I can say Aakash instead of Aasman, right?
Yes, they are synonyms.
Yes you can
I think this can also be translated as "There are stars in the sky".
What's the difference between सितारा and तारा?
None. They are synonyms. The only difference is that सितारा is a loanword from Persian and तारा is a native word from Sanskrit.
"Stars are in the sky" would more like तारे आसमान में हैं
Wiktionary says that तारा can be both masculine (with plural तारे) and feminine (with plural ताराएँ). Is one more common than the other?
The moon is my uncle
That felt weird