There are many Puranas and there are even new ones being created to this day. However, there are about 19 main ones about to the main Hindu deities like Agni, Vayu, Vishnu, Shiva and Brahma. The Bhagavad Purana is about Krishna. There are also Puranas about Vishnu's avatars (like the Matsya Purana).

Mahabharata & Ramayan are epic poems of Hinduism. While puranas are description of Vedas.

The Puranas are a collection of Hindu scriptures which glorify particular deities by listing their special qualities and supernatural feats. In some instances it is suggested that the Puranas were created to provide support for the Vedas.

Bharatavarsha

I am not aware of any astronomical object called "Puranas". Perhaps you may want to check the spelling, or otherwise clarify what you are talking about.