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Name: Anuj M.
Status: student
Grade: other
Country: India
Date: Summer 2012

What exactly governs the boundary of a galaxy? Where does one galaxy end and another begin? And do lone Star systems exist which are not part of any galaxy? (In the sense that if there is a huge gap between two galaxies and a solar system exists between them balanced by the gravitational pull of the neighboring galaxies).

Thank you for your message,. You have a good question. I think that the main boundary refers to the sheer size of the galaxy. They are huge-- some have trillions of stars-- and there is a huge amount of space between them. It is even better than "what defines the boundary between one human and another?"

There may be lone stars that wander through space unrelated to galaxies. In fact, that is the very first question that I asked at a Star Night way back in 1960. They would be rare, but quite possibly present wand wandering alone through space.

Two very good questions! Sincerely David H. Levy

The boundary of a galaxy is not a sharp cutoff. Having said that, it is possible to estimate the boundary. All of the stars in a galaxy move together (more or less). So all of them will have approximately the same “red shift”, that is, the same Doppler shift. So that collection of stars can be “lumped together” as belonging to the same galaxy. In addition not all stars belong to a galaxy. They can be single stars, binary systems, and small multiples. It is gravity that keeps them together, or keeps them separated.

Vince Calder

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