How is it decided where one galaxy ends and another begins

Question

How is it decided where one galaxy ends and another begins

Answer 1

That's like asking where New York City ends and Chicago begins. The galaxies do not merge into each other, they are separated by millions of lightyears of space.

Answer 2

Well, pretty much almost all galaxies are separated by many light years of practically empty space, much like islands of land surrounded by oceans of water.  There pretty much isn't any confusion as to where one galaxy ends and another begins.

Answer 3

Galaxies are easily distinguished by their shape and the separation fro other galaxies.  In the case of galaxies that are starting to merge, you can still see the two individual galaxies.  When they have merged, they are no longer separate galaxies.

"Historically, galaxies have been categorized according to their apparent shape (usually referred to as their visual morphology). A common form is the elliptical galaxy, which has an ellipse-shaped light profile. Spiral galaxies are disk-shaped assemblages with curving, dusty arms. Galaxies with irregular or unusual shapes are known as peculiar galaxies, and typically result from disruption by the gravitational pull of neighboring galaxies. Such interactions between nearby galaxies, which may ultimately result in galaxies merging, may induce episodes of significantly increased star formation, producing what is called a starburst galaxy. Small galaxies that lack a coherent structure could also be referred to as irregular galaxies.

There are probably more than 100 billion (1011) galaxies in the observable universe.  Most galaxies are 1,000 to 100,000 parsecs in diameter and are usually separated by distances on the order of millions of parsecs (or megaparsecs).  Intergalactic space (the space between galaxies) is filled with a tenuous gas of an average density less than one atom per cubic meter. The majority of galaxies are organized into a hierarchy of associations called clusters, which, in turn, can form larger groups called superclusters. These larger structures are generally arranged into sheets and filaments, which surround immense voids in the universe."

Answer 4

Zoning laws

Answer 5

There's millions of light years of emty space between them, that's how.

Answer 6

http://antwrp.gsfc.nasa.gov/apod/ap04030...

Take a look at the picture. With a few exceptions, each of those blobs of light is a galaxy. It's pretty clear where one ends and another begins.

Answer 7

The aliens have told us so don't worry about it, they are always right.

Answer 8

Everything Orbits around a centre of mass. In our galaxy it is the sun. You can tell you are in a different galaxy when the stars orbit around a different centre of mass.