Emission nebulae

[jonnydreads]

Can someone just clarify something for me. When talking about emission nebulae regarding imaging and filters. Does the emissions part refer to what gases a particular nebula is made up of? 

Sorry if this is a silly question 

Jon 

[vlaiv]

Basic two types of nebulosity out there:

1. reflection

2. emission

They differ in the way light that we see / record is generated. With reflection nebulae it is light from nearby stars that gets reflected from dust/gas in nebula - hence name reflection. It's a bit like shining torch at smoke / fog during the night - you will see smoke / fog because it has light shone on it. Because star light is broad band - so is the reflection of it from the gas and dust.

Emission nebula don't have separate light source to power their glow. It's the gasses that form the nebula that get excited (temperature / magnetic fields / gravity / whatever) - that makes them shine. They shine because electrons in excited atoms jump back into lower energy state and photon is emitted. Different atoms have different energy states and energy difference between two states is always the same - this is why we talk about H alpha and beta - meaning Hydrogen atom, jump from second orbital to first one and from third to first. Similarly other elements emit their light - OIII, SII and so on ... These all have well defined frequency of light and this is why we can use narrow band imaging - filters designed for particular wavelength. This process is very similar to neon lights that we regularly use.

 

[jonnydreads]

That makes sense thanks.

So is reflection nebula the more common nebula that we can see from earth..... I'm saying that because almost all nebula I see pictures of have stars all around it or can that be a perspective illusion, or is this just one of the many stupid questions I have lined up for this forum 

[vlaiv]

1 minute ago, jonnydreads said:

That makes sense thanks.

So is reflection nebula the more common nebula that we can see from earth..... I'm saying that because almost all nebula I see pictures of have stars all around it or can that be a perspective illusion, or is this just one of the many stupid questions I have lined up for this forum 

Not sure about proportion of reflection to emission ones, but do remember that most stuff out there between the stars is in form of gasses like hydrogen, helium and such. These have potential to glow in emission manner.

I think that we observe reflection nebulae only "close" to us, we are however able to record and observe emission regions in other galaxies.

You can also think of it like this: With reflection nebulae we have one or couple of stars (can be nebulosity around cluster) that emit light, and since dust in outer space is very sparse / low in density (couple of atoms in cubic meter or there about) how many of the photons that star emitted happen to scatter of that few atoms?

On the other hand emission nebulae - every atom can emit light if it gets excited.

This reasoning tells me that emission nebula tend to be much brighter and can be seen from further away (like Halpha regions of other galaxies). This would also imply that most of nebulosity that we see in the night sky is related to emssion nebulae.

[jonnydreads]

I suppose it becomes part and parcel of the same thing in the end, the more gases are reflected by stars the more excited they would get as well... 

 

[teoria_del_big_bang]

50 minutes ago, vlaiv said:

Basic two types of nebulosity out there:

1. reflection

2. emission

They differ in the way light that we see / record is generated. With reflection nebulae it is light from nearby stars that gets reflected from dust/gas in nebula - hence name reflection. It's a bit like shining torch at smoke / fog during the night - you will see smoke / fog because it has light shone on it. Because star light is broad band - so is the reflection of it from the gas and dust.

Emission nebula don't have separate light source to power their glow. It's the gasses that form the nebula that get excited (temperature / magnetic fields / gravity / whatever) - that makes them shine. They shine because electrons in excited atoms jump back into lower energy state and photon is emitted. Different atoms have different energy states and energy difference between two states is always the same - this is why we talk about H alpha and beta - meaning Hydrogen atom, jump from second orbital to first one and from third to first. Similarly other elements emit their light - OIII, SII and so on ... These all have well defined frequency of light and this is why we can use narrow band imaging - filters designed for particular wavelength. This process is very similar to neon lights that we regularly use.

 

Great explanation 

Steve

[vlaiv]

Have a look here:

https://www.syfy.com/syfywire/reflection-and-emission

(just a thing I found in a quick search) - they often come in pairs, but for emission part - you either need hot dense gas (in atomic form) or strong UV source to ionize it (or other means of ionization).

Reflection bit is mostly of dust - which is larger in size (like not individual atoms but tiny dust grains). Sometimes dust is just there not reflecting anything (because no nearby stars) - this gives so called dark nebulae - just dust blocking starlight from behind - looking like dark patch of the sky.

[jonnydreads]

Thanks thats hit home quite easily. You've taught me something today ??

[ollypenrice]

Often the reflection nebulae are blue for the same reason that the daytime sky is blue. https://en.wikipedia.org/wiki/Rayleigh_scattering

Modern cameras also make it possible to capture extremely faint reflection nebulae scarcely brighter than the background sky. 

Olly