 |
| Two copies of the same publication? Not. On the left is the traditional Royal Astronomical Society of Canada (RASC) Observer's Handbook, in its 2018 edition. On the right is something new as of 2018, a USA version. Promotional writeups may be found on my Society's Web server at https://www.rasc.ca/handbook. |
Quality assessment:
On the 5-point scale current in Estonia, and surely in nearby nations, and familiar to observers of the academic arrangements of the late, unlamented, Union of Soviet Socialist Republics (applying the easy and lax standards Kmo deploys in his grubby imaginary "Aleksandr Stepanovitsh Popovi nimeline sangarliku raadio instituut" (the "Alexandr Stepanovitch Popov Institute of Heroic Radio") and his grubby imaginary "Nikolai Ivanovitsh Lobatshevski nimeline sotsalitsliku matemaatika instituut" (the "Nicolai Ivanovich Lobachevsky Institute of Socialist Mathematics") - where, on the lax and easy grading philosophy of the twin Institutes, 1/5 is "epic fail", 2/5 is "failure not so disastrous as to be epic", 3/5 is "mediocre pass", 4/5 is "good", and 5/5 is "excellent"): 4/5. Justification: Kmo had time to develop the necessary points to reasonable length.
Revision history:
All times in these blog "revision histories" are stated in UTC (Universal Coordinated Time/ Temps Universel Coordoné, a precisification of the old GMT, or "Greenwich Mean Time"), in the ISO-prescribed YYYYMMDDThhmmZ timestamping format. UTC currently leads Toronto civil time by 5 hours and currently lags Tallinn civil time by 2 hours.
On the 5-point scale current in Estonia, and surely in nearby nations, and familiar to observers of the academic arrangements of the late, unlamented, Union of Soviet Socialist Republics (applying the easy and lax standards Kmo deploys in his grubby imaginary "Aleksandr Stepanovitsh Popovi nimeline sangarliku raadio instituut" (the "Alexandr Stepanovitch Popov Institute of Heroic Radio") and his grubby imaginary "Nikolai Ivanovitsh Lobatshevski nimeline sotsalitsliku matemaatika instituut" (the "Nicolai Ivanovich Lobachevsky Institute of Socialist Mathematics") - where, on the lax and easy grading philosophy of the twin Institutes, 1/5 is "epic fail", 2/5 is "failure not so disastrous as to be epic", 3/5 is "mediocre pass", 4/5 is "good", and 5/5 is "excellent"): 4/5. Justification: Kmo had time to develop the necessary points to reasonable length.
Revision history:
All times in these blog "revision histories" are stated in UTC (Universal Coordinated Time/ Temps Universel Coordoné, a precisification of the old GMT, or "Greenwich Mean Time"), in the ISO-prescribed YYYYMMDDThhmmZ timestamping format. UTC currently leads Toronto civil time by 5 hours and currently lags Tallinn civil time by 2 hours.
- 20170103T2001Z/version 2.3.0: Kmo made some small changes of substance, most notably by (i) expanding his discussion of the "+9P" or "+8P" flag for our Sun (we need to take account of large Kuiper Belt objects, including larger Kuiper Belt objects farther out than Pluto), and (ii) giving a better estimate of the lifetime of the Sun, were the Sun to be powered by the sadly transient Victorian mechanism of gravitational contraction rather than by the robust modern mechanism of nuclear fusion. - Kmo reserved the right to make tiny, nonsubstantive, purely cosmetic, tweaks over the coming 48 hours, as here-undocumented versions 2.3.1, 2.3.2, 2.3.3, ... .
- 20170103T0158Z/version 2.2.0: Kmo corrected various small points, on the boundary between the obvious typo and the misleading error-of-substance. - He reserved the right to make tiny, nonsubstantive, purely cosmetic, tweaks over the coming 48 hours, as here-undocumented versions 2.2.1, 2.2.2, 2.2.3, ... .
- 20170103T0140Z/version 2.1.0: Kmo corrected an embarrassing small mistake. He had wrongly asserted the South Celestial Pole to move along the horizon for an observer on the Earth's equator. In reality, the South Celestial Pole is for all observers stationary (falling below the horizon for observers north of the Earth's equator, and sitting somewhere above the horizon for observers south of the Earth's equator). - Kmo reserved the right to make tiny, nonsubstantive, purely cosmetic, tweaks over the coming 48 hours, as here-undocmented versions 2.1.1, 2.1.2, 2.2.3, ... .
- 20180103T0100Z??/version 2.0.0: Kmo, running a bit late, finished converting his point-form outline into coherent full-sentences prose. He reserved the right to make tiny, nonsubstantive, purely cosmetic, tweaks over the coming 48 hours, as here-undocumented versions 2.0.1, 2.0.2, 2.0.3, ... .
- 20180102T0311Z/version 1.0.0: Kmo had time to upload a polished point-form outline. He hoped to finish converting this outline into coherent full-sentences prose, through a series of incremental uploads, by UTC=20180102T2000Z.
