BP's Astronomy Unanswered questions...

[gradientcurl]

hi, not sure the technical details bt i guess that Wien's law can't be applied for WR cos WR's r not exactly black bodies. their outer atmospheres are quite significant and are losing mass at a much higher rate than the usual stars, thats why ngc 5139 says that its in a class of their own (ie: the usual laws applied to usual stars cannot be applied for wr). black bodies are such that the emitting radiation from the surface does not affect the equilibrium state within the star, whereas W r unstable as they r near the end of their lives (u can try a google)

[Jin Peng]

Hello to those from Bukit Panjang Government High School,

I am from republic polytechnic astronomy CCA, and expresses interest to visit one of your sessions, mind having a cross astronomical exchange? Hope to able work with you guys =)

Have a nice day,
Republican astronomers(RP)

[Q]

Jin Peng: Please try to stay within topic.

[chrisyeo]

Hi, I want to ask,

Are WR stars really green in colour? I've been trying to find photos of actual stars and my textbooks don't say anything on wolf-rayet stars.


But I suspect that gradientcurl is right in saying that Wien's law doesn't apply to WRs because their radiation is too different from a black body's.

[weixing]

Hi,

Are WR stars really green in colour? I've been trying to find photos of actually stars and my textbooks don't say anything on wolf-rayet stars.

Go and look at the Gamma Velorum in constellation Vela when it become visible a few months later lor... But I don't think it is green in colour, since Wolf-Rayet stars are extremely hot (hot star should look blue or white).

Anyway, read the below web if you got time... me, too lazy to read... ha ha ha =P =P =P
http://www.peripatus.gen.nz/Astronomy/WolRaySta.html

By the way, the above link got a image of a WR Star and it look like blue or white.

Have a nice day.

[ALPiNe]

Hi,

If indeed what gradientcurl mentioned are true, then this statement of

whereas WR r unstable as they r near the end of their lives

only goes to show that

Wolf-Rayet stars, being green are hotter than blue stars.

is absolutely flawed!!! :o It has long been known that dying stars are at a stage where they gradually cool over a period of time. Hence, regardless of the colour of the star, Wolf-Rayet stars will never be hotter than blue stars? This is very contradicting… :?

1) Why are green stars hotter than blue stars? According to Wien's law, blue stars should have a hotter temperature, but Wolf-Rayet stars, being green are hotter than blue stars. This is a fact.

In this question, I get the impression that ALL Wolf-Rayet stars are green. If indeed there are any green stars at all, let alone those green Wolf-Rayet (WR) stars, it does not necessarily mean that their colour will have any effect, if any at all, on the temperature either. For a start, I think the main concern here would be, “Are there any green stars?” To address this underlying problem, I have found a website relating to sci.astro which has provided a good explanation on this. For convenience, I have placed the discussion below:

-------------------------------------------------------------------------------
Subject: G.02 Are there any green stars?
Author: Paul Schlyter <pausch@electra.saaf.se>,
Steve Willner <swillner@cfa.harvard.edu>

The color vision of our eyes is a pretty complicated matter. The
colors we perceive depend not only of the wavelength mix the eye
receives at a perticular spot, but also on a number of other factors.
For instance the brightness of the light received, the brightness and
wavelength mix received simultaneously in other parts of the field
of view (sometimes visible as "contrast effects"), and also the
brightness/wavelength mix that the eye previously received (sometimes
visible as afterimages).

One isolated star, viewed by an eye not subjected to other strong
lights just before, and with very little other light sources in the
field of view, will virtually never look green. But put the same
star (which we can assume to appear white when viewed in isolation)
close to another, reddish, star, and that same star may immediately
look greenish, due to contrast effects (the eye tries to make the
"average" color of the two stars appear white).

Also, stars generally have very weak colors. The only exception is
perhaps those cool "carbon" stars with a very low temperature---they
often look quite red, but still not as red as a stoplight. Very hot
stars have a faint bluish tinge, but it's always faint---"blue" stars
never get as intense in their colors as the reddest stars. Once the
temperature of a star exceeds about 20,000 K, its temperature doesn't
really matter to the perceived color (assuming blackbody
radiation)---the star will appear to have the same blue-white color no
matter whether the temperature is 20,000, 100,000 or a million degrees K.

Old novae in the "nebular" phase often look green. This is because
they are surrounded by a shell of gas that emits spectral lines of
doubly ionized oxygen (among other things). Although these object
certainly look like green stars in a telescope---the gas shell cannot
usually be resolved---the color isn't coming from a stellar
photosphere.
-------------------------------------------------------------------------------
Taken from http://sciastro.astronomy.net/sci.astro.7.FAQ

WR's r not exactly black bodies. their outer atmospheres are quite significant and are losing mass at a much higher rate than the usual stars

If these lost mass in WR’s result in something which is similar to the “nebular” phase of old novae where it creates a shell of gas emitting spectral lines of doubly ionized oxygen surrounding the dying WR star, then there is a high possibility that those WR’s will look like green stars in an observing instrument. However, like what was mentioned in the discussion, it has to be noted that the color may not come from a stellar photosphere.

Taking a step further, I found that in the website weixing gave, the surface composition of WR stars “is extremely exotic, being dominated by helium rather than hydrogen.” If helium is indeed the most abundant here, and the fact that helium emission is blue, I don’t see the reason why ALL Wolf-Rayet’s stars have to be green.

Therefore, in this perplexity on:

1) Why are green stars hotter than blue stars?

, the question is somewhat vague or if not, doesn’t make much sense at all given that the presence of such ‘green stars’ has not yet been verified even with current day science technology. *Quickly squats down and covers own head to defend against ‘flying’ rotten eggs!* =P Well, I guess if a solution to this question is to be reached, maybe the best lies with only our own imagination afterall…


Cheers,
:cheers:
- ALPiNe

[chrisyeo]

Yup. Waiting for quantumgravity to point us to the info on green WR stars.

Not sure how to read spectra properly, but the following website seems to show spectra which is dominantly green in the visible wavelengths: http://home.freeuk.com/m.gavin/wr5.htm

Maybe I'm reading it wrongly. Someone explain?

Cheers,
Chris

[weixing]

Hi,
I search the internet and this is what I found:
http://curious.astro.cornell.edu/question.php?number=14

Have a nice day.

[ChaosKnight]

Yup. Waiting for quantumgravity to point us to the info on green WR stars.

Not sure how to read spectra properly, but the following website seems to show spectra which is dominantly green in the visible wavelengths: http://home.freeuk.com/m.gavin/wr5.htm

Maybe I'm reading it wrongly. Someone explain?

Cheers,
Chris

Interesting link you got there. After looking through it, here's my take on the topic.

The spectra is mostly green because it is emission lines. This colour is caused by the elements in the star, not by the temperature.

Lets use our sun as an example. We know our sun is yellow, which agrees well with blackbody curve. Suppose you took emission spectra of the sun, it will show strong red lines due to hydrogen. But we can't say the sun is red, and therefore its colour doesn't agree with its temperature.

I suppose the Wolf-Rayet stars will show its true colour if absorbtion spectra is taken and intensity at each wavelength measured. Anyone can provide information on this?