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King Creole
2009-Apr-25, 01:36 PM
I'm trying to get a visual idea of where we are in relation to the nearest (20 ly or less) stars and our motion around the galaxy.

So, for example, where is up (galactic north) from our viewpoint on Earth? Which of our neighboring stars lie between us and the galactic center? Which are ahead of and behind us as we orbit the galaxy?

Also, how fast are we moving and are any local stars moving closer to us or farther away?

Peter B
2009-Apr-26, 11:43 AM
G'day King Creole, and welcome to the BAUT Forum.

I can't provide much of an answer to your question, except to say the following:

1. I seem to remember that the plane of the Solar System is inclined at about 60 degrees to the plane of the Milky Way galaxy;

2. I seem to remember that the centre of the galaxy is in the direction of the constellation Sagittarius; and

3. I seem to remember that a program called Celestia would be able to show you the nearest stars to Earth in relation to the galaxy and our motion through it.

Others with more knowledge than me might like to correct my mistakes...

loglo
2009-Apr-26, 11:47 AM
You need the Atlas. (http://www.atlasoftheuniverse.com/12lys.html)

King Creole
2009-Apr-26, 01:25 PM
Thanks Peter B & loglo. Celestia is a beautiful looking program; I'm looking forward to learning how to navegate the nearby galaxy.

As far as the atlas goes, that is nicely done as well. One thing that is not clear: Are we moving in a counterclockwise direction, from a celestial north perspective, just as the solar system does? (For that matter, is Celestial North the same as Galactic North?)

Also, I would assume that that our star is moving faster than the stars farther from the galactic center, just as the Earth does compared to the outer planets. Is this difference in velocity significant or are we so relatively close that the velocities are fairly similar?

Wizard From Oz
2009-Apr-26, 01:51 PM
I just tried to google the information for you, but cant recall the exact name of what I am looking for

We know the sun is moving towards the constellation of Herculese.

The stars in the galaxy dont behave the same way the planets do. This is the reason the whole discussion about dark matter and such stems from. The galaxy behaves as if surrounded by a halo of matter we can not see.

On a smaller scale we know many stars move in streams or rivers. Some point to this as evidence of our galaxy absorbing other smaller galaxies.

The constellation of the Big Dipper is an example of one of these streams. Virually all the stars are traveling together, just passing us by

grant hutchison
2009-Apr-26, 02:30 PM
As far as the atlas goes, that is nicely done as well. One thing that is not clear: Are we moving in a counterclockwise direction, from a celestial north perspective, just as the solar system does? (For that matter, is Celestial North the same as Galactic North?)We're moving clockwise around the galaxy. Galactic north is a long way from celestial north, because of the tilt in the plane of the solar system that Peter B mentioned. The galactic north pole lies in the constellation Coma Bernices.
Relative to the Local Standard of Rest (which is the mean velocity of nearby stars) we're moving upwards out of the galactic plane, a little faster around the galaxy than most of our surrounding stars, and inwards towards the galactic centre.


Also, I would assume that that our star is moving faster than the stars farther from the galactic center, just as the Earth does compared to the outer planets. Is this difference in velocity significant or are we so relatively close that the velocities are fairly similar?The galaxy's a big place, so we don't notice the shear effect in our own little neighbourhood. Instead, the local stars behave a little like a swarm of bees: all heading briskly clockwise around the galaxy (at a couple of hundred kilometres per second), but each with their own small relative motions (measured in kilometres per second), so that their relative positions change over time. Exceptions to that rule are halo stars, like Arcturus, which happen to be passing through the plane of the galaxy in our neighhourhood and don't share the common motion in the galactic plane of the rest of the "swarm".

By the way, Celestia won't show you the rotation of the galaxy or the relative movements of the stars. The new version (1.6) will plot a galactic grid for you, however, so you can see how far the Earth and solar system are tilted relative to the plane of the galaxy. Or you can just look at a sky map and notice how the Milky Way circles through both northern and southern constellations.

