Relative to this simple drawing, can someone explain to me,
- in which direction is the sun and its solar system travelling,
- which star (irrespective of distance) is closest to that direction,
- and in which direction the milky way center is.
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$\begingroup$ Closest to that direction is kind of a misnomer here... The sun's closest stellar neighbors are three stars in the Alpha Centauri system, moving at 18.6±1.64 km/s. We're moving at 19.1 km/s. But with expansion and other factors, I don't know, someone smarter can talk about "direction", which I'm assuming you mean "direction relative to the center of our galaxy". $\endgroup$– Magic Octopus UrnNov 30, 2018 at 15:27
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$\begingroup$ @MagicOctopusUrn Expansion doesn't have any effect at such scales. $\endgroup$– RIanGillisDec 3, 2018 at 17:12
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3 Answers
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The direction of solar motion is referred to as the solar apex. It is at around RA=18h28m and Dec=+30d in the constellation of Hercules (you can see a map on the page linked to). Note that this is the direction in which you would see the "star wars" effect of onrushing stars diverging from the apex point. i.e. It demonstrates the sun's motion with respect to the local stellar population.
On your 2-dimensional cartoon of the Earth's, the circle is the ecliptic plane and the Earth's northern hemisphere (at winter solstice) is tilted away from the Sun at an angle of 23 degrees. The celestial coordinate system defined above is geocentric and the Sun is at 18h -23d at the winter solstice. The apex point is therefore towards the Sun but 57 degrees above it.
Look at the map below (produced by Christian Ready), you can judge where the solar apex is with respect to the Sun and ecliptic plane (green line), which is roughly the path (drawn in your diagram) that the Earth takes around the Sun.
The very bright star Vega is reasonably close to the solar apex direction.
The Milky way centre is at RA=17h45m, Dec=-29d in the constellation of Sagittarius - i.e. almost where the Sun is in December. In Galactic coordinates (see Best approximation for Sun's trajectory around galactic center? ) the Sun moves at about 10 km/s towards the Galactic centre, about 5 km/s faster than the average circular speed (which is itself about 220 km/s) of local stars in the tangential direction and 7 km/s upwards out of the Galactic plane.
Thus the motion of the Sun with respect to the Galactic centre, as opposed to the local stellar population, is basically at $\sim 230$ km/s towards a point that is slightly less than 90 degrees around the Galactic plane and slightly above it. This would be at 9h30m -50d in the constellation of Vela. I think that is into the page and down about 30 degrees in your cartoon.
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$\begingroup$ so, our direction of travel is "just" 60 degrees off from the center of the milky way? that means we are moving towards it? if our movement was split in two vectors, how fast are we moving towards the center vs perpendicular to it? $\endgroup$– AlondaNov 30, 2018 at 16:39
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$\begingroup$ @Alonda You have misunderstood what the solar apex is. It shows the velocity relative to local stars. I have added to the answer $\endgroup$– ProfRobNov 30, 2018 at 18:48
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$\begingroup$ actually I realized that from reading the solar apex page. guess I didnt specify the question well enough. but your addition provided the exact answer I was after. my original question was much about which part of the sky is the "nose" of our solar system "spaceship", where we smash into new galactic debris. $\endgroup$– AlondaNov 30, 2018 at 18:59
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3$\begingroup$ @Alonda The orbit is not Keplerian and not elliptical. Roughly, the sun is on a circular orbit with epicycles in and out with a period of 150 Myr and up and down on a period of 70 Myr. Roughly, $\sim 10$ km/s inwards for 50 Myr is 500pc. So nowhere near the GC at 8000pc. $\endgroup$– ProfRobNov 30, 2018 at 20:05
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1$\begingroup$ "star wars" effect? What you describe sounds more similar to what is shown during warp travel in stark trek than star wars. $\endgroup$ Dec 1, 2018 at 2:05
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Relative to your drawing, the sun is moving south-ish:
The solar system is tilted about 60 degrees of the milky way plane, on which it's moving around like everyone else.
Here is another example/explanation.
To know which star is closest, you'd have to define what "closest" mean. Are we setting a straight line and seeing if it hits any star? If a start is 50 light years away from that straight line and 100 light years away from the sun, is that closer than a start that's 5 light years away from line but 800 light years away from the sun? The video above says we're moving towards Vega, I don't know how satisfactory this information is to you.
The milky way center is to the right of the path the sun is moving on. But I don't know the location when earth is at winter solstice, and you didn't specify northern hemisphere's winter or southern hemisphere's. ;-)
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1$\begingroup$ the animation is a bit confusing. after rob jeffries answer i thought Sol was travelling 60deg off from earths north pole, the animation seems to show the opposite? by closest star I meant, if you had a laser pointer aimed at the direction of travel, which star or stars would be visually closest to the beam. it seems vega is a good answer for me. $\endgroup$– AlondaNov 30, 2018 at 23:45
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$\begingroup$ My understanding is that the solar system is tilted 60 degrees from the milky way plane. I'm not sure relative to earth's north pole, since earth is also tilted itself relative to the solar system plane. But the animation doesn't show in which way earth is tilted either, so you can't say the animation is following or not the tilt relative to earth's north pole. ;) $\endgroup$– msbNov 30, 2018 at 23:56
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1$\begingroup$ I assume the planets orbit the sun counterclockwise... that would make the sun in the animation move in the direction of its south pole. unless my brain malfunctioned. $\endgroup$– AlondaDec 1, 2018 at 0:44
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$\begingroup$ Oh! That's a good point. Not what I understood from your first comment, but I see what you mean now. Either the sun is travelling south, or the animation is wrong, then either the plane is tilted 60 degrees to the wrong side or the planets are rotating to the wrong side. Good catch! I'm at work now, I'll do a better research later and answer this properly. I'm quite curious about it too. :D Rob Jeffries's answer is very informative, but not straight to the point relative to your questions, hopefully I'll get the straight answers we want. ;) $\endgroup$– msbDec 1, 2018 at 0:49
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1) and 2) The direction that the Sun is moving is known as the Solar Apex. This Wikipedia article shows a star map, so you can decide which star you want to say is closest. Solar Apex
3) The center of the Milky Way is toward the constellation of Sagittarius.
answered Nov 30, 2018 at 15:52
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1$\begingroup$ The solar apex marks the trajectory of the Sun with respect to the local stars, not with respect to the Galactic frame of reference. $\endgroup$– ProfRobApr 21, 2019 at 19:23