How Stars Die

How Stars Die A star will spend about 90% of its life in the main sequence phase, in which hydrogen nuclei fuse into helium nuclei in its core. A common misconception is that a more massive star has more “fuel”, and will have a longer lifetime. But in fact, because the core of a massive star is hotter and denser, it burns much faster and has a shorter life. The most massive stars will use up the fuel in their cores in about one million years. For less massive stars, their main-sequence lifespans could be up to tens of billions of years.

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The Death of Low Mass Stars

We are first going to discuss the fate of a star with a mass less than, or about equal to, that of our Sun. When no hydrogen is left in the star’s core, the fusion reaction stops. The helium core of the star starts to collapse. Its core keeps on contracting and heating until it is hot enough for a helium fusion reaction to occur. In this reaction, three helium nuclei fuse together to form a carbon nucleus. The gravitational contraction will heat up the hydrogen envelope surrounding the core. Fusion therefore begins in the envelope, and the star’s envelope expands. As the star becomes very large - up to 100 times its previous size or more - its surface temperature decreases, and the star’s surface appears red. Although its surface is cool, the star’s core temperature is still very high, and the star’s total luminosity is also high because its surface area is so large. This kind of large, bright, cool, red star is called a red giant. All red giant stars were once on the main sequence. The Sun, which formed about 4.6 billion years ago with the rest of our solar system, has a main-sequence lifetime of about 10 billion years. Thus, in about five billion years, our Sun will enter its red giant stage. The inner planets Mercury, Venus, Earth, and possibly Mars, will be swallowed into the expanding sun and will not survive its red giant stage. Size comparison of the Sun as a main sequence and red giant star (left); internal structure of a red giant (right) A red giant star’s helium-rich core and hydrogen-burning shell do not produce energy in a stable and steady manner. All red giants are variable stars. Such stars pulsate, and eventually, will expel their outer envelope of material into interstellar space, creating a “planetary nebula” like the Ring Nebula (M 57) in Lyra. Meanwhile, the star’s helium-rich core collapses again. But now the core is not massive enough to heat up sufficiently to fuse helium into carbon. Gravity crushes the star’s core until finally even the electrons making up a its atoms are smashed together. The star is supported by electron degenerate pressure. As it collapses, the star grows hotter but much smaller and fainter. It has become a white dwarf. The Ring Nebula, M 57 (left), and Sirius and its white dwarf companion (right); both imaged by the Hubble Space Telescope. A typical white dwarf is slightly smaller than the Earth, but with about the same mass as our Sun. Its density is about 300,000 times that of rock. A white dwarf has no source of energy, so its luminosity comes from residual heat in the core. After it has radiated away all of its residual energy, it becomes a black dwarf. However, the time required for this is much larger than the age of the universe. So, we believe there are as yet no black dwarfs in the cosmos. The most famous of all of the white dwarfs is the companion of Sirius. It is visible in a modest-size telescope. The bright star Procyon also has a white dwarf companion. If a star is less massive than about 0.4 solar mass, its life will be quite uneventful. It will quietly and steadily burn its hydrogen into helium and become a white dwarf without ever entering the helium-burning red giant stage.

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The Death of High Mass Stars

