Question for astronomers, nebula - light emission

[Bruce]

I've seen photographs of nebula that do not appear to have stars. I presume it is because there is not enough mass in the nebula to form stars. My question is, how are these nebula emitting light? I presume the light is coming from ionized gas, but where is the energy coming from to ionize the gas?

Examples:
http://www.bergoiata.org/fe/divers01/Cloud Nebula.jpg
http://www.bergoiata.org/fe/espace05/HST Photos, Galactic Nebula NGC 7635 (1024).jpg
http://antwrp.gsfc.nasa.gov/apod/image/0202/orion_gendler_big.jpg

I would think that the only way to ionize huge, galaxy-sized regions of gas would be to ionize it with the energy from a star. If that is the case, I would only expect to be able to see ionized gas immediately surrounding stars, maybe within a few light years. I would also expect the intensity of the glowing gas to diminish as you go further away from the star. Instead, nebula appear to be fairly homogenous. Why is that?

 

[wollery]

The first is the Eagle Nebula, a star forming region in Serpens, and is lit by the stars forming inside the dense dust cloud.

The second is the Bubble Nebula in Cassiopeia (NGC7635), a planetary nebula with an 8th magnitude OB star at the centre, which illuminates it.

The third is the Orion Nebula, also a star forming region lit by the stars it contains.

None of these are lit by ionization of the gas, all are lit by reflected or diffused starlight.

 

[Bruce]

The first is the Eagle Nebula, a star forming region in Serpens, and is lit by the stars forming inside the dense dust cloud.

The second is the Bubble Nebula in Cassiopeia (NGC7635), a planetary nebula with an 8th magnitude OB star at the centre, which illuminates it.

The third is the Orion Nebula, also a star forming region lit by the stars it contains.

None of these are lit by ionization of the gas, all are lit by reflected or diffused starlight.

What is reflecting it? And do you mean refracting it since there are colors? Non-ionized hydrogen and helium does not reflect light. That is why it is invisible.

 

[RecoveringYuppy]

Some of the gas is ionized in at least some of those nebulas but I don't think it would be accurate to identify the ionization as the source of the lighting.

BTW none of those neblulas are anywhere near galaxy sized.

 

[TheDoLittle]

Also, some of the photos (I'm not sure which ones) are "false color" photographs, which means they use wavelengths outside of the visible spectrum, namely infrared and x-ray, to enhance the image of the nebula. This is a very common practice with dark objects like nebula that emit little or no light.

 

[Bruce]

Also, some of the photos (I'm not sure which ones) are "false color" photographs, which means they use wavelengths outside of the visible spectrum, namely infrared and x-ray, to enhance the image of the nebula. This is a very common practice with dark objects like nebula that emit little or no light.

Doesn't matter what wavelength. It's still emitting light. It's emitting light that can be seen from many light years away, which means lots of energy. And, if it's emitting X-rays, that means even more energy than visible light. Name a gas that spontaneously emits X-rays.

Sorry, haven't seen a convincing answer yet. Where is the energy in nebula's coming from?

 

[RecoveringYuppy]

What makes you think the energy being emitted can't be explained by the sources Wollery listed?

 

[wollery]

What is reflecting it? And do you mean refracting it since there are colors? Non-ionized hydrogen and helium does not reflect light. That is why it is invisible.

It's not just hydrogen and helium, and I never said they weren't ionized, just that ionization isn't what lights them up. A reasonable amount of the matter in star forming nebulae is dust, which both refracts and reflects light, and also there's other elements and molecules. Add to this that pre-stellar, and proto-stellar objects kick out huge amounts of energy in the UV, and you have a lot of energy to light up your nebula.

The planetary nebula is mostly gas, but contains heavier elements, since it's the remnants of the outer shell of a star that's been going through the last stages of its life, the central star is what illuminates it.

Most of the colours you see on astronomical images are false, although approximated to true colours a lot of the time. This is because professional astronomical instruments take monochrome images in specific wavelength ranges. These monochrome images can then be added together, with false colour, to reveal more detail.

 

[Dancing David]

Doesn't matter what wavelength. It's still emitting light. It's emitting light that can be seen from many light years away, which means lots of energy. And, if it's emitting X-rays, that means even more energy than visible light. Name a gas that spontaneously emits X-rays.

