# If Kapteyn's star has a parallax angle of 0.255 arcsec, a proper motion of 8.67 arcsecs per year and a radial velocity of +246 km/s, What is the star's tangential velocity?

Feb 1, 2018

${v}_{t} = 183.0 \dot{3} k m$ ${s}^{- 1}$
${v}_{t}$ is given by the formula: ${v}_{t} = \frac{4.75 \mu}{p}$, where:
• $\mu$ = proper motion (arc seconds per year)
• $p$ = parallax (arc seconds)
${v}_{t} = \frac{4.75 \cdot 8.67}{0.225} = 183.0333 \ldots = 183.0 \dot{3} k m$ ${s}^{- 1}$