The motion of the sun in the galaxy.

[ElPelado]

Yesterday i saw a TV program called SPACE, the episode that I saw was called "staying alive". it talked about asteroids and many other things that put us in denger.
one of that things was the motion of our Sun in the Milky Way. they say that the sun not only orbits the center of the galaxy like the earth orbit the sun(in an almost circular orbit), but it also moves up and down form the galatic plane. and they said that when the sun is in the galactic plane, where are in great danger, because of the density of stars in that part of the galaxy.
but i dont understand what the risk is: our sun can crash with other star? the orbits of the planets can change becasue of the precense of another star? can someone explain this?

btw: they said that now we are not in the galactic plane so there is no risk. and they said that it takes 30 million years to the sun to complete a circle around the galactic plane(starting under it, crossing it, goinf up, crossing it again and finishing under it again).

[Evil Dr Ganymede]

Yesterday i saw a TV program called SPACE, the episode that I saw was called "staying alive". it talked about asteroids and many other things that put us in denger.
one of that things was the motion of our Sun in the Milky Way. they say that the sun not only orbits the center of the galaxy like the earth orbit the sun(in an almost circular orbit), but it also moves up and down form the galatic plane. and they said that when the sun is in the galactic plane, where are in great danger, because of the density of stars in that part of the galaxy.
but i dont understand what the risk is: our sun can crash with other star? the orbits of the planets can change becasue of the precense of another star? can someone explain this?

The risk of actually crashing into a star are very very small - the space between stars is MUCH greater than the size of the stars themselves. However, it's quite possible for the sun to pass relatively close (within a couple of lightyears) to other stars, which can cause gravitational disturbances the cloud of comets around the sun.

On a related note, IIRC there is a theory that every time the sun passes through the galactic plane, it is more likely to pass through interstellar gas/dust clouds, which might bring about mass extinctions on Earth (because the sun gets dimmed by the dust).

btw: they said that now we are not in the galactic plane so there is no risk. and they said that it takes 30 million years to the sun to complete a circle around the galactic plane(starting under it, crossing it, goinf up, crossing it again and finishing under it again).

IIRC it takes the sun about 250 million years to orbit the galactic core, so that 30 Ma number sounds like it must be how long it takes to go from its highest point to its lowest point.

[granthutchison]

Yes, the Sun goes once around in 240 Myr, but cycles up and down through the plane of the galaxy with a period of 66Myr. (The difference in periods is because of the planar distribution of much of the galaxy's mass.) The overall effect is that the Sun rises and falls in its orbit, like a horse on an old-fashioned carnival merry-go-round. So once every 33Myr, the Sun passes through the median plane of the galaxy.
The theory is that it is more likely to encounter giant molecular clouds at these times, which would exert significant drag on the orbits of bodies in the Oort cloud, potentially dropping a dinosaur-killer into the inner solar system.
Unfortunately for that theory, calculations of the dynamics apparently indicate that molecular cloud drag would tend to eject Oort cloud residents into interstellar space, rather than drop them into the inner solar system.

Grant

[ElPelado]

what is the Oort cloud? i think that i know, but not sure. is the cloud where we are, isn't it?

[granthutchison]

what is the Oort cloud? i think that i know, but not sure. is the cloud where we are, isn't it?

No, sorry, I should have been more precise. The Oort is the cloud of very distant icy bodies that are assumed to orbit in a shell about 40,000AU from the Sun, and which supplies the reservoir for long-period comets (comets that arrive in near-parabolic orbits). Some theories of the dinosaur-killer comet/asteroid imply that the Oort cloud is periodically perturbed, releasing a shower of such comets into the inner solar system. This might be by a dim companion star (the Nemesis theory) or by the close passage of a neighbouring star, or by the drag of a molecular cloud, as I've just mentioned.

Grant

[Evil Dr Ganymede]

what is the Oort cloud? i think that i know, but not sure. is the cloud where we are, isn't it?

