Our current models describe stars as huge, hot gasballs, bloated and made luminous by a fusion furnace deep inside that burns hydrogen into helium and releases energy in the process. A kind of internal thermostat keeps them stable, so our planet enjoys a comfortable environment in its orbit around our star, the sun. In about 6,000 million years or so, all available fuel will be burned up, and as the fuel gets low, the sun will bloat, then shrink until it is 100 times smaller than it is now, becoming a white dwarf star. Written inside, in the ashes of the furnace, will be its nuclear history. We have pieced together this story by looking at many different stars, which last much longer than we do, but we cannot see inside any of them.
Stars are very luminous yet thoroughly opaque.
By a very fortunate circumstance, we have found that some white dwarf stars vibrate internally with something akin to earthquakes, all the time. We then believe that the parameters in our model tell us about the physics inside the white dwarf star. We can then start to read the history written there. The basic idea is nifty, but the practice is a bit complicated. The models have many parameters, not all independent of one another, and we are not completely sure we have all the physics right.