Hi, everyone!

I'm writing a science fiction story which features the nearby star 18 Scorpii. In doing research for this I have caught the following:

http://www.solstation.com/stars2/18sco.htm
I'm not sure what to make of this. I can find no other references to this possibility and the links don't provide enough context for my untrained interpretation. Is it that there is a faint, distant companion?

Thanks for any input!

I've examined the POSS2 photographic plates -- there is no object at a distance of about 26 arcseconds. There is no object at that distance on the K-band (infrared 2.2 microns) image of 2MASS, either.

The Tycho-2 catalog contains a notice about duplicity which states:

My conclusion: no companion, don't worry, write your story.

Welcome to BAUT, QuackDamnYou! Did your nick originate from the Mythbusters episode exploring the echo of quacking ducks?

__________________
"Ignorance more frequently begets confidence than does knowledge" -- Charles Darwin
"Your right to hold an opinion is not being contested. Your expectation that it be taken seriously is." -- Jason Thompson
Meet the OOONG TOE.

18 Sco was regarded as the best solar twin, but I think another was announced a year or so ago that was slightly better. It is very similar to the Sun in almost every respect.

Contrary to the article's color statement, "18 Scorpii is a yellow-orange main sequence dwarf star", 18 Sco is white, like the Sun. But don't call it a white dwarf.

__________________
Lighten up! This is a stellar board!

Thanks for the info! I will continue researching so I don't make any big mistakes, but that one little niggle was killing me.

And slang, yes, it's from Mythbusters. I'm a huge fan but can hardly find time to watch it any more.

Indeed no! The Sun is a G type star & these are generically known as yellow dwarfs. This is more a star type label than a reflection of the stars actual colour.

My apologies if I'm belabouring the obvious here & esp. for those already clear on this but for those who aren't aware the system for stellar classification for main-sequence*or dwarf stars runs like this :

0 & B stars = blue dwarfs

A & some F stars = Sirian dwarfs (white stars)

F dwarfs = Procyonese / Yellow-white dwarfs

Some F plus G stars & some K dwarfs = yellow dwarfs / sunlike stars

K dwarfs = orange dwarfs

Some K dwarfs & M dwarfs = Red dwarfs

T & L dwarfs = brown dwarfs

The general colour tags here are not strictly accurate & star colours are subtle. Often *very* subtle esp. for fainter stars where the Human eye doesn't percieve much colour for biological reasons. Brown dwarfs are (apparently) actually magneta or mauve-ish in hue, many A type stars such as Vega and Sirius are frequently described as blue~ish in their tint and one or two stars appear green at least sometimes and to some observers. (Beta Librae or Zubeneschamali is one example of this rare colour - but not everyone sees it as such esp. not all the time.)

White dwarfs are actually not true stars at all but "stellar corpses" that have finished nuclear "burning" and are just gradually cooling down - these are not all white with some having cooled down to the yellow or even red end of the spectrum. Eventually, they will lose their last bit of heat and become cold black inert crystals - "black dwarfs" but this is thought to take place over so many trillions of years that there hasn't been enough time for one such "star" type to form yet. (A black dwarf would be exceedingly hard to detect anyhow.)

To avoid confusion with white dwarfs A type dwarfs are sometimes termed Sirian after the brightest example of this class** & F type stars likewise termed Procyonese after the nearest example** of this class.

Also I have in at least one source (P.13, Peter Lancaster Brown, 'Star & Planet Spotting', Sterling, 1974.) seen the very hottest Wolf-Rayet or W type stars described as greenish or green hot! But these are very rare and unusual "evolved" stars that are not main-sequence dwarfs and can be set aside for now. The most vividly coloured stars are probably some of the giants of classes M or C. (N, R, S, & C are "chemical": rather than "temperature" spectral classes and mostly overlap with K & M type giants.)

It also really unlocks the code of stars to know the Luminosity classes which are :

0 Hypergiant stars - the most extreme examples of high mass, huge stars, eg. Eta Carinae, Pistol Star, Rho Cassiopiae.

I supergiants - subdivided by Ia bright and Ib less bright with other variants eg. Iab seen too. Eg. Betelgeux, Rigel, Antares.

II bright giant - self-explanatory really & I sometimes wonder whether this is that necessary a class but still, it is. Eg. Scheat (Beta Pegasi) & GaCrux (Gamma Crucis)

III Giant - what our Sun will become millions of years hence when it uses up thits core hydrogen and has to fuse the helium ash. Eg. Mira, Arcturus & Aldebaran

IV Sub-giant - Transition stage from dwarf to giant. Eg. Some astronomers believe Procyon is beginning this stage. Also Errai (Gamma Ceti) and Alpha Mensae.

