PX158 - A1 - introduction

SI units are used in astronomy, but there are also many unique units:
- $M_{⊙} \equiv$ mass of the sun $\approx 2 \times 10^{30} k g$
  - ' $⊙$ ' represents solar
- $R_{⊙} \equiv$ radius of the sun $\approx 6.9 \times 10^{8} m$
  - $R_{j u p i t e r} \approx \frac{1}{10} R_{⊙}$
  - $R_{e a r t h} \approx \frac{1}{100} R_{⊙}$
- $A . U . \equiv$ astronomical unit (average earth-sun distance) $\approx 1.5 \times 10^{11} m$
- $p c \equiv$ parsec $= 3.086 \times 10^{16} m$
  - $k p c \equiv 1000 p c$ - used for the size of galaxies
  - $M p c \equiv 10^{6} p c$ - used for the distance to galaxies
- $a r c s e c o n d (^{"})$ : $1^{"} = (\frac{1}{60 \times 60}) °$