Planetary Nebulae and emission lines

[knowlewj01]

Homework Statement

A planetary nebula is larger when imaged in H$$\alpha$$ than when observed in a Helium line, Why?

Homework Equations

The Attempt at a Solution

I understand that a planetary Nebula gives an emission spectrum and that If the nebula looks larger in a H line there is obviously more Hydrogen present, or there is more ionized Hydrogen than helium, which suggests that the nebula is not hot enough to give an emission spectrum for heavier elements. Am I on the right lines here?

 

[Redbelly98]

I am not 100% certain, but this has gone 2 days without a response. You bring up temperature, and I suspect that is the key to answering this.

Do planetary nebulas tend to have a uniform temperature throughout, or would the temperature be different in different parts of it (i.e., in the center vs. the outer parts)?

 

[kerimek]

Yes, you are on the right track. Planetary nebulae are formed from the outer layers of a dying star being ejected into space. These layers primarily consist of hydrogen and helium, with smaller amounts of heavier elements. When the star's core runs out of hydrogen to fuse, it begins to fuse helium, creating a hotter and denser environment. This leads to the ejection of the outer layers and the formation of a planetary nebula.

In the H\alpha line, we are observing the emission from ionized hydrogen atoms. This line is much stronger and more easily detectable than the emission from helium atoms. This is because the outer layers of the planetary nebula are primarily composed of hydrogen, so there is a larger amount of ionized hydrogen present. This leads to a larger and more easily observable planetary nebula in the H\alpha line.

In the helium line, we are observing the emission from ionized helium atoms. However, because there is a smaller amount of helium present in the outer layers of the nebula, the emission is weaker and the nebula appears smaller.

Overall, the difference in size between the H\alpha and helium lines is a result of the different abundances of hydrogen and helium in the outer layers of the planetary nebula.