14.7 Old Age of Massive Stars

Stars that begin their lives with more than 8 solar masses do not become planetary nebulas or whitedwarfs. Their great mass compresses and heats their cores enough to ignite carbon and allows them tokeep burning when their helium is gone. A variety of other nuclear reactions may also occur that createheavy elements and at the same time supply energy for the star. Astronomers call the formation of heavyelements by such nuclear burning processes nucleosynthesis. Nucleosynthesis in stars was discussed indetail by E. Margaret Burbidge, Geoffrey R. Burbidge, William A. Fowler, and Fred Hoyle, whoproposed in 1957 that all the chemical elements in our Universe heavier than helium were made in thisway. Their idea is one of the triumphs of stellar evolution theory, and an enormous amount of theory andobservation support it.

Formation of Heavy Elements: Nucleosynthesis

Heavy elements form when two or more light nuclei combine and fuse into a single heavier one. The newnucleus must have a number of protons + neutrons equal to the sum of the numbers found in its parentnuclei.*For example, suppose 12C and 16O fuse. Carbon has 6 protons and 6 neutrons, and oxygen has 8protons and 8 neutrons. Their fusion makes a nucleus with 6 + 8 = 14 protons and 6 + 8 = 14 neutrons(that is, an atom with 14 protons and 28 particles total in the nucleus). Reference to a table of theelements shows that such an atom is silicon, 28Si. Table 14.1 shows some of the primary reactions thatmake heavy elements in high­mass stars. The table also lists the approximate temperatures needed for thereactions and their duration in a 25 M⊙ star.

d TABLE 14.1SEQUENCE OF MAJOR FUSION REACTIONS IN MASSIVE STARSFusion Reaction Minimum TemperatureDuration in 25­M⊙ Star4 1H → 4He 5,000,000 K 7,000,000 years3 4He → 12C 100,000,000 K12C + 4He → 16O 200,000,000 K 700,000 years12C + 12C → 20Ne + 4He 600,000,000 K 300 years

20Ne + 20Ne → 24Mg + 16O1,500,000,000 K 8 months16O + 16O → 28Si + 4He 2,000,000,000 K 3 months28Si + 28Si → 56Fe 2,500,000,000 K 1 day KEY: H, hydrogen; He, helium; C, carbon; O, oxygen; Ne, neon; Mg, magnesium; Si, silicon; Fe, iron.Page 381

In massive stars, these reactions provide enough energy to support the stellar mass against gravity'sunceasing pull. As each fuel—hydrogen, helium, carbon—is exhausted, the star's core contracts and heatsby compression. The higher temperature then allows the star to burn still heavier elements, the “ashes” ofone set of nuclear reactions becoming the fuel for the next set. Oxygen, neon, magnesium, and,eventually, silicon are formed, but because progressively higher temperatures are needed for each newburning process, each new fuel is confined to a smaller and hotter region around the star's core. Thus, thestar develops a layered structure, as illustrated in figure 14.21. The star's surface, where the temperatureis too low for nuclear reactions to occur, remains hydrogen, but beneath the surface lies a series of nestedshells, each made of a heavier element than the one surrounding it. Helium surrounds a shell of carbon,which in turn surrounds a shell of neon, and so on to the core. By the time the star is burning silicon intoiron, its core has shrunk to a diameter smaller than the Earth's, and the core's temperature is about 2.5billion K. A very massive star (20 M⊙ or more) may take less than 10 million years from its birth to forman iron core. This brief lifetime results because (1) the star burns its fuel rapidly to offset the energy lostbecause of its high luminosity, and (2) the structure of atomic nuclei is such that fusion of elementsheavier than hydrogen yields much less energy than the fusion of a similar amount of hydrogen, and somore fuel must be burned to supply the same amount of energy.

Figure 14.21The layered structure of a massive star as it burns progressively heavier fuels in its core. (These innerzones are enormously magnified for clarity.)Core Collapse of Massive Stars

The formation of an iron core signals the end of a massive star's life. Iron cannot burn and release energy:the iron nucleus turns out to be the most tightly bound of all nuclei. As a result, attempting to fuseadditional protons or neutrons to it weakens the bonds and absorbs energy, rather than releasing it. Thus,nuclear fusion stops with iron, and a star with an iron core is out of fuel.

Fuel exhaustion causes a star's core to shrink and heat. But in high­mass stars, the shrinkage presses theiron nuclei so tightly together that a new reaction can occur: protons and electrons may themselves“merge,” neutralizing their charge and becoming neutrons. The shrinking core is thus transformed from asphere of iron into a sphere of neutrons, with catastrophic results for the star. Most of the pressure thatsupported the core was supplied by electrons, but they have been absorbed by protons. Thus, the star'score pressure suddenly drops. Nothing remains to support the star, and so its interior begins to collapse.However, because the matter is so dense, its force of gravity is immense, and it crushes the core. In lessthan a second, the core drops from an iron ball the size of the Earth to a ball of neutrons about 10kilometers (about 6 miles) in radius. The star's outer layers, with nothing to support them, plummetinward like a tall building whose first floor is suddenly blown away. The plunging outer layers of the starstrike the neutron core, crushing it still more, while the impact heats the infalling gas to billions ofdegrees. The pressure surges and lifts the outer layers away from the star in a titanic explosion in which astar can briefly shine as brightly as all of the stars in a galaxy combined (fig. 14.22). Astronomers callthis remarkable event a supernova explosion.

