Satellites

billybob884 · Post #1by **billybob884** » 10.05.2003, 02:19

Ok, I know satellites have been very popular lately, so I found this page that lists like 50 satellite missions: http://seds.lpl.arizona.edu/nineplanets ... craft.html

billybob884 · Post #2by **billybob884** » 11.05.2003, 03:40

here is a nice site for nebulas: http://www.seds.org/billa/twn/

Darkmiss · Post #3by **Darkmiss** » 12.05.2003, 00:21

Wow i really like the Crab Nebula,

and the Veil nebula

Brina1 · Post #4by **Brina1** » 19.05.2003, 03:57

one thing that always puzzled me... exactly what illuminates nebulae? I know it's stars, but we're talking about gas clouds that I believe (not sure) sometimes reach several lightyears in diameter, and yet they're always iluminated so brightly all around...

I mean, there's a nebula that Sol's located inside of... Not much of one but it's there... And we can't see IT... and the Crab Nebula, correct me if I'm wrong, it comes from a star that went supernova and the light from it was seen in daylight by humans back in the 900s CE. The Chinese called it "The Visitor" I think... So if there's no star there anymore what's lighting it up? Sure there's a pulsar there, but aren't pulsars more like the radio version of a very BIG lighthouse? I wasn't aware they produced much light...

Post #5by **selden** » 19.05.2003, 04:21

Brina1,

The nebulae that give off light are illuminated by extremely brilliant stars. Many other nebulae are dark dust clouds that absorb light.

Do a Web search for "emission nebulae," "reflection nebulae" and "absorption nebulae" for details.

Matt McIrvin · Post #6by **Matt McIrvin** » 24.05.2003, 16:06

Brina1 wrote:one thing that always puzzled me... exactly what illuminates nebulae? I know it's stars, but we're talking about gas clouds that I believe (not sure) sometimes reach several lightyears in diameter, and yet they're always iluminated so brightly all around...

They're not always illuminated brightly all around. Those are just the parts you can see. And those pictures are taken with telescopes, which collect light over a large area and concentrate it, and sometimes with long exposure times-- they're not really that bright. Consider that some of these nebulae cover enough of the sky that they'd be quite prominent to the naked eye if they really were that bright!

The Veil Nebula looks to me like the basis for a really ambitious 3D surface modeling project! Maybe if Rassilon has a month or so free...

Matt McIrvin · Post #7by **Matt McIrvin** » 24.05.2003, 16:16

Brina1 wrote:So if there's no star there anymore what's lighting it up? Sure there's a pulsar there, but aren't pulsars more like the radio version of a very BIG lighthouse? I wasn't aware they produced much light...

By the way, the answer to your question about the Crab Nebula is really cool. It mostly isn't being illuminated by the pulsar. Rather, there are fast-moving electrons in that gas, and the intense magnetic field from the pulsar bends their paths into a helical shape. The tremendous acceleration necessary to stay on the helix causes them to shed energy in the form of electromagnetic waves, some of which are visible light.

Physicists call this synchrotron radiation, because it also happens in the circular particle accelerators known as synchrotrons (where it usually manifests at higher wavelengths than the visible-- it can be intense ultraviolet or X rays, for instance). Normally this is a nuisance because it wastes the energy that is put into the particles. But today this same radiation is sometimes put to good use, for X-ray diffraction to discover the atomic structure of materials, medical imaging, and even cancer therapy, and there are synchrotrons specially built just to produce it.

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Satellites

Satellites

The light's better there...

Synchrotron radiation