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G1.9+0.3 lies 25,000 light years away from Earth, in the Sagittarius constellation, meaning it is only 1,000 light years away from the galactic nuclei. This also means that the supernova explosions that occurred 140 years ago, in reality took place 25,140 years ago. It is a Type Ia supernova, thus its progenitor is a white dwarf, with an estimated mass of about 1.4 times that of the Sun.
The time in the past at which the light we now receive from a distant object was emitted is called the look-back time. When astronomers discuss events in distant objects, they take for granted that the actual event occurred earlier because of light travel time. It is similar to finding a series of photographs of a child in a 300 year-old time capsule. We could see how the child was developing 300 years ago, even though he/she would no longer be alive.
Originally posted by AnAbsoluteCreation
This is from a real popular thread.