The tree star is the planet Jupiter. Imagine Jupiter covered in very strange trees — the kind that are able to take root in gas! It's a pretty weird image, but will hopefully help you to remember this word. Alternatively, think about how the clouds around Jupiter make patterns that look a lot like wood grain.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
Have you ever seen Jupiter?
What's between Mars and Jupiter?
Jupiter and Saturn were born like that.