02-02-2009, 01:44 PM
Fellas, a couple of observations. First, just because a black body radiator (a star) is 2 million light years away doesn't mean it isn't "there" any longer once we detect its light. Everyting is in relative motion, but some of that motion is purely radial from our vantage point...always in the same line of sight but either coming toward us or receding. Others have tangential motion, or proper motion, and they do move perpendicularly to our line of sight when we first see them. Those stars are no long "there", but the vast majority of them still exist after a mere 2 million years have passed.
Secondly, the younger stars, wherever they are, and at whatever distance from us, are sure to be there less as a likelihood than the older, hydrogen rich, metal poor dwarf stars that have been burning for billions of years. For example, the very ancient Population II stars in globular clusters have been "burning" for billions of years and will go on burning for billions more, while their nephews and nieces, Pop I stars, much larger and undergoing core fusion at much higher rates, will have their entire lives measured in several hundreds of millions of years...or less. These newer stars are more massive, which means more density at their cores due to the pressure of their more massive outer envelopes crushing in on the core. More compression = more heat= more/faster nuclear synthesis= shorter time to the iron core or core collapse to neutron density or black hole...every star's eventual demise.
Or so my learning went many years ago.
Secondly, the younger stars, wherever they are, and at whatever distance from us, are sure to be there less as a likelihood than the older, hydrogen rich, metal poor dwarf stars that have been burning for billions of years. For example, the very ancient Population II stars in globular clusters have been "burning" for billions of years and will go on burning for billions more, while their nephews and nieces, Pop I stars, much larger and undergoing core fusion at much higher rates, will have their entire lives measured in several hundreds of millions of years...or less. These newer stars are more massive, which means more density at their cores due to the pressure of their more massive outer envelopes crushing in on the core. More compression = more heat= more/faster nuclear synthesis= shorter time to the iron core or core collapse to neutron density or black hole...every star's eventual demise.
Or so my learning went many years ago.