13.2: Evolution of Massive Stars

The Death of Large Stars

Thanks to mass loss, stars with starting masses of at least 8 M_Sun probably end their lives as white dwarfs. But we know stars can have masses as large as 150 M_Sun. They have a different kind of fate in store for them.

Fusion of Heavy Elements

An example of a high-mass star is the red supergiant Betelgeuse. It is 500 times the size of our Sun and is less than 100 million years old. Within 10 million years, it will reach the end of its life and follow a path different from low-mass stars. In a massive star, the weight of the outer layers is sufficient to force the carbon core to contract until it becomes hot enough to fuse carbon into oxygen in a process called the CNO cycle. This cycle of contraction, heating, and the ignition of another nuclear fuel repeats several more times. After each of the possible nuclear fuels is exhausted, the core contracts again until it reaches a new temperature high enough to fuse still-heavier nuclei. The products of carbon fusion can be further converted into neon, magnesium, silicon, sulfur, calcium, argon, and, finally iron. In really massive stars, some fusion stages toward can take only months or even days, a much shorter time than the millions of years they spend in the main-sequence stage.

At this stage of its evolution, a massive star resembles an onion with an iron core. As we get farther from the center, we find shells of decreasing temperature in which nuclear reactions involve nuclei of progressively lower mass elements. In Figure \(\PageIndex{1}\), the iron core is surrounded by layers of silicon and sulfur, oxygen, neon, carbon mixed with some oxygen, helium, and finally hydrogen. Outside the core, the composition is mainly hydrogen and helium. This diagram is not precisely to scale but is just meant to convey the general idea of what such a star would be like.

Figure \(\PageIndex{1}\) : Structure of an Old Massive Star. Before the end, the internal structure of an old, high-mass star consists of layers which made of elements that increase in mass with depth ending with an iron core. (CC BY; Fraknoi, et al. via Openstax Astronomy 2nd ed.) Alternative description of Figure \(\PageIndex{1}\).

In a high-mass star, fusion ends with the iron. Remember that the energy of nuclear fusion comes from light nuclei losing some binding energy when they fuse into a larger, more stable nucleus. The energy released by fusion counterbalances the gravity of the star, preventing the star from collapsing. Iron is the most tightly bound and most stable of all nuclei. The fusion of nuclei larger than iron absorb energy instead of releasing it. If iron in the core of a star fuses into larger nuclei, it will remove some of the energy that prevents stellar collapse.

A Ball of Neutrons

Similar to a white dwarf, when nuclear reactions stop in a high-mass star, its core is supported by a degenerate gas of electrons. For stars that begin their evolution with masses of at least 10 M_Sun, this core is likely made mainly of iron. For stars with initial masses in the range 8 to 10 M_Sun, the core is likely made of oxygen, neon, and magnesium, because the star never gets hot enough to form elements as heavy as iron. The exact composition of the cores of stars in this mass range is very difficult to determine because of the complex physical characteristics in the cores, particularly at the very high densities and temperatures involved. We will focus on the more massive iron cores in our discussion.

While no energy is being generated within the white dwarf core of the star, fusion still occurs in the shells that surround the core. As the shells finish their fusion reactions and stop producing energy, the products of the last reaction fall onto the white dwarf core, increasing its mass. A higher mass means a smaller core. The core can contract because even a degenerate gas is still mostly empty space. The electrons at first resist being crowded closer together, and so the core shrinks only a small amount. Ultimately, however, the iron core reaches a mass so large that even degenerate electrons can no longer support it. When the density reaches 4 × 10¹¹ g/cm³, or 400 billion times the density of water, some electrons are squeezed into the atomic nuclei, where they combine with protons to form neutrons and neutrinos.

Some of the electrons are now gone, so the core can no longer resist the crushing mass of the star's overlying layers. The core begins to shrink rapidly. More and more electrons are now pushed into the atomic nuclei, which ultimately become so saturated with neutrons that they cannot hold onto them. At this point, the neutrons are squeezed out of the nuclei. Like electrons, neutrons strongly resist being in the same place and moving in the same way. The force that can be exerted by such degenerate neutrons is much greater than that produced by degenerate electrons, so unless the core is too massive, they can ultimately stop the collapse.

This means the collapsing core can reach a stable state as a crushed ball made mainly of neutrons, which astronomers call a neutron star. We don't have an exact number, similar to a Chandrasekhar limit, for the maximum mass of a neutron star, but calculations predict that the upper mass limit of a body made of neutrons might only be about 3 M_Sun. If the mass of the core were greater than this, then neutron degeneracy would not be able to stop the core from collapsing further. The dying star must end up as something even more extremely compressed, a black hole.

Supernova

When the collapse of a high-mass star's core is stopped by degenerate neutrons, the core is saved from further destruction, but the rest of the star is literally blown apart. A supernova is an explosion of a star that briefly outshines an entire galaxy, radiating as much energy as an ordinary star like the Sun over its entire lifetime. Note that the plural of supernova is supernovae due to its Latin origin, with the ending -ae sounding like the -ee in bee. Let's review the process that leads to this spectacular light show.

The star's core collapses rapidly when electrons are absorbed into the nuclei due to the immense pressure and gravitational attraction. In less than a second, a core with a mass of about 1 M_Sun, which originally was approximately the size of Earth, collapses to a diameter of less than 20 kilometers. The speed with which material falls inward reaches one-fourth the speed of light. The collapse halts when the density of the core exceeds the density of an atomic nucleus, the densest material known to science. The neutron degenerate core strongly resists further compression, abruptly halting the collapse. The shock of the sudden jolt initiates a shock wave that starts to propagate outward. The energy produced by the shock wave is quickly absorbed by atomic nuclei in the dense, overlying layers of the star, where it breaks up the nuclei into individual neutrons and protons.

