APOD: Ultraviolet Rings of M31 (2015 Jul 24)

[Animal of Stone]

[size=150][color=#0000FF]The Flanderomeda Nedula[/color][/size]

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Always good to greet the neighbors.
When i see this image it always reminds me of deep space.
Who knew we were so close

Speaking of arms and rings, have there ever been any artist illustrations of what Andromeda would look like from a face-on view like ours has been illustrated from a face-on perspective?

Am still laughing

[Ann]

While I enjoyed the Flanderomeda Nedula fun, and while I agree with the (light-hearted) descriptions of Art and Geck, I prefer your musings, Ron-Astro Pharmacist.

You wrote:

The center of the Andromeda galaxy looks like it is emitting photons at a variety of wavelengths or are those photons coming from matter that just varies in temperature? I know I've struggled with this before but to understand an image constructed from a single type (within the range of UV photons) leaves me wondering if I'm looking at a variety of types of matter or the same type of matter under different physical conditions.

The way I understand it, yes, it is the temperature of an emitting object that determines what sort of photons it emits.

But we should remember that a hot object will always emit some "cool" (red and infrared) photons too. I think it is considerably harder and maybe impossible for a cool object, like a red giant star, to emit ultraviolet photons from its photosphere. But a cool star may have other ways of producing ultraviolet light than emitting it from its photosphere. The cool star Mira is an example.

And the center of a galaxy like the Andromeda will contain all sorts of stars of different temperatures. Yes, most of them will be cool stars that emit most of their light at red and infrared wavelengths. But there will certainly be a lot of ultraviolet-bright white dwarfs there, too. There will be other stellar beasts as well, such as blue stragglers. And we must expect neutron stars and pulsars and all kinds of weird binaries, including black hole ones.

So the reason why the center of the Andromeda galaxy emits photons at a variety of wavelengths is because there are all kinds of stars in there.

But in my opinion, a photon has no "memory". A ultraviolet photon emitted from our own G2V-type Sun will be no different from an ultraviolet photon of the same wavelength emitted from the photosphere of a searingly hot O-type star, and no different from an ultraviolet photon of the same energy emitted from the blazingly hot accretion disk of a supermassive black hole.

Ann

[Chris Peterson]

The way I understand it, yes, it is the temperature of an emitting object that determines what sort of photons it emits.

There is only one "kind" of photon, and it is characterized by its wavelength (equivalent to its energy) and momentum. The temperature of a body determines the wavelength of the photons it generates. When we're discussing stars, the bodies are well approximated by blackbodies, and therefore produce a continuum- photons with every possible wavelength. The peak wavelength is determined by temperature, as is the total energy output.

But we should remember that a hot object will always emit some "cool" (red and infrared) photons too. I think it is considerably harder and maybe impossible for a cool object, like a red giant star, to emit ultraviolet photons from its photosphere.

Although it may not be apparent from the above graph (but is from Planck's law, which describes that graph),


there is no temperature above absolute zero which produces zero energy at any wavelength. So cool objects produce UV wavelength photons, they just don't produce very many.

But in my opinion, a photon has no "memory". A ultraviolet photon emitted from our own G2V-type Sun will be no different from an ultraviolet photon of the same wavelength emitted from the photosphere of a searingly hot O-type star, and no different from an ultraviolet photon of the same energy emitted from the blazingly hot accretion disk of a supermassive black hole.

I'm not sure what you mean by "memory". Certainly, the photons you describe from these different sources will have the same energies. The wave vectors that characterize them will be different, simply because of their different directions of propagation. But for most purposes, we can consider them identical.

[Chris Peterson]

Why are there delineations between forms of light but not within sets of the same types? We have given names to defined ranges for the thing we call electro-magnetic radiation or can they be uniquely different from one another when created under different conditions? Is it just for convenience we call all UV photons that all fall within certain wavelengths, frequencies and energies a "UV photon" or is there some way to differentiate it between another UV photon that was created under slightly different circumstances.

It's all just photons with different energies. The names we have for parts of the spectrum are partly historical, and partly practical. There is no physical distinction between these regions, other than the obvious fact that the photon energies are different. We categorize the spectrum based on things like the kind of detectors we use, the kind of transmitters we can make, the kind of natural sources that exist, and many other factors. Also, keep in mind that different regions were discovered at different times, without the connection being made that they were all the same thing- electromagnetic radiation.

The center of the Andromeda galaxy looks like it is emitting photons at a variety of wavelengths or are those photons coming from matter that just varies in temperature? I know I've struggled with this before but to understand an image constructed from a single type (within the range of UV photons) leaves me wondering if I'm looking at a variety of types of matter or the same type of matter under different physical conditions.

What do you mean by "types of matter"? What you're seeing here is matter radiating under different physical conditions, primarily at different temperatures.

[neufer]

...there is no temperature above absolute zero which produces zero energy at any wavelength.
So cool objects produce UV wavelength photons, they just don't produce very many.

• That's not quite true.

33º C human beings typically have a net radiative heat loss of about 100 W peaking at about 9500 nm.

But the Planck curve falls off SO rapidly at short wavelengths
that ALL the 33º C humans that have EVER lived have never cumulatively
radiated a single photon of GALEX UV (135 to 280 nm) light (thru BB radiation).

[Chris Peterson]

...there is no temperature above absolute zero which produces zero energy at any wavelength.
So cool objects produce UV wavelength photons, they just don't produce very many.

• That's not quite true.

