Hey, don’t forget about the lunar eclipse tonight!
Somehow I missed this one at the AAS meeting I attended in January: a team of astronomers found that our galaxy is twice as thick as previously thought!
The Milky Way is a spiral, meaning it’s a flat disk. Face on, you can see the glorious spiral arms and the central bulge, with the disk spreading across an impressive 100,000 light years or so.
But edge-on, it’s much thinner. The exact thickness depends on what you’re looking at, though, making this a bit dicey. High-mass stars tend to stick right to the mid-plane, the line dividing the galaxy in half (like a layer of icing through a cake). Lower mass stars can reach farther above and below this plane (lighter weight stars get tossed around more easily). Other methods produce different results yet.
The astronomers in question here looked at warm gas in the galaxy… sorta. What they did was examine pulsars, dead, dense stars that send out short, sharp radio signals. As those signals pass through warm gas, they get spread out in wavelength, so that the longer wavelengths arrive a wee bit later than the shorter ones. The amount of dispersion of wavelength tells you how much gas the signal passed through.
The astronomers looked at pulsars where the distances were known, and the pulsars themselves were well away from the galactic mid-plane. When they did this, they could determine the density of the warm gas between us and the pulsars, and where that gas stops.
The results: the warm gas stretches 6000 light years above and 6000 light years below the mid-plane. Before, it was thought the galaxy was only half this thickness.
Like I said before, this doesn’t mean the galaxy itself is twice as thick as we thought in general, because the thickness depends on what you measure. But this does throw a monkey wrench in some ideas about the size of the galaxy, and the distribution of gas within it. Why is the gas spread out so much more than previously thought? How did it get there?
Surprises like this are generally important. It means we get to revise theories, and that in turn means we’re edging closer to the truth. That’s what science is all about.
Tip o’ the dew shield to BABloggee Rich for sending this along!
February 20th, 2008 at 11:41 am
Interesting. It still looks a very thin disk to me. By the way, we were told in high school (and even uni) that the Sun lies in the outer regions of the Milky Way. According to the picture you posted, it’s approximately halfway along the radius. This was news to me as well.
By the way, what is Admiral Ackbar doing down there?
February 20th, 2008 at 11:50 am
That’s a really old diagram of the galaxy. If you click on the galaxy, you’ll see that the copyright is from 1991.
We now know that the Milky Way is a moderately barred spiral. Here’s one somewhat more accurate image.
http://en.wikipedia.org/wiki/Image:Milky_Way_Arms.svg
February 20th, 2008 at 11:56 am
The grade-school version I remember hearing was that the galaxy was about 100,000 light years across, and about 10,000 LY thick, which is only a 20% change. Was that figure measured from the bulge, or just inaccurate (or inaccurately remembered?)
February 20th, 2008 at 11:58 am
because the thickness depends on what you measure.
I’m just not going to touch that one.
February 20th, 2008 at 12:00 pm
The image, that is not the Milky Way I know. It shows a wrong type of galaxy. Our galaxy is a barred spiral. Although the image is only 17 years old it is outdated. Tells something about the speed astronomy advances at these days…
For much better reconstructions, please see indescribably cool website An Atlas of the Universe: http://www.atlasoftheuniverse.com/galaxy.html
February 20th, 2008 at 12:00 pm
Yeah, I thought the Sun was about 2/3rds the way from the center to the edge. It shows it being only a quarter of the way.
And haven’t we discovered that it’s a barred spiral?
February 20th, 2008 at 12:02 pm
But we’re still going to pass through the exact center of the galactic disk on Dec. 21, 2012, right?
February 20th, 2008 at 12:22 pm
But we’re still going to pass through the exact center of the galactic disk on Dec. 21, 2012, right?
Absolutely not.
February 20th, 2008 at 12:52 pm
I thought that Earth was at the centre? Just where Jeebus left it
February 20th, 2008 at 12:55 pm
As others have noted, observations using the Spitzer Space Telescope showed a few years ago that the Milky Way is a barred spiral galaxy.
February 20th, 2008 at 1:05 pm
Dr. BA,
Could you do me a great big favor with your graphic?
I’m writing a sci-fi novel, and in it I’d like to go visit Nancy Leider’s home planet. Could you label Zeta Ridiculous for me so I can visualize it for navigation purposes?
Thanks
February 20th, 2008 at 1:48 pm
I don’t think this observation changes the scale height of the various star populations; it seems to apply only to the interstellar medium. And if I remember correctly the ISM is only 5% or so of the (baryonic) mass of the galaxy.
