A rare galaxy type, polar ring galaxies have a substantial population of stars, gas, and dust orbiting in rings nearly perpendicular to the plane of the galactic disk.
极环星系是种稀有的星系类型，它们有着几乎和银盘平面垂直，呈环形公转的大量的恒星、气体和星尘。
R This one is a double integral. So, if you are doing it, say, on a disk, you would have both R and theta if you're using polar coordinates.
不是变量，这是在一个圆上。，R，is，not，a，variable。，You，are，on，the，circle。,这个是二重积分，如果你们这么做的话，在圆盘上，如果你们用极坐标的话,就需要用到R和θ
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