What can be learnt from star colors?

[Ann]

Dario Giannobile has posted a fantastic collection of well-known bright stars and their colors. To me, this collection is a wonderful demonstration of what can and what can't be learnt from star colors. Also, for me, it is so satisfying to see stars having basically the same hue as I have seen in a telescope.

For more or less unreddened stars, color does say a lot about the temperature of the star - up to a point, that is. In Dario Giannobile's star compilation, there is very little difference in hue between the A-type and the B-type stars. Indeed, A-type Castor looks every bit as blue as B-type Rigel (if not bluer), and B8-type Algol looks as blue as the hot Orion's Belt stars.

But for cooler stars, fantastic variations can be seen. The pale pure yellow color of Arcturus is exactly as I remember it from a telescope. The brighter yellow of Aldebaran and the still more golden yellow color of Betelgeuse also agree perfectly with my memories of them from my telescopic sessions. The startlingly orange color of Antares is also close to what I remember. The egg-white color of Capella is just so, and the "cold white color" of Mirfak is a little bluer than it looked like in the telescope, but still very fine. The extremely pale yellow hue of Pollux is perfect.

I must conclude that stars that are at least 2,000K hotter than the Sun typically look bluish in a telescope. Stars that are close to the temperature (or at least close to the B-V index) of the Sun look white. Cooler stars start out looking very pale yellow (like Pollux), then when they are still cooler they get more purely yellow (like Arcturus) and then even yellower, until they look golden like cool Betelgeuse. The orange color of Antares may be due to the fact that Antares must be dust-reddened, and for me Antares is always quite low in the sky.

So it is not always possible to judge from the color of a star if it is an early A-type star or a B-type one, but for cooler stars there are all sorts of shades of yellow, all strongly dependent on the temperature of the star.

Ann

[BDanielMayfield]

A star's temperature, first and foremost, but of course you knew that Ann. And knowing a star's temp might tell what type it is, or constrain the possibities to a few places on the HR diagram. That might tell you something of the star's age. But you had to know these things. Did you have something else in mind?

I think that if you'd asked, 'What can be learnt from star spectra?' you might have garnered more interest in your topic.

[Beyond]

I wish the color yellow was removed from the Asterisk* Font spectra. It's hardly visible.

[Chris Peterson]

A star's temperature, first and foremost, but of course you knew that Ann. And knowing a star's temp might tell what type it is, or constrain the possibities to a few places on the HR diagram. That might tell you something of the star's age. But you had to know these things. Did you have something else in mind?

I think that if you'd asked, 'What can be learnt from star spectra?' you might have garnered more interest in your topic. :ssmile:

Agreed. Spectrum is much more interesting. "Color" as applied to stars roughly classifies temperature, and little else. It's a coarse metric, and is becoming somewhat obsolete except for broad surveys (because it's so trivially determined). It's not something anybody pays much attention to when examining any star in detail.

[Ann]

A star's temperature, first and foremost, but of course you knew that Ann. And knowing a star's temp might tell what type it is, or constrain the possibities to a few places on the HR diagram. That might tell you something of the star's age. But you had to know these things. Did you have something else in mind?

I think that if you'd asked, 'What can be learnt from star spectra?' you might have garnered more interest in your topic.

Well... I meant... if you are looking at stars with a telescope, and it is a pretty clear night, and you just observe the colors of the stars without knowing the names or designations or "identities" of the stars, then you can really guess at the spectra of those stars just by looking at their colors.

Still, there is only so much you can know. Usually you can't really say if a bluish star is an early A-type star of a B-type star. Yellow stars do come in such subtly different shades that you can begin to guess at their spectral classes. Could this one be a G8 star? A smallish K0 giant? A bigger K3 giant? An M0 star? It is possible to make an educated guess from the star color alone.

But if you see a red star, then it is reddened by something, either by dust between us and the star, or by "soot" in the star's own atmosphere. The red color does not, therefore, reflect the temperature of the star, certainly not its bolometric energy production. All right: I believe that brown dwarfs are really red in color, but they are so faint that we can't even see them in (optical) telescopes.

Of course spectra will tell you so very much more than just star colors. Still, the colors reflect the spectral classes, at least when it comes to the temperature of unreddened stars.

Ann

[owlice]

But you had to know these things. Did you have something else in mind?

Ann posed the question for herself, not for someone else to answer.