Figure 14.22A supernova explosion in a distant galaxy observed in 2002 shines brightly compared to the billions ofstars in the galaxy.Page 382Supernova Explosions

A supernova explosion marks the death of a massive star*, and we chart its complete life in the H­Rdiagram shown in figure 14.23. We can see that its path through time is far simpler than that of a low­mass star. The massive star begins its life on the upper main sequence as a blue star. As its core hydrogenis consumed, it leaves the main sequence, swelling and cooling to become a yellow supergiant while itbegins to burn helium in its core. When the helium there is consumed, the star switches to other fuels, allthe while contracting and heating its core and expanding and cooling its outer layers to become a redsupergiant. Overall, its luminosity stays approximately constant as the star ages, but some massive stars,depending on how many heavy elements are incorporated in them at their birth, may return partway tothe main sequence as they switch to new fuels. The resultant heating may turn them into blue supergiantsbefore they explode as supernovas. Such was the behavior of Supernova 1987A, whose explosion isdocumented in figure 14.24.

Figure 14.23H­R diagram of a massive star's evolution.

Figure 14.24Photograph of a portion of the Large Magellanic Cloud (a small, nearby galaxy) before Supernova1987A exploded, and a second photograph taken during the event.

For the star, the supernova explosion is a quick and glorious death. In a few minutes, it releases moreenergy than it has generated by nuclear burning during its entire existence. In a matter of hours, the dyingstar brightens to several billion times the luminosity of the Sun, radiating as much energy in its deaththroes as the Sun emits over its entire lifetime.

A supernova emits more than just visible light: most of the energy of its blast is carried by a burst ofneutrinos, the same tiny, highly penetrating particles generated in the Sun as it converts hydrogen intohelium. Just as the Sun's neutrinos escape freely into space, so the supernova's neutrinos escape as well.A pulse of such neutrinos was detected in February 1987 when Supernova 1987A blew up in the LargeMagellanic Cloud. Despite its enormous distance, more than a trillion neutrinos from that explosionpassed through each person on Earth.

Page 383Supernova Remnants

The explosion of a massive star mixes the elements synthesized by nuclear burning during its evolutionwith the star's outer layers and blasts them into space. This incandescent spray expands away from thestar's collapsed core at more than 10,000 kilometers per second. Depending on the star's mass, 10 or sosolar masses of matter may be flung outward, ultimately to mix with surrounding material and, in time, toform new generations of stars. Interstellar gas is thereby enriched in heavy elements, the atoms needed tobuild the rock of planets and the bones of living creatures. Indeed, the supernova outburst itself generateseven more heavy elements. The explosion creates free neutrons that rapidly combine with atoms in thestar to build up heavy and rare elements such as gold, platinum, and uranium.

Gas ejected by the supernova blast plows through the surrounding interstellar space, sweeping up andcompressing other gas that it encounters. The huge, glowing cloud of stellar debris—a supernovaremnant—steadily expands. At the end of one year, it is 300 billion kilometers (about 0.03 light­years)across. At the end of one century, it is several light­years in diameter, but its expansion slows as it runsinto more surrounding gas. Figure 14.25 shows pictures of several supernova remnants of different ages.Notice how ragged the remnants in Figures 14.25 look compared with the smoothly ejected bubbles ofplanetary nebulas—evidence that massive stars die more violently than low­mass stars.

Figure 14.25Several supernova remnants arranged according to their age: (A) Chandra X­ray image of Cassiopeia Asupernova remnant, which is about 300 years old. In this image, the red on the left is radiation from hotiron atoms and the bright green is radiation from silicon and sulfur atoms. (B) Hubble Space Telescopeimage of the Crab Nebula, which is about 1000 years old. (C) Hubble Space Telescope image of asupernova remnant that is several thousand years old.

One such violent outburst was seen about 1000 years ago by astronomers in China and elsewhere in Asia.On a date that according to our calendar was July 4, 1054, the records describe a “guest star” thatappeared in the evening sky and that was visible even in broad daylight for several weeks. Today, witheven a small telescope, you can still see the glowing gases ejected from that dying star. Figure 14.25B isa picture of this perhaps most famous of all supernova remnants, now known as the Crab Nebula. Inchapter 15, we will learn that the star's core survived the blast and lies in the expanding debris.

Supernova remnants eventually slow down, cool, and mingle with the interstellar clouds around them.But the remnant's gas is rich in heavy elements that were formed in the core of the star that blew up as asupernova. Thus, when the remnant mixes with an interstellar cloud, the cloud is enriched in heavyelements. Eventually the enriched cloud may collapse and turn into a new generation of stars. These newstars will, therefore, contain more heavy elements than earlier generations of stars. In this way, theamount of heavy elements in stars increases from generation to generation.

What remains when the supernova remnant dissipates? For stars that began their lives with less thanabout 20 M⊙, the core of the star—a few solar masses—survives as a ball of neutrons (a neutron star).Astronomers think that more­massive stars may end up as even more compressed bodies—black holes—as we will discover in chapter 15.