So the sun is moving?

[sja785]

Hi,

I am new to the whole subject of astronomy and space in general but got interested in it and started reading some articles here and there. I now have some questions and thought of asking them here.

On Wikipedia I read that the sun is not static, but rather goes round what is called a galactic core. Is this right? I mean does this mean that the sun is constantly moving in a particular direction (similar to how earth and the other planets move around the sun)?

Regard,
sja785

 

[MeteorWayne]

That's correct, the sun is in orbit around the center of mass of the galaxy, so follows an elliptical path around the center, just as the earth orbits the center of mass of the solar system.

Welcome to Space.com!

Wayne

 

[sja785]

That's correct, the sun is in orbit around the center of mass of the galaxy, so follows an elliptical path around the center, just as the earth orbits the center of mass of the solar system.

Does the Sun pull every other planet with it?

Welcome to Space.com!

Thanks

 

[bushwhacker]

yes the planets follow the sun in its path around the galaxy

 

[MeteorWayne]

I know this is kind of technical, but the sun isn't what everything in the solar system orbits. In fact the sun, as well as all the planets, dwarf planets, asteroids, and comets orbit the barycenter (center of mass of the solar system). So the sun isn't really the center, in fact it orbits the barycenter.

Along the same lines, the solar system as a whole orbits the barycenter (center of mass) of the galaxy.

 

[neilsox]

MW is correct as usual, but the barycenter is inside the Sun and the galaxy barycenter is also very close to the center of the galaxy, so this is almost irrelevant. Neil

 

[MeteorWayne]

MW is correct as usual, but the barycenter is inside the Sun and the galaxy barycenter is also very close to the center of the galaxy, so this is almost irrelevant. Neil

Not always correct. When Jupiter and Saturn are on the same side of the solar system, the barycenter is outside the surface of the sun.

 

[origin]

MW is correct as usual, but the barycenter is inside the Sun and the galaxy barycenter is also very close to the center of the galaxy, so this is almost irrelevant. Neil

Not always correct. When Jupiter and Saturn are on the same side of the solar system, the barycenter is outside the surface of the sun.

Thanks Wayne, I didn't know that. Yet another fact I can wow the ladies with (that kind of thinking is one of the reasons I will never leave my wife - that and she's a hellava cook).

 

[weeman]

Hi,

I am new to the whole subject of astronomy and space in general but got interested in it and started reading some articles here and there. I now have some questions and thought of asking them here.

On Wikipedia I read that the sun is not static, but rather goes round what is called a galactic core. Is this right? I mean does this mean that the sun is constantly moving in a particular direction (similar to how earth and the other planets move around the sun)?

Regard,
sja785

A great beginner question! Wayne has already answered it well, now I can elaborate to give your brain something to really ponder. The sun, our solar system, and all other star systems in the galaxy orbit the galactic center much like planets orbit stars.

Our solar system sits roughly 2/3 of the way out from galactic center. To give you an idea of the size of the galaxy, the Sun is orbiting about the galactic center at a rounded speed of 200 km/sec. Yet, at this speed it still takes around 230 million years to make one galactic orbit. Think about how different Earth was 230 million years ago! That was well before the extinction of the dinosaurs.

 

[duluthdave]

Yes, the sun is in orbit around the center of mass of the galaxy at over 200 km/s, but why stop there? The whole galaxy is moving at over 500 km/s relative to the cosmic microwave background.

 

[SpaceTas]

The Milky Ways motion against the microwave background is in the direction of Leo.

 

[crazyeddie]

On Wikipedia I read that the sun is not static, but rather goes round what is called a galactic core. Is this right? I mean does this mean that the sun is constantly moving in a particular direction (similar to how earth and the other planets move around the sun)?

And if you are wondering what that particular direction is in the sky, it is towards a spot in the constellation Hercules.

 

[csmyth3025]

The Milky Ways motion against the microwave background is in the direction of Leo.

I just looked up this thread and am wondering if the CMB constitutes a rest frame of reference for everything (stars, galaxies, ect.)

Chris

 

[neilsox]

Likely the microwave background can be a virtual rest frame, as defining the direction and speed of stationary is difficult with everything moving with respect to almost everything else. Neil

 

[csmyth3025]

Likely the microwave background can be a virtual rest frame, as defining the direction and speed of stationary is difficult with everything moving with respect to almost everything else. Neil

As I understand it, the CMB bathes us with a more-or-less even distribution of microwave energy coming from all directions. We detect a dipole anistropy in this radiation which we interpret as our motion against (or through) this background at ~630 km/sec.

