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Cosmic Challenge: Pease 1
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Cosmic Challenge: Pease 1
Of the more than 130 globular clusters gravitationally linked to our Milky Way galaxy, only four are known to contain planetary nebulae. The best known example of a planetary/globular pairing is Pease 1 found within M15 in Pegasus. That's this month's Cosmic Challenge.
Above: Summer star map from Star Watch by Phil Harrington.
Above: Finder chart for
this month's Cosmic Challenge. Chart adapted from Cosmic Challenge by Phil Harrington.
Pease 1 was the first planetary nebula to be detected within a globular cluster. That was back in 1927, when Francis Gladheim Pease first spotted it on a photograph taken with the 100-inch Hooker reflector at Mount Wilson Observatory in California. In his report "A Planetary Nebula in the Globular Cluster Messier 15" published the following year in the Publications of the Astronomical Society of the Pacific, Pease observed that "through the 'Pulkowa ultra-violet' color filter, the star Küstner No. 648, photographic mag. 13.78, appeared very bright as compared with the surrounding stars." Those surrounding stars appeared identical to Küstner
The Küstner referred to in Pease's statement was German astronomer Friedrich Küstner, who had published a paper entitled "Der kugelfoermige Sternhaufen Messier 15" ("The Globular Star Cluster Messier 15"; Veröffentlichungen der Universitäts-Sternwarte zu Bonn, No. 15, Bonn: F. Cohen, 1921) seven years earlier that detailed the internal structure of M15. Küstner inventoried many of the cluster's stars, but failed to recognize the unique qualities of star number 648. That's why, although the proper designation of the planetary is Pease 1 (or if you prefer PK 65-
Is Pease 1 a true member of M15 or just a line-of-sight alignment? Such a nonexistent team is not without precedent; recall the planetary nebula NGC 2438 situated in front of the open cluster M46 in winter's Puppis. But unlike that chance encounter, studies of the radial velocities of the Pease 1 and stars in M15 show that all are traveling at nearly the same speed. That infers that they lie at the same distance. Supporting this are additional spectroscopic analyses that demonstrate the abundance and types of elements in Pease 1, including notable lack of metals, match those of the cluster stars. Based on these results, it is now generally conceded that Pease 1 is gravitationally bound within M15. Skeptics argue that a planetary nebulae couldn't possibly survive -- at least not for long -- the gravitational interplay of all the nearby cluster stars. Gravity should have pulled the gossamer cloud apart almost immediately. Why that hasn't happened is still unknown.
Finding M15 is easy, since it is just 4° northwest of Enif [Epsilon (ε) Pegasi, the star at the tip of Pegasus's nose. You'll find a 6th-magnitude star just to the cluster's west, which also helps to mark its location. In fact, through finderscopes, the pair almost seems to create a double star, although M15 will look fuzzy even at low powers.
Spotting Pease 1 within that vast stellar metropolis is another matter altogether. While the nebula is bright enough to be visible through 15-inch (38cm) and larger telescopes under naked-eye limiting magnitude 5.0 skies, trying to pick it out from among all of the stars is the challenge. Fortunately, Pease 1 is slightly offset from the cluster's dense center. The finder chart above, adapted from Doug Snyder's former web site, blackskies.org, will help us in our quest. (As an aside, for those familiar with Snyder's "Planetary Nebula Home Page," you may know the web site has been down for a while. Snyder wrote earlier this year that he is in poor health, but hopes to reboot his web site something in the future. The link here takes you to the archive.org "Wayback Machine" capture of the web site on March 27, 2014.)
To help you in your "pursuit of Pease," here's a photograph of M15 taken by Adam Block, which I reoriented to match the chart above and annotated with the same field stars.
Credit: Adam Block/NOAO/AURA/NSF
Pease 1 takes no less than
300x to be seen, so unless the sky is both exceptionally transparent and
steady, wait for another night. Take your time to orient the chart to match
what you see through your eyepiece and to get a feel for the scale of the chart
versus what your telescope is showing you.
With everything set, look about halfway out toward the northwestern edge of the cluster for the highlighted trapezoid of 14th-magnitude stars, labeled A on the chart. (The star labels on the chart here match the convention shown on Snyder's web site to make cross-reference a little easier.) See it? If so, then look just to its east for a right triangle of faint stars, shown as B, C, and D. Extend a line from star A in the trapezoid through star D to arrive at Star E found about 20 arc-seconds to the southeast. Continue that line past star E another 28" to a small knot of stars.
Once there, call in an O-III filter. By passing the filter in and out of view, the stars in the cluster will dim enough for the planetary to stand out from the crowd. Doing just that allowed me to see the planetary from naked-eye limiting magnitude 5.0 skies, but it remained perfectly stellar through my 18-inch even at 514x. Using the in-and-out filter method (discussed in the August 2016 installment of this e-column) should leave no doubt in your mind as to which point is the planetary.
favorite challenge object of your own? I'd love to hear about it, as well
as how you did with this month's test. Contact me through my web site or post to this
month's discussion forum.
Until next month, remember that half of the fun is the thrill of the chase. Game on!
Phil Harrington's Cosmic Challenge is copyright 2016 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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