Whatever luminaries we see in the sky is within this one universe.
In Bhagavad-gita (10.21) Krishna says, naksatranam aham sasi: “Of stars, I am the moon.” This indicates that the moon is similar to the other stars. The Vedic literature informs us that within this universe there is one sun, which is moving. The Western theory that all the luminaries in the sky are different suns is not confirmed in the Vedic literature. Nor can we assume that these luminaries are the suns of other universes, for each universe is covered by various layers of material elements, and therefore although the universes are clustered together, we cannot see from one universe to another. In other words, whatever we see is within this one universe. In each universe, there is one Lord Brahma, and there are other demigods on other planets, but there is only one sun.
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Fifth Canto, Chapter 21 – Text 11
