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View Full Version : Do we need the OBAFGKM sequence any more?



fagricipni
2011-Oct-19, 08:11 AM
What I am thinking is that as I understand it that these designations are simply a another way of stating the "surface" temperature of the star; as I understand it, they were based on the strength of particular spectral absorption lines in the star's light, but now we know that is determined by the "surface" temperature of the star only. There are a few "odd" stars that have other spectral absorption lines; eg, W, or WC -- I don't recall if that is also true of the S, N, and R stars --, but my question is about the primary sequence, not the special types. (Though, I'd not object to being told about the special types in addition to the primary sequence.)

antoniseb
2011-Oct-19, 09:08 AM
Need it? maybe not. Is it handy? I think so. I think that our vocabulary for discussing stars includes these designations, and will for a long time.

StupendousMan
2011-Oct-19, 12:06 PM
Spectral types denote properties of a star including temperature and surface gravity and chemical composition and magnetic field and stellar rotation, at times. Astronomers find that a description such as "G2V" to be a lot more compact and convenient than the words which would be needed to describe temperature, surface gravity, etc., etc.

ngc3314
2011-Oct-19, 12:34 PM
Also, as with the magnitude system - the stellar spectral classifications are what they are, derived pretty directly from observed properties such as ratios of absorption lines and their widths. Nothing about them changes if the calibration to temperature/surface gravity is revised, so they remain equally useful should there be an update to these translations to physical quantities.

Hornblower
2011-Oct-19, 08:31 PM
Be advised that when all types are included, it is not a simple sequential function of temperature. For example, type S, which often has technetium lines in the spectrum, is in roughly the same temperature range as ordinary type M red giants.

fagricipni
2011-Oct-19, 11:40 PM
What gets me about the primary types is that a type A, for instance, describes Sirius B, a white dwarf; Altair, a main sequence star; and Deneb, a supergiant. Now, I find it hard to think of these stars as being of the same "type" in any real sense (I know that the "odd" types likely describe stars more similar that the primary types); given that a primary type like A describes three such very different stars and that the letters are no longer even in a logical order, I was thinking that one could replace the primary type designations by a simple description of "surface" temperature. I know that it is unlikely that it will be actually done.

StupendousMan
2011-Oct-20, 01:04 AM
You asked a question. Astronomers answered your question.

Yes, one could stop using the existing spectral classification in favor of a new one based on temperature.

No, the astronomical community is not going to make the change you suggest.

EDG
2011-Oct-20, 06:37 AM
I don't think there's any need to be so snippy at the OP.

He raises a good point I think, but the spectral type is only part of the description and isn't an awful lot of use on its own - as he points out himself, the size is the other part. And also, the OBAFGKM sequence is a handy shorthand to describe the star - e.g. An M V star is a red dwarf, a K III is a red giant.

fagricipni
2011-Oct-20, 06:53 AM
For some reason, though, I have never seen anyone talk about a "V class" star, whereas, I have often seen talk about "G class" stars; ie, I have seen the bare spectral type talked about, but not the bare I-V types talked about; even though, it would seem that they are more important to describing a star. At any rate, it seems that the answer is that the "primary" types are just an awkward shorthand for referring to "surface" temperature (awkward because the now-known-to-be illogical ordering), just as I had thought.

StupendousMan
2011-Oct-20, 11:27 AM
Astronomers don't say "V class" star, they say "dwarf star" or "main sequence star". Likewise, we say "subgiant star" instead of "IV class."

I can't argue with the statement that the ordering of spectral classes is illogical. So is the spelling of English words, the relationship of feet to miles, the US tax system, the layout of the city streets in Boston and London, and just about every other aspect of human life. Some people deal with things as they are, others try to make things more logical. Apparently, astronomers are among those who deal with things as they are.

Hornblower
2011-Oct-20, 05:18 PM
Do we need it? No, we could reassign the letters so that those seven would be in order of descending temperature. But with human nature being what it is, there is comfort associated with something historically familiar, even though archaic. As a practical matter, sticking with the same system when referring to old records that use it helps to eliminate possible confusion. This sequence is easy to read and to remember, at least for my feeble brain. I memorized it quickly as an inquisitive child, without the aid of what I consider to be silly mnemonics.

The familiar sequence originally was based on the strength of the Balmer absorption lines of excited hydrogen, along with the lines of other substances that occur in stellar spectra. Type A has the strongest hydrogen lines and little else. B has weaker hydrogen along with neutral helium. F has similarly weaker hydrogen without the helium but with the K line of ionized calcium. As we progress through G, K and M the hydrogen continues to weaken, the calcium gets stronger, neutral metals appear, and finally titanium oxide in K and especially M. So far this is a logical sequence. Rare stars with weak hydrogen but strongly ionized helium and metals are classified as O. As classification work proceeded, many intervening letters were dropped and numbers used to denote intermediate types. Thus we have G2 for the Sun, about 2/10 of the way from the F-G boundary to the G-K boundary.

Much of this work was done with photographic plates which were sensitive only to blue, violet and near-ultraviolet. This gave very little from which to infer temperature. Later, as panchromatic plates became available, the blackbody signature could be recorded and we discovered that O and B stars were actually hotter than A stars.

The Roman numerals for the luminosity class are based on the width of the spectral lines, which tend to be narrow for giants and broad for main sequence stars. Using Roman numerals for this item avoids confusion with the numerical subdivisions. Unlike the long strings on many copyright dates and Super Bowls, they are easy to read.
Thus we have the term G2V for the Sun, where G2 is near the hotter end of the G range, and V indicates a main sequence star.

I see no compelling need to change this.

fagricipni
2011-Oct-20, 06:11 PM
As a practical matter, sticking with the same system when referring to old records that use it helps to eliminate possible confusion.

Yes, there is always the cost of changing the system; which is a often a consideration where a new system would be better than an old system, but there is difficulty in using the old system and new system at the same time.


Much of this work was done with photographic plates which were sensitive only to blue, violet and near-ultraviolet. This gave very little from which to infer temperature.

The Roman numerals for the luminosity class are based on the width of the spectral lines, which tend to be narrow for giants and broad for main sequence stars. Using Roman numerals for this item avoids confusion with the numerical subdivisions.

I was aware of the fact that hydrogen lines were a significant factor in the ordering of the old system; ie, it wasn't in an illogical order at the time it was invented; the changes in knowledge after in was well in place caused that issue. I was unaware of the other details that you mentioned; I find it particularly interesting that the limitations of the instruments available at the time the system was invented prevented the discovery of the fact that most of the spectral lines were strongly correlated with the "surface" temperature of the stars.

My actual question was whether there was anything other that "surface" temperature determining a "primary" type and has been answered; the rest is just details of why I thought of the the initial question.