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10

It's almost 100% stars. In good conditions, you can see perhaps 2000 stars. (There are about 6000 naked-eye visible stars; of these, 3000 are above the horizon at any time, and about 1000 are hidden because they're too close to the horizon and blocked by the atmosphere.) The number of non-star objects you can see without assistance is tiny in comparison: ...


5

From this webpage, I have a few statistics regarding required density and temperature to burn a specific element. Burning phase; Required temperature; Required mean density; Duration Hydrogen burning: $4 \times 10^7 \text{ degrees K; } 5 \text{ gm per cubic cm; } 7,000,000 \text{ years}$ Helium burning: $2 \times 10^8 \text{ degrees K; } 700 \text{ gm ...


1

This is only a rough estimate. But the number of supernovae in the Milky Way is approximately 2-5 / 100 years (here and here). There is approximately $3 \times 10^{11}$ stars in the Milky Way. Provided the supernovae rate did not change much through the history (which is a big if that probably doesn't work), the total number of supernovae would be ...


1

I'm no expert on stellar atmospheres, so I have a limited idea of how things like $\log g$ affect the lines. But I work with stellar models, so I can take a stab at that part. The overall principle is that computing stellar model ages is a kind of optimization problem. We model the structure of stellar interiors by constructing a system of differential ...



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