Why do very hot and cool stars show weak hydrogen lines?

[nmsurobert]

Im reading that very hot stars and very cool stars have weak hydrogen lines. With that being said, how do we know that these very hot stars contain high quantity of hydrogen if we can't see it in the spectra?

 

[Vanadium 50]

Weak does not mean absent.

 

[nmsurobert]

Weak does not mean absent.

i get that. but how do we know that these super hot stars have just as much hydrogen than cooler (10,000K) stars?

 

[Arman777]

i get that. but how do we know that these super hot stars have just as much hydrogen than cooler (10,000K) stars?

http://www.astronomynotes.com/starprop/s12.htm
This might help. When we are trying to determine the tempature of the star we don't just look the Hydrogen spectrum but also other elements to determine the tempature. So If we have weak hydrogen line we can't be certainly sure about the tempature of the star. (hope this helps cause I didnt quite understand what you have asked for)

 

[nmsurobert]

http://www.astronomynotes.com/starprop/s12.htm
This might help. When we are trying to determine the tempature of the star we don't just look the Hydrogen spectrum but also other elements to determine the tempature. So If we have weak hydrogen line we can't be certainly sure about the tempature of the star. (hope this helps cause I didnt quite understand what you have asked for)

thanks. ill look through that. my question is: if we see weak hydrogen lines, how do we know these stars have an abundance of hydrogen?

i understand the physics of why these weak lines exist but, to play devils advocate, what if the weak lines mean absence of hydrogen?

 

[Arman777]

if we see weak hydrogen lines, how d o we know these stars have an abundance of hydrogen?

If it's hot tempature star, like Class O, it doesn't mean that It have a high hydrogen level since it produces high amount of energy ?

what if the weak lines mean absence of hydrogen?

If it's cold then, like class K, it doesn't mean that it have few hydrogen ?

 

[Arman777]

We can't tell the amount of hydrogen just by looking at the spectrum. We need another information about the star like tempature in this sense.

 

[davenn]

i understand the physics of why these weak lines exist but, to play devils advocate, what if the weak lines mean absence of hydrogen?

that's a contradiction
if there an absence of hydrogen, there would be no hydrogen line, aye

as V50 said earlier ... weak DOESNT mean absence

 

[stefan r]

i get that. but how do we know that these super hot stars have just as much hydrogen than cooler (10,000K) stars?

Put some pure hydrogen ions in a magnetic container. Start heating. record the intensity of every frequency at each temperature.

 

[Drakkith]

thanks. ill look through that. my question is: if we see weak hydrogen lines, how do we know these stars have an abundance of hydrogen?

i understand the physics of why these weak lines exist but, to play devils advocate, what if the weak lines mean absence of hydrogen?

If you mean the amount of hydrogen in the entire star, then we know that they have to initially have lots of hydrogen because H is by far the most common element in the universe. This means that new stars are always formed primarily of hydrogen. Knowing that, and applying the known laws of physics, we've developed models that make testable predictions about what these stars should look like and what they should be composed of at various stages of their lifecycle. It turns out that there is currently no known way for a massive, hot star to not have a significant amount of hydrogen for most of its life.

Furthermore, the strength of a spectral line is determined not only by its abundance, but also by the temperature. The temperature itself determines what energy states the electrons of each atom will occupy, which then directly determines the exact spectral line seen. For very hot stars, the temperature is so high that electrons can rarely bind with a proton for long enough to absorb any light. This causes their hydrogen spectral lines to be very weak, even though there is an abundance of hydrogen in the star.

Knowing how stars form, knowing the temperature of the star, and knowing that most of the hydrogen cannot absorb any light at that temperature, we can reasonably conclude that there is still an abundance of hydrogen in the star.