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Why H alpha?

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gasman's picture
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Why H alpha?

Hi guys

Following on from Tony`s recent post regarding the emphasis on H alpha observations with spectroscopy.

Since becoming interested in spectroscopy I have often wondered why most if not all spectroscopists seem to focus their attention on the presence (or absence) of Hydrogen alpha. Does the H alpha signature give more information about whats happening in a star`s atmosphere than other elements?.

regards

Steve

andrew.j.smith1905's picture
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Why H alpha?

I think the simple answer is yes. Not only the atmosphere but also any surrounding shell or ring of gas which is normally mostly hydrogen. It is not always the case but there has been a strong focus on Be stars where the hydrogen alpha line has been the main focus as it shows the most variation.

Other lines are important for radial velocity measurement and in spectral classes where hydrogen lines are very weak or absent.

Regards Andrew

gasman's picture
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H alpha plus others!

Thanks Andrew. Does the amount of H alpha tell us anything different to say H beta or H gamma for instance?. I suppose we are delving into complex physics here!. I remember many years ago doing an OU course (Matter in the Universe) involving the C N O cycles within stars but H alpha didn`t seem to be discussed?.

Steve

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Hydrogen lines

Hydrogen alpha emission is generally stronger than hydrogen beta, and occurs at a more conveniently observable wavelength. Hydrogen beta is bluer than H-alpha, and suffers more atmospheric attenuation as well as being a weaker line.

Hydrogen alpha isn't actually the strongest line that ionised hydrogen atoms produce - that's Lyman-alpha. But it's in the ultraviolet and can't be observed from the ground, so once again practicalities get in the way.

The ratios of all these lines are interesting, but if you're going to measure one single line, H-alpha is where you're going to get the strongest detection.

The CNO cycle is a mechanism which produces energy in stars which are somewhat more massive than the Sun. It requires a temperature of about 17 million kelvin, whereas the Sun's core is only 15 million K. Even in stars which are powered by the CNO cycle, it's restricted to the very centre of the star. The visible photosphere will be only a few thousand K and what you'll see is mostly H and He.

Robin Leadbeater's picture
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H alpha

The H alpha emission line profile is very useful for indicating the presence of and tracking the movement of material in the neighborhood of the star such as outflowing winds, eg in P Cygni, accretion inflows eg in T Tauri, rotating discs eg in Be stars, explosions eg in novae and type II supernovae, transfer of material between stars eg in symbiotics.  It is not the only useful line though. eg the narrower metal lines are better if you want to measure a star's radial velocity and some systems don't have hydrogen eg WR stars where the winds can be tracked using eg C III, IV lines (as in the current WR140 colliding wind campaign) or type 1a supernovae where the Si absorption line is the key one to look for. In low temperature systems you might look for low excitation lines like the 7699A K line in eps Aur which I used to track the density and velocity of the material in the cool eclipsing disc.

Robin

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Balmer decrement and interstellar redenning

Also the ratio of H alpha/H beta emission in nebulae for example is useful for measuring the amount of interstellar extinction eg as described here

https://web.williams.edu/Astronomy/research/PN/nebulae/exercise2.php

In fact there are a number of interesting measurements you can make based on measuring the relative intensity of various lines in planetary nebulae using just a low resolution spectrograph, as described in Francois Teyssier's low resolution spectroscopy observers guide

http://www.astronomie-amateur.fr/Documents%20Spectro/SpectroscopieBasseR...

and detailed on his website here

http://www.astronomie-amateur.fr/feuilles/Spectroscopie/NGC2392.html

Robin