What change in a stars properties would indicate it is starting to die?

Answer 1

This "answer" comes with some caveats;

1. We haven't been able to study any other stars in detail to really examine the local conditions before and after a star's death.

2. All of our mathematical models on how a star works when it's a star often fail to really predict how the star works and what changes, so our models of stellar "death" are even less refined.

With that said; one of the changes that we believe may indicate "incipient" stellar demise is a shrinking of the star's physical size. A star is in a tenuous balance between the nuclear fusion energy that powers the star and attempts to blow it apart, and the gravitational forces that would crush it into a point. When the star's fusion energy begins to wane - for example, when the proportion of helium "ash" in the stellar core increases to the point where it interferes with hydrogen fusion, the gravity will begin to tip the balance and the star will collapse.

This collapse causes the pressure and temperature of the Star to increase. At some point, the temperature and pressure become SO high that the helium "ash" becomes fuel for a more energetic fusion of helium into carbon, nitrogen and oxygen. This new burst of energy can cause the star to expand into a red giant, or blow it apart as a nova.

For example, the star Betelgeuse, the red giant at the shoulder of Orion, appears to be shrinking. We suspect that Betelgeuse will go supernova "Real Soon Now", which in stellar terms means "Probably less than 100,000 years". When it does, it will probably outshine the full Moon - for a month or so. Fortunately for us, it's over 500 light-years away, so it should be safe enough for us to enjoy the fireworks.

Answer 2

When a star dies, there is a flash of light, then the star dissapears. Its really a very quick process, so you will just be lucky if you see one.