V Dwarf / Main-sequence* - This means in essence that the star is fusing hydrogen (H) into helium (He) at its core. 90% of all stars fall into the main-sequence. Eg.. Sun, Sirius, Alpha Centauri, Altair, Regulus, Vega & Zubeneschamali.

VI subdwarf* - are very metal poor stars which are otherwise like main-sequence dwarfs & which also fuse H into He at their cores. Also denoted by the prefix sd. Eg. I think Kapteyn's Star, Groombridge 1830 & Mu Cassiopiae. See : http://en.wikipedia.org/wiki/Subdwarf_star for more.

VII white dwarf - White dwarfs are actually not true nuclera fusing stars at all but "stellar corpses" that have finished nuclear "burning" and are just gradually cooling down. Eg. The Pup or Sirius B**, Procyon B or the Lil' Puppy**, Van Maanen's Star.


Hope that summary was useful & worthwhile for at least some folks here. If folks want to learn more I would highly recommend James Kaler's stars website : http://stars.astro.illinois.edu/sow/star_intro.html


--------------------------------------------------

* This means in essence that the star is fusing hydrogen (H) into helium (He) at its core. 90% of all stars fall into the main-sequence and exceptions are white dwarfs (cooling embers that have finished their active stellar lives also neutron stars and Black Holes) supergiants, giants and sub-giants that are fusing helium and other heavier elements instead of H at their cores & sub-dwarfs that are very metal poor stars which are otherwise like main-sequence dwarfs & which also fuse H into He at their cores.

** Sirius is also the brightest star in the night sky or so it appears from Earth, as well as the nearest A type star. It is also known as the Dogstar and Alpha Canis Majoris. Procyon (Alpha Canis Minoris) a.k.a. the "little Dogstar" is co-incidentally the nearest F-type star, and the brightest F type dwarf (Bright giant or supergiant F star Canopus or Alpha Carinae is brighter and the brightest F star overall although I have seen it listed as A9 too.) Weirdly enough both Sirius and Procyon have white dwarf companions which were theoretically predicted before being spotted. The stars are unrelated though Procyon being either 1.7 or three Billion yeras old and Sirius around 250 million years old. However, its a curious thing that, purely by co-incidence, the "Barking quartet" of Dogstar & Pup & Little dogstar and Puppy have so much in common!

__________________

Why is this? Shouldn't the color label represent the color?

No apologies necessary. I undestand where you're coming from and it is likely me that is belabouring the issue. I do it partly to have some fun with it because I honestly think it is something somewhat trivial, yet I think it doesn't hurt to recognize true color, when available, just for the sake of knowledge, if nothing else. Even though color is often subjective, I suspect that this is not the case for the Sun; there is enough objective evidence for an air-tight case against the Sun being yellow.

Yes and colors are perceived slightly different by different people.

Yes. Often some colors are pereceived to be different than their class because of the color of the adjacent star. This is part of our color processing within the brain. The cooler stars that have far more molecular compounds will allow a much greater variation in color, I think. Grant introduced a T-class star that is maroon (crimson).

I had thought that most white dwarfs were hotter and I have not seen a white dwarf cool enough to be red, but, of course, once they do cool down, they should become red in color. Are there some red ones known? I hope so.


That seems like a nice way to handle that and I had not seen this before. Thanks. This is somewhat important since how will we change the yellow dwarf Sun to a white dwarf Sun and avoid confussion. [I honestly don't think we will always in the future call the non-yellow Sun a yellow dwarf, but I could be wrong.]

I appreciate such a thorough post. Though it's not a big deal, the Sun, however, remains not a yellow star.

__________________
Lighten up! This is a stellar board!

Actually its based more on stellar surface temperature -this affects colour eg. hotter = bluer, cooler = redder but there a real whole lot of complicating factors such as dust which reddens them, the sensitivity of the human eye and the subtlety of star colours generally.

There could easily be (& perhaps already is) a whole other thread on this "colour of the stars" topic.

Thanks. Star types - their spectral & luminosity classes - are one of my favourite areas & I'm always happy to discuss 'em.

I'm pretty sure the colour of our Sun was discussed by the BA in his first book - 'Bad Astronomy' I think twas called - but, unfortunately, I can't seem to find my copy anywhere -think I've lent it out & have not had it returned. :-(

From what I recall, the Sun is usually described & commonly perceived as yellow esp. in artwork but is in fact white & would appear white if we went above earth's atmosphere - although it actually puts out most of its energy at the "green" wavelength.

[quote] I had thought that most white dwarfs were hotter and I have not seen a white dwarf cool enough to be red, but, of course, once they do cool down, they should become red in color. Are there some red ones known? I hope so. [quote]

In his '100 Greatest Stars' (Copernicus Books, 2002) book Kaler includes the fiantest white dwarf ESO 439-26 noting on page 75 :

"We might expect the faintest white dwarfs to be coolest, but while ESO 439-26's luminosity may be lowest, its temperature is not. At 4560 K, it is warmer than the current record of 3500 for a somewhat brighter star called WD 0346+246. (At these temperatures, white dwarfs are reddish showing the danger in naming clases of objects before they are understood.)"

