Less than one month after my Origin's first light, I began capturing star fields for educational purposes - my education! For example, this image captured the beautiful Double Cluster in Perseus, NGC 869 and NGC 884. It's just one photograph like thousands of others; no big deal. What is a big deal is all of the astronomy scattered throughout this 1.27" x 0.85" field of view just waiting to be explored.
After surviving a stroke, my new Celestron Origin Astrograph is playing a significant role in my rehabilitation. It was important that I learn to read again, comprehend what I was reading, and re-learn everything that I no longer knew about astronomy, physics, mathematics, and science in general. It's been a hard journey but I am proud to say that I finally, after more than a decade, read a book cover to cover - The Perfect Machine: Building the Palomar Telescope, Ronald Florence, HarperPerennial, New York, 1994. I am currently working my way through an advanced university level textbook filled with mathematics from simple geometry to calculus - Fundamental Astronomy, 6th Edition, Hannu Karttunen, Pekka Kröger, Heikki Oja, Mark Poutanen, and Karl Johan Donner, Editors, Springer-Verlag, Berlin, 2017.
So, let's analyze this photograph using all of the knowledge I have gained over the last year and a half, making good use of astronomy books, magazines, online surveys, and apps.
Date & Time: August 12, 2024, 10:04 p.m.
Location: Kempt Shore, Nova Scotia, Canada, 45°N
Conditions: Clear
Moon: First quarter (54%)
Moonset: 11:38 p.m.
Technical: 10-sec exposures, 30-min integration, line power, no filter, no post-processing
One of the most obvious features of this 1.27° x 0.85° field of view photograph is the presence of several red supergiant stars. I have labeled seven of them. These huge stars have variable visual magnitudes, some periodic and others without regular periods. Let's have a look at their visual magnitudes, spectral types, and effective temperatures (from Ref.1). All seven were candidate members of the Perseus OB1 Association at that time.
Reference 2 lists 13 M-type supergiants, the seven above plus the six below. Data is from SIMBAD, AAVSO, SkySafari 7 Pro, and Refs. 1 & 2.
1. "Galactic OB Associations in the Northern Milky Way Galaxy. I. Longitudes 55° to 150°," C.D. Garmany and R.E. Stencel, Astron. Astrophys. Suppl. Ser. 94, 211-244 (1992).
2. "The M-type supergiant members of the Double Cluster in Perseus," W.P. Bidelman, Astrophysical Journal, 105, 492 (1947).
There are many types of variable stars in this field of view. I have labelled a small sample of seven. All of these variables are members of the Perseus OB1 Association (Ref. 1).
All 21 stars have identification numbers originally assigned by A. Van Maanen (Ref. 3) and studied by R.E. Schild (Ref. 4). They are listed below with GAIA and other useful catalogue numbers as well as star type.
*Note that the Fig.1 finding chart in Ref. 4 has an error. Star 2264 should be 2262 to match the identification number in Table 1 of the same paper.
3. "The proper motions of 1418 stars in and near the clusters h and χ Pers.," A. Van Maanen, Rech. Astr. de l'Obs. d'Utecht, 5 , 1911.
4. "Spectral classification in h and χ Persei," Rudolf E. Schild, Astrophysical Journal 142, 979-982 (1965).
All 31 stars have identification numbers originally assigned by A. Van Maanen (Ref. 3) and studied by R.E. Schild (Ref. 4). They are listed here with GAIA and other useful catalogue numbers as well as star type.
*Note that the Fig.2 finding chart in Ref. 4 does not show star 1041 listed in Table 2 of the same paper.
Stars in congested areas that are not labelled in the photograph include the following:
29. 976, GAIA DR3 458377743274229504 (Star)
30. 980, GAIA DR3 458377743274231936 (Star)
31. 1041*, GAIA DR3 458374788336736512 (Star)
As a first approximation, we can estimate the distance to the h and χ Persei using the distance modulus formula, Equation 4.12 from Reference 5;
m-M = 5log(r/10 pc), (1)
where m is the apparent magnitude, M is the absolute magnitude, m-M is the distance modulus, 10 parsecs is the reference distance, and r is the distance to the object of interest in parsecs. Extinction by the interstellar medium due to reddening, scattering, absorption, and the Earth's atmosphere is assumed to be negligible in this equation. Note that 1 parsec (pc) = 3.26 light-years (lys).
