In which direction is the sun travelling?

  • enter image description here

    Relative to this simple drawing, can someone explain to me,

    1. in which direction is the sun and its solar system travelling,

    2. which star (irrespective of distance) is closest to that direction,

    3. and in which direction the milky way center is.

    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".

    @MagicOctopusUrn Expansion doesn't have any effect at such scales.

  • ProfRob

    ProfRob Correct answer

    3 years ago

    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.

    Celestial sphere

    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.

    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?

    @Alonda You have misunderstood what the solar apex is. It shows the velocity relative to local stars. I have added to the answer

    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.

    now I also find the vectors interesting... as I am not good at math, if I may ask... our solar system is in an elliptical orbit around the galaxy center, where we are currently moving towards our galactic perigee? how long will it take before we get there and how close to the galactic center will we get? and how far out is the apogee?

    @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.

    "star wars" effect? What you describe sounds more similar to what is shown during warp travel in stark trek than star wars.

    @RobJeffries, I'm not saying it's not measurable. I'm saying that your description of "onrushing stars diverging from the apex point" is depicted in Star Trek and not in Star Wars and should probably be called a "star trek" effect instead of a "star wars" effect.

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