I think what my question comes down to is: Does this background radiation represent the co-moving "stationary" background of our "local" space? Also, in regard to the expansion of the universe, how much volume (or radial distance) is generally considered to be the "local" space of a galaxy (or star, if that's appropriate)?

Chris

 

[ramparts]

The CMB rest frame can be a useful one, but it's not a special frame; special relativity still applies, all inertial frames are equally valid, only people can give meaning to particular ones So saying our galaxy is moving with respect to the CMB is qualitatively different from saying that the Earth is moving around the Sun and the Sun is moving around the galactic center, because in the latter two cases, there's acceleration involved so you know there's motion. Also, there's gravity. Neither of these things are quite true with the motion of the galaxy w.r.t. the CMB.

 

[ZenGalacticore]

Everything in the Universe is in motion. Nothing stays "still".

 

[csmyth3025]

Everything in the Universe is in motion. Nothing stays "still".

I'm wondering about "local" space and the way it fits in with the expansion of the universe. As has been pointed out many times in this forum, the limiting speed of any massive particle or object is c. This speed limit only applies to the "local" space since there are obviously distant galaxies receding from us at speeds greater than c.

As I understand it, this is possible because these distant galaxies are more-or-less stationary in their "local" space - but their "local" space, itself, is receeding from us. I believe this is explained by postulating that the space in which these distant galaxies are located was originally receeding at less than c, and that the light from these galaxies was subsequently "stretched" by the expansion of space during the multi-billion year journey to us.

I'm guessing that the CMB looks the same to observers in these distant galaxies as it does to us. Further, I'm thinking that these observers will notice that they have a peculiar motion against (or through) the CMB in their "local" space, as do we, and that the temperature of the CMB that they measure will be the same as what we measure.

I haven't read any articles touching on this, so I'm going with the isotropic principle. Is any of this right?

Chris-

 

[ramparts]

Think of local space as being like the patch of Earth you can see that looks flat - you know globally the Earth is curved, but locally it acts almost exactly like a bit of flat space. How small a patch we call "local" depends, of course, on how good your measuring devices are It's not really an exact concept.

Now when we throw in expansion, we have two types of "motion": motion of objects against the background, and motion of the background itself. It's the former type of motion which can never go faster than c, and the latter type (that is, the expansion of space) which can be as fast as it damn well pleases. Motion against the background is usually on small enough scales that we often refer to it as local motion.

I would expect observers elsewhere in the Universe (assuming the properties there are the same as here) to observe roughly the same CMB with the same temperature, differing mainly in the dipole moment which measures peculiar velocity relative to the CMB.

 

[csmyth3025]

Thanks Ramparts.

Am I correct that to the limit of our observations so far, the space we can see is flat to a very high degree. I believe this means that either the universe (even beyond our horizon) is flat or, as in your analogy, we are seeing just a small patch of an enormously larger curved spacetime.

Chris

 

[SpeedFreek]

Am I correct that to the limit of our observations so far, the space we can see is flat to a very high degree. I believe this means that either the universe (even beyond our horizon) is flat or, as in your analogy, we are seeing just a small patch of an enormously larger curved spacetime.

Chris

Yes, you've got it.

 

[ramparts]

Thanks Ramparts.

Am I correct that to the limit of our observations so far, the space we can see is flat to a very high degree. I believe this means that either the universe (even beyond our horizon) is flat or, as in your analogy, we are seeing just a small patch of an enormously larger curved spacetime.

Chris

Yeah, either one is possible. It's the same as with the Earth; noticing that the Earth looks flat on small scales tells you that the Earth isn't a really small sphere, but says nothing about whether it's a really big sphere or a really big flat rock. You can do a ton of experiments to test the large-scale curvature of the Earth; one really easy one (if you have some really long rope and a lot of zoning permits) is to make a big triangle by drawing lines between three widely-separated points, measure the angles the ropes make at those points and sum them up. If it's 180 degrees, you're more or less flat (within experimental error). If it's greater than 180, then congratulations on discovering that the Earth is round. Similar tests have been done on cosmological scales, but they've turned up no evidence of large-scale curvature, so that's no help.

Of course, physicists like to believe the simplest thing, and the Universe being globally flat is the simplest possibility, mainly because it accords with the cosmological principle. A flat universe is a natural outcome of inflation, which is motivated by multiple theoretical concerns, so if inflation happened similarly around the Universe then you'd expect it to be more or less flat all around. But it's a complicated question... we can certainly envision other patches of the Universe which have different physics, maybe didn't have inflation for quite as long, and have stronger curvature. Patches which are strongly-enough curved can even have collapsed by now ("now"). But those patches would be beyond the observable Universe so it could be accurate to call them entirely different universes in themselves, since they can't be observed. There's quite a bit to say on this