[CAUTION: A bug in the blogger server-side software has in some past months shown a propensity to insert inappropriate whitespace at some points in some of my posted essays. If a screen seems to end in empty space, keep scrolling down. The end of the posting is not reached until the usual blogger "Posted by Toomas (Tom) Karmo at" appears. - The blogger software has also shown a propensity, at any rate when coupled with my erstwhile, out-of-date, Web-authoring uploading browser, to generate HTML that gets formatted in different ways on different downloading browsers. Some downloading browsers have sometimes perhaps not correctly read in the entirety of the "Cascading Style Sheets" (CSS) which on all ordinary Web servers control the browser placement of margins, sidebars, and the like. If you suspect CSS problems in your particular browser, be patient: it is probable that while some content has been shoved into some odd place (for instance, down to the bottom of your browser, where it ought to appear in the right-hand margin), all the server content has been pushed down into your browser in some place or other. - Finally, there may be blogger vagaries, outside my control, in font sizing or interlinear spacing or right-margin justification. - Anyone inclined to help with trouble-shooting, or to offer other kinds of technical advice, is welcome to write me via Toomas.Karmo@gmail.com.]
In almost every year from 2001 or so onward, I have helped the Royal Astronomical Society of Canada (RASC) produce annual updates of its Observer's Handbook "Brightest Stars" article. (The sole exception was one exceptionally harsh year in the David Dunlap Observatory and Park (DDO&P) conservation casework, in which time and energy seemed to run out.)
For much of this long period, I was the junior literary partner of the late Prof. Robert F. Garrison. This is the mourned authority whose rich scientific career I examine in a blogspot posting of 2017-08-14 or 2017-08-15, headed "Prof. Robert F. Garrison Remembered (1936-05-09/2017-08-13)". In the last couple of years, with Prof. Garrison's illness and death, I have been lead author, supported under this new arrangement by Prof. Richard Gray (in the Department of Physics and Astronomy of Appalachian State University) and Fr (Dr) Chris Corbally (in the Vatican Observatory Research Group, I presume juridically anchored in Città del Vaticano, but in practical terms anchored in Arizona).
For 2017, and again for 2018, our work in the printed Handbook has been supplemented by an online publication, which in its 2018 version can be downloaded for free as https://www.rasc.ca/sites/default/files/Bright_Stars_extra_rows_2018.pdf. In the current arrangement of our work, the printed Handbook deals in a necessarily cursory way with 286 stars, while a table within the PDF deals in a more extended way with those 286, while adding a further 28.
Both the 286-star set and the extended (286+28 =) 314-star set fairly represent what can be seen, without gross effort, by a reasonably healthy naked, perhaps bespectacled, eye from some reasonably secluded suburban park, even in such a light-polluted suburb as Richmond Hill. From the David Dunlap Observatory lawn in its present state (a subdivision, however, might be coming), you can perhaps push down a little beyond our self-imposed 314-star limit, if the moon is new or scant and your eyes are satisfactory. You cannot, however, push much further down without reaching for binoculars.
I used to put it to myself approximately thus: Here I stand, on the DDO lawn or on the DDO dome catwalk, quite isolated from the lights even of Hillsview Drive, and with the bulk of Richmond Hill largely behind a generous 600, 700, or 1000 metres of field and woodland. Under these not-too-good and not-too-bad conditions, I can make out, as pretty much the limit of what is to my bespectacled eyes feasible, eta Cassiopeiae. This modest star "η Cas", at a visual magnitude somewhere between 3.4 and 3.5, is about as faint as stars get in our 286-star (printed-edition) selection. It is a little off the sprawling, blazingly bright, reclining chair which we casually think of as Cassiopeia, lying beside the short arc linking reclining-chair α Cas to reclining-chair γ Cas. (You have to populate the sky with acronyms, folks, so as to remember those Bayer letters which come up everywhere in astronomical literature and astronomical maps. The celebrated reclining chair is "BAGD", for β-α-γ-δ Cas, or with a slight increment in visual effort "BAGDE", for β-α-γ-δ-ε Cas. If you a little perversely add in the modest star now under contemplation, however, you get "BAηGDE" - in a reasonable Estonian-or-Latin pronunciation "baa- eig-de").
****
The Handbook is venerable, having first appeared as a 108-page volume under the editorship of Prof. Clarence Augustus Chant (1865-1956; in the 1930s, Prof,. Chant became the Founding Director of DDO, going into retirement upon his facility's May 1935 opening ceremony). As of 2018, the Handbook as a printed volume comprises just over 350 pages.