(Brackets original.)

I think Van Maanen's Star - the third nearest after Sirius B and Procyon B and nearest single white dwarf in our skies is also extremely cool, small and ancient.

Afraid I'd have to disagree on that last line there - I think the term is too well established to be changed and, as noted, its based on the surface temperature idea more than the colour one anyhow. I suppose I can see the possibility of 'main-sequence' or 'normal' star taking over from "dwarf star" with the latter then applying to white dwarfs & maybe brown dwarfs only, but I'm not sure that even the chances of that are high. Just my gut feeling there. Once terms are established it is very hard to change them as there's a large amount of pyschological terminological inertia to overcome.

My pleasure! :-)

Great link there too complete with true colour picture of Sun & planets & 18 Scorpii image too.
"Who knows what color lurks hidden behind the blaze of Sun? The heliochromologist knows!" could make a great signature line! ;-)

Yeah, I'd have to agree in the sense that our wouldn't appear yellow or at least very yellow-ish although in the classification sense it is! ;-)

__________________

Agreed. Color can be taken more generally and as a convention. But yellow is still problematic for the Sun because it has no yellow; not a smidgen. A white or yellow Sun does not change physics; it's true color is a small issue. Yet since we now have a pair of tweasers, why not finally remove the little colorful thorn?

You bet, and there have been a number of threads on this, though more on the Sun.

Ah, 'bout time you showed up!

Yep, he was a big reason why I got interested in this topic. He said the Sun is white because sunlight is white. Most of his short time on the topic was more about how we perceive it terrestrially and why it looks yellow. He made some comments after his book that kept the door open on whether or not it was more a yellow-white color.

Yes, given proper attenuation.

Oops. I've seen this a time or two. The peak energy is actually, well..... pick your data set:

Wehrili '85 .... 450.5 nm ..... between indigo and blue
Thuiller '02 ... 456.6 nm ..... Blue or indigo-blue
SORCE ......... 480 nm ........ Cyan-blue
Planck dist.... 495.4 nm ...... border of cyan and green [using 5850K]

No greens. The BA did mention that green was the peak temp., but he refered to photon flux, in lieu of energy distribution. This is the correct way to address color perceiption, no doubt.

Unfortunately, whatever his source was, it was very likely wrong. Using the same data sets, but adjusting for photon flux ( E = hf), hear are the peak photon flux wavelengths:

Wehrili '85 .... 582.5 nm ..... Yellow (can you believe it??)
Thuiller '02 ... 605 nm ........ Orange
SORCE ......... 580.7 nm ..... Yellow (again)
Planck dist.... 625 nm ........ orange-red

I am curious how the green color arose, because I've heard this before.

[There was an impressive article, before the BA's book, that claimed the Sun was a green star. Perhaps that triggered some green thinking, or maybe it was Kermit the frog. ]

[quote] I had thought that most white dwarfs were hotter and I have not seen a white dwarf cool enough to be red, but, of course, once they do cool down, they should become red in color. Are there some red ones known? I hope so. [quote] Yes, that's my understanding too.

Both of these would be, essentially, white or yellow-white. They are much hotter than a tungsten light filament.

Obviously, ESO 439-26 is somewhat smaller than the warmer one to be dimmer.

Yes. It's like the old joke about how many psychologists it takes to change a light bulb?... only one, but the bulb has to want to change. For star color taxonomy, it's tough to change because there isn't an easy fix for a clean alternative.

You've said it better.

__________________
Lighten up! This is a stellar board!

I should have mentioned that the values given in the prior post are peak values only and not representative of the total flux for each color portion of the visible spectrum. The following pie chart shows the photon flux distribution using the Wehrli 1985 data set. Of course, the boundary wavelengths for each color is somewhat debateable especially given the fact that we sometimes see color slightly differently than one another.

The wavelengths (nm) used here are:
Blue:........ 455 to 489
Cyan:....... 490 to 509
Green:..... 510 to 545
Lt. Green: 546 to 570
Yellow:.... 571 to 590
Orange:... 591 to 619
Red:......... 620 to 700


Even if we combine blue with cyan and green with light green, red is still the winner in most number of photons entering our eye if we are in space. What is not taken in consideration next is the spectral sensitivity of our eye, but when it is, white is the result.
.
.

Attached Images

Solar Flux Pie.jpg (129.0 KB, 1 views)

__________________
Lighten up! This is a stellar board!

I wonder how many people get this one?

__________________
Lighten up! This is a stellar board!