When the sky is starless, there are no stars in it.
A starless sky can feel dark and empty.
Examples:
The night was so dark and starless.
She looked up at the starless sky and felt alone.
In the starless night, we couldn't see anything in the sky.
The starless sky made the night seem extra spooky.
The starless sky stretched out endlessly above us.
The starless night was like a big black blanket over the world.