The Life Cycle of Stars

White dwarfs are the stars near the lower left that are small in radius and appear white in color because of their high temperatures. A main-sequence star’s mass determines both its luminosity and its surface temperature. More massive stars live much longer lives because they fuse hydrogen at a much greater rate. Giants and supergiants are stars that are nearing the ends of their lives. White dwarf stars are the cooling embers of stars that have exhausted their nuclear fusion fuel. Stars are born in cold, dense clouds of gas whose pressure cannot resist gravitational contraction. A molecular cloud fragment heats up as gravity makes it contract, producing a protostar at its center. Conservation of angular momentum ensures that protostars rotate rapidly and are surrounded by spinning disks of gas. Stars are born deep in molecular clouds, cold (10 – 30 K) dense nebulae, so cold that H₂ can exist. A cold cloud can fragment, gravity overcomes thermal pressure in dense regions, and these regions (cores) become more dense and compact. Stars more massive than about 100Msun blow off their outer layers, while protostars smaller than 0.08Msun become brown dwarfs that never get hot enough for efficient hydrogen fusion. A high-mass star lives a short life, rapidly fusing its core hydrogen into helium via the CNo cycle. All low-intermediate-mass stars follow life stages similar to those of our sun, while high-mass stars live short but brilliant lives and die in supernova explosions. White dwarfs are stable due to gravity vs. electron degeneracy pressure. They generate no new energy. As they radiate their heat into space, they get cooler and fainter. They are very dense; 0.5 - 1.4 M_¤ packed into a sphere the size of the Earth! 1 million times as dense as water! 100,000 times Earth’s surface gravity. If a white dwarf is in a close binary, matter from its companion can be accreted onto the WD. The matter forms a disk around the WD, and friction in the accretion disk heats it, emitting visible, UV, and even X-ray light. If matter falls onto the WD, H fusion begins, and the WD temporarily gets brighter. Novae typically increase in brightness considerably for a few days, then fade. An accretion disk is a rotating disk of gas orbiting a star, formed by matter falling onto the star.