Uranus
noun
The heaven king star is the planet Uranus. Knowledge of Greek mythology will definitely help you here, because the English name is a reference to the Greek god of the skies, and the Japanese name is a direct translation of that!
てんのうせい
This is a jukugo word and uses the on'yomi, but over time the reading for 王 changed from おう to のう, to make it easier to pronounce. You'll still see that listed as an alternative reading in some dictionaries, but it's pretty much never used nowadays, so we'll only accept the てんのうせい reading. Here's a mnemonic to help you remember:
Who predicted the discovery of Uranus? Nostradamus (のう), of course!
これは天王星の画像です。
This in an image of Uranus.
天王星の発見者はウィリアム・ハーシェルです。
The discoverer of Uranus is William Herschel.
天王星は太陽のまわりを回るのに八十四年かかります.
Uranus takes eighty-four years to go around the sun.