Learn Hardcore Arabic (Modern Standard): النجوم في السماء.

النجوم is written with الـ, but because ن is a sun letter, the l sound of الـ is not pronounced. So it sounds like an-nujūm, not al-nujūm.
السماء works the same way: س is also a sun letter, so it sounds like as-samāʾ, not al-samāʾ.
The ū in nujūm is a long vowel.
The final ء in السماء is a hamza, a small glottal stop.

If you pause at the end of the sentence, you will often hear:

an-nujūm fī as-samāʾ

with the final case endings not pronounced.

Why is there no word for are in this sentence?

Because Arabic often uses nominal sentences in the present tense without a verb meaning to be.

So instead of saying something like The stars are in the sky, Arabic simply says:

Together, that naturally means The stars are in the sky.

This is very normal in Arabic. In the present tense, is / are / am is usually not stated.