We will now discuss what happens to a main sequence star with a mass over about 5 solar masses. Because of the star’s great mass, its core temperature and density are higher. The star’s surface is hotter and bluer, so the star is of spectral type O, B or A. Like other main sequence stars, hydrogen nuclei fuse to form helium nuclei in its core. However, the massive star will have a shorter life span. A 15-solar-mass star will deplete its hydrogen after only about 10 million years. But because of its large mass, the temperature and pressure at its core is high enough to trigger helium fusion into carbon. The helium will burn steadily, and its higher energy production rate will heat up the surface. The star will swell to a size even larger than a red giant, and we have a red supergiant. A typical red supergiant can be about 100 times larger than a red giant. Its surface temperature is low while its total luminosity remains high - up to a million times that of our Sun. The stars Betelgeuse in Orion and Antares in Scorpius are both examples of red supergiants. Evolutionary tracks of 1, 5, and 10 solar mass stars after the main sequence on the H-R diagram. As with helium fusion, the strong gravitational forces at the star’s core controls the carbon fusion. Light atoms fuse into heavier and heavier atoms. We believe all of the heavy elements found on Earth were made in a star somewhere a long time ago by this mechanism. Carbon (C) fuses to oxygen (O), nitrogen (N), and silicon (Si), until finally silicon is fused into iron (Fe). Iron is, in fact, the dead end of nuclear fusion. To fuse iron into even heavier elements, we have to supply more energy than the reaction generates. This is also why we can produce energy when we split an atom heavier than iron - like uranium - into several smaller ones. The internal structure of a red supergiant on its last day. Supernovae After enough iron accumulates in a supergiant star’s core, the pressure there decreases rapidly. In less than a second, the inner core collapses and heats up dramatically. All fuel, if not yet spent, will fuse to iron and nickel. The outer core collapses along with the inner core. The upper limit of nuclear density prevents the inner core from compressing too far, so the collapsing inner core bounces back outwards. The out-going inner core collides with the in-coming outer core. The collision sends off shock waves and creates heavy elements, like uranium. The outer layers of the star are thrown off into space. This is a supernova explosion. A supernova is extremely violent. The brightness of the star will increase by up to 15 magnitudes, and it may outshine its entire galaxy for a few days or weeks. It is a spectacular astronomical event. The most recent supernova visible to human eyes was SN 1987A, located in a small nearby galaxy, the Large Magellanic Cloud. Another famous supernova was recorded in 1054 A.D. by Chinese astronomers in the Sung dynasty. They discovered a “guest star” in the constellation we now call Taurus. That star was visible in the day time and remained visible for two months. The remnant of that supernova, which contains the material ejected from the exploded star, became the Crab Nebula, or M 1. The Crab Nebula, visible through a small telescope today, is still expanding, and will eventually dissolve into the surrounding interstellar medium. The Crab Nebula, M 1, imaged by the Hubble Space Telescope. Neutron Stars and Pulsars What happens to the remains of the star after the supernova explosion? It depends on the mass of the core that is left over. After the explosion, if remaining mass is less than 1.4 solar masses, a white dwarf will form. But if the remaining mass is more than 1.4 solar masses, then electron degenerate pressure is not strong enough to support the star against further collapse. The electrons are squeezed into the nuclei, and are combined with the protons to form neutrons. Then neutron degenerate pressure will stop the star from collapsing further. The star will contract to a size even smaller than a white dwarf. A neutron star is formed. A neutron star is composed mostly of neutrons (about 95% - 99%), with trace amounts of electrons and protons. Its typical size is about 8 to 16 km in radius, which is roughly the size of New York City. The gravitational field on the surface of a neutron star is millions of times stronger than at the surface of the Earth. Another very important property of a neutron star is its strong magnetic field. It ranges from 108 to 1015 times the magnetic field strength at the surface of the Earth. When electrons move in spirals around magnetic lines of force, they produce radio waves which radiate out along the two magnetic poles of the star. Usually, the magnetic poles do not align with its rotational axis, so the radio beams will sweep around like the beam of a lighthouse. What we observe on Earth is pulses of radio waves with very fast but very stable period. This is a pulsar - one can be found at the center of the Crab Nebula. How a pulsar works (left). Image of Crab Nebula pulsar (right) by the Chandra X-Ray Observatory satellite. As the mass of a supergiant collapses to form a neutron star, its rotation rate increases rapidly. This is caused by conservation of angular momentum, exactly the same reason as a twirling ice-skater with outstretched arms will spin faster when she brings her arms in. Many pulsars - spinning neutron stars - rotate as fast as 1000 times per second or more, and are among the most regularly-ticking “clocks” in the heavens. Due to their large masses, their rotational periods are very regular. We can specify the periods of some pulsars up to more than ten decimal places. When they were discovered in the 1960s, pulsar radio emissions were first thought to be signals from intelligent life. Astronomers now use pulsars to calibrate their measurements of the rotation of the Earth.

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Black Holes

For a star more massive than about 4 times the mass of the Sun, not even neutron degenerate pressure is strong enough to halt the star’s final collapse. The star’s core becomes a black hole - a mysterious object so dense that not even light can escape the intense gravitational pull at its surface. Because no light can escape a black hole, we cannot observe such an object directly. However, we can observe matter falling into the black hole, at speeds approaching the speed of light. Under these conditions, matter emits X-rays and other high-energy wavelengths. The most famous black hole in the sky, Cygnus X-1, was first the first black hole to be observed from its X-ray emissions.