Why are the emission lines in the Crab Nebula split into two?

[judi6070]

I'm doing a project on the Crab Nebula for my first-year astronomy class and I am finding the spectroscopy a bit puzzling. What I have read is that the normal emission line for a particular gas is split into two, one red shifted and one blue shifted. This of course is accounted for by the Dopler Shift, as parts of the gas cloud is approaching us and part is receding. What I can't fully comprehend is why the lines would be split in two and why wouldn't we see a wider band of colour. Is there not parts of the cloud that remain still, and parts that are receding or approaching faster than others?

 

[xAbsoluteZerox]

This is kind of hard to explain, but if you consider a rotating body composed of many, many particles, the only place where there will be no red/blue shifting will be on a linear center of the nebula. However, in spectra, you can kind of think of it as an average. Yes, there is actually a gradient which composes each of the emission lines. However, because there are many more particles moving away/towards us than going close to perpendicular to our line of sight, the bands appear darkest where the majority of the particles are red/blueshifting, therefore you get what looks like 2 lines, however these lines are are somewhat of a brightness/darkness gradient, where the dark part of it is very broad compared to the rest. This is pretty hard to explain by typing, so if you need further clarification just say so.

 

[judi6070]

Alright that makes sense, but won't there still be "intermediate" lines, where the particles are moving outwards on a vector that isn't directly in our line of sight. Then, we will only be able to account for only the component of their velocity that IS in our line of sight, making it appear to be red/blue shifted slightly less than a particle that is moving directly in our line of sight?

 

[xAbsoluteZerox]

Alright that makes sense, but won't there still be "intermediate" lines, where the particles are moving outwards on a vector that isn't directly in our line of sight. Then, we will only be able to account for only the component of their velocity that IS in our line of sight, making it appear to be red/blue shifted slightly less than a particle that is moving directly in our line of sight?

Yes, there will be intermediate lines, sort of. There will be an extremely large amount intermediate lines, thus creating this sort of gradient, which will be extremely small and compacted, so for all practical purposes, it is just a tiny, unnoticeable part of the strong blue/red shift line.

 

[SpaceTiger]

This is kind of hard to explain, but if you consider a rotating body composed of many, many particles, the only place where there will be no red/blue shifting will be on a linear center of the nebula.

The Crab Nebula is a supernova remnant, so the Doppler shift will be due to expansion, not rotation. The precise emission line structure will depend on the particular emission line you're looking at. The conditions in a supernova remnant vary dramatically from the edge to the center, the edges usually being defined by a shock front with colder, denser gas. Emission lines that trace this gas at the edges will be much more cleanly split into two lines than emission lines that trace the hotter gas towards the center.

 

[Tido611]

I am also studing the Crab Nebula/Pulsar but i was wondering a different question about the spectral images. I noticed that there is one major picture of the nebula in the X-ray region and it is purple but there is one X-ray picture that i have seen far fewer times that is orange i was wondering what determines the color of these things or even if the color means anything at all, especially since we are dealing in x-rays not visible so how would there be visible color anyways?

 

[SpaceTiger]

I am also studing the Crab Nebula/Pulsar but i was wondering a different question about the spectral images. I noticed that there is one major picture of the nebula in the X-ray region and it is purple but there is one X-ray picture that i have seen far fewer times that is orange i was wondering what determines the color of these things or even if the color means anything at all, especially since we are dealing in x-rays not visible so how would there be visible color anyways?

If the color is indicating X-ray energy or intensity, then the scale is pretty arbitrary. It's just a way of visualizing the distribution of X-ray radiation and has no correspondence to visible colors. Usually, the colors mean something, but it varies from picture to picture.

 

[Tido611]

ok so the colors in the optical wavelength picture would be the true colours in the visible spectrum? Or would they mean somthing else, if so what?

 

[SpaceTiger]

ok so the colors in the optical wavelength picture would be the true colours in the visible spectrum?

Yes, most of the time, though you will occasionally see things in "false color", indicating that they've changed the color scale to something prettier or more convenient.

 

[xAbsoluteZerox]

The Crab Nebula is a supernova remnant, so the Doppler shift will be due to expansion, not rotation. The precise emission line structure will depend on the particular emission line you're looking at. The conditions in a supernova remnant vary dramatically from the edge to the center, the edges usually being defined by a shock front with colder, denser gas. Emission lines that trace this gas at the edges will be much more cleanly split into two lines than emission lines that trace the hotter gas towards the center.

Right, I was trying to give an overview of a rotating body. I didn't go specifically into the crab nebula because I was trying to be general, but I agree with you on the expansion causing the doppler shift in this instance.