4 ms·

I don't think that's true. Other galaxies don't appear as thin or as flat as paper. (Examples: http://apod.nasa.gov/apod/ap031008.html http://apod.nasa.gov/apod

I don't think that's true. Other galaxies don't appear as thin or as flat as paper. (Examples: http://apod.nasa.gov/apod/ap031008.html http://apod.nasa.gov/apod/ap031008.html http://apod.nasa.gov/apod/ap020703.html http://apod.nasa.gov/apod/ap020703.html http://apod.nasa.gov/apod/ap060612.html http://apod.nasa.gov/apod/ap060612.html) Typically they're about 100x wider than they are thick.

ars 16y ago
http://www.astronomycafe.net/qadir/q546.html http://www.astronomycafe.net/qadir/q546.html "....the thickness of the Milky Way is 50/100000 or 5/10000 of its maximum size making it flatter than a sheet of writing paper." You can not calculate how flat something is by estimating from such a photo!
- AngryParsley 16y ago
  Wikipedia says the stellar disc of the Milky Way is 1,000 light-years thick and cites http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980317b.html http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980317b.... . The core is thicker. Even the image on the page you linked to shows an object that is much thicker than a piece of paper.
  - ars 16y ago
    Did you miss the part where I said: "Depending on how you define the thickness" ? The milky way does not have a clear cut end, so you need some kind of definition on where to stop, and there are many ways to do so. And you should stop estimating thickness by looking at photos. Brightness often masquerades as thickness in photos.
    - gjm11 16y ago
      Yes, there are many definitions. None of them, however, gives the result that the Milky Way is flatter than a piece of paper. Not even the (rather silly) one used by the author of the web page you quoted; his numbers are wrong.
- gjm11 16y ago
  1. He says 150 light years out of 100,000 light years. Then he writes 50/100000, just losing a factor of 3 for no reason at all. 2. A typical sheet of writing paper is 10um or so in thickness and 20cm or so in extent. The ratio is 1/20000, which is 10x smaller number than even his (wrong) 50/100000. 3. His figure of 150ly is based on the distribution of molecular clouds. This seems an absolutely ridiculous way of measuring the thickness of the galaxy, about as sensible as measuring the dimensions of a human being by the distribution of their stem cells or something. It seems that the choice was motivated by wanting the smallest possible thickness figure. 4. The figure of 150ly for molecular cloud heights is too small. See figure 3 of http://iopscience.iop.org/1538-4357/619/2/L159/pdf/18343.web.pdf http://iopscience.iop.org/1538-4357/619/2/L159/pdf/18343.web...; the right-hand portion of the figure shows average heights, which indeed are in that range, but as you can see from the left-hand portion there's a lot of variation and there are plenty of molecular clouds well above 150ly. (Note that the vertical scale in that figure is in parsecs; a parsec is about 3.26ly.) So. The author of that page (1) selected the "thinnest" measure he could find, (2) used too low a value for it, (3) dropped a factor of 3, and (4) still got a result 10x thicker than a typical piece of paper. (That's assuming that "flat" means "thin". The Milky Way is not particularly close to being planar, either.)

ars 16y ago

http://www.astronomycafe.net/qadir/q546.html http://www.astronomycafe.net/qadir/q546.html "....the thickness of the Milky Way is 50/100000 or 5/10000 of its maximum size making it flatter than a sheet of writing paper." You can not calculate how flat something is by estimating from such a photo!

AngryParsley 16y ago
Wikipedia says the stellar disc of the Milky Way is 1,000 light-years thick and cites http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980317b.html http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980317b.... . The core is thicker. Even the image on the page you linked to shows an object that is much thicker than a piece of paper.
- ars 16y ago
  Did you miss the part where I said: "Depending on how you define the thickness" ? The milky way does not have a clear cut end, so you need some kind of definition on where to stop, and there are many ways to do so. And you should stop estimating thickness by looking at photos. Brightness often masquerades as thickness in photos.
  - gjm11 16y ago
    Yes, there are many definitions. None of them, however, gives the result that the Milky Way is flatter than a piece of paper. Not even the (rather silly) one used by the author of the web page you quoted; his numbers are wrong.
gjm11 16y ago
1. He says 150 light years out of 100,000 light years. Then he writes 50/100000, just losing a factor of 3 for no reason at all. 2. A typical sheet of writing paper is 10um or so in thickness and 20cm or so in extent. The ratio is 1/20000, which is 10x smaller number than even his (wrong) 50/100000. 3. His figure of 150ly is based on the distribution of molecular clouds. This seems an absolutely ridiculous way of measuring the thickness of the galaxy, about as sensible as measuring the dimensions of a human being by the distribution of their stem cells or something. It seems that the choice was motivated by wanting the smallest possible thickness figure. 4. The figure of 150ly for molecular cloud heights is too small. See figure 3 of http://iopscience.iop.org/1538-4357/619/2/L159/pdf/18343.web.pdf http://iopscience.iop.org/1538-4357/619/2/L159/pdf/18343.web...; the right-hand portion of the figure shows average heights, which indeed are in that range, but as you can see from the left-hand portion there's a lot of variation and there are plenty of molecular clouds well above 150ly. (Note that the vertical scale in that figure is in parsecs; a parsec is about 3.26ly.) So. The author of that page (1) selected the "thinnest" measure he could find, (2) used too low a value for it, (3) dropped a factor of 3, and (4) still got a result 10x thicker than a typical piece of paper. (That's assuming that "flat" means "thin". The Milky Way is not particularly close to being planar, either.)