Do You Know Why Stars Have Different Colors?

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In summary, the conversation discusses the issue of newly minted physics PhDs giving incorrect answers when asked about the different colors of stars. The correct answer is that stars have different temperatures, which is directly related to their energy frequency and can be observed through astronomical spectroscopy. The conversation also touches on the uncertainty of the perceived color of stars and the role of mass and composition in determining a star's properties. The Russell-Vogt theorem states that for most of a star's lifetime, its parameters are primarily determined by its mass. However, measuring the spectrum of stars can be a complex process with potential errors and uncertainties.
  • #1
gmax137
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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?
 
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  • #2
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
Newly minted physics PhDs all know about the Planck function for blackbody emission.
 
  • #4
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 possesses 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 therefore color of any observed star.
 
  • #5
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
It's strictly a function of mass for a main sequence star.
 
  • #7
B.M.Gray said:
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 :)

Eh, it depends on the question asked and how much the person asking already knows.
 
  • #8
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.
http://casa.colorado.edu/~ajsh/colour/Tspectrum.html
Wouldn't an ND filter used on a regular camera, from the space station say, show the true color?
 
  • #9
Solon said:
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.
http://casa.colorado.edu/~ajsh/colour/Tspectrum.html
Wouldn't an ND filter used on a regular camera, from the space station say, show the true color?

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

please correct me but would that be mass AND composition according to Russell-Vogt theorem?
 
  • #12
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.
 
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  • #13
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.

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:
http://lasp.colorado.edu/sorce/news/2008ScienceMeeting/doc/Session1/S1_04_Thuillier.pdf
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...)
 
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  • #14
Solon said:
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?

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.

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?

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

And lastly, what about Stark or Zeeman shifting, does those come into play in these measurements?
(Dons flak jacket and stands well back...)

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
 

1. Why do stars have different colors?

Stars have different colors because of their surface temperature. The color of a star is determined by how hot it is, with hotter stars appearing blue or white and cooler stars appearing red or orange. This is due to the fact that stars emit different wavelengths of light, and the hotter the star, the shorter the wavelength of light it emits, which our eyes perceive as blue. Cooler stars emit longer wavelengths of light, which our eyes perceive as red.

2. How does the temperature of a star affect its color?

The temperature of a star affects its color because it determines the wavelength of light that the star emits. As mentioned before, hotter stars emit shorter wavelengths of light, which our eyes perceive as blue, while cooler stars emit longer wavelengths of light, which our eyes perceive as red. This is why stars with different temperatures appear to have different colors.

3. Do all stars emit the same color of light?

No, not all stars emit the same color of light. As mentioned before, the color of a star is determined by its surface temperature, and since stars have different temperatures, they emit different colors of light. This is why we see stars with different colors in the night sky.

4. Can a star's color change over time?

Yes, a star's color can change over time. This usually happens when a star runs out of fuel and starts to cool down. As the temperature of the star decreases, its color may change from blue to white, then to yellow, orange, and finally red. This change in color can take millions of years to occur and may signal the end of a star's life cycle.

5. Are there any other factors that can affect a star's color?

Yes, there are other factors that can affect a star's color. For example, a star's composition can also play a role in its color. Stars with higher amounts of elements like helium and hydrogen tend to appear bluer, while stars with more carbon and nitrogen may appear redder. Additionally, the presence of dust and gas in a star's surrounding environment can also affect its color. This is because these materials can absorb and scatter different colors of light, making the star appear a different color to us.

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