Plasma and TVs


When I think of television, I picture the grungy old 70s trash-rescue we had at home when I was a kid (complete with pinkish fringes from an ill-advised magnet demonstration). But it seems TV technology has advanced significantly: DVB, HDTV, LCD screens, plasma screens... there is an interesting link between the last of these and some of the brightest "stars" in the sky.

A plasma screen is a large array of cells full of low-pressure gas, with electrodes on either side, and phosphors coating the inside of the front. To turn on a pixel, you apply a voltage across that cell, ionizing the gas. The flowing current accelerates the ions, which crash into the phosphors, which then light up. (A similar process occurs in the old CRT TV I grew up with, except there it was electrons hitting the phosphors, and they were focused into a beam that sept across all the pixels.)

What interests me, though, is that some of the early plasma screens had memory: you could put an image up on them, and then simply supply a holding voltage, and the image would stay indefinitely. By contrast, CRTs (normally) need the image constantly refreshed. Not a problem for computers, of course, but memory is extremely tricky to build. This is why you shouldn't leave a VCR on pause for too long: in order to keep the picture on screen, the head needs to keep spinning against the same piece of tape, constantly rereading the same picture.

Aside for younger readers: that box under an old TV that looks like a toaster on its side is a VCR. It records video on a spool of magnetic tape. Under no circumstances should you put bread in it.

These early plasma screens were on/off - no shades of gray, each pixel was either on or off. The way the memory worked was this: a high voltage will strip electrons off their atoms and turn a gas into a plasma. But once the electrons are loose, the plasma is very conductive, and a quite modest voltage will keep a current flowing, which keeps the plasma ionized. But if you shut off the voltage, the electrons will recombine with their atoms and you get a non-conductive gas - across which you can apply exactly the same modest voltage and get no current. So the information about the brightness of the pixel is stored in the ionization state of the gas.

What does this have to do with astronomy? Well, there are a number of kinds of "nova", astronomical events that make what looks like a new star appear in the sky. The one I'm going to talk about is a "dwarf nova". This is actually two stars orbiting very close to one another. One is a white dwarf, incredibly small and dense, and the stars are so close together that the matter on the surface of the bigger (less-dense) star actually "overflows" and falls on the white dwarf. Since the white dwarf is so small, it has a very deep gravity well and this process releases a tremendous amount of energy.

It turns out it's hard for matter to actually fall onto a white dwarf (or a neutron star or black hole). In spite of the strong gravity, the matter can't just fall in because of conservation of angular momentum: as the matter moves inward, if it keeps its angular momentum it must go around more quickly, which tends to move it outward. In order for matter to fall in, it must get rid of its angular momentum somehow.

It turns out that if the gas falling in isn't ionized, then there's no way to get rid of that angular momentum quickly enough to make the bright novas we see. But if you ionize the gas, well, there's still considerable debate about exactly what happens, but since the plasma is conductive, you open the door to all kinds of magnetic effects. In particular, magnetic fields can connect adjacent regions of the inflowing material, providing enough viscosity to transport lots of angular momentum outward.

In some systems, it looks like this is what's going on: you have a disk of infalling material, it's ionized, and so the material flows steadily and rapidly down onto the surface of the star, producing a reasonably steady, bright, source. (I recently heard John Thorstensen describe this as "a flat, gravity-powered star", which I think is a very nice way to think of it.) But what happens when the companion cannot keep up with the amount of matter that the disk is gobbling up?

Well, here we return to the plasma TVs: what you get is a system with two states. In one state (most of the time), gas is overflowing into the disk, but the disk isn't ionized, so not much gas can fall in onto the white dwarf, and the system is dim. But as this continues, the density of the disk increases, and eventually the trickle releases enough energy to ionize some of the disk. This ionized material then starts falling in rapidly, releasing more energy, so that the whole disk is soon ionized. Now the system becomes very bright, with the ionized matter pouring down onto the white dwarf, and we see a "nova". But since the companion can't supply much gas, soon the disk will empty out, cool off, and stop being ionized.

So: a two-state system, stable (for a while) in either ionized or non-ionized states. When ionized it's bright; when the ionization shuts off it's dim. Just like a plasma TV? Well, maybe not. For one thing plasma TVs don't emit nearly as many X-rays.

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