冒牌自然老師

• Why do astronomers believe that vast quantities
of dark matter surround our Galaxy?

利用銀河盤面旋轉曲線， 我們可估計，本銀河系40 kpc 半徑以內的質量約有6* 10¹¹太陽質量。因為發光星體的半徑大約終止在15 kpc，所以顯然本銀河物質的沿伸範圍，遠大於發光或可見的物質所標出的區域。

• Approximately how many times has the solar
system orbited the Galactic center since the Sun
and planets were formed?

Radius=8.5kpc

Period=240×10⁶years

Solar system age=5×10⁹years

(5×10⁹)/( 240×10⁶)=20 times

• The visible disk of the Galaxy is about 100,000
ly in diameter and 2000 ly in thickness. If about
5 supernovae explode in the Galaxy each century,
how often on the average would one expect to
see a supernova within 1000 ly of the Sun?

共可以分成50個方圓1000ly的cube

所以共有5/50的機會

落在距離太陽系1000ly之內

(5/50)×1000=100years