Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Side Star Eyed

  1. Sep 29, 2008 #1
    I'm extremely new to all of this, so excuse me if this question is so stupid you dribble some tea over the keyboard. Though I have tried to google it, and had quite a look around the forums.

    I've read about Doppler Shifts, Red/blue waves.. however, in the night sky, why is it all stars seem to be merely moving away from us.. why can't I see more of a visible sideways movement to some stars, and not others? You see it with comets and them leaving a tail of sorts.. yet you don't with stars. Despite the fact that some of the stars light we're seeing now must of been very erratic during the early part of the universe. (I say fact, I'm just presuming it was messy at the start).

    I feel like my ignorance deserves double apologies to anyone suffering this :)
  2. jcsd
  3. Sep 29, 2008 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    To the best of my knowledge there are NO stars with motion that is detectable by the naked eye. Any motion you perceive is scintillations due to the atmosphere.

    A comet is not a star, it is a body orbiting the sun as is the earth and all planets.

    Stars are much further away then the planets or comets. Stars orbit the center of the galaxy as does our sun and solar system.

    With careful measurements a few stars have measurable parallax. That is their position shifts with respect to more distant stars as the earth moves in its orbit about the sun. This motion is the origin of the term "parsec" or parallax second. This is the distance from the earth which results in a parallax shift of 1 sec arc.
  4. Sep 29, 2008 #3
    First off, welcome! I’m not sure that I understand your question entirely, but here goes. Actually, there are only about two to three thousand stars that are visible to the naked eye in the night sky (plus the Andromeda Galaxy). These stars are thousands to millions of times father away than a comet might be. So you’re not going to see them actually move with respect to the Earth in one night, because they are so very far away. (The Earth’s rotation will of course cause them to come into and then out of view due to planet fall.) There was an astronomer in something like 320 B.C. who mapped the night sky. When his map was found in the Renaissance, nearly two thousand years later, it was discovered that some of the stars had indeed changed their location in respect to other stars. So there is an imperceptible movement, each night.
  5. Sep 29, 2008 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    To give an idea of how small this movement is, Arcturus, a star with one of the highest proper motions moves about 2.27 arcsec/year. The moon is about 1800 arcsec across, so it would take Arcturus almost 8 centuries to travel the width of the moon.
  6. Sep 29, 2008 #5
    Thank you, all, for your time and answers.

    I didn't mean to confuse a comet with a star, more I've seen what happens and it was the only way of describing it with my limited knowledge (I've always loved the night sky, and delve in to books like Olaf Stapledons Star Maker and Hyperspace by Michio Kaku when and where I can). Though this still puzzles me. I understand I'm not going to see anything visually by looking - though considering how fast I imagine them to be moving, while we're also moving at such a tremendous rate just makes it seem very static when viewing it.

    The Earths tilt changes 0.47 arc-seconds a year, combined with 30km/sec for the Earth revolving around the Sun, 300 km/sec for the speed of the Milky Way moving in relation to local Galaxies.. is it just me or does it all seem quite static considering all of the visible stars are also moving in various directions, at varying speeds. While the actual position of them seems to have remained stable (noticeable movement over 2000 years seems.. too calm!).

    I know they're a great distance away, one that I barely even begin to comprehend. It just seems as though that everything in the Universe is moving with itself to a major degree and I wondered why that was, and why it's not more visible, even with a telescope? :-)

    Perhaps I'm just letting my imagination run a bit too wild (as always). Though Janus - what determines "a star with one of the highest proper motions.."? Would that be just the most visual points of movement to us from our position of earth?

    Are there any videos/online simulations showing the expansion from the big bang until now (and after?)? May be quicker to appease my needs and curiosities visually! (thus saving my family having to listen to my constant pondering!)
  7. Sep 29, 2008 #6


    User Avatar

    Staff: Mentor

    Those speeds are nothing compared with the vast distances between the stars. 300 km/sec is 1/1000th the speed of light and the nearest star is 4.5 lyr away. So if it were moving that fast perpendicular to our line of sight (impossible), it would move 3 arcmin per year (1/10th the diameter of the moon).
  8. Sep 29, 2008 #7


    User Avatar
    Science Advisor
    Gold Member

    Barnard's star has the highest proper motion of all known stars. It is still miniscule. The average distance between stars is incredibly large, even within our own galaxy.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook