by Ted Frimet
man bites dog
There comes a time in every amateur astronomers life, when they have to come to terms with real astronomy. I used to be fond of saying, from time to time, that “real men eat quiche”. Now-a-days, I am fine tuning my hind-brain to accommodate a phrase, “real amateur astronomers understand what makes a cepheid variable, variable”.
Too long? Yeah, I get that all the time. In a Dale Carnegie course, I was taught to make it simpler, smaller, and with fewer words. How about, “helium sucks light”? That might be more akin to “man bites dog”. For Astronomers, anyway, it entices you to read on. Where is Hubble, when you really need him? Probably lurking in the shadows of my discontent, stroking along with Schroedinger’s cat. Meow.
At one or more points in time, over the last two years, I managed to get confused between type 1A supernovas and Cepheid variables. I know, I know – “how is that even possible”, you ask? It is because in the ven diagram of my mind, I see them both as astronomical distance measurement tools. And now, sitting down to one, or the other, to study, I choose to make short work of the cepheid variable. I do this, because I don’t want to visit fermi pressures on compressed star matter before it nova’s. It is simply not in my wheel house, this fine Fourth of July morning. You know, on second thought, a nova is more akin to galactic fireworks display. And truth be told, it is simply more like fireworks, to me.
However, today I am in a particularly gaffy mood. I have not been able to coordinate two telescopes for a parallax study. Hence, I am not entirely pumped up for a fireworks display. And this is to your benefit, as we get to get a closer look into not what a cepheid variable is, but the more pedantic reason as to the how, or why the light intensity is periodic in the first place. And when we’re done, please do not think the lesser of me, when I suggest, now, that Cepheid Variables are reminiscent of first light in the Universe. Oh, come on now! Keep an open mind for my cat’s’ sake?
I started to draw upon reference to laboratory exercises, on Cepheid Variables and the Cosmic Distance Scale (1). And within their reprint, lurking on a page, I find Shapley’s Calibration. A formula of M = m + 5 – 5 log d. I can’t express my happiness in finding such consolation in a mathematical formulae. That is, having used it, previously in magnitude calculations essays’ of old, it is like stumbling across an old friend. I never knew that math could be that way. I understand, at least from the froth of a good cup of coffee, why professional mathematicians get all gawky eyed. Like adolescents at their first dance party, they see and appreciate simple quantifications.
I’ve read the article, and didn’t want to mess up the virgin grid, provided in the fold-out. Hence, I didn’t forge forward to Baades’ Calibration. Frankly, I didn’t want to get further distracted by how the early distances to Andromeda were poorly estimated. I recall, from recent memory, how I’ve been off by more than 20 percent, in my own error. (Ok, for those of you that remember my early asteroid VT calculations, it had more error than our ancestral astronomers, not named above.) And why should I drag you to the depths of our earliest primordial history, too? Instead, let’s hop over to “The Universe Around Us”, by Sir James Jeans.
Sir Isaac Newtown, gave us gravity with an explanation left to the likes of Einstein, 228 years later. Sir James, it appears, went the same route. “Whatever their mechanisms may be [cepheid variables], observation shews that these stars possess a certain definite property, which proves to be of the utmost value.” (2) Published September 1929, perhaps we are making some progress, as our musing are now only 90 years apart? The Universe(s) may be vast, and the study of its Cosmology appears to us in the blink of an eye. Two shakes of a lambs tail? A blink of thine eye? No, just kidding. Almost a century passes us by, and still it falls upon the shoulders of the amateur to breach the dam, bypass the scholastic, and come to the point! If ye be bored, tarry here, no more. I’ve got more interstellar molasses to wade thru, before I blow up my birthday balloon.
It is a tantalizing read, that is, on the light curves of variables. Especially so, when I muse out loud, “As we go from the ultraviolet to the red end of the spectrum for any particular Cepheid, the light curves flatten out, indicating that the range of variation in luminosity is much wider in the ultraviolet than it is in the infrared” (3). Ah, music to my ears, a delight to the mind to read. Almost, we get the tantalizing taste of a spectrum of color, from the less human visuals of ultraviolet (bugs be damned, as they can sense UV quite well!) to the lovely warm cloaca of infrared. And as we stumble across the work of A.H. Joy, on the interesting characteristic of the period of W Virginis stars (2 to 75 days – ibid p214), a probability occurs to that author. That some “material” is going thru an expansion and contraction. Oh, so close! So close, indeed.
As I peruse The AAVSO Variable Star Atlas (4), a tome, titled, “Sky and Telescope”, pops out onto my table, before me. Ever so hopeful to study matter, over time. Mattei, Mayer and Baldwin, bring to my attention that more than 25,000 stars are variable. (as of the time of their editing, circa 1980). And their time scales ranged from fewer than minutes to centuries. I would be remiss not sharing with you my opinion, that time does not matter, here. That the curtain of gas, that periodically hides the light from my telescope, is what is of importance. Here, it is matter, and not time that I seek. And what is that, which matters most? How do I find it? Do we set the stage, and let Shakespeare determine both our entrance and exit? Let us move to the beginning, and bring to life the Boatswain. He sweeps us aside during the tempest, and conveys that we have enough room to run around the storm!
I crawl from under the weight of my books, my tempest, and key into Wikipedia, https://en.wikipedia.org/wiki/Cepheid_variable (5). It’s author(s), brings to light so simply that it is the veil of Helium that obscures our light. That during the due course of ionizing helium, the ionized gas becomes opaque, even more so, when both electrons are stripped off. The trapping of heat, that is an increase in temperature, causes an expansion. And with this expansion comes a subsequent cooling; it becomes less ionized, permitting contraction, and allowing starlight to escape. It is known as the Eddington Valve (or kappa-mechanism).
I leave you with the pedantic solution we agreed upon, at the beginning, where I suggested that Helium sucks light. If you were to stay the course, with me, to this point – I shall now beg your forgiveness over the time-length of the essay, as the books, were truly so heavy that they imbued their own gravitational effect and were a cause of space-time dilation. Just ask Einstein.
Citations and References:
1) Pasachoff, J. M., & Goebel, R. W. (1979). Laboratory Exercises in Astronomy – Cepheid Variables and the Cosmic Distance Scale. In Reprinted from Sky and Telescope (pp. 241-244). Cambridge, MA: Sky Publishing. p243 referenced, in the essay.
2) Jeans, J. (1934). The Universe Around Us p62 (3rd ed.). NY: The MacMillan Company.
Sir James Jeans, M.A., D.Sc., Sc.D., LL.D., F.R.S.
3) Motz, L., & Duveen, A. (1967). Essentials of Astronomy. p415, Belmont, CA: Wadsworth Publishing Company.
4) Scovil, C. E., & Robinson, L. J. (1980). The AAVSO Variable Star Atlas (2nd ed.). Cambridge, MA: Sky Publ., with The “Sky and Telescope” Guide to the Heavens, Edited by Leif J. Robinson, Sky Publishing Corporation, 1980.
5) Cepheid variable. (2018, June 27). Retrieved July 4, 2018, from https://en.wikipedia.org/wiki/Cepheid_variable. detail: Smith, D. H. (1984). “Eddington’s Valve and Cepheid Pulsations”. Sky and Telescope. 68: 519. Bibcode:1984S&T….68..519S. Eddington’s Valve and Cepheid Pulsations Smith, D. H. Abstract. Please note: No abstract found. Publication Sky and Telescope, Vol. 68, NO. 6/DEC, P.519, 1984 Pub Date: December 1984 Bibcode 1984S&T….68..519S