Sorry, haven't seen a convincing answer yet. Where is the energy in nebula's coming from?

Please read what Wollery wrote and then look at these :
http://antwrp.gsfc.nasa.gov/apod/ap070202.html
Ionizing hydrogen in Orion nebula
http://antwrp.gsfc.nasa.gov/apod/ap070125.html
Stars in Orion nebula
http://antwrp.gsfc.nasa.gov/apod/ap061211.html
reflection nebula in Orion
http://antwrp.gsfc.nasa.gov/apod/ap061208.html
hydrogen, sulpher and oxygen emission in Orion nebula



And please remember that light does not get tired as it travels in space. to see the Andromeda galaxies does nor require extra strong photons. just photons, like the ones that come off a light bulb. they can and do travel all that way.

 

[wollery]

Although the total amount of photons from a given object is a constant, it's spread over the surface of a sphere, and therefore drops as the square of the distance from that object.

However, having said that, the vast majority of such nebulae are to be found within our own Galaxy, so the light isn't travelling that far in the grand scheme of things.


Oh, by the way, in order for the gases to be ionized they must have a source of ionizing radiation, i.e. a central star.

 

[Bruce]

I A reasonable amount of the matter in star forming nebulae is dust, which both refracts and reflects light, and also there's other elements and molecules.

What exactly do astronomers mean by "dust"? I know it's not the same dust that's been gathering on the top of my PC, but what is it? Bits of rock? A random scattering of all elements? Methane? Ether? Dark matter? Fragments of atoms such as quarks? All of the above?

If there's that much floaty bits, I suppose I could believe that the glow of nebula is coming from refracted lights of stars and ionization of several gases, but how did all those little floaty bits find their way into a cloud after the Big Bang scattered all matter in all directions? Gravity, I suppose, but the strength of gravity is proportional mass. What we see in the universe is matter gathering into pools to form nebula and galaxies. Seems that the Big Bang wasn't exactly a homogeneous explosion, correct? If it were, I would expect the universe to look like a balloon, with all matter scattered evenly around the perimeter.

Bah. The Universe is too huge. I'm sticking to my tiny little world of chemistry where everything I'm trying to study is contained in a manageable space.

 

[Bruce]

And please remember that light does not get tired as it travels in space. to see the Andromeda galaxies does nor require extra strong photons. just photons, like the ones that come off a light bulb. they can and do travel all that way.

Matter shouldn't get terribly tired either, which leads me to another question. Why do nebula look like clouds? Clouds on earth are shaped the way they are by our atmosphere and all the forces within that act on them. What forces are in play in space other than gravity? Seems like nebula should predominately be spherical, not poofy and lumpy.

 

[wollery]

What exactly do astronomers mean by "dust"? I know it's not the same dust that's been gathering on the top of my PC, but what is it? Bits of rock? A random scattering of all elements? Methane? Ether? Dark matter? Fragments of atoms such as quarks? All of the above?

Dust is anything which is composed of more than a single molecule. I.e., stuck together by Van der Waals (for example) forces rather than by chemical bonding.

If there's that much floaty bits, I suppose I could believe that the glow of nebula is coming from refracted lights of stars and ionization of several gases, but how did all those little floaty bits find their way into a cloud after the Big Bang scattered all matter in all directions? Gravity, I suppose, but the strength of gravity is proportional mass. What we see in the universe is matter gathering into pools to form nebula and galaxies. Seems that the Big Bang wasn't exactly a homogeneous explosion, correct? If it were, I would expect the universe to look like a balloon, with all matter scattered evenly around the perimeter.

Indeed, not a bad summary. The star forming regions in our Galaxy are the remnants of long dead stars, which is why they contain heavier elements and dust. The non-homogeneities in the Big Bang are very small, just a tiny fraction of a percent, but enough to allow matter to collect in lumps and streams, from where it collapses into galaxies under self gravity.

Bah. The Universe is too huge. I'm sticking to my tiny little world of chemistry where everything I'm trying to study is contained in a manageable space.

