Stars are usually born together out of giant hydrogen clouds like the Orion Nebula. The gas cloud condenses under its own gravity and spawns dozens to even hundreds of stars until it's spent and the remaining gas dissolves into space. What's left is a cluster of stars that during their childhood remain together, bound by their mutual gravity. But after a while, the much stronger gravitational force of our galaxy will break their bond and disperse the stars, like going through a blob of paint with a thick brush. The stars will then each go their own way, just like children that've grown up will leave their parents' home to build a life of their own. A very fine example of this is probably the most famous constellation of them all: Ursa Major, the "great bear" or "big dipper". All of the stars in the "big dipper", apart from two (Dubhe and Alkaid), share the same origin and they were born together some 500 million years ago, together with a few dozen others among which Alpha Coronae Borealis (Gemma or Alphecca), Beta Aurigae (Menkalinan) and Delta Aquarii (Scheat).
What I'm showing you on this sketch is a star cluster containing about 30 members, NGC6633, that's breaking up. The young, hot stars are leaving the nest and will start their individual journeys through space very soon. They've already moved so far away from each other that with my big telescope I was almost looking through them. Definitely this lovely cluster's best enjoyed with smaller instruments at low power or even with a pair of binoculars... for as long as we still can. Within another 100-200 million years the stars will be scattered and the cluster will be no more.
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