We start with distance estimates to h Persei (NGC 869) and χ Persei (NGC 884) by R.J. Trumpler (Ref. 6). His distance moduli using Eq. 4.12 were 11.4 and 11.9, respectively. Inverting Eq. 1 and solving for the distance r in parsecs gives
r = 10{10^[(m-M)/5]}. (2)
Thus, his distances to h and χ Persei were approximately 1900 and 2400, respectively. Trumpler simply takes the average of these two distances to conclude that both clusters are about 2200 parsecs away (~ 7200 lys).
Trumpler also attempted to correct for interstellar absorption using an absorption constant k as follows
m-M = 5log(r/10 pc) +kr, (3)
where his value for k is 0.79 magnitudes per 1000 parsecs. This leads to corrected distance estimates of 1350 parsecs (~4400 lys) for both clusters. These distances do not compare well with predicted distances by follow-on research teams whose dereddened distance moduli estimates became more sophisticated. Here are some examples from the academic literature. Separate distances for the two clusters are noted, otherwise the authors concluded that they are at the same distance. I have converted published dereddened distance moduli to distances in parsecs and lightyears using Eq. 1 above.
References
5. Fundamental Astronomy, 6th Edition, Hannu Karttunen, Pekka Kröger, Heikki Oja, Markku Poutanen, and Karl Johan Donner, Editors, Springer-Verlag, Berlin, 2017, p. 96.
6. "Preliminary results on the distances, dimensions, and space distributions of open star clusters," Robert J. Trumpler, Lick Observatory Bulletin ; No. 420, University of California Press, 1930, pp. 154-188.
7. " A spectroscopic study of the region of the double cluster in Perseus," William P. Bidelman, Astrophysical Journal, 98, 61-81 (1943).
8. "Ages and structures of strain the h and χ Persei association," Rudolph Schild, The Astrophysical Journal 148, 449-458 (1967).
9. "Photometric study of the double cluster h & χ Persei," A. Marco and G. Bernabeu, Astronomy & Astrophysics 372, 477-494 (2001).
10. "UBVI and Hα photometry of the h and χ Persei cluster," Stefan C. Keller, Eva K. Grebel, Grant J. Miller, and Kenneth M. Ross, The Astronomical Journal 122, 248-256 (2001).
11. "CCD uvbyβ photometry of young open clusters: I. The double cluster h and χ Persei," G. Capilla and J. Fabregat, Astronomy & Astrophysics 394, 479-488 (2002).
12. "Membership in the region of the double cluster h and χ Persei working from proper motions: distance moduli and extinction in that galactic direction," Antonio Uribe, José-Alejandro García-Varela, Beatriz-Eugenia Sabogal-Martínez, Mario A. Higuera G., and Eduardo Brieva, Publications of the Astronomical Society of the Pacific 114, 233-248 (2002).
13. "The star formation history and mass function of the double cluster h and x Persei," Catherine L. Slesnick, Lynne A. Hillenbrand, and Philip Massey, The Astrophysical Journal 576, 880-893 (2002).
14. "Fitting the young main-sequence: distances, ages and age spreads," N.J. Mayne and Tim Naylor, Monthly Notices of the Royal Astronomical Society 386(1), 261-277 (2008).
15. "The stellar population of h and χ Persei: Cluster properties, membership, and the intrinsic colors and temperatures of stars," Thane Currie, Jesus Hernandez, Jonathan Irwin, Scott J. Kenyon, Susan Tokarz, Zoltan Balog, Ann Bragg, Perry Berlin, and Mike Calkins, The Astrophysical Journal Supplement Series 186, 191-221 (2010).
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