A ______ is a group of millions and billions of stars held together by their own gravity.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for a term that describes a large group of stars that are held together by gravity.

2. **Identifying Key Concepts**:
- Stars: Luminous celestial bodies made of gas that produce light and heat.
- Gravity: A force that attracts two bodies towards each other.
- Grouping of Stars: When stars are grouped together, they form larger structures.
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