 

[wollery]

Matter shouldn't get terribly tired either, which leads me to another question. Why do nebula look like clouds? Clouds on earth are shaped the way they are by our atmosphere and all the forces within that act on them. What forces are in play in space other than gravity? Seems like nebula should predominately be spherical, not poofy and lumpy.

As you noted earlier, they're very big, and they consist of knots of dense material, embedded in less dense material. Part of this is due to the gravitational influence of other objects in their surrounding, and differential collapse rates in different parts of the cloud. And once the stars start to form in the nebulae they emit a lot of energy which blows some of the material out, also altering the shapes. And since these protostars are moving around, also due to gravity, they drag a lot of the dust and gas around with them. It's a very chaotic system.

 

[Bruce]

The non-homogeneities in the Big Bang are very small, just a tiny fraction of a percent,

Really? So could this mysterious "dark matter", the stuff that causes woos and science fiction novelists to drool on themselves, simply be methane, hydrogen, helium, etc. scattered homogeneously throughout the universe, thus unable to gather into pools due to gravity and form stars, and thereby invisible to our prying eyes for all eternity?

Takes the mystery out of the term "dark matter", doesn't it?

 

[Bruce]

As you noted earlier, they're very big, and they consist of knots of dense material, embedded in less dense material. Part of this is due to the gravitational influence of other objects in their surrounding, and differential collapse rates in different parts of the cloud. And once the stars start to form in the nebulae they emit a lot of energy which blows some of the material out, also altering the shapes. And since these protostars are moving around, also due to gravity, they drag a lot of the dust and gas around with them. It's a very chaotic system.

I've heard biologist say, "We owe our existence to asymmetry in nature". Based on what you just said, I would think astronomers would say the same.

 

[wollery]

Really? So could this mysterious "dark matter", the stuff that causes woos and science fiction novelists to drool on themselves, simply be methane, hydrogen, helium, etc. scattered homogeneously throughout the universe, thus unable to gather into pools due to gravity and form stars, and thereby invisible to our prying eyes for all eternity?

Takes the mystery out of the term "dark matter", doesn't it?

No.

The dark matter is concentrated in galaxies and clusters of galaxies, and we can measure the effects it has on the visible matter in these systems. It was precisely the effects that it has (such as galactic rotation curves) that cannot be explained by the visible matter that make us think it's there.

I've heard biologist say, "We owe our existence to asymmetry in nature". Based on what you just said, I would think astronomers would say the same.

Not asymmetry so much as inhomogeneity.

 

[Dancing David]

Matter shouldn't get terribly tired either, which leads me to another question. Why do nebula look like clouds? Clouds on earth are shaped the way they are by our atmosphere and all the forces within that act on them. What forces are in play in space other than gravity? Seems like nebula should predominately be spherical, not poofy and lumpy.

Cool question.

Molecular clouds are effected by gravity, tidal forces from other objects and friction I would assume.

My thinking is that the gravity of a molecular cloud is weak, so it holds it together rather looseley and gathers it slowly, so passing stars and other forces will tend to distort it.

Check this out

http://antwrp.gsfc.nasa.gov/apod/ap950911.html

 

[Bruce]

Check this out

http://antwrp.gsfc.nasa.gov/apod/ap950911.html

Sweet.

 

[RecoveringYuppy]

Molecular clouds are effected by gravity, tidal forces from other objects and friction I would assume.

My thinking is that the gravity of a molecular cloud is weak, so it holds it together rather looseley and gathers it slowly, so passing stars and other forces will tend to distort it.

Whether or not a cloud will collapse is determined by three factors: radius, temperature and density. The formula for figuring it out exactly is the "Jean's length" formula. Temperature determines the velocity of particles in the cloud and if they are below the clouds escape velocity they are stuck there. Density determines the gravity field and escape velocity.

If a cloud is below it's Jean's length then it isn't big enough to collapse. If it's above it's Jean length it will collapse and fragment into multiple clouds as it collapses.

Clouds which will collapse are usually very cool which means they don't emit much light. Cloud collapse is pretty straight forward until the point where the cloud becomes so dense that it can't radiate away the heat generated by collapse effectively.

I'd have to look it up but I'd think the nebula mentioned in the OP are too hot to collapse. The two star forming regions are almost guaranteed to be at the point where they can't radiate away heat effectively.