This 2018 New Year marks a significant new stage in Prof. Chant's venerable enterprise, over and above the already-mentioned production of an online supplement. Now, in 2018 for the first time, the print version is being issued in a pair of parallel editions. Alongside the traditional print version, as much used in Canada, there is this year a "USA Edition". Where the Canadian print version contains, ahead of its many properly technical chapters, a "Guest Editorial: The RASC at 150", the USA print version features in that same place a more USA-appropriate guest editorial, by John J. Goss, promoting the "Astronomical League" which serves America as its loose RASC equivalent. There are similar divergences, created with a view to the particular needs of American readers, in a list of star parties in the two print editions. I suspect there are similar divergences in some other presentations of resources for hobbyists and educators.
The bulk of the content is however, of necessity nation-neutral. Our long table of the brightest stars covers the entire sky, right down to the South celestial pole (stationary on the horizon, as the night advances, should you be on the Equator - for instance, on the open sea just south of Singapore - but stationary all through the night below it if you are in Canada, in the United States, or in Mexico). Such things as lists of eclipses, and lists of meteor showers, are likewise suited to the needs of a global readership. The table of planetary occultations, common to the two print editions, concentrates on events visible from North America and Hawai'i.
Over the last few months, I put a lot of effort not only into updates and expansions of the 314 entries in our online brightest-stars table, but additionally into something we have not attempted in previous years - namely, into an online essay highlighting the specially unrepresentative character of the visually brightest stars.
It is sometimes said, in a kind of popular-science National Geographic or USA Public Broadcasting System spirit, that our own Sun is an "average star". This is misleading. Our Sun, being a stable core-hydrogen-fuser of temperature type "G", is in its visible outer layers (much) hotter than the visible outer layers of what is believed the most common of the temperature types, "M".
Here is an adaptation of the traditional temperature-types mnemonic, arranged from the hottest of the traditionally familiar types down to the coldest of the traditionally familiar types: OABFGKM - "Oh Be A Fine Gymnast, Kiss Me". (Well, strict tradition did unfortunately say "Girl".)
"O" remains nowadays the hottest. But at the cool end there are as additions, accommodating recently discovered brown dwarfs, "L", "T", and (most recently of all, and I think so far only thinly populated by discoveries) "Y". Further, there are special traditional classification letters "R" and "N" (more recently consolidated under the single letter "C") and "S". These denote certain not-very-hot stars presenting chemical anomalies, notably (in the case of the erstwhile R and N, or nowadays C) an excess of visible-layers carbon. It is thus necessary to replace the old mnemonic with the following: "Oh Be A Fine Gymnast, Kiss Me (Right Now, Smack; or to be more up-to-date Chomp, Smack), Like This, Yowee."
A really full mnemonic would additionally have to accommodate the hot, heavily mass-ejecting, Wolf-Rayets, or "W" stars ("Wow! Oh Be A Fine...").
But I must put these details of temperature types (strictly speaking, of temperature-driven spectroscopy features) aside. My main point here is only that our familiar Sun, plus our familiar visually bright night-time stars, as prominent from what I remember of the DDO lawn and catwalk, are in a statistical sense unrepresentative. In the online essay, as I wrote it to supplement the two parallel 2018 printed Handbook editions, I develop my point at length.
I would like to reproduce the greater part of the essay here, changing a few words and adding quite a few further comments. These are such comments as can reasonably be added in the informal and relaxing setting of the blogosphere, while being less appropriate in the more formal setting of the Handbook.
I will use boldface italics, like this, for the essay as it appears online, and [ordinary lightface type in square brackets, like this] for my various additions or overwrites.
I will leave out here just one major thing, a (perhaps for my present readership somewhat dry) discussion of traditional star names and the current International Astronomical Union star-names legislation.
This sample of 314 stars, our own Sun included ("Sample S"), is found to lie in a region, around 3000 [light years] in radius, essentially confined to the sandwich-filler, or "thin disk", part of the overall galactic disk.
[Our Galaxy is a "barred spiral", dominating our "Local Group" of a couple of dozen galaxies, along with the similarly dominant glorious small-telescope spiral galaxy which is M31. Our Galaxy - similar in its size to M31, and larger than many - comprises a sandwich of two thin pita breads, with an exceptionally star-rich, yet exceptionally thin, filling. Around this buzzes a big, rarefied, spherical halo of distinctively ancient stars. There is additionally invisible, or "dark", matter, readily deduced from the unexpectedly speeds of the stars we see, as each of them orbits the centre-of-mass of the aggregate mass its particular closed-curve orbit encloses: the pull of gravity, and therefore the observed speeds, are found to be too high if all the matter in our Galaxy is presumed visible.]
Of the few Sample-S interlopers born outside the sandwich filling, and now temporarily passing through it on orbits oblique to the thin disk, the best known is α Boo. [This is the prominent star, "Arcturus", reached by following the arc of the Big Dipper Handle. As one says in DDO dark-lawn lectures, "Arc to Arcturus" (and thereafter, as one leaves Bootes for Virgo, closer to the Richmond Hill horizon, "Straight on to Spica").]