No, sorry, I should have been more precise. The Oort is the cloud of very distant icy bodies that are assumed to orbit in a shell about 40,000AU from the Sun, and which supplies the reservoir for long-period comets (comets that arrive in near-parabolic orbits). Some theories of the dinosaur-killer comet/asteroid imply that the Oort cloud is periodically perturbed, releasing a shower of such comets into the inner solar system. This might be by a dim companion star (the Nemesis theory) or by the close passage of a neighbouring star, or by the drag of a molecular cloud, as I've just mentioned.

Grant

I thought some astronomers had recently spotted a suspicious common aphelion distance of several comets in the Oort Cloud, implying there was something big out there... do you happen to know any more about this?

[granthutchison]

I thought some astronomers had recently spotted a suspicious common aphelion distance of several comets in the Oort Cloud, implying there was something big out there... do you happen to know any more about this?

Various matched parameters of a (small) number of cometary orbits, suggesting a brown dwarf orbiting in the outer Oort. http://www.ucs.louisiana.edu/~jjm9638/MS7292.pdf

Grant

[ElPelado]

I can't donwload/open the .pdf file. is there an error maybe?

[granthutchison]

I can't donwload/open the .pdf file. is there an error maybe?

The link works. Do you have Adobe Acrobat Reader installed on your machine? You'll need that to read a pdf file.

Grant

[ElPelado]

1) i do have the acrbat reader
2) i tryed to viwe it online and also i wanted to download it, but didnt work
3) not it works

[granthutchison]

I click, it opens. Not much I can think of to help you, I'm afraid.
You can see the abstract of the article here:
http://www.aas.org/publications/baas/v31n4/dps99/2.htm

I find a Google cached html version of the document at:
http://216.239.37.100/search?q=cache:L-H5HX_atVUJ:www.ucs.louisiana.edu/~jjm9638/MS7292.pdf
though the equations are turned into gibberish.

Grant

[ElPelado]

I AM REALLY SORRY.
IN THE POINT 3 I WANTED TO SAY:
"3) NOW it works".

[jamarsa]

ElPelado, don't worry about that. I tried too last night and the link didn't work then. I think the server was down at that time, it's up again now.

I have a question for Grant. Could you give me a link about how the measurements of galaxy rotation and Sun movement time were made? I'm just curious at how accurate could be the estimation given the little time (in astronomical terms) we have been observing.

[granthutchison]

I can't offer a link, I'm afraid. But here's a summary.

The likely movement of the Sun around the galaxy is worked out from two things:
1) An estimate of its distance from the centre of the galaxy, based on calculations of the distance to various structures dotted around the galactic disc and halo - it comes out to be ~27000ly.
2) Measurements of the velocity of movement of various gas clouds at various distances from the galactic centre. By averaging those at a given radius, you can get an idea of the orbital velocity at that radius. Looking at the way orbital velocity changes with distance from the galactic centre gives an estimate of the mass distribution within the galactic disc.

With those two bits of information, you can effectively build a scale model of the galaxy and calculate how the Sun would move, given its current velocity. There's something called the "Local Standard of Rest", which is based on averaging out the velocities of the stars around the Sun - Doppler studies of other galactic structures show the LSR is moving around the galaxy at an orbital velocity of ~200km/s, which as you might expect is the appropriate circular velocity for our distance from the galactic centre.
But the Sun is moving around the galaxy faster than the LSR by about 14km/s, and towards the galactic centre at about 10km/s, which means it must be approaching perigalacticon. It is also moving towards galactic north at 7km/s, and so is currently rising out of the galactic plane. From these figures, and the model of the mass distribution of the galaxy, it's possible to predict its future movements with reasonable accuracy, without having actually observed more than a tiny fraction of its orbit.

(The figures I give here are from a publication by astronomer Frank Bash, of which I have read a summary, but I confess to not having seen the original work.)

Grant

[jamarsa]

Thanks for your summary, Grant; very succint and comprehensive. I hadn't thought on the Doppler effect as a good measure of speed (given that I'm not astronomer, it's somehow understandable ).