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Cosmic Challenge: Palomar 2


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Cosmic Challenge: Palomar 2

 

February 2023

Phil Harrington

This month's suggested aperture range:

15-inch (38-cm) telescopes and up

 

 

 

 

Target

Type

RA

DEC

Constellation

Magnitude

Size

Palomar 2

Globular
cluster

04h 46.1m

+31° 22.9'

Auriga

13

2'

 

When it was released more than half a century ago, the Palomar Observatory Sky Survey showed the universe at a level of detail never before achieved.  This major photographic survey covers the sky from the north celestial pole to -30° declination and recorded stars down to an average of magnitude 22.

 

Astronomers at the time immediately turned to the Survey's 900-plus photographic plates to examine known objects in exquisite detail as well as to discover other sights that earlier studies had missed.  For instance, 86 planetary nebulae were discovered on the plates by American astronomer George O. Abell. (Footnote: Two of Abell's planetaries were already in the NGC catalog (Abell 50=NGC 6742 and Abell 75=NGC 7076), while another pair were already in the IC catalog (Abell 37=IC 972 and Abell 81=IC 1454.)

 

During a survey of the Survey, Abell, along with Halton Arp, Walter Baade, Edwin Hubble, Fritz Zwicky, A.G. Wilson, and others, cataloged 15 "unknown" Milky Way globulars. As with four of the planetary nebulae, two Palomar globulars -- Palomar 7 and Palomar 9 -- were previously known. The former is listed as IC 1276, while the latter is NGC 6717. But regardless, each represents a suitably daunting test for today's most diehard deep-sky observers. 

 

Palomar 2, discovered by A.G. Wilson in 1955, is a particularly nasty cluster to spot because it is partially concealed behind a dark nebula in Auriga.  Barnard 221 spans ¾° and is centered just 32' northwest of Palomar 2.  The dark nebula may not be readily evident by eye, but it has a stifling influence on Palomar 2.

 

Above: Evening star map showing the location of this month's Cosmic Challenge.

 

Credit: Map adapted from Star Watch by Phil Harrington

 

Above: Finder chart for this month's Cosmic Challenge.

 

Credit: Chart adapted from Cosmic Challenge by Phil Harrington
Click on the chart to open a printable PDF version in a new window

 

 

Most globular clusters associated with the Milky Way are positioned around the galactic nucleus, and so are referred to as "inner-halo globulars."  There is a second family, however, whose members lie far beyond the Galaxy's center and so are known as "outer-halo globulars."  Of all the outer-halo globulars known, Palomar 2 is the most extreme, located almost directly opposite the Galactic center in Sagittarius, separated by 85,400 light years.  A 1997 analysis entitled Unveiling Palomar 2: The Most Obscure Globular Cluster in the Outer Halo conducted by Professor William E. Harris and colleagues from McMaster University in Hamilton, Ontario, found that Palomar 2 is it is one of the brightest and most massive clusters in the outer halo.

 

That distinction goes completely unappreciated through our telescopes, however, because Palomar 2 is also one of the most heavily obscured.  As a result, we see only a vague inkling of the cluster's true self.  In many images, the dark nebula's presence also reddens the stars' hues.

 

To find it, scan 2° southwest of Iota (ι) Aurigae, the southwestern star in the Auriga pentagon.  Keep an eye out for a triangle of three 6th- to 7th-magnitude stars.  The brightest and westernmost star in the triangle, SAO 57441, is 40' due east of the cluster.  A close-set pair of 12th- and 13th-magnitude stars lying halfway between the two makes a useful checkpoint along the way.  Another close stellar duo, shining faintly at magnitudes 13 and 14.5, is just 2.3' northeast of the cluster.

Above: Palomar 2 through the author's 18-inch (46-cm) reflector.

 

Below: The view of Palomar 2 as captured by the Hubble Space Telescope.

Credit: ESA/Hubble & NASA

 

For most readers, see Palomar 2 will take the firepower of at least a 15-inch (38-cm) scope to pull out its tiny glow from the surroundings.  Palomar 2 has a listed magnitude of 13, and so may be visible through smaller telescopes if attempted under very dark skies.  Through my 18-inch (46-cm) reflector under suburban skies, it is an extremely dim, but doable, smudge measuring no more than 1' across, as shown in the rendering above.  Averted vision is a must no matter which eyepiece I use, although I have found the best view was at 171x.  The overall impression is more reminiscent of a distant galaxy than a globular cluster.  Indeed, it was this vague appearance that led Boris Vorontsov-Velyaminov and V. P. Arkhipova to misclassify Palomar 2 as a galaxy in their Morphological Catalog of Galaxies (MGC) compiled by in the 1960s.  Even today some references cross-list Palomar 2 as MCG +05-12-1.

 

Have a favorite challenge object of your own?  I'd love to hear about it, as well as how you did with this month's challenge.  Contact me through my website or post to this month's discussion forum.

 

Remember that half of the fun is the thrill of the chase.  Game on!



About the Author:

Phil Harrington writes the monthly Binocular Universe column in Astronomy magazine and is the author of 9 books on astronomy.  Visit his web site at www.philharrington.net to learn more.

Phil Harrington's Cosmic Challenge is copyright 2023 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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