February 20th, 2008 at 1:52 pm
Of course, the Milky way has more than one disk - As well as the thin disk of stars, there is also a kinematically distinct thick disk. The different scale heights reflect the formation of the components and their interaction with the gravitational potential of the MW. It’s not suprising that the gas disk has a different scale height to stars.
Also, the fact that the MW has a bar was discovered long before Spitzer even got of the ground. See
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1994ApJ%2E%2E%2E429L%2E%2E73S&db_key=AST
and no - we went through the mid-plane of the MW 3 Myrs ago - not another passage for another ~30Myrs
http://www.nature.com/nature/journal/v316/n6030/abs/316706a0.html
February 20th, 2008 at 2:22 pm
When I look into the sky and see the “Milky Way,” am I seeing the Perseus Arm, the Sagitarius Arm, both or neither? Does it depend on the season? Is it perhaps the Orion Arm seen edge on (assuming that is the section of the galaxy we live in.).
February 20th, 2008 at 3:35 pm
Norma arm?
February 20th, 2008 at 3:58 pm
When talking about the thickness of one’s galaxy, it’s important to add a few light years to impress the ladies.
February 20th, 2008 at 4:13 pm
According to MPFC:
The galaxy itself contains a hundred billion stars;
It’s a hundred thousand light years side to side.
It bulges in the middle; 16,000 light years thick,
But out by us it’s just 3,000 light years wide.
So, does Eric have to re-write the song?
February 20th, 2008 at 4:45 pm
> So, does Eric have to re-write the song?
Sigh… No - There is more than one disk - different populations have different scale heights - the new result is the gaseous disk, not the stellar disk(s)
February 20th, 2008 at 5:20 pm
Does anyone know WHY it’s flat, as opposed to, say, spherical? What’s the force that makes everything stick to the same plane?
February 20th, 2008 at 5:41 pm
> Does anyone know WHY it’s flat, as opposed to, say, spherical? What’s the force that makes everything stick to the same plane?
Yes - angular momentum of the cloud the Milky Way formed from - then you look at cooling, spinning gas, the rapid collapse is along the poles of the angular momentum vector, forming a disk.
Ellipticals didn’t have this angular momentum and so didn’t collapse into a disl
February 20th, 2008 at 5:53 pm
Desctructor > Does anyone know WHY it’s flat, as opposed to, say, spherical? What’s the force that makes everything stick to the same plane?
The slow work of the super massive black hole jets dissipating angular momentum? Billions of years of mergers with prefered planes of rotation? Anyone else?
February 20th, 2008 at 6:06 pm
>The slow work of the super massive black hole jets dissipating angular momentum? Billions of years of mergers with prefered planes of rotation? Anyone else?
No and a little (in the inital stages of formation)
February 20th, 2008 at 6:54 pm
Good grief! It’s bad enough that Americans are getting obese, now the whole frakkin’ GALAXY is fatter!
J/P=?
February 20th, 2008 at 6:58 pm
[ Paradox - I always find it funny that several countries I have visited - US, Canada, UK and Australia - all scream “We have the fattest kids in the world” - I wonder what the truth is]
February 20th, 2008 at 7:21 pm
>> The slow work of the super massive black hole jets dissipating angular momentum? Billions of years of mergers with prefered planes of rotation? Anyone else?
Well that just raises the question of why THOSE things are disks and not spheres. Cusp has the correct answer above.
February 20th, 2008 at 8:21 pm
Slightly off subject, Is the solar system part of a zodiac?
February 20th, 2008 at 8:38 pm
Edward- As seen from the aforementioned star Zeta Rediculus our sun makes up the South end of the North bound zodiac known there as Bovinus. Or so I heard.
February 20th, 2008 at 8:54 pm
Cusp,
Let me just say this about that:
Not a scientific survey, small sample, subject to confirmation bias, ymmv, and tax and license not included, but I’ve traveled from the States to London on two occasions, a year apart. It’s also true that there’s a lot of walking going on in the city that might not be going on out in the country, and we only ventured outside of London for a day trip once on each occasion. (Stonehenge, Bath, and Windsor tour each time, lovely, highly recommended but spend the night in Bath if you can.)
But.
We noticed very few people there with what I would consider major weight problems, and without exception, the ones who did had American accents.
Ditto rude people, as an aside, the one having no correlation to the other.
Just sayin’.