The Milky Way Galaxy is a collection of gas, dust, and about 200 or so billion stars. It’s got three main parts: the central bulge (sometimes called the “hub”), the disk, and a spherical halo of stars. Our Galaxy is a spiral, because the disk appears to have great sweeping spiral arms.
Surprisingly, we know a lot about the disk. It’s hard to map it out because we’re in it; imagine being deep in the woods and trying to figure out what shape the forest boundary is. Still, using various different methods, astronomers have been pretty successful getting a feel for it.
The inner part is different, though. There’s 25,000 light years worth of dust, gas, stars, and assorted junk between us and it, so even seeing those stars gets to be pretty hard.
Enter the Spitzer Space Telescope. This is an observatory designed to see infrared light, invisible to the eye. You can think of the light as being redder than red, if that helps. While thick clouds of gas and dust block our view of the Galaxy’s core in visible light, infrared can pass through. So stars near the center can be seen by Spitzer.
We’ve known for a long time that our Galaxy has a bar in the center, a rectangular collection of stars. What’s different now is that this new data from Spitzer clearly show the bar, and also show it to be about 27,000 light years long, which is pretty big. It’s also very well-defined. They put together an artist’s illustration:
But what forms the bar? As the stars orbit the center, they feel a pull from the rest of the Galaxy. This can distort their orbits from a circle into truly weird shapes, including ellipses, curved diamonds (like a kite outline), and other odd paths. This effect gets amplified, and the gravitational physics works out to produce this long rectangular feature.
Most spiral galaxies have one, though it’s more pronounced in some galaxies than others. I suppose, if you care to phrase it this way, that if galaxies are cities in space, than ours just so happens to have a pretty good bar downtown.
August 16th, 2005 at 11:24 pm
Clearly, a better title for this post would have been “Milky Way Bar.”
August 17th, 2005 at 3:31 am
The large version of that pic is nifty.( http://www.news.wisc.edu/newsphotos/images/Milky_Way_galaxy_sun05.jpg )
Instead of “sun” the sign should have said “You are here”. =D
August 17th, 2005 at 4:53 am
I always wanted to see our own galaxy for myself… It’s just too bad we can’t go very far away and take a picture! But hey, artist visions are good for that!
Astronomy Picture of the Day already featured an artist vision of the Milky Way too!
http://antwrp.gsfc.nasa.gov/apod/ap050104.html
But the one you posted is much prettier.
August 17th, 2005 at 5:42 am
The Milky Way, Defined
The Milky Way, due to observations from the Spitzer Space Telescope, is now a more defined. Instead of a flat disk like I’ve always visioned, it’s apparently a disk within a disk. Much better descriptions and discussion can be
August 17th, 2005 at 7:23 am
> This is an observatory designed to see infrared light, invisible to the eye. You can think of the light as being redder than red, if that helps.
I’m not sure that explanation actually helps most people. How can something be redder than red? Can something be greener than green? Yes for violet, but not for yellow. What?
Understanding that comment requires understanding looking at visible light as a directional chart, red on one end and violet on the other. Thus there is a “red” direction and a “violet” direction. That is how something can be “redder” than red – it is farther in the red direction of the scale.
That’s not very intuitive, and I don’t think it will make much sense to most people. YMMV.
August 17th, 2005 at 10:39 am
When gravity is taught in physics class, it’s generally assumed that masses act instantaneously upon each other: when an orbiting satellite moves one meter in its orbit, it instantly “knows” the Earth’s new position relative to it, and reacts accordingly. This is good enough for everyday calculations.
But when you’re dealing with galaxies, I’m guessing that it’s not safe to assume that gravity is propagated instantaneously. Presumably Sol is feeling the pull not of the center of the Milky Way where it is today, but where it was 30,000 years ago.
Am I completely off-base here? And if not, does this have much to do with the shapes of galaxies, including bars and spiral arms?
August 17th, 2005 at 10:50 am
>But when you’re dealing with galaxies, I’m guessing that it’s not safe to ?>assume that gravity is propagated instantaneously. Presumably Sol is >feeling the pull not of the center of the Milky Way where it is today, but >where it was 30,000 years ago.
>
>Am I completely off-base here? And if not, does this have much to do >with the shapes of galaxies, including bars and spiral arms?
If you would see gravity as an effect that operates at light speed…
Gravity has if I’m not mistaken been observed as a distortion in the
fabric of time and space, and not a kind of particle pull subject to
a maximum speed..
The Earth orbits the Sun relative to where the Sun is now, not where
it was eight minutes ago.. correct me if I’m wrong, because I can’t post
any reference at the moment.
August 17th, 2005 at 11:03 am
IIRC gravity does indeed travel at the speed of light, and the sun does indeed orbit the place where the sun was 8 minutes ago.
August 17th, 2005 at 12:02 pm
this was the first useful hit I got on google..
http://www.ldolphin.org/vanFlandern/gravityspeed.html
perhaps you can find the flaw in this guy’s explanation as these
)
formula’s give me kind of a headache..(math class has been a
long time ago for me
August 17th, 2005 at 1:30 pm
I wouldn’t pay too much attention to VanFlandern — he’s a “fringe” guy. For instance, he apparently thinks Valles Marineris on Mars was created by a crashed (crashing?) moon rolling across the martian surface…
Meanwhile, I can’t recall what Einstein predicted the “speed of gravity” would be, but if memory serves, a number of experiments are planned (or underway) in attempts to measure it.
August 17th, 2005 at 2:48 pm
Well, a bit of Googling later, it appears that there is currently no definitive answer on the speed of propagation of gravity. One Sergei Kopeikin has made some observations that he believes show that gravity travels at the speed of light, but other people claim that he’s wrong. AIUI the basic problem is that we don’t have instruments sensitive enough to measure the speed of gravity, at least not with ordinary occurrences like Jupiter passing in front of a quasar.
PS: Phil, do you know about the error messages generated when one submits a comment here?
August 17th, 2005 at 7:02 pm
I’ll have my shaken – not stirred
August 17th, 2005 at 8:51 pm
From what I’ve read, astronomers are pretty sure that there’s a black hole near/at Sagittarius A*, which would account for the “water going down a bathtub drain” look (from above) of our spiral galaxy. I was surprised to read about the bar, though, as I would have thought a black hole would have precluded such a thing.
Oh, I’ll take some Stella Artois in a glass, thanks.
August 18th, 2005 at 8:54 am
If we could only just get “someone” from the Andromeda Galaxy to send us a tight laser-beamed digital CCD of the Milky Way!
August 18th, 2005 at 9:59 pm
VanFlandern is a PhD Astronomer who believes that the
“Face on Mars” is artificial and not natural.
A “fact” that we know now to be untrure it’s natural and more of a product of using less magnification. At higher magnification the “face” turns into a Mesa of hills & valleys.
He used to post regularly on Sci.Astro. Amateur.
Thank God he is gone!
August 19th, 2005 at 5:20 am
Someone correct me if I’m wrong, but this is what I know about gravity:
According to General Relativity, any body will warp space-time in such a way as to cause surrounding objects to “fall” toward it. For an object which is stationary, the way space-time is warped is relatively simple, and could be imagined to be just like the shape the fabric of a trampoline would make with a bowling ball in it. For an object moving with a constant acceleration, however, spacetime will be warped in such a way that everything surrounding it will always experience a force of gravity toward the point in space that the first object will be by the time the “ripple” reaches it. For example, right now, the earth is pulled toward where the sun is right now because eight minutes ago, the sun caused a distortion in spacetime that, when it reached the earth just now, would cause it to experience a force toward where the sun would be 8 minutes from then (right now.) And right now, the sun is causing a distortion that in 8 minutes will cause the earth to experience a force toward where the sun will be in 8 minutes. I’m not sure if that makes any sense to anyone else, though.
August 19th, 2005 at 5:03 pm
This very article in the JPL website prompted me in my blog to rejoice that we now have somewhere to go for a post-prandial drink after we have eaten in the “Restaurant at the End of the Universe”!
You are doing a great job, Phil, it seems we need every skeptic we can find at the moment.
August 22nd, 2005 at 7:40 am
BB, please elaborate on where your understanding of projected gravitation derives. It does not jive with my understanding of General Relativity.
August 24th, 2005 at 10:51 am
Too bad that guy turned out to be a fraud… I never heard of his name
before.. but then again, I thought that the question of whether gravity
was instantaneous or delayed wasn’t one open for debate as I would
think that it shouldn’t be too hard to calculate if you knew the location
and motion of the objects around you, the amount of redshift, the
earth’s momentum, etc.. surely the answer should be known by now,
or at least one of the possibilities been plausibly ruled out…
August 25th, 2005 at 2:13 pm
It’s amazing, isn’t it? We can view hundreds of galaxies in one image at about a billion or so parsecs away (e.g. the Hubble Deep Field), yet we are still discovering things about the basic structure of our own galaxy. These things are only 30,000 light years away.
Doesn’t sound like much, when you say it like that, does it? I think it’s around 176 quadrillion miles from here to the galactic centre. Taxi!
Anyway, make mine a single-malt scotch. Something western, but not too vigorous. Say, a 12-year-old Bunnahabhain. Straight up. No ice.
August 29th, 2005 at 10:46 am
Let’s say we discover clear signals from ET in M31 today, perhaps via optical laser. Let’s say that we transmit the question right away. Let’s say they get it, figure out what it means,
and send us an image right away. Its going to take 2,250,000 years to get the request there, and another 2,250,000 years to get the answer. So, in 4,500,000 years we’ll get an image that’s 2,250,000 years old. I’d like to be around for the answer. Really, i’ll wait.
Still, the new image would make a better T shirt – You are here.
August 31st, 2005 at 11:39 pm
how do we know what out galaxy looks like from within it? just wondering.
May 11th, 2007 at 7:13 pm
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