Which type of star group appears as a tight ball and can contain up to 1 million stars?

The correct choice is globular cluster because these structures are characterized by their dense concentration of stars, often appearing as a tight, spherical grouping. Typically containing hundreds of thousands to over a million stars, globular clusters are among the oldest known star populations and mainly reside in the halo of galaxies. Their stars are usually closely packed together, which contributes to their distinctive appearance.

In contrast, open clusters are more loosely organized and contain significantly fewer stars, typically numbering in the dozens or hundreds instead of millions. Binary stars are pairs of stars that orbit each other and do not form clusters at all; they represent a different stellar formation. Irregular galaxies, on the other hand, are much larger and more chaotic in their structure, containing stars in a much less cohesive arrangement, which further distinguishes them from the tight-knit grouping seen in globular clusters.