It is convenient here to use the term "Population P" for the ensemble of non-brown-dwarf, non-white-dwarf, stars in the much larger, 3000-[light-year] radius, subdisk-of-the-thin-disk from which our (tiny) Sample S is drawn. This P-region is itself only a (tiny) fraction of the overall galactic thin-disk region, [around 50,000 light years in radius]. [On this scale, our own Solar system fades into insignificance, with the Moon a mere light-second away from Earth, the Earth a mere eight light-minutes away from the Sun, and Jupiter a mere 40-odd light-minutes away from the Sun. Typical prominent DDO lawn-or-catwalk stars are a mere few tens or hundreds of light-years away. It is, to be sure, possible to impress DDO tourists by exhibiting the "Summer Triangle" of Deneb, Vega, and Altair, and remarking that whereas so-prominent Vega and Altair are on the order of 20 light-years away, Deneb - it looks scarcely less bright than Vega and Altair! - lies at an uncertain, yet hefty, distance, being somewhere between 1000 and 3000 light-years away. It is also helpful to remember that M31, so easily seen from even blazingly bright downtown Toronto on autumn evenings through even the poorest binoculars, is about 2.5 million light-years away. - Our Galaxy, and M31, and with it our whole Local Group, diminish into insignificance on a cosmic scale, since the visible universe runs outward to a radius of around 10 thousand million light-years. Beyond that prodigious radius - if, as has since the 1980s been thought, the Big Bang was swiftly followed by a dramatic cosmic "Inflation" - is a part of the universe presently invisible, and with every passing second coming incrementally more and more into the range of visibility. - Our own Galaxy is thought to harbour a hundred thousand million or more, but not as many as a million million, stars. The number of galaxies in the currently visible universe is considered comparable (by sheer coincidence) to the number of stars in our own (anomalously big?) Galaxy.]
Sample S, being formally defined by an apparent-magnitude cutoff as opposed to a distance cutoff, is itself far from statistically representative of Population P. (a) In P, the O stars are vanishingly rare. A tabulation by Glenn Ledrew, in JRASC 95 (2001), pp. 32ff, suggests an O-star frequency within P of just 0.00003%. By contrast, O stars comprise a hefty 2% of S. A similar overrepresentation occurs for the B, A, F, G, and K stars, with Ledrew’s tabulation suggesting that these [temperature types] might have a respective frequency within P of 0.1%, 0.6%, 3.2%, 8.0%, and 12.9%. (A small caveat: unavoidable rounding errors make our various percentages, throughout this discussion, capable of adding up to 99% instead of 100%, or to 99.9% instead of 100.0%.) By contrast, these five types comprise 28%, 19%, 9%, 13%, and 21%, respectively, of S. (b) In P, something on the order of 76% or 78%–different authorities are perhaps mildly discrepant–must be M stars. (Ledrew’s tabulation, in particular, suggests an M-star frequency of 78.2%.) Only a few of these (the Ledrew tabulation suggests 0.04%) have evolved to beyond the main-sequence stage of stable core hydrogen fusion. By contrast, the M stars comprise just 7% of S. All of them have evolved beyond the main sequence, having started their lives as types hotter than M or K.
[Folks, I would like to do this better. "JRASC" is our very own publication, the Journal of the Royal Astronomical Society of Canada. But more careful work would involve digging into the properly authoritative catalogue of our solar region - I think that these days it is Gliese-Jahreiss - and punching buttons on one's own calculator.]
The statistically anomalous character of S is further illustrated by the fact that in S, in each of the Big Six [temperature types] hotter than M [that is to say, in each of the six "Oh Be A Fine Gymnast, Kiss" types], the stars that have ended stable core hydrogen fusion (and so have evolved [into one of the specially high-power-output luminosity classes] are in the numerical majority. In Ledrew’s tabulation, the percentages of evolved stars in F, G, and K, as a percentage of the overall respective F, G, and K populations, are just 2.0%, 2.5%, and 3.8%. Consistently with this, the 1991 Gliese-Jahreiss catalogue of the nearest 1000 stars (containing, we admit, [even white dwarfs]) assigns less than 1% of its population to [the specialliy high-power-output luminosity classes].
[It is perhaps helpful here to interject that there are two foundational aspects stellar taxonomy, so far as basic spectroscopic observation (as at the pre-2008 DDO) can go: there are on the one hand the temperature "types", on the other hand the luminosity, or intrinsic-power-output-rating, "classes". For present blogspot purposes, it is enough simply to remark that the power-output-rating classes get denominated V, IV, III, II, Ib, and Ia, with additionally "0" for very rare, exceptionally powerful, stars - and with VI for a class of anomalously low-power stars. I will here leave out the white dwarfs, and also those major discoveries of the last 20 years which are the brown dwarfs. Our Sun is technically of temperature type G (more precisely, "G2", being in one of the hotter subdivisions of "G") and luminosity class V. Stars begin their stable, core-hydrogen-fusing lives in class V. In later age, they temporarily evolve into some higher-power-output class, as their stable fusing of core hydrogen comes to an end. Stars of types K and M fuse their hydrogen so slowly, in a low-temperature regime, that however close the time of their birth may happen to be to the Big Bang, they have nevertheless not yet managed to evolve out of class V.]
