When we observe astronomical objects that are located thousands of light-years away, we are essentially seeing them as they were thousands of years ago, due to the finite speed of light. This raises the question of how we can be sure those objects still exist in their observed state when we see them.
There are a few factors that contribute to our confidence in the continued existence of distant astronomical objects:
Light travel time: As mentioned, the light we receive from distant objects takes thousands of years to reach us. However, the vast majority of astronomical objects have lifetimes that are much longer than this light travel time. For example, stars have lifespans of millions or billions of years, so the chances of a star ceasing to exist within a few thousand years are extremely low.
Stellar populations: Astronomers study entire populations of stars and other celestial objects within a given region of space. By examining a large sample of stars, they can infer the overall characteristics and evolution of stellar populations. This allows them to make predictions about the behavior and lifespan of individual stars, even if they are observed at different points in time.
Consistency with models and theories: Our understanding of astrophysics and cosmology is based on mathematical models and theoretical frameworks. These models have been tested and refined over time, providing us with a reliable understanding of stellar evolution, galaxy formation, and other astrophysical processes. When we observe distant objects and find that their properties align with our theoretical expectations, it gives us confidence that they are still present.
Long-term observations: Some astronomical objects, such as variable stars or active galactic nuclei, exhibit periodic or recurrent behaviors. By studying these objects over an extended period, astronomers can track their changes and confirm their ongoing existence.
While there is always a degree of uncertainty in any scientific observation, the combination of these factors allows astronomers to have confidence in the continued existence of astronomical objects located thousands of light-years away.