I think that’s where negative images with the stars or in this case galaxies in black on a white background come in very useful and handy for revealing faint detail too.

For instance, see the discovery image for the tiny irregular moonlet Trinculo :

http://en.wikipedia.org/wiki/Trinculo_(moon)

orbiting Ouranos as one example.*

&

http://apod.nasa.gov/apod/ap030617.html

for a colour negative and see :

http://nedwww.ipac.caltech.edu/level5/March01/Israel/Israel3.html

FIgure [sic] 6. Deep negative B-band image of NGC 5128 shows the system of optical shells at the edges of the galaxy. This system of shells is one of the clearest indications that NGC 5128 has undergone mergers in its past.

for a third.

Aussie astrophotographer David Malin has taken some great examples but alas not too many that I can easily find online.

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* Trinculo was discovered in 2001 making this year (August 13th to be precise!) the tenth anniversary of its discovery – and also the discovery of no less than eleven small eccentric orbiting moons of Jupiter : Autonoe, Thyone, Hermippe, Eurydome, Sponde, Pasithee, Euanthe, Kale, Orthosie, Euporie & Aitne. Grand and poetic names for these tiny and obscure shards of rock or ice in tilted and tumbling paths around their gas giant primaries. Anyone seeking a magical sounding incantation (for whatever reason) could do worse than chanting the names of some of these lesser knowns moons or moonlets! Oh & while the Jovian moonlets are named from Greek mythology, Trinculo was named for a drunken jester from a Shakespearean play.

…well, a spectrum made up of distinct emission peaks, but with the intensity of each of those peaks following what you’d expect for that frequency were you dealing with a continuous blackbody spectrum. Is that incorrect?

I think that’s incorrect but there are other people who read this blog better able to comment than I can. But the intensity of a spectral line is not due to temperature but due to the number of atoms (or molecules) making the transition. Raising the temperature, i.e. energy, will promote more atoms or molecules to a higher energy level and therefore different spectral lines, but that won’t increase the energy of a given spectral line. If there are fewer atoms at a given energy level to decay, because an increase temperature caused electrons to move to higher energy levels, then the intensity of that line will decrease with temperature.

Increasing temperature will increase the width of a spectral line but won’t increase the intensity.
In fact astronomers use the ratio various lines of the same atom to determine the temperature of a body.

By the way electron transitions are not the only source of spectral lines used in Astronomy. Molecules give rise to spectral lines. Molecules rotate. The angular momentum of the rotation is quantized. That is how radio astronomers detect inter stellar molecules.

Also the electrons of the molecules vibrate back in forth like they were connected to the molecule with springs. That vibration is quantized. The vibrational transitions are primarily of use in infra red astronomy but are also important in radio astronomy.

George

In fact, in your superposition, the hydrogen arms seem to fit very well with and continue, the visible arms. This has to be fascinating to somebody that (unlike me) actually understands some of this. Obvious possibilities: One, the spiral structure is composed entirely of hydrogen, and the visible stars merely outline it (because the stars formed from gas that was already moving in the spiral dance and retained that motion); Two, whatever fizzicks create the spiral structure operates equally on stars and on diffuse cold gas; Three, the stars AND the gas are merely passengers on a spiral structure that is formed in something else, e.g. dark matter.

And it’s quite possible I am confused there; my understanding is that the black body curve is what you’d get for a material capable of absorbing and emitting light across infinitely many different wavelengths (effectively what you’d get if you looked at the light emitted by, say, a plasma that was capable of absorbing and emitting light translationally, yes?), and that for something like electronic transitions you’d still get…well, a spectrum made up of distinct emission peaks, but with the intensity of each of those peaks following what you’d expect for that frequency were you dealing with a continuous blackbody spectrum. Is that incorrect?

Thinking about what I quoted from Ronam @6, I think that post the post @6 showed some confusion between spectral lines and the Planck black body curve.

George

Hrm. Magnificent photo, but I’m puzzled by your statement that once the hydrogen is warmed up, it doesn’t emit as strongly at 21 cm. With an increase in temperature, I would have thought that although the peak emissions would shift to a shorter wavelength, the intensity of the 21 cm emissions would still increase.

The 21 cm line of hydrogen arises from a “spin flip” of the electron in the ground state of the hydrogen atom. The intensity of the radiation is due only to the number of hydrogen atoms in the line of sight. The velocity of the hydrogen atoms relative to the line of sight shifts the observed frequency.

If the hydrogen is warm enough to say emit the visible alpha line, electrons moving from the second exited state to the first exited state, then there will be very many fewer hydrogen atoms in the ground state so the 21 cm line will be much weaker if detected at all; few or no electrons in the ground state to flip spin.

George

Any idea of percentage/amount of mass they contain…?

It seems like the only way you could get such low-energy electron transitions would be if the electrons were already at quite a high energy level, and were only passing from that level to one just above or below them–which would imply ridiculously high temperatures for such “cold” gas, but cold and hot can be awfully relative terms, so I guess that works. And they wouldn’t be likely to be disturbed by much, out there in the void, so maybe such excited states could be relatively stable even at really low temperatures.

A long-exposure optical photo taken to the sky background limit might show much more faint optical emission. Of course, it would totally white out the central regions – not pretty.

The pink regions in my image are caused by hot hydrogen. They mark were active star formation is currently going on much like in our Orion Nebula. Though these regions are likely far grander than the Orion Nebula as I did not use a hydrogen alpha filter to bring out these regions.

is it possible to determine how big the milky way is, including “cold hydrogen” arms?

do these cold hydrogen arms extend into or beyond the calculated dark matter halos for spiral galaxies?