Sample S – so rich, we stress, in varieties of star statistically infrequent within Population P – harbours physical extremes. Although the extremes are for the most part not written into our table, they can be studied easily, from such sources as Prof. James Kaler’s http://stars.astro.illinois.edu/sow/sowlist.html:
Around 58 of our 314 each radiate, across the full spectrum from X-ray through [ultraviolet] and optical to [infrared] and radio, at least as much power as is radiated by 10,000 Suns. The most dramatic is ζ Ori, with a [full-spectrum power output] of 375,000 Suns – making ζ Ori notable not within S alone, but even within the overall Galaxy. Several others are not far behind, among them ζ Pup (360,000 Suns, suggests Kaler, as of 2008 July revising his earlier, circa-1999, suggestion of ~750,000 Suns). Just two stars in Sample S, nearby τ Cet and nearby α Cen B, radiate more feebly than our Sun, each at about 0.5 of the Sun’s [full-spectrum power output]. – The principal determinant of stellar [power output], for any given phase in stellar evolution, is mass, with even small variations in mass translating into large variations in [power]. The exceptional luminosities of ζ Ori and ζ Pup, in particular, are a consequence of their exceptionally high respective masses, 20 [times the solar mass] and 40 [times the solar mass]. (Kaler now suggests 40 [solar masses] for ζ Pup, while having previously suggested 60 [solar masses]. He additionally notes from the literature the lower suggested value of 22.5 [solar masses]. – Theory does predict, although our small Sample S does not manage to illustrate, the possibility of masses up to the Eddington stellar mass limit, somewhere above 100 [solar masses], and even of some "super-Eddington" stars. Eddington's limit is by definition attained when luminosity rises to high as to make the outward radiation push, tending to tear a star apart, exceed the inward gravitational pull.)
We use the flags "+nP" (n = 1, 2, ... ) for companions of sub-stellar mass, such as have been found outside our solar system, in an accelerating tempo of discovery that has eventually reached even the tiny Sample S, from the 1990s onward. Such companions are typically planets, but could in principle also be brown dwarfs. We do not attempt here to define formally the difference between a planet and a brown-dwarf companion. [In the 2018 online table, as interested readers can see for themselves, are two occurrences of "+2P" and ten occurrences of "+1P". Our own Sun would traditionally have flown the flag "+9P", but here (in light of the 2006 August International Astronomical Union demotion of Pluto from "planet" to "dwarf planet") is instead flagged "+8P". Admittedly, in the case of the Sun one could argue - disregarding, in this suggested line of argument, mere asteroids on the scale of Ceres or smaller - that Pluto AND other big Kuiper-belt objects count as "companions of sub-stellar mass", and that therefore an appropriate flag is something like "+10P" or " or "+11P" or (cautiously) "+>9P".]
/.../
I may as well finish with an expression of puzzlement, which some prof somewhere might some day be able somehow to address - perhaps even by posting a comment through the present blogspot interface, in conformity with the commenting rules as laid out here on 2016-04-14 under the heading "Background FAQ, regarding purpose and conduct of this blog".
The core of those rules is the requirement that any commenter give, within the comment body text, her or his personal name, her or his municipality of residence and nation of residence, and a functioning personal e-mail address. I would reserve the right to authenticate the identity of the commenter, by sending a mail to the offered e-mail address, in those conceivable cases in which the commenter's real identity seems somehow uncertain, in other words in which there is some fear of impersonation.
Here is my puzzle: why does type M appear to be more heavily populated than types L, T, and Y? What guarantee have we that the M stars really predominate?
The problem seems particularly acute once we note that the brown dwarfs in L, T, and Y are hard to observe, and therefore hard to tally for statistical analysis. To make matters worse, they are liable to a fading-out which may, for all I myself know at this stage, be early and severe. In contrast with stars in the hotter types, stars in L, T, and Y obtain their (feeble) power output in just one of two ways:
The first process is notorious for not lasting long. If it was this that was the source of our Sun's heat (such was one prominent opinion in Victorian, and therefore pre-nuclear, astrophysics discussions), our Sun could shine for only on the order of 10 million years. The accepted actual age of the Sun is 4,600 million years.
Even the second process (say I a little timidly this week, subject to professorial correction) does not last long on the cosmic scale.
So how do we know that the cosmos is not populated with an abundance of not-yet-observed brown dwarfs, each of them by now quite finished releasing energy from the (notoriously brief) process of gravitational contraction, and by now likewise quite finished with the fusing of its (scanty, rare) deuterium?
I am not reproaching the professionals with not knowing. My hunch is that they do know. But - ask I, in a student's puzzlement - how (through, that is, what perhaps subtle combination of observations and deductions) do they know it?
[This is the end of the current blog posting.]
