How do stars move in the Galaxy?
How do stars move? I do not mean the daily rising and setting of the stars in the sky, but the movement of stars in the galaxy...if there is any. I would really like to know more about this as I am very curious about it. I am a 19 year old student who is very interested in the study of astronomy. Thank you very much.
Our Galaxy, like all other spiral galaxies is rotating. The stars move on orbits around the centre of the Galaxy. It was the motions of stars in external galaxies that lead to the idea of dark matter in the universe - their motions indicated that there was more mass within their orbit than could be accounted for by visible matter alone. This is also true in our Galaxy.
At the distance of the Sun from the centre of the galaxy (about 8 kpc or 24 thousand light years) we move at an orbital speed of about 220 km/s and take about 230 million years to make one revolution around the centre of the Galaxy.
Stars also have some random motions - they don't orbit the galaxy in exact circles. This random motion usually amount to a few tens of km/s in some direction.
As observers we see this motion of the stars as what's called 'proper motion' - the projection of their velocity onto the plane of the sky, and 'radial motion', which is the projection of their velocity along our line of sight. We can dectect radial motion by looking at the wavelength shifts it creates in the spectrum of the star. Proper motion we detect by plotting the position of the star over time and seeing how it moves relative to more distant "fixed" objects. The star with the largest proper motion is Barnard's star which moves about 10 arcseconds (0.003 degrees) per year. A more typical proper motion is about 0.1 arcseconds a year.
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