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Spiral Arms of the Milky Way
Curious...
Our sun and other stars in the spiral arm we are in, (Orion's Arm), alternately pass through other spiral arms and inter arm regions . So my question is, do regions where stars form in, like the Orion Nebula, move in the same way as the sun? If the stars are dynamic in position over time, does this mean Orion's belt and stars like Theta, at the heart of Orion, will someday be part of another arm within our galaxy? |
#2
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Spiral Arms of the Milky Way
lanky_lx5 wrote:
Curious... Our sun and other stars in the spiral arm we are in, (Orion's Arm), alternately pass through other spiral arms and inter arm regions . So my question is, do regions where stars form in, like the Orion Nebula, move in the same way as the sun? Roughly speaking, yes. The Galaxy is known to be rotating and we and Orion are rotating with it in such a way that the Orion Nebula will appear to be where we see it now for a long time to come. If the stars are dynamic in position over time, does this mean Orion's belt and stars like Theta, at the heart of Orion, will someday be part of another arm within our galaxy? In the Universe there are motions within motions within motions -- more localized motions matter in this case. Due to perturbations by other masses (stars, gas clouds, dark matter (maybe), the local motion of a given body will likely be less regular than its larger motions (rotation with the rest of the stars of the Galaxy, movement of the Galaxy itself within the Local Group, and on up the hierarchy.) The constellations are ephemeral; Orion has looked the way it does today for a very long time, but in the distant past a hypothetical observer on Earth would have seen it differently; indeed, if the observer lived long enough he would have seen Orion taking shape. This goes for the future, as well; Orion (and the rest of the sky) will look pretty much the same for a very long time, but not forever. If our observer continues his very long life he will see Orion break up. Possibly the most visible short term (circa 26,000 yrs) change is that the pole star is inconstant due by precession of the Earth's polar axis. It has not always been Polaris, and it will not always be Polaris. Please see "Precession" at http://www.seds.org/~spider/spider/ScholarX/coord_ch.html for a good explanation of some of this motion. This motion, and the motions of nearby stars are easily detectable in the course just a few years -- well within a human lifetime (also see "Parallax" at the above URL.) None of this will require you to adjust your polar alignment in the course of a night's observing, however... Davoud -- usenet *at* davidillig dawt com |
#3
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Spiral Arms of the Milky Way
In article , Davoud wrote:
In the Universe there are motions within motions within motions -- more localized motions matter in this case. Due to perturbations by other masses (stars, gas clouds, dark matter (maybe), the local motion of a given body will likely be less regular than its larger motions (rotation with the rest of the stars of the Galaxy, movement of the Galaxy itself within the Local Group, and on up the hierarchy.) The constellations are ephemeral; Orion has looked the way it does today for a very long time, but in the distant past a hypothetical observer on Earth would have seen it differently; indeed, if the observer lived long enough he would have seen Orion taking shape. This goes for the future, as well; Orion (and the rest of the sky) will look pretty much the same for a very long time, but not forever. If our observer continues his very long life he will see Orion break up. Perhaps we should but some numbers on the time scales here, because what's "very long" from a human time perspective will be just a blink of an eye in a cosmic time perspective. A few millennia will be enough to notice that some stars are moving, even without special instruments. That's how the first star was detected to have a proper motion: the star Arcturus was noticed to be situated one or two degrees differently relative to the background stars a few centuries ago, compared to the time of the ancient Greeks. After tens of millennia, many constellations will have changed their shapes quite noticeably. After hundreds of millennia, most of today's constellations will be recognizable only with difficulty - some not at all. And after millions of years or more, today's constellations will be almost completely unrecognizable. Some of the more massive stars will by then even have evolved significantly - Betelgeuze may have gone nova and then become a white dwarf, for instance. Consider a swarm of mosqitoes flying by you. Also, imagine that you were a creature with an extremely short life time, living for perhaps only 1/100 second or so. During your very short life you see this swarm of mosqitoes as more or less stationary - perhaps you even recognize patterns among their apparent positions, and invent "constellations" of mosqitoes to be able to recognize them. These constellations would be there all through your life and a number of generations afterwards. But after hundreds of generations (i.e. after a few seconds in real time), the mosquitoes will have moved quite noticeably, and those "constellations" will have changed, or even vanished, just to be replaced by new constellations, invented by future generations. You can view the sky in a similar way: there's a huge of stars there, all moving relative to one another and relative to the Sun. Now, humans live some 100 years instead of 1/100 of a second - but otoh stars live and move over a much longer time scale. So the effect will be similar: our constellations are nothing but a snapshot of this swarm of stars, or, more correctly, of that part of the swarm which happens to be close enough to our own solar system fo the moment. These constellations will vanish after a suffucuently long time (thousands of human generations), and perhaps our descendants in the distant future will invent other constellations of their own? -- ---------------------------------------------------------------- Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN e-mail: pausch at stockholm dot bostream dot se WWW: http://stjarnhimlen.se/ |
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Halley, Ptolemy, proper motion
Paul Schlyter wrote:
A few millennia will be enough to notice that some stars are moving, even without special instruments. That's how the first star was detected to have a proper motion: the star Arcturus was noticed to be situated one or two degrees differently relative to the background stars a few centuries ago, compared to the time of the ancient Greeks. It's true that Halley cited the position of Arcturus (and of Sirius and Procyon) in the Almagest as support for the idea of proper motion, but those positions really weren't accurate enough to constitute evidence. The positions for both Arcturus and Procyon are wrong by half a degree of latitude, and this is about average for the Almagest star catalog. It's more accurate to say that Halley became convinced that proper motion occurred, and that he looked for and claimed to find evidence of this in the Almagest star catalog. A more convincing case was built, by Halley and others, using data from much more recent and accurate observations, starting with those of Tycho. A fine point and not very germaine to the OP's question, but... - Ernie http://home.comcast.net/~erniew |
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