Just a spectroscopic binary seen nearly exactly edge on By measuring how long it takes the stars to pass in front of one another we can determine their sizes. By measuring their velocity and how long it takes the stars to pass in front of one another we can determine their sizes. If we can determine a star's apparent magnitude, spectral class, and luminosity class we determine what its absolute magnitude should be and therefore we can find its distance. A star can remain in equilibrium as long as it can generate heat at its core. Bigger stars live fast and short.
Smaller stars live longer.
The sun will have about an 8 billion year lifetime. The sun is stable Gravity tends to pull it in and cause it to collapse The nuclear reactions in its core heat it and try to cause it to expand The heat generated in the core it transported radiatively to the outer zones of the Sun. From there it is transported by convection to the surface where the heat energy is released as light. It will take the heat generated in the core of the Sun a few million years to reach its surface. The star will settle onto the main sequence where it will spend most of its life time Nebula Emission nebulae Pinkish due to H a and H b lines.