Grant Hutchison

Cougar
2009-Apr-26, 04:59 PM
...we're moving... a little faster around the galaxy than most of our surrounding stars, and inwards towards the galactic centre.

Excuse me? :eek:

grant hutchison
2009-Apr-26, 06:02 PM
Excuse me? :eek:No worries. We'll reach perigalacticon in a few million years and then start moving outwards again. The cycle repeats. We follow a sort of precessing elliptical path while simultaneously oscillating up and down through the galactic plane.

Grant Hutchison

Cougar
2009-Apr-26, 06:14 PM
No worries. We'll reach perigalacticon in a few million years and then start moving outwards again. The cycle repeats. We follow a sort of precessing elliptical path while simultaneously oscillating up and down through the galactic plane.

Ah, good, thanks. :cool: It was that "inwards towards the galactic centre" that surprised me a bit. So it must not be by much. Nevertheless, we must be picking up a little speed....

grant hutchison
2009-Apr-26, 07:04 PM
Ah, good, thanks. :cool: It was that "inwards towards the galactic centre" that surprised me a bit. So it must not be by much. Nevertheless, we must be picking up a little speed....About 10km.s-1, according to Frank Bash's figures. There are newer estimates, but as I recall they're in the same ballpark.

Grant Hutchison

chornedsnorkack
2009-Apr-27, 08:10 AM
As far as the atlas goes, that is nicely done as well. One thing that is not clear: Are we moving in a counterclockwise direction, from a celestial north perspective, just as the solar system does? (For that matter, is Celestial North the same as Galactic North?)


Does the direction of Milky Way rotation switch due to precession of Earth?

grant hutchison
2009-Apr-27, 09:22 AM
Does the direction of Milky Way rotation switch due to precession of Earth?I'm guessing that you're asking if precession will ever carry the conventional galactic north pole into the southern celestial hemisphere.
It won't. The ecliptic latitude of the galactic north pole is close to thirty degrees. Since the celestial north pole precesses around the ecliptic north pole within a circle 24.5 degrees in radius, it can never stray far enough from the galactic north pole to reverse its celestial latitude.

Grant Hutchison

chornedsnorkack
2009-Apr-27, 09:42 AM
Roughly where do the poles of Earth pass closest to the Milky Way?

Regarding the star streams, the Big Dipper is supposed to be an open cluster, like Hyades and Praesepe... How long do open clusters last?

King Creole
2009-Apr-27, 09:59 AM
Okay, then I think I'm somewhat back to where I started. The Atlas provided by loglo shows Celestial North, which is oriented to Earth, not to the galaxy. Maybe Celestia 1.6, with the galactic grid visualization, will get me closer to what I'm trying to understand, but a quick Google search shows a lot of discussion about it but no place to download.

So, I now know that we're moving counterclockwise, toward the constellation Herculese and that Sagitarrius points toward the center.

Isn't there some kind of 3D map of local stars that shows the grid of the galactic plane and velocities? Pity if there isn't. You'd think that would be something everybody would want to visualize, just as we do the solar system.

grant hutchison
2009-Apr-27, 11:17 AM
Okay, then I think I'm somewhat back to where I started. The Atlas provided by loglo shows Celestial North, which is oriented to Earth, not to the galaxy. Maybe Celestia 1.6, with the galactic grid visualization, will get me closer to what I'm trying to understand, but a quick Google search shows a lot of discussion about it but no place to download. There's a download link for the Windows Release Candidate 1 in this thread (http://www.shatters.net/forum/viewtopic.php?f=2&t=13710) on the Celestia forum.

Grant Hutchison

grant hutchison
2009-Apr-27, 12:42 PM
Roughly where do the poles of Earth pass closest to the Milky Way?The galactic equator comes within thirty degrees of the ecliptic north pole close to the point at which the Milky Way crosses the border between Cygnus and Cepheus. The celestial north pole will precess through this region, making its closest approach to the galactic equator, about six thousand years from now. At the same time, the celestial south pole will be making a close approach to the galactic equator in the constellation Vela.

