"hot" interstellar gas

[Kamienka]

I am a total newbie here, although I have been on APOD for years just as an ordinary reader.
My question is this: In many APOD pictures comments a "hot gas" is mentioned, referring to interstellar (molecular) gas, just like it is mentioned in the February 13th picture explanation - "visible light is white, while the X-ray light is blue and indicates the unusual presence of very hot gas".
Well, as far as I know, normal temperature of interstellar space is very near absolute zero, right? So, I suppose that anything "warmer" than that for about 100 degrees K should be considered "hot". Or am I totally wrong here?
Could anyone explain this? What is considered "hot" or "very hot" in interstellar space, dust or molecular gas? How hot must that gas be to be seen as a red glowing cloud by us?
Thank you.

[starnut]

Could anyone explain this? What is considered "hot" or "very hot" in interstellar space, dust or molecular gas? How hot must that gas be to be seen as a red glowing cloud by us?
Thank you.

"Hot" in interstellar/intergalactic space is not the same as "hot" you feel on good old terra firma. The heat we feel is primarily infrared heat coming from the sun, the surrounding air, the parking lot on a hot day, and your room heater. The heat in the cold of space is not what you would feel on your skin. It is generated by the energy of the photons zipping along in vacuum and hitting the gas atoms floating around. It can only be sensed by instruments.

The photons have minimal effects on dust unless the dust is close to a star whose radiation and wind would push it away. The red glow comes from hydrogen atoms whose electrons were knocked out by the ultraviolet radiation from the stars and then recombined with the protons. The false colored x-ray glow may have come from gas accelerated to very high speed and colliding with each other, giving off x-ray. At least, that is what I thought. Someone please correct me.

[neufer]

I am a total newbie here, although I have been on APOD for years just as an ordinary reader.
My question is this: In many APOD pictures comments a "hot gas" is mentioned, referring to interstellar (molecular) gas, just like it is mentioned in the February 13th picture explanation - "visible light is white, while the X-ray light is blue and indicates the unusual presence of very hot gas".
Well, as far as I know, normal temperature of interstellar space is very near absolute zero, right? So, I suppose that anything "warmer" than that for about 100 degrees K should be considered "hot". Or am I totally wrong here?
Could anyone explain this? What is considered "hot" or "very hot" in interstellar space, dust or molecular gas? How hot must that gas be to be seen as a red glowing cloud by us?

8,000º K would help.
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A red hot glowing poker must be:

Code: Select all

Color  	Approximate Temperature
-----------------------------
Faint Red _____     770K
Blood Red __        855K
Dark Cherry         910K
Medium Cherry 	   965K
_______Cherry 	  1020K
Bright Cherry __   1060K

but, at least, a poker is approximately a black body.
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OTOH, 1060º K atomic hydrogen (and to somewhat a lesser extent molecular hydrogen)
is virtually invisible and does not absorb OR radiate (except at radio frequencies).

Hydrogen atoms must be excited to close to 8,000º K before they are in an excited enough state to effectively absorb or radiate the Balmer red H-α 656.3 line

<<The first line in the ultraviolet spectrum of the Lyman series was discovered in 1906 by Harvard physicist Theodore Lyman, who was studying the ultraviolet spectrum of *electrically excited hydrogen gas*. In stars, the Balmer lines are usually seen in absorption, and they are "strongest" in stars with a surface temperature of about 10,000º kelvin (spectral type A).>>

Check out: http://en.wikipedia.org/wiki/Interstellar_medium

http://casswww.ucsd.edu/public/tutorial/ISM.html

Code: Select all

   Principal Gas Constituents of the ISM (The Interstellar Medium)
---------------------------------------------------------------------------------------
____________________  Total Mass(M) _"Cloud" Mass(M)__ Density(cm-3)__Temperature(K)

Molecular Clouds  H2 gas 	1-5 x 10^9 	10^5-10^6 	10^3-10^5 	~10
Neutral Hydrogen  HI gas 	~5 x 10^9 	_______  	0.1-10 __      100-1000
Ionized Hydrogen  II gas 	__________  	100-1000 	10^3-10^4 	10,000

------------------------------------------------------------------------------------

[Qev]

Also bear in mind the difference between heat and temperature.

Heat is the total amount of (thermal) energy contained in a substance. Temperature is the average kinetic energy of the particles making up a substance.

'Hot' interstellar gas is 'hot' in terms of temperature; the individual particles are moving with great speed with respect to each other. However, the gas is so incredibly diffuse that any given volume contains very little heat.

[Arramon]

ahh.. this seems to answer my question from another topic about today's APOD image, the red and blue nebulae creating a purplish glow where it mixes. But is that what's really going on in a false-color image? Is red and blue making purple? Would yellow and blue make green?

Or are the colors from a mixture of different wavelength images being combined?

[bystander]

ahh.. this seems to answer my question from another topic about today's APOD image, the red and blue nebulae creating a purplish glow where it mixes. But is that what's really going on in a false-color image? Is red and blue making purple? Would yellow and blue make green?

Or are the colors from a mixture of different wavelength images being combined?

By "today's APOD", I assume you mean http://antwrp.gsfc.nasa.gov/apod/ap080215.html. In this false color photograph, colors have been assigned to represent relative temperatures of the star field. http://www.spitzer.caltech.edu/Media/re ... -03a.shtml.

But to answer your other question, red and blue still combine to make purple. However, blue and yellow do not make green, green and red make yellow (additive combination of colored light, http://en.wikipedia.org/wiki/Primary_colors).