[cosmo_uk]

Good question Ann. Some of you guys are being a bit dismissive of colour, a colour magnitude diagram (basically HR) can tell you a lot. As you all rightly say the colour is due to the temperature (in the main) as stars are approximately Black Body emitters (OK not all of them and strong absorption features can affect this approximation) where a BB has a colour (or peak wavelength) determined only by its temperature. A colour magnitude diagram of stars in globular clusters etc in the halo of the Milky Way or even galaxies out into the wider local group can give you a pretty handy approximate distance to their host by assuming the tip of the Red Giant Branch always occurs at the same luminosity.

As with Ann's discussion of stars with different temps appearing to have the same colour this may also have something to do with how the human eye works as well.

[cosmo_uk]

Also as Annmentions you can use the average colours of stars in different regions of the sky to make a pretty decent statistical dust map of the Milky Way (using a very large dataset like SDSS)

[Chris Peterson]

As with Ann's discussion of stars with different temps appearing to have the same colour this may also have something to do with how the human eye works as well.

An issue that can't be ignored is that "color" means many different things. In stellar photometry, it is a scalar typically derived as the ratio or difference in intensity across two adjacent parts of the spectrum (e.g. B-V). These are what we see plotted in H-R diagrams. But these "colors" are not the same as the "colors" we see visually. This is made most obvious by the example of the Sun, which is perfectly white visually, but is classified as a yellow star photometrically. In short, our eyes are poor characterizers of photometric color.

One reason that photometric color is slowly losing favor as an important metric compared with spectroscopic analysis is that photometric color is easily confounded by dust. The reddening caused by short wavelength scatter from dust results in a significant miscategorization of stellar temperatures.

[cosmo_uk]

"An issue that can't be ignored is that "color" means many different things. In stellar photometry, it is a scalar typically derived as the ratio or difference in intensity across two adjacent parts of the spectrum (e.g. B-V)"

Well indeed but I decided not to go into that!

[BDanielMayfield]

I wish the color yellow was removed from the Asterisk* Font spectra. It's hardly visible.

While I agree that yellow on white can be hard for some to see, I woudn't want any color to be completely removed, but it would make sense to remove all the Font colour button's color redundancies and make the function have all colors. It would also be nice to be able to change the background too, at least to black.

[Chris Peterson]

I wish the color yellow was removed from the Asterisk* Font spectra. It's hardly visible. :thumb_down:

While I agree that yellow on white can be hard for some to see, I woudn't want any color to be completely removed, but it would make sense to remove all the Font colour button's color redundancies and make the function have all colors. It would also be nice to be able to change the background too, at least to black.

I'd be happier just eliminating the option for font colors completely. All it can really do is make things harder to read.

That said, however, there are no limitations on the color. The palette just shows web safe colors- those that can be displayed with with a card in 8-bit color mode, something that nobody does anymore. But if there are any colors you'd prefer to use, nothing is stopping you from putting their values in.

[geckzilla]

Don't tell them you can search for automatic rainbow text generators...

[BDanielMayfield]

Don't tell them you can search for automatic rainbow text generators...

Show off! And telling Beyond that he doesn't need to be posting pictures too! What are you trying to do, keep all the mad skills to yourself

[Chris Peterson]

Don't tell them you can search for automatic rainbow text generators...

How about that. 20 times the storage requirements (that's a 2.9 KB line!), and 20 times harder to read...

[geckzilla]

Yeah. I suppose I should give myself a short ban for that.

[BDanielMayfield]

Yeah. I suppose I should give myself a short ban for that.

And you know, now that you let the cat out of the bag, someone is bound to colorize a multi page rant that way.

[geckzilla]

If someone does that I'll not only take away the color bbcode but reduce the emoticon limit again from three to one.

[BDanielMayfield]

If someone does that I'll not only take away the color bbcode but reduce the emoticon limit again from three to one.

Whoa now, that would be a horrible unforeseen effect of Ann's topic about the wonderful colors of stars. Please don't take our crayons away, please.

[geckzilla]

Don't make me do it!

[BDanielMayfield]

Don't make me do it!

I won't, in case you were worried that I might. You have my signed word on it.

Bruce D. Mayfield

[Beyond]

So, what can be learned learnt from star colors? Apparently lots of things, including... don't make a mess with your crayons, or the school marm ( ) will take then away from you.