Grant Hutchison

AndrewJ
2009-Apr-27, 03:18 PM
The galactic north pole lies in the constellation Coma Bernices.
Relative to the Local Standard of Rest (which is the mean velocity of nearby stars) we're moving upwards out of the galactic plane, a little faster around the galaxy than most of our surrounding stars, and inwards towards the galactic centre.


The galactic equator comes within thirty degrees of the ecliptic north pole close to the point at which the Milky Way crosses the border between Cygnus and Cepheus. The celestial north pole will precess through this region, making its closest approach to the galactic equator, about six thousand years from now. At the same time, the celestial south pole will be making a close approach to the galactic equator in the constellation Vela.

Encyclopedic knowledge! Are you translating this info from a database or are you just very familiar with the geography of the Milky Way?

grant hutchison
2009-Apr-27, 03:38 PM
Encyclopedic knowledge! Are you translating this info from a database or are you just very familiar with the geography of the Milky Way?Bit of both. :)
I've answered questions about the orientation and movement of the solar system within the galaxy often enough that I recall the details.
I pulled up Celestia 1.6 to find the constellations in which the galactic equator comes closest to the ecliptic poles, and then just happened to know that the celestial pole will precess past Cepheus around 8000 CE.

(You can check that I'm doing it right by looking at the sky map here (http://zebu.uoregon.edu/~imamura/121/oct6/precession.html), and noticing the close approach to the Milky Way just after 8000 CE.)

Grant Hutchison

AndrewJ
2009-Apr-27, 04:36 PM
I've answered questions about the orientation and movement of the solar system within the galaxy often enough that I recall the details.

I rate that.

I have often thought that there's a gap in the market between kids' guides to the solar system and spotter's guides to constellations or abstruse stuff on black holes for a book or website detailing the direction we, the Sun and the Galaxy are moving through space.

When I first got interested in this sort of thing I googled the direction the Sun was travelling around the MW as seen from above its north pole - about 40% of the results claimed anti-clockwise. I still get frustratingly conflicting views on whether our Local Group is approaching or receding from the Virgo Cluster.

King Creole
2009-Apr-27, 06:47 PM
So, grant, if I may be so familiar ... Using this atlas (http://www.atlasoftheuniverse.com/12lys.html) provided by loglo, would it be safe to say that the circles correspond to the galactic plane and that we are moving toward the 90 coordinate and away from the 270 one? And, if so, are the velocities of all the surrounding stars in that 12.5 ly radius more or less the same?

grant hutchison
2009-Apr-27, 07:59 PM
So, grant, if I may be so familiar ... Using this atlas (http://www.atlasoftheuniverse.com/12lys.html) provided by loglo, would it be safe to say that the circles correspond to the galactic plane and that we are moving toward the 90 coordinate and away from the 270 one?It's a little difficult to tell, since the author of the page hasn't paid us the courtesy of explaining the coordinates on the page itself. But it looks more galactic than anything else. In which case, yes, the sun is moving roughly towards ninety degrees. Though, as I say, we're moving a little inwards and upwards: so we're actually moving towards about sixty degrees in the galactic plane, and about twenty degrees northwards.


And, if so, are the velocities of all the surrounding stars in that 12.5 ly radius more or less the same?More or less, since they're all orbiting the galactic centre. So there's a mean motion at a couple of hundred kilometres per second towards ninety degrees galactic longitude. But individual stars deviate from that by (typically) tens of kilometres per second and (occasionally) more than a hundred kilometres per second. From the Gliese catalogue data, for instance, Barnard's Star is clipping along at 135 km.s-1 relative to the Local Standard of Rest, compared to the sun's 20 km.s-1 and Alpha Centauri's 30 km.s-1.

Grant Hutchison

eburacum45
2009-Apr-27, 09:03 PM
It's a little difficult to tell, since the author of the page hasn't paid us the courtesy of explaining the coordinates on the page itself. But it looks more galactic than anything else. Yes, that is the galactic plane in that image. All the maps on that site, of stars within our galaxy, use the galactic plane as a reference. But, as you remarked, that is not made explict anywhere.