The bulk of the content is however, of necessity nation-neutral. Our long table of the brightest stars covers the entire sky, right down to the South celestial pole (stationary on the horizon, as the night advances, should you be on the Equator - for instance, on the open sea just south of Singapore - but stationary all through the night below it if you are in Canada, in the United States, or in Mexico). Such things as lists of eclipses, and lists of meteor showers, are likewise suited to the needs of a global readership. The table of planetary occultations, common to the two print editions, concentrates on events visible from North America and Hawai'i.
****
Over the last few months, I put a lot of effort not only into updates and expansions of the 314 entries in our online brightest-stars table, but additionally into something we have not attempted in previous years - namely, into an online essay highlighting the specially unrepresentative character of the visually brightest stars.
It is sometimes said, in a kind of popular-science National Geographic or USA Public Broadcasting System spirit, that our own Sun is an "average star". This is misleading. Our Sun, being a stable core-hydrogen-fuser of temperature type "G", is in its visible outer layers (much) hotter than the visible outer layers of what is believed the most common of the temperature types, "M".
Here is an adaptation of the traditional temperature-types mnemonic, arranged from the hottest of the traditionally familiar types down to the coldest of the traditionally familiar types: OABFGKM - "Oh Be A Fine Gymnast, Kiss Me". (Well, strict tradition did unfortunately say "Girl".)
"O" remains nowadays the hottest. But at the cool end there are as additions, accommodating recently discovered brown dwarfs, "L", "T", and (most recently of all, and I think so far only thinly populated by discoveries) "Y". Further, there are special traditional classification letters "R" and "N" (more recently consolidated under the single letter "C") and "S". These denote certain not-very-hot stars presenting chemical anomalies, notably (in the case of the erstwhile R and N, or nowadays C) an excess of visible-layers carbon. It is thus necessary to replace the old mnemonic with the following: "Oh Be A Fine Gymnast, Kiss Me (Right Now, Smack; or to be more up-to-date Chomp, Smack), Like This, Yowee."
A really full mnemonic would additionally have to accommodate the hot, heavily mass-ejecting, Wolf-Rayets, or "W" stars ("Wow! Oh Be A Fine...").
But I must put these details of temperature types (strictly speaking, of temperature-driven spectroscopy features) aside. My main point here is only that our familiar Sun, plus our familiar visually bright night-time stars, as prominent from what I remember of the DDO lawn and catwalk, are in a statistical sense unrepresentative. In the online essay, as I wrote it to supplement the two parallel 2018 printed Handbook editions, I develop my point at length.
I would like to reproduce the greater part of the essay here, changing a few words and adding quite a few further comments. These are such comments as can reasonably be added in the informal and relaxing setting of the blogosphere, while being less appropriate in the more formal setting of the Handbook.
I will use boldface italics, like this, for the essay as it appears online, and [ordinary lightface type in square brackets, like this] for my various additions or overwrites.
I will leave out here just one major thing, a (perhaps for my present readership somewhat dry) discussion of traditional star names and the current International Astronomical Union star-names legislation.
****
This sample of 314 stars, our own Sun included ("Sample S"), is found to lie in a region, around 3000 [light years] in radius, essentially confined to the sandwich-filler, or "thin disk", part of the overall galactic disk.
[Our Galaxy is a "barred spiral", dominating our "Local Group" of a couple of dozen galaxies, along with the similarly dominant glorious small-telescope spiral galaxy which is M31. Our Galaxy - similar in its size to M31, and larger than many - comprises a sandwich of two thin pita breads, with an exceptionally star-rich, yet exceptionally thin, filling. Around this buzzes a big, rarefied, spherical halo of distinctively ancient stars. There is additionally invisible, or "dark", matter, readily deduced from the unexpectedly speeds of the stars we see, as each of them orbits the centre-of-mass of the aggregate mass its particular closed-curve orbit encloses: the pull of gravity, and therefore the observed speeds, are found to be too high if all the matter in our Galaxy is presumed visible.]
Of the few Sample-S interlopers born outside the sandwich filling, and now temporarily passing through it on orbits oblique to the thin disk, the best known is α Boo. [This is the prominent star, "Arcturus", reached by following the arc of the Big Dipper Handle. As one says in DDO dark-lawn lectures, "Arc to Arcturus" (and thereafter, as one leaves Bootes for Virgo, closer to the Richmond Hill horizon, "Straight on to Spica").]
