Chapter 13: Star Birth and Death

At the end of its life, a massive red giant has consumed as much fuel as possible and any remaining fusion has worked its way into the outer parts of the star. There is no point in the core where there is both potential fuel and the possibility to create a hot enough temperature to ignite it. The star has an "onion skin" structure, with the heavier elements concentrated toward the center. Since iron is the most stable element, it represents an insurmountable obstacle to further fusion. In other words, the iron "slag-heap" core will not ignite, even though it gets extremely hot and dense. In a star with an initial mass of 6 solar masses, the core may be the central 1.1 solar masses. In a star with an initial mass of 8 solar masses, the core may encompass 1.4 solar masses. In a larger star, the eventual core mass can exceed what is called the Chandrasekhar limit. What is the fate of such a massive core?

Three rings of glowing gas around supernova 1987a

The advanced evolutionary stages of a massive star represent a crescendo of activity. After millions of years of fusing hydrogen and helium, each of the subsequent fusion stages (creating carbon, neon, oxygen, and silicon) takes less than 1000 years. The conversion of silicon and sulfur to iron takes only a few days. Iron has the most stable nuclear configuration, so no more energy can be released by rearranging the nucleus. The core collapses at about a quarter of the speed of light, and the gravitational energy of the collapse is released as a prodigious burst of energy. The outburst of energy holds off the collapsing outer layers of the star and, with the help of a hot neutrino wind from the newly created neutron star at the center, blows off all the outer layers of the star in a titanic explosion called a supernova (plural: supernovae).

 

A supernova represents a rare astronomical event of unimaginable violence. Astrophysicists have gained insight from supercomputer models, but the details are uncertain. Calculations suggest that the core collapse only takes a few seconds! The density rises by a factor of a million as a volume the size of the Earth shrinks to a radius of about 50 kilometers. Temperature also rises and the iron in the core that took so many millions of years to create gets disintegrated into protons and neutrons! As the core gets compressed to the density of an atomic nucleus, forces between particles cause it to rebound, and on its way out it meets the outer layer material that is still falling in. When matter meets matter traveling at supersonic speeds, the result is a shock wave, giving compression and a rapid temperature rise up to billions of degrees. The energy of the explosion is easily sufficient to eject the outer envelope of the star and to momentarily take the star to 10 billion times the luminosity of the Sun! When we observe distant supernovae, they can outshine their entire parent galaxy.

Iron seems to be a dead end in the fusion chain, but we know heavier elements exist. Although helium capture in massive stars does not make elements heavier than iron, there is another mechanism that can. Many of the nuclear reactions described so far release floods of neutrons that strike nearby nuclei. Since neutrons have no electric charge, heavy nuclei can be built up by neutron capture. In evolved stars neutrons are slowly or gradually added to nuclei to build still heavier elements. These are the s-process reactions ("s" for slow), and they can make elements as massive as bismuth. However, a more spectacular form of neutron capture occurs in the white heat of a supernova explosion. These are the r-process reactions ("r" for rapid), and they make the heaviest elements such as radium, uranium, and plutonium.

Supernovae are wonderful devices for creating and recycling heavy elements. The death of a massive star takes only seconds, but it has long-term consequences. The shock wave of the explosion allows the fusion of nuclei more massive than iron by explosive nucleosynthesis. The high flux of free neutrons leads to a rapid form of neutron capture, and the extra energy provided by the explosion allows the iron barrier to be overcome. Moreover, the explosion flings all the heavy elements formed in the star's brief lifetime deep into interstellar space. Supernovae are the source of rare elements and precious metals in the world: the silver in the coins in our pockets and the gold in the jewelry on our bodies (although we now know that neutron stars colliding make many of these elements too). The rarity of supernovae explains the rarity of the heaviest elements. Atoms of precious metals like gold and platinum are a million times less abundant than iron and ten million times less abundant than carbon, nitrogen, and oxygen.

The next time you look at a gold ring or earring, consider the exotic origin of those atoms. Many of these atoms were forged from lighter elements in the cauldron of a distant supernova and then surfed a blast wave into interstellar space. Or they may have been ejected after the cataclysm of two neutron stars colliding. Then the atoms floated in deep, frigid space for millions of years before they became part of a collapsing gas cloud. Far from the center of the cloud, the atoms became part of a rocky mass, which churned for billions of years in the sluggish magma. Eventually, they were lifted in a concentrated nugget to the surface, where they came within the grasp of inquisitive human fingers. Also think of the way that the lighter elements on which life depends, such as carbon, nitrogen, and oxygen, get shot into space by supernovae to be incorporated into a new generation of stars.

Composite image of the Crab pulsar in optical (red) and x-ray (blue)

Peering into the heart of the Crab Nebula

Another outcome of a supernova is a neutrino burst. Neutrinos are the weakly interacting particles that are produced in nuclear reactions of many kinds, including those in the Sun's core. Unlike photons that slowly migrate to the surface, neutrinos are not scattered by nuclei in a star and escape the core immediately. During the core collapse, protons and electrons are forced to merge, creating a sea of neutrons and a huge number of neutrinos. The neutrinos flood into space at the speed of light, carrying away a huge amount of energy, about 10⁴⁷ Watts, equivalent to the mass-energy of 50 Earths. For a brief few seconds, a supernova exceeds the luminous intensity of the entire rest of the universe! The final result comes months after the star itself fades. During the explosion, an expanding cloud of gas is launched at about 10,000 kilometers per second (22 million miles per hour!). Initially, it is too close to the star to be seen from Earth, but years later the site will be marked by a colossal, expanding nebula, called a supernova remnant.

 

 

Petroglyph of the Chaco Canyon which is suspected to represent the historic supernova SN 1054 at the time of its conjunction with the moon in the morning of 5th July.

Many supernovae have been close enough to the Solar System to produce temporarily prominent new stars that were recorded by ancient people. The ancient Chinese called them "guest stars." Some astronomers have suggested such an explanation for the Star of Bethlehem. The most famous supernova was the explosion that produced the Crab Nebula. It was visible in broad daylight for 23 days in July 1054 and at night for the subsequent six months. It was recorded in Chinese, Japanese, and Islamic documents, and perhaps in Native American rock art. The nebula is the expanding, colorful gas shot out of this supernova. Other supernova remnants are scattered throughout our galaxy.

What are the chances of seeing a supernova in your lifetime? In recorded history, there have been 14 supernovae in our galaxy, and a careful examination of the statistics leads us to expect an average of one every 40 to 50 years (although many of these will be invisible from Earth due to dust obscuration in the plane of the Milky Way). By this reckoning, we are long overdue, as the last supernova in the Milky Way was nearly 400 years ago. This might be bad news because a supernova within about 15 parsecs would produce enough high-energy radiation to destroy all life on the Earth! The nearest massive star to us that might one day explode is Spica in the constellation of Virgo. Luckily, it is 80 parsecs away, which is probably a safe distance. Considering stars at larger distances, but still within our galaxy, any one of us might live to see a supernova bright enough to be visible in daylight.