I had difficulties in drawing thin lines in PowerPoint.

@JohnRennie I think Fraunhofer did an independent but detailed work after the first one. So the credit goes to him. But I also like the idea - two people worked on the same thing without knowing each others work.

@GuruVishnu I've edited his answer to change emission to absorption :-)

@GuruVishnu that happened a lot in the days before fast communication.

@JohnRennie Oops. I didn't mean the answer. I meant the comment.

The answer was totally fine previously sir. It was emission only!

@JohnRennie That's a comment by Pieter. Not the answerer.

Sir, I think there must be a rollback to v1;

Use that diagram instead.

@JohnRennie It seems it would change the entire question's context but not its meaning sir.

@GuruVishnu you've confused me now. The only comment is Pieter's. Niels just answered - he didn't post any comments.

@JohnRennie Yes sir. Rollback mandatory :-)

@GuruVishnu well you're asking about astronomical observations, and these are almost always of absorption lines not emission lines.

@JohnRennie Is that so sir? I didn't know that. Do they wait for Stellar occultation every time?

If so it's even more difficult.

You don't get lines from planets. Just from stars.

@JohnRennie Reflected light?

In fact you can't see planets at all outside the solar system except in very unusual circumstances.

@JohnRennie Yes sir. Transit method.

@GuruVishnu the light reflected from planets tends to have very broad lines that aren't any use for detailed analysis. You can see broad features but almost never fine details.

Now caught it. It's the leftover of absorption spectrum aka emission spectrum. What about stars?

With stars you always get absorption lines.

Stars themselves produce black body radiation so it's a continuous spectrum.

@JohnRennie Ok sir. Will that do good if I change Planet X to Star X?

I would probably make clear you mean the spectrum from stars, because in 99.9999999% of cases that is what the astronomers are measuring.

@JohnRennie Ok sir. I think I've made a big mistake in asking the question on the main site.

:-(

No, the question looks fine to me.

@JohnRennie It seems I haven't understood something based on which I typed the question sir. I think it requires tremendous edit to get that right.

I'm unable to delete that because of an answer and the system warns of possibility of getting banned from asking.

No, just take your fourth paragraph and change planet to star.

And possibly make the lines in your diagram dark.

That's the only changes needed.

Otherwise it's a perfectly good question.

@JohnRennie Process 1 of 2 complete.

@JohnRennie I'm going to make a few changes to the original diagram sir. The spectrum has reduced its intensity as I used a filter.

@JohnRennie Thank you sir.

This is a very minor suggestion and can be ignored, but I might change Let us assume Star X is made of two elements to Let us assume Star X contains two elements

The star is mostly made of hydrogen and helium

The lines we see are due to small amounts of the other elements.

@JohnRennie: Ok sir. I'll change that. Thank you for your suggestion.

@JohnRennie: May I ask for a last help?

@GuruVishnu yes?

But thank you. You've already helped me using the comment you made :-)

Thank you very much sir.

I thought that comment would land my question closed as personal theory questions.

No, I think the question is fine. It isn't at all obvious how you separate out the different elements from a huge mess of Fraunhofer lines.

@JohnRennie Thank you sir. I'm making some changes to the original diagram. It should be done in about 5 minutes.

@JohnRennie: Are you free now sir?

@GuruVishnu yes

@JohnRennie Is that post now scientifically accurate sir?

This one: physics.stackexchange.com/q/526618/238167

Yes it looks fine.

It's a lot clearer now.

What do you want to discuss?

Your original question was basically fine. Everyone would have known what you meant.

@JohnRennie Sir. I thought we would be identifying elements in stars and planets using their emission spectrum, only now I realised it is the absorption spectrum used for planets. But still I don't understand why it's wrong to say we use emission spectrum for stars. I understood the case for planets as we use transit method. But the star case is slightly confusing me.

A star does produce emission lines, but it also produces a huge amount of black body radiation that is just smooth and has no emission lines.

So you would be trying to see an emission line against a massively bright background.

In theory it could be done but in practice the black body radiation is so bright it totally swamps any emission lines that may be present.

@JohnRennie Ok sir. I think we could also say the same for absorption spectrum - we'll see small dark lines in between bright parches of light.

Which are also difficult to find.

No, because it's much easier to see the dark line against the bright background.

When an atom emits a photon it emits it in a random direction.

@JohnRennie Ok sir. I thought nearby bands would prevent us from seeing the dark band.

So of all the photons the atom emits only a tiny fraction end up reaching us here on Earth, because most of the emitted photons go off in directions that don't reach us.

OK so far?

Yes sir.

Intensity falls with distance. Is that what you're willing to say?

But when photons are absorbed by the atoms every photon that is absorbed doesn't reach us.

@JohnRennie Or don't some get absorbed and rest don't find time and just come happily?

So if you compare the contrast i.e. the change in intensity, absorption lines have a much greater contrast than emission lines.

@JohnRennie Fine sir. Thank you very much. If it's a fact I'll accept it. Today I got really surprised. I need to check a NASA video which I viewed a long time back about MAVEN spacecraft which uses setting star as a source of light to conduct spectroscopic studies. I think I saw an emission spectrum. But it must be an absorption spectrum.

MAVEN is a bit of a special case because it is orbiting the planet that it is observing.

It's not looking at a star umpteen light years away.

@JohnRennie Yes sir. But the way they used spectroscopy to analyse the atmosphere really fascinated me :-)

@JohnRennie I think we're just assuming the star to be of a continuous spectrum for this purpose. I don't want to combine both of the questions.

I hope this might help:

But there is a graph and not a spectrum diagram. But the principle remains the same. Again it's very genius idea according to me.

Can you link the article?

svs.gsfc.nasa.gov/20223

@JohnRennie I think its a video from this page:

Aha!! Its an absorption spectrum for sure.

Yes. It's assuming the light from the star is basically white and measuring how much of the light from the star is absorbed.

@JohnRennie Yes sir. Isn't that really interesting? By this they can calculate the variation of composition from the top to the bottom of the atmosphere.

The absorption spectrum looks surprisingly broad, but maybe the graph in the video isn't showing the fine detail.

@JohnRennie Yes sir. I agree with you. Maybe let me check absorption spectrum of methane.

I found this one:

Most of the absorption takes place at the infrared region. But the spectrograph on MAVEN is ultraviolet specific according to the linked video.

The IR spectrum is measuring vibrational transitions while the UV spectrum measures electronic transitions. Both are good ways of measuring the amount of methane present.

@JohnRennie I haven't learnt the terms in this message. But I could somewhat understand what your are trying to say sir.

@JohnRennie So is the graph in that video somewhat accurate?

I don't know. I don't think it's possible to tell from the video. You'd have to look at the publications from the MAVEN team. They would give the results in more detail.

@JohnRennie Ok sir. I'll like to do that after JEE :-)

@JohnRennie: Thank you very much for your time and support sir. I'm going out to walk sometime. We'll meet either today or tomorrow :-)

Good bye sir.

@GuruVishnu Bye :-)

:-)

@JohnRennie Hello

@pi-π hi. I'm just starting lunch, but if you want to post a question I can look at it later.

@JohnRennie Ah it's okay. We will discuss tomorrow.

@pi-π OK. See you tomorrow.