t stars in the sky will no longer exist. Deneb and Rigel, for example, will explode as supernovae. Sirious will swell into a red giant and puff out a planetary nebula. Alpha Centauri, currently the sun's closest neighbor, will recede from the sun, and its apparent brightness will fade below the threshold of naked-eye visibility.As the next billion years unfold, Earth will face pressing problems because of sun's activity. So far, the sun has fused approximately half of its central store of hydrogen into helium. As inert helium ash accumulates in the solar core, the efficiency of nuclear processes steadily decreases, and gravity impels the center of the sun to contract and heat up. Contraction and heating in the solar center raise the overall power output at the surface. As a result, in ten galactic years sun will be 10 percent brighter than it is today. As a result of this increase in sun's luminosity will induce "unwelcome" weather on Earth (the authors do not give the details about the kind of weather). Today's global climate models indicate that Earth may experience a "moist greenhouse effect". Earth will lose its water and presumably its entire life forms and will wind up as a twin of the planet Venus. According to computer models of stellar evolution, the hydrogen fuel in the sun's central core will be exhausted in 6.2 billion years. At that time, the sun will be 2.2 times brighter than it is now, and Mars will be receiving roughly the same ray of illumination that Earth currently enjoys. The exhaustion of its central hydrogen will mark the end of our sun's 11-billion-year role as a main sequence star. As the hydrogen level becomes exhausted, energy will leak out of the sun, and pure helium core remaining at the center of the sun will be unable to support the crushing weight of the layers above. Thus, the core will be forced to contract and heat up. This extra heat will cause the thinner outer layers of the sun to s...
