# Star color

1. Oct 4, 2012

### gmax137

In the Letters section of the August issue of Physics Today, a reader claims that when he asks 'newly minted physics PhDs' to explain why stars have different colors, that 75% of them answer incorrectly. How would you answer the question?

2. Oct 4, 2012

### Drakkith

Staff Emeritus
They are different temperatures. But unless you are asking someone who has had astronomy classes before, they may not realize that it's that simple, so the reader's claim may be correct, but irrelevant.

3. Oct 4, 2012

### cepheid

Staff Emeritus
Newly minted physics PhDs all know about the Planck function for blackbody emission.

4. Oct 5, 2012

### B.M.Gray

They are different temperatures with different frequencies (colors) of stars with various chemical compositions. Separate chem. comps give stars a wide spectrum of frequencies depending on the chemical composition of said star.
Stars possess a wide array of temperatures from red (red is a low frequency on the spectrum and therefore red giants are relatively cool stars), to blue to voilet (these stars have high energy frequencies and are thus the hottest stars in the cosmos).
From this we can conclude that temperatures are proportionally related to a stars energy frequency, and vise-versa. So the color of a star is directly related to its energy frequency which astronomical spectroscopy is used to observe and deduce the frequencies, heat, energy, and therfore color of any observed star.

5. Oct 5, 2012

### B.M.Gray

Sorry if I posted too much on this topic. I posted before reading the other responses and noticed everyone is trying to keep it simple. I will do this when possible from now on and not get too technical unless asked to. lol :)

6. Oct 5, 2012

### Chronos

It's strictly a function of mass for a main sequence star.

7. Oct 6, 2012

### Drakkith

Staff Emeritus

8. Oct 11, 2012

### Solon

There seems to be much uncertainty about the color of our own star, so how is it possible to be certain about the color of stars many light years away? I see white, yellow, blu-ish and even pink are suggested.
Wouldn't an ND filter used on a regular camera, from the space station say, show the true color?

9. Oct 11, 2012

### Drakkith

Staff Emeritus
There is no uncertainty of the spectrum of our own star. The uncertainty is in the perceived color, which will be different depending on who you ask and how you are looking at it.
Even digital cameras don't have the exact same standards for their bayer filters, so the color will be very slightly different.
When we talk about "star colors" we actually mean the spectrum that it emits. We are very easily able to measure this spectrum to a very very high accuracy.

10. Oct 11, 2012

### Antiphon

The original question is ill posed and can have correct answers on many levels. Since we're not given the alleged incorrect ones, I conclude (and without having read it) that it's a BS pop-sci article and each of the newly minted PhD's likely gave valid answers in the domain they understood the question to fall into.

Why do stars have different colors?

Here are some valid answers to the moronically contextless question:

Because the stars have widely varying masses with different nuclear cycles and energy outputs.

Because the Planck equation says that objects of different temperatures emit different spectra.

Because the human eye has sensors that respond differently to various wavelengths of light.

And on and on through wide swaths of nuclear chemistry, photometry, cosmology, psychology, ophthalmology, etc.

Let's ask newly minted journalism majors what makes popular science articles look different to various readers and see how many get the wrong answer.

11. Oct 12, 2012

### goldsax

please correct me but would that be mass AND composition according to Russell-Vogt theorem?

12. Oct 12, 2012

### Chronos

from http://abyss.uoregon.edu/~js/ast222/lectures/lec04.html [Broken]

"Russell-Vogt Theorem:

Despite the range of stellar luminosities, temperatures and luminosities, there is one unifying physical parameter. And that is the mass of the star. Hot, bright stars are typically high in mass. Faint, cool stars are typically low in mass. This sole dependence on mass is so strong that it is given a special name, the Russell-Vogt Theorem.

The Russell-Vogt Theorem states that all the parameters of a star (its spectral type, luminosity, size, radius and temperature) are determined primarily by its mass. The emphasis on primarily' is important since we will soon see that this only applies during the normal' or hydrogen burning phase of a star's life. A star can evolve, and change its size and temperature. But, for most of the lifetime of a star, the Russell-Vogt Theorem is correct, mass determines everything."

The 'normal', hyrogen burning phase of a star's life is the main sequence phase.

Last edited by a moderator: May 6, 2017
13. Oct 13, 2012

### Solon

You make it sound easy, but from a quick look into how star colors are measured, it doesn't look so easy. I looked at this site to start with:
http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_colour.html [Broken]
Then I looked at the SOLAR payload on board the ISS, the SOLSPEC experiment in particular:
http://www.nasa.gov/mission_pages/station/research/experiments/Solar-SOLSPEC.html
and a more detailed pdf file:
I'm no scientist so excuse my perhaps naive questions. SOLSPEC does have error margins, and needs regular calibration. It is also looking at the nearest star, so is quite large. The next nearest star though, and all the others, are only going to resolve to 1 pixel even from our most powerful instruments, aren't they? How can a similar accuracy be claimed? I'm not saying it can't, but if there is indeed an easy explanation, I'd be interested to know.
Secondly, if we are examining spectra, through filters, how is it determined that the spectra are thermal in origin rather than from ionisation of elements in a stars electric field? If we look through a red filter, how do we know that we are not seeing a Balmer line of hydrogen?
And lastly, what about Stark or Zeeman shifting, does those come into play in these measurements?
(Dons flak jacket and stands well back...)

Last edited by a moderator: May 6, 2017
14. Oct 13, 2012

### Drakkith

Staff Emeritus
No, to measure the spectrum of a star we use a spectrograph. This spreads the light out like a prism instead of focusing it onto one spot. Spectrographs must be calibrated of course, but that is not terribly difficult as far as I know.

A spectrograph shows you which wavelengths are being emitted and absorbed.

Of course. The spectrum of a star is not a perfect black body, it will have all kinds of things that make it slightly different. These are all seen the in spectrum as different emission/absorption lines, broadening of the lines, etc.

http://en.wikipedia.org/wiki/Spectroscopy