33º C human beings typically have a net radiative heat loss of about 100 W peaking at about 9500 nm.

But the Planck curve falls off SO rapidly at short wavelengths
that ALL the 33º C humans that have EVER lived have never cumulatively
radiated a single photon of GALEX UV (135 to 280 nm) light (thru BB radiation).

That's not quite true. Your argument is a statistical one. Certainly, it's extremely unlikely that any human has ever emitted a (thermal) photon of such short wavelength. But the fact remains, there's a small but finite radiant flux in the UV for 33°C humans (who are dead, BTW, if that's supposed to be their core temperature!). So we don't know with absolute certainty that a human hasn't emitted a UV photon. Indeed, I think I just emitted one now. But I'm a statistical anomaly. Just last week I absentmindedly walked through a door without any of my atoms interacting with those of the door. What are the odds of that?

[ta152h0]

scientists are comedians too ?

[alter-ego]

...there is no temperature above absolute zero which produces zero energy at any wavelength.
So cool objects produce UV wavelength photons, they just don't produce very many.

• That's not quite true.

33º C human beings typically have a net radiative heat loss of about 100 W peaking at about 9500 nm.

But the Planck curve falls off SO rapidly at short wavelengths
that ALL the 33º C humans that have EVER lived have never cumulatively
radiated a single photon of GALEX UV (135 to 280 nm) light (thru BB radiation).

That's not quite true. Your argument is a statistical one. Certainly, it's extremely unlikely that any human has ever emitted a (thermal) photon of such short wavelength. But the fact remains, there's a small but finite radiant flux in the UV for 33°C humans (who are dead, BTW, if that's supposed to be their core temperature!). So we don't know with absolute certainty that a human hasn't emitted a UV photon. Indeed, I think I just emitted one now. But I'm a statistical anomaly. Just last week I absentmindedly walked through a door without any of my atoms interacting with those of the door. What are the odds of that?

It depends on the door material and thickness, but I believe the probability is roughly 1 part in 10⁸⁰ give or take 10's of orders of magnitude. I gave up trying after about 100 attempts.

[ta152h0]

can you scrub my above comment, It was meant elsewhere where it would make sense. Thank you in advance

[neufer]

33º C human beings typically have a net radiative heat loss of about 100 W peaking at about 9500 nm.

But the Planck curve falls off SO rapidly at short wavelengths
that ALL the 33º C humans that have EVER lived have never cumulatively
radiated a single photon of GALEX UV (135 to 280 nm) light (thru BB radiation).

That's not quite true. Your argument is a statistical one. Certainly, it's extremely unlikely that any human has ever emitted a (thermal) photon of such short wavelength. But the fact remains, there's a small but finite radiant flux in the UV for 33°C humans
(who are dead, BTW, if that's supposed to be their core temperature!).

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[b][color=#0000FF]Much of a person's energy is radiated away in the form of infrared light. Some materials are transparent in the infrared, but opaque to visible light, as is the plastic bag in this infrared image (bottom). Other materials are transparent to visible light, but opaque or reflective in the infrared, noticeable by darkness of the man's glasses.[/color][/b]

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<<Skin temperature is about 33°C = 91.4°F, but clothing reduces the surface temperature to about 28°C = 82.4°F when the ambient temperature is 20°C = 68°F.>>

So we don't know with absolute certainty that a human hasn't emitted a UV photon. Indeed, I think I just emitted one now. But I'm a statistical anomaly.

• You're quite sure that was a UV photon

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Marge: What about Bart [for the baby's name]

Homer: Let's see. Bart, cart, dart, e-art...

• Nope can't see any problems with that.

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Just last week I absentmindedly walked through a door without any of my atoms interacting with those of the door.

What are the odds of that?

• Was the door open?

[Beyond]

What delightfully brilliant light this thread is radiating. It sure brightens up my day.

[DavidLeodis]

I have tried to find out but I am not fully certain so I would be grateful if someone can please confirm or not that the large fuzzy area below M31 is the M110 galaxy and that a small fuzzy area just on the left upper edge of M31 is M32. My uncertainty is because if it is M32 it looks even smaller than in other APOD images of M31 that I've looked up and seems also to likely be too small to have influenced M31.

[geckzilla]

Here ya go, David. Used Aladin to make sure.

[DavidLeodis]

Thanks geckzilla. Your help is appreciated.

[chuckster]

I've also read that Andromeda is actually huge in our sky, but it is dim because of "low surface brightness". Does that mean dust is hiding it, in optical wavelengths, from human eyes ?

It doesn't have particularly low surface brightness. It's a fairly typical spiral galaxy, quite like our own, and has a similar surface brightness. That is, visually, M31 looks very much like the Milky Way in brightness. A patch of light somewhat larger than the Moon that resembles an isolated patch of Milky Way glow. This is true for many galaxies, but most are too small for our eyes to clearly see without some magnification. Our acuity is low for dim objects, so they need to be large in order to be seen.

As Chris said, the limitation is the eye. With the naked eye under dark skies you can see a small fuzzy spot where the brightest central core is. But the actual angular size is about 6 full moons wide.

Here is a photo-shopped image that is a good comparison:
http://www.slate.com/blogs/bad_astronom ... e_sky.html

Thanks for the great answers, and to Chris and Ann also. I've had my consciousness raised. I think APOD once showed a picture of a house at night, with a lot of sky showing above it, with a few nebulae shown, to scale, how they would appear if our eyes saw in a wider spectrum. Huge !