Main Sequence Stars - Tau Ceti. If protostars accumulate sufficient mass as they develop, collapsing gas and dust may reach temperatures where hydrogen fuses into helium. Once hydrogen fusion occurs, stars become stable between the competing forces of fusion and gravity and are considered main sequence stars. Both our Sun and Tau Ceti are currenthy in this stage (both are considered yellow dwarf stars). For most stars, the main sequence staoe lasts the longest (with the amount of mass inversely proportional to how fast the star burns Which type of star will have the longer main sequence stage, and therefore, "live the longest? a. high-mass star (greater than 8x the mass of our Sun) b. intermediate- to low-mass stars (between 0.8x to 8x the mass of our Sun)

The correct answer is: b. intermediate- to low-mass stars (between 0.8x to 8x the mass of our Sun).

Explanation:

Once the hydrogen in the nucleus is finished, the hydrostatic equilibrium is broken, and gravity is able to overcome radiation. This causes the star's core to contract, thus increasing its internal temperature and density. Meanwhile, due to convection, the outer layers expand, so the external temperature decreases. The star increases its external size becoming what we know as the Red Giant.

By increasing the temperature in the core, the necessary conditions are given to begin to fuse Helium and convert it into Carbon. So the core is getting denser and the outer layers of the star expand even more.

The end of our star is marked by the moment when the helium in the nucleus ends. In addition, the outer layers are so far apart that they no longer exert sufficient pressure to compress the core and trigger carbon fusion. In parallel, the star is so large that the outer layers of the star escape the gravity of the star, pushed by radiation. In this way, the interstellar medium of helium, carbon and a little oxygen is enriched. Leaving behind a White Dwarf, the nucleus of the star in which Helium has become Carbon.

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