It is convenient here to use the term "Population P" for the ensemble of non-brown-dwarf, non-white-dwarf, stars in the much larger, 3000-[light-year] radius, subdisk-of-the-thin-disk from which our (tiny) Sample S is drawn. This P-region is itself only a (tiny) fraction of the overall galactic thin-disk region, [around 50,000 light years in radius]. [On this scale, our own Solar system fades into insignificance, with the Moon a mere light-second away from Earth, the Earth a mere eight light-minutes away from the Sun, and Jupiter a mere 40-odd light-minutes away from the Sun. Typical prominent DDO lawn-or-catwalk stars are a mere few tens or hundreds of light-years away. It is, to be sure, possible to impress DDO tourists by exhibiting the "Summer Triangle" of Deneb, Vega, and Altair, and remarking that whereas so-prominent Vega and Altair are on the order of 20 light-years away, Deneb - it looks scarcely less bright than Vega and Altair! - lies at an uncertain, yet hefty, distance, being somewhere between 1000 and 3000 light-years away. It is also helpful to remember that M31, so easily seen from even blazingly bright downtown Toronto on autumn evenings through even the poorest binoculars, is about 2.5 million light-years away. - Our Galaxy, and M31, and with it our whole Local Group, diminish into insignificance on a cosmic scale, since the visible universe runs outward to a radius of around 10 thousand million light-years. Beyond that prodigious radius - if, as has since the 1980s been thought, the Big Bang was swiftly followed by a dramatic cosmic "Inflation" - is a part of the universe presently invisible, and with every passing second coming incrementally more and more into the range of visibility. - Our own Galaxy is thought to harbour a hundred thousand million or more, but not as many as a million million, stars. The number of galaxies in the currently visible universe is considered comparable (by sheer coincidence) to the number of stars in our own (anomalously big?) Galaxy.]
Sample S, being formally defined by an apparent-magnitude cutoff as opposed to a distance cutoff, is itself far from statistically representative of Population P. (a) In P, the O stars are vanishingly rare. A tabulation by Glenn Ledrew, in JRASC 95 (2001), pp. 32ff, suggests an O-star frequency within P of just 0.00003%. By contrast, O stars comprise a hefty 2% of S. A similar overrepresentation occurs for the B, A, F, G, and K stars, with Ledrew’s tabulation suggesting that these [temperature types] might have a respective frequency within P of 0.1%, 0.6%, 3.2%, 8.0%, and 12.9%. (A small caveat: unavoidable rounding errors make our various percentages, throughout this discussion, capable of adding up to 99% instead of 100%, or to 99.9% instead of 100.0%.) By contrast, these five types comprise 28%, 19%, 9%, 13%, and 21%, respectively, of S. (b) In P, something on the order of 76% or 78%–different authorities are perhaps mildly discrepant–must be M stars. (Ledrew’s tabulation, in particular, suggests an M-star frequency of 78.2%.) Only a few of these (the Ledrew tabulation suggests 0.04%) have evolved to beyond the main-sequence stage of stable core hydrogen fusion. By contrast, the M stars comprise just 7% of S. All of them have evolved beyond the main sequence, having started their lives as types hotter than M or K.
[Folks, I would like to do this better. "JRASC" is our very own publication, the Journal of the Royal Astronomical Society of Canada. But more careful work would involve digging into the properly authoritative catalogue of our solar region - I think that these days it is Gliese-Jahreiss - and punching buttons on one's own calculator.]
The statistically anomalous character of S is further illustrated by the fact that in S, in each of the Big Six [temperature types] hotter than M [that is to say, in each of the six "Oh Be A Fine Gymnast, Kiss" types], the stars that have ended stable core hydrogen fusion (and so have evolved [into one of the specially high-power-output luminosity classes] are in the numerical majority. In Ledrew’s tabulation, the percentages of evolved stars in F, G, and K, as a percentage of the overall respective F, G, and K populations, are just 2.0%, 2.5%, and 3.8%. Consistently with this, the 1991 Gliese-Jahreiss catalogue of the nearest 1000 stars (containing, we admit, [even white dwarfs]) assigns less than 1% of its population to [the specialliy high-power-output luminosity classes].
[It is perhaps helpful here to interject that there are two foundational aspects stellar taxonomy, so far as basic spectroscopic observation (as at the pre-2008 DDO) can go: there are on the one hand the temperature "types", on the other hand the luminosity, or intrinsic-power-output-rating, "classes". For present blogspot purposes, it is enough simply to remark that the power-output-rating classes get denominated V, IV, III, II, Ib, and Ia, with additionally "0" for very rare, exceptionally powerful, stars - and with VI for a class of anomalously low-power stars. I will here leave out the white dwarfs, and also those major discoveries of the last 20 years which are the brown dwarfs. Our Sun is technically of temperature type G (more precisely, "G2", being in one of the hotter subdivisions of "G") and luminosity class V. Stars begin their stable, core-hydrogen-fusing lives in class V. In later age, they temporarily evolve into some higher-power-output class, as their stable fusing of core hydrogen comes to an end. Stars of types K and M fuse their hydrogen so slowly, in a low-temperature regime, that however close the time of their birth may happen to be to the Big Bang, they have nevertheless not yet managed to evolve out of class V.]
