’s a planetary nebula or a galaxy at 1:00 on the perimeter in the Hubble photo? (One would assume it’s a galaxy, given that the only Palomar noted to have a planetary so far is Pal 6.)
I'm considering an attempt to image them all, and the Terzan clusters too..
Will I be able to succeed with 8" Celestron and Hyperstar?
Dave
Yes, though it will be very small in a field of stars. Here is a fast 200mm scope image (not mine).
https://www.astrobin...rz/?q=Palomar 4
Jon
Looks like a galaxy to me, although it didn't seem to be identified in NED. Probability would be heavily against this being a PN considering how sparse the globular is, planetary nebula frequency would be an order of magnitude less.
Here is one of the best write-ups I have found:
https://www.cloudyni...3772-palomar-4/
Interesting that Rick pondered that the bright orange stars at the forefront may not be part of the globular.
Here are the full set of images Rick produced on his legacy website:
http://images.mantra.../PAL4/index.htm
Jon
The bright orange stars are the red giants near the tip of the red giant branch. These are in the upper 17 magnitude range and beyond. The CMD (color magnitude diagram) from 1986 (link) shows the corresponding stars and magnitudes. The magnitudes and coordinates on the image are given.
It looks like they rejected some of the brighter visible stars, although that might have been due to overly stringent criteria. When one looks at the B - V these are on the far red end, and from what I can tell that is where the red giants are most variable. Variability greater than 0.03 mag was a basis for rejection in the CMD study, but that doesn't mean that some were not members. Indeed, looking at the field, it seems unlikely that there would be a clump of stars of this magnitude and color overlaying the cluster. One interloper is likely (the 16.7 V mag / 1.6 B-V example), but the next one at 17.7 V with 1.635 B-V looks like a plausible cluster member. Remember, they were looking for a "clean" CMD and that was part of their selection process. However, it doesn't appear that addition of the 17.7 mag star would do any damage to the CMD, and instead looks consistent with the tip.
Hard to say until someone does a modern study of the cluster with more detail.
I have observed and made drawing of all 15 Palomar GC between 1996 and 2000.
Most difficult was Pal 13. I used a 16" F5 to complete the challenged.
I forgot to add that I observed Pal 4 on the 28 Jan 2000 in not the best sky conditions.
Faint glow at 160x in Nagler 13, 14 or so mag star close to the glow.