King Creole
2009-Apr-27, 09:20 PM
Okay, that does a lot for me. Thanks.

135 km.s-1 compared to 20 km.s-1 is quite a difference. That would indicate it's moving away from us at 115 km.s-1, if we're going in the same direction. Guess it would be one of the more difficult stars to get to, if that day ever arrives.

For that matter, and on a slightly different subject, does that mean that since we are moving in a direction directly opposite that of Sirius, that a gravity assist would be impossible if we were one day ready to send a probe? As I understand it, a gravity assist only works in the direction of the moving body. Or, would said probe use the Sun to decelerate --or basically come to a stop-- and wait for Sirius to come to it?

Sorry for getting a little off topic.

grant hutchison
2009-Apr-27, 10:21 PM
For that matter, and on a slightly different subject, does that mean that since we are moving in a direction directly opposite that of Sirius, that a gravity assist would be impossible if we were one day ready to send a probe? As I understand it, a gravity assist only works in the direction of the moving body. Or, would said probe use the Sun to decelerate --or basically come to a stop-- and wait for Sirius to come to it?I'm not sure how you feel the gravity assist is working in this situation, so I'm afraid I can't respond.
But Sirius is not moving in the opposite direction to the Sun: according to the Gliese dataset, it is moving around the galaxy with about the same velocity as ours; moving towards to the galactic centre rather faster; and descending towards the galactic plane rather than rising away from it as we are. The relative velocity is about 19 km.s-1.

Grant Hutchison

King Creole
2009-Apr-27, 10:54 PM
Sorry, I guess I phrased that poorly. I meant that Sirius is moving toward us while we are moving away from Sirius, though I see from your explanation that we are wandering in slightly different directions.

What I was saying about the gravity assist is that, as I understand it, if the Sun is moving to the right, an object could only use the assist to accelerate to the right. If it were to try and go left, the object would simply slow down.

So, if Sirius and Sol are both moving to the right around the galaxy, --with Sol ahead of Sirius-- and we wanted to send an object to Sirius (that is, to the left), essentially we would be using the Sun to decelerate the object's velocity around the galactic plane, effectively causing the object to come to a standstill while it waited for Sirius to continue on its path toward the object. Is that how it would work?

grant hutchison
2009-Apr-28, 12:55 AM
If you want to use a gravitational slingshot to change velocity relative to the sun, you can't use the sun itself for the manoeuvre. Conservation of energy dictates that anything you send inwards on a ballistic trajectory will come back out again at the same speed, albeit going in a different direction: you might as well just launch it in that direction in the first place.

But if you have the capability of completing a journey between stars in anything short of millennia, then you're not going to worry about a paltry few tens of kilometres per second relative velocity.

Grant Hutchison

King Creole
2009-Apr-28, 07:12 AM
In using a planet or the Sun for a slingshot, the object will come out at the same speed relative to the body's surface, but will pick up the velocity of the body relative to a stationary observer.

Or, so says This Wiki (http://en.wikipedia.org/wiki/Gravitational_slingshot#Explanation) article.

Further down, it also states:

"An interstellar slingshot using the Sun is conceivable, involving for example an object coming from elsewhere in our galaxy and swinging past the Sun to boost its galactic travel. The energy and angular momentum would then come from the Sun's orbit around the Milky Way."

Of course, this goes way beyond my pay scale, but I don't see why the object has to come from elsewhere in the galaxy rather than further out in the solar system.

eburacum45
2009-Apr-28, 05:45 PM
Of course, this goes way beyond my pay scale, but I don't see why the object has to come from elsewhere in the galaxy rather than further out in the solar system.
Because if the object comes from 'further out in the solar system' it is already in orbit around the Sun, so shares the Sun's orbital motion around the Galaxy. This means that nothing you can do will allow any gain or loss in velocity after a slingshot manoevre around the Sun.

Around another star, yes, that is a possibility, but the maximum gain or loss is about a thousandth of any respectable interstellar cruising speed.