Sample S – so rich, we stress, in varieties of star statistically infrequent within Population P – harbours physical extremes. Although the extremes are for the most part not written into our table, they can be studied easily, from such sources as Prof. James Kaler’s http://stars.astro.illinois.edu/sow/sowlist.html:
Around 58 of our 314 each radiate, across the full spectrum from X-ray through [ultraviolet] and optical to [infrared] and radio, at least as much power as is radiated by 10,000 Suns. The most dramatic is ζ Ori, with a [full-spectrum power output] of 375,000 Suns – making ζ Ori notable not within S alone, but even within the overall Galaxy. Several others are not far behind, among them ζ Pup (360,000 Suns, suggests Kaler, as of 2008 July revising his earlier, circa-1999, suggestion of ~750,000 Suns). Just two stars in Sample S, nearby τ Cet and nearby α Cen B, radiate more feebly than our Sun, each at about 0.5 of the Sun’s [full-spectrum power output]. – The principal determinant of stellar [power output], for any given phase in stellar evolution, is mass, with even small variations in mass translating into large variations in [power]. The exceptional luminosities of ζ Ori and ζ Pup, in particular, are a consequence of their exceptionally high respective masses, 20 [times the solar mass] and 40 [times the solar mass]. (Kaler now suggests 40 [solar masses] for ζ Pup, while having previously suggested 60 [solar masses]. He additionally notes from the literature the lower suggested value of 22.5 [solar masses]. – Theory does predict, although our small Sample S does not manage to illustrate, the possibility of masses up to the Eddington stellar mass limit, somewhere above 100 [solar masses], and even of some "super-Eddington" stars. Eddington's limit is by definition attained when luminosity rises to high as to make the outward radiation push, tending to tear a star apart, exceed the inward gravitational pull.)
We use the flags "+nP" (n = 1, 2, ... ) for companions of sub-stellar mass, such as have been found outside our solar system, in an accelerating tempo of discovery that has eventually reached even the tiny Sample S, from the 1990s onward. Such companions are typically planets, but could in principle also be brown dwarfs. We do not attempt here to define formally the difference between a planet and a brown-dwarf companion. [In the 2018 online table, as interested readers can see for themselves, are two occurrences of "+2P" and ten occurrences of "+1P". Our own Sun would traditionally have flown the flag "+9P", but here (in light of the 2006 August International Astronomical Union demotion of Pluto from "planet" to "dwarf planet") is instead flagged "+8P". Admittedly, in the case of the Sun one could argue - disregarding, in this suggested line of argument, mere asteroids on the scale of Ceres or smaller - that Pluto AND other big Kuiper-belt objects count as "companions of sub-stellar mass", and that therefore an appropriate flag is something like "+10P" or " or "+11P" or (cautiously) "+>9P".]
/.../
****
I may as well finish with an expression of puzzlement, which some prof somewhere might some day be able somehow to address - perhaps even by posting a comment through the present blogspot interface, in conformity with the commenting rules as laid out here on 2016-04-14 under the heading "Background FAQ, regarding purpose and conduct of this blog".
The core of those rules is the requirement that any commenter give, within the comment body text, her or his personal name, her or his municipality of residence and nation of residence, and a functioning personal e-mail address. I would reserve the right to authenticate the identity of the commenter, by sending a mail to the offered e-mail address, in those conceivable cases in which the commenter's real identity seems somehow uncertain, in other words in which there is some fear of impersonation.
Here is my puzzle: why does type M appear to be more heavily populated than types L, T, and Y? What guarantee have we that the M stars really predominate?
The problem seems particularly acute once we note that the brown dwarfs in L, T, and Y are hard to observe, and therefore hard to tally for statistical analysis. To make matters worse, they are liable to a fading-out which may, for all I myself know at this stage, be early and severe. In contrast with stars in the hotter types, stars in L, T, and Y obtain their (feeble) power output in just one of two ways:
- they undergo gravitational contraction, translating the gravitational potential energy lost upon falling-inward into the increased kinetic energy of their atoms or molecules (and therefore into raised temperatures: the faster the atoms and molecules are moving, the hotter is the star, and therefore the more power it is radiating - in the case of L, T, and Y, always largely at infrared, rather than at even yellow, orange, or red wavelengths);
- they fuse deuteriuim (as opposed to the more common, no-neutron, isotope of hydrogen) into helium.
The first process is notorious for not lasting long. If it was this that was the source of our Sun's heat (such was one prominent opinion in Victorian, and therefore pre-nuclear, astrophysics discussions), our Sun could shine for only on the order of 10 million years. The accepted actual age of the Sun is 4,600 million years.
Even the second process (say I a little timidly this week, subject to professorial correction) does not last long on the cosmic scale.
So how do we know that the cosmos is not populated with an abundance of not-yet-observed brown dwarfs, each of them by now quite finished releasing energy from the (notoriously brief) process of gravitational contraction, and by now likewise quite finished with the fusing of its (scanty, rare) deuterium?
I am not reproaching the professionals with not knowing. My hunch is that they do know. But - ask I, in a student's puzzlement - how (through, that is, what perhaps subtle combination of observations and deductions) do they know it?
[This is the end of the current blog posting.]
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