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View Full Version : Solar sibling radial velocity

Tom Mazanec
2018-May-21, 11:36 AM
I conjecture that HD 162826 https://en.wikipedia.org/wiki/HD_162826 has a radial velocity of (110/4,600,000,000)*299792.458 ie essentially zero.
Am I correct? If not, why the nonzero velocity?

Hornblower
2018-May-21, 03:09 PM
I conjecture that HD 162826 https://en.wikipedia.org/wiki/HD_162826 has a radial velocity of (110/4,600,000,000)*299792.458 ie essentially zero.
Am I correct? If not, why the nonzero velocity?

First things first. That expression is not in units of velocity. It appears that you took lightyears per year and multiplied it by the speed of light in km per second, for reasons that are a mystery to me. The result has units of velocity squared.

The initial relative velocity of the two stars is a result of initial turbulence in the gas cloud that formed the cluster, followed by the galaxy's overall gravitational gradient which tends to disperse the cluster. The Sun and the other star are 110 lightyears apart after 4.6 billion years, but it probably was not a linear progression to the present state. The stars have made roughly 20 orbits around the galaxy's center since their formation. They most likely are in somewhat different eccentric orbits which would make them alternately separate and converge many times over that period. We just have a lucky break in being near this one at the present time, when the cluster is long since dispersed far and wide.

Hornblower
2018-May-22, 01:18 PM
According to the Sky Catalogue 2000.0, the radial velocity of this star is about +2 km/sec. The mean value to separate by 110 lightyears in 4.6 billion years is about 4 m/sec, or about 1/500 of the current rate.

This star is close enough, and as a result bright enough, to make it easy to get a good high-resolution spectrum to evaluate the amounts of the various metals. The challenge is to get similar data on the reputed siblings that are much farther away.