High-mass stars evolve much faster than their low-mass counterparts. The more massive a star, the more ravenous is its fuel consumption and the shorter its main-sequence lifetime. The Sun will spend a total of some 10 billion years on the main sequence, but a 5—solar mass B-type star will remain there for only a hundred million years. A 10—solar mass O-type star will depart in just 20 million years or so. This trend toward much faster evolution for more massive stars continues even after the main sequence. All evolutionary changes happen much more rapidly for high-mass stars because their larger mass and stronger gravity generate more heat, speeding up all phases of stellar evolution.

Stars leave the main sequence for one basic reason—they run out of hydrogen in their cores. As a result, the early stages of stellar evolution beyond the main sequence are qualitatively the same in all cases: main-sequence hydrogen burning in the core (stage 7) eventually gives way to the formation of a nonburning, collapsing helium core surrounded by a hydrogen-burning shell (stages 8 and 9). A high-mass star leaves the main sequence on its journey toward the red-giant region with an internal structure quite similar to that of its low-mass cousin. Thereafter, their evolutionary tracks diverge.

Figure 20.15 compares the post—main-sequence evolution of three stars having masses 1, 4, and 15 times the mass of the Sun. Note that, whereas stars like the Sun ascend the red giant branch almost vertically, higher-mass stars move nearly horizontally across the H—R diagram after leaving the upper main sequence. Their luminosities stay roughly constant as their radii increase and their surface temperatures drop.

Figure 20.15 Evolutionary tracks for stars of 1, 4, and 15 solar masses (shown only up to the point of the helium flash in the low-mass cases). Low-mass stars ascend the giant branch almost vertically, whereas high-mass stars move roughly horizontally across the H—R diagram from the main sequence into the red-giant region. The most massive stars experience smooth transitions into each new burning stage. No helium flash occurs for stars more massive than about 2.5 solar masses. Some points are labeled with the element that has just started to fuse in the inner core.

In stars having more than about 2.5 times the mass of the Sun, helium burning begins smoothly and stably, not explosively—there is no helium flash. Calculations indicate that the more massive a star, the lower its core density when the temperature reaches the 10⁸ K necessary for helium ignition and the smaller the contribution to the pressure from degenerate electrons. As a result, above 2.5 solar masses, the unstable core conditions described earlier do not occur. The 4—solar mass red giant in Figure 20.15 remains a red giant as helium starts to fuse into carbon. There is no sudden jump to the horizontal branch and no subsequent reascent of the giant branch. Instead, the star loops smoothly back and forth near the top of the H—R diagram.

A much more important divergence occurs at approximately 8 solar masses—the dividing line between high and low mass mentioned in Section 20.1. A low-mass star never achieves the 600 million K needed to fuse carbon nuclei, and so it ends its life as a carbon—oxygen white dwarf. A high-mass star, however, can fuse not only hydrogen and helium but also carbon, oxygen, and even heavier elements as its inner core continues to contract and its central temperature continues to rise. The burning rate accelerates as the core evolves. Evolution proceeds so rapidly in the 15—solar mass star whose evolution is shown in Figure 20.15 that the star doesn't even reach the red-giant region before helium fusion begins. The star achieves a central temperature of 10⁸ K while still quite close to the main sequence, and its evolutionary track continues smoothly across the H—R diagram, seemingly unaffected by each new phase of burning.

With heavier and heavier elements forming at an ever-increasing rate, the high-mass stars shown in Figure 20.15 are very close to the ends of their lives. We will discuss their fate in more detail in the next chapter, but suffice it to say here that they are destined to die in a violent explosion soon after carbon and oxygen begin to fuse in their cores. These stars evolve so rapidly that, for most practical observational purposes, high-mass stars explode and die soon after they leave the main sequence.