Each time an electron and a proton in the star's core merge to make a neutron, the merger releases a neutrino. In the initial second of the star's explosion, the power carried by the neutrinos, 10⁴⁶ watts, is greater than the power put out by all the stars in over a billion galaxies. While neutrinos ordinarily do not interact very much with ordinary matter, the matter near the center of a collapsing star is so dense that the neutrinos release their energy in the layers of the star just outside the core. This huge, sudden input of energy reverses the infall of the outer layers, the majority of the mass of the star, and drives them explosively outward. Most of the mass of the star is then ejected outward into space. Supernova explosions occur for stars of at least 8 M_Sun. Its neutron core has a mass of at most 3 M_Sun, so at least 5 M_Sun of material is ejected into space.

There are at least two different types of supernova explosions. The kind we have been describing, the collapse of a massive star, is called a type II supernova. The II is the Roman numeral for two, so it is read as type two supernova. We will describe the other type in Binary Star Systems. Figure \(\PageIndex{2}\) includes examples of supernovae. The arrows in the top row of images point to the supernovae. The bottom row shows the host galaxies before or after the stars exploded. Each of these supernovae exploded between 3.5 and 10 billion years ago. When they first explode, a supernova can be as bright as an entire galaxy.

Figure \(\PageIndex{2}\) : Five Supernova Explosions. Supernova are observed as brief, bright flashes in these examples from five different galaxies. (CC BY; Fraknoi, et al. via Openstax Astronomy 2nd ed.) Alternative description of Figure \(\PageIndex{2}\).

Stellar Finale

Table \(\PageIndex{1}\) summarizes the discussion so far about what happens to stars of different initial masses at the ends of their lives. This list represents a progress report because it is the best astronomers can do with the present models and observations. The mass limits corresponding to various outcomes may change somewhat as models are improved.

Stars with initial masses less than 10 M_Sun end their lives as a white dwarf. The composition of that white dwarf depends on the stars initial mass, because higher mass stars produce higher mass elements. The lowest mass white white dwarfs are mostly helium, while the highest mass ones are made of oxygen, neon, and magnesium. High-mass stars have two possible ending scenarios, depending on the star's original mass. If the initial mass of the star is less than 40 solar masses, or 40 M_Sun, it will end as a neutron star. If the initial mass of the star is greater than 40 M_Sun, then it ends as a black hole.

Table \(\PageIndex{1}\): Final States of Stars

Initial Mass (Mass of Sun = 1)	Final State at the End of Its Life
0.08 to 0.25	White dwarf made mostly of helium
0.25 to 8	White dwarf made mostly of carbon and oxygen
8 to 10	White dwarf made of oxygen, neon, and magnesium
10 to 40	Supernova explosion that leaves a neutron star
> 40	Supernova explosion that leaves a black hole

Supernova Aftermath

The death of each massive star is an important event in the history of its galaxy. The elements built up by fusion during the star's life are now recycled into space by the explosion, making them available to enrich the gas and dust that form new stars and planets. The supernova explosion produces a flood of energetic neutrons that can be absorbed by iron and other nuclei where they can turn into protons. This creates elements that are more massive than iron, including gold, silver and uranium. Supernovae are one of the only ways these elements can form. Supernovae play a crucial role in enriching their home galaxy with heavier elements, allowing, among other things, the chemical elements that make up earthlike planets and the building blocks of life.

Figure \(\PageIndex{3}\) includes the expanding remains of a supernova explosion, which was first seen about 400 years ago. The bubble-shaped shroud of gas and dust is now 14 light-years wide and is expanding at 2,000 kilometers per second. The remnant emits energy at wavelengths from X-rays, in blue and green to visible light in yellow and into the infrared in red. The expanding shell is rich in iron, which was produced in the star that exploded. The main image combines the individual single-color images seen at the bottom into one multi-wavelength picture.

Figure \(\PageIndex{3}\) : Kepler Supernova Remnant. The material from this supernova is difficult to detect in a visible image, but includes complex structure in infrared and X-ray light. (CC BY; Fraknoi, et al. via Openstax Astronomy 2nd ed.) Alternative description of Figure \(\PageIndex{3}\).

Part of the energy released from supernova is in the form of high energy particles. This may be the source of some of the high-energy cosmic ray particles observed by astronomers. Trapped by the magnetic field of the Galaxy, the particles from exploded stars continue to circulate around the vast spiral of the Milky Way. The high energy particles from supernova could be a threat to life. An average high-mass star supernova within 50 light-years of Earth would pose a danger to life on Earth due to the damage done to living cells, possibly wiping out all life on Earth. If life develops on a planet it is dependent on the energy from its star. It therefore requires a long-lived, lower-mass, star. The continued existence of that life would also depend on there being no supernova in its neighborhood.

The good news is that there are at present no massive stars that promise to become supernovae within 50 light-years of the Sun. This is in part because the kinds of massive stars that become supernovae are overall quite rare. The massive star closest to us, Spica, is about 260 light-years away, probably a safe distance, even if it were to explode as a supernova in the near future.

Examples and Exercises

Example: Extreme Gravity

In this section, you were introduced to some very dense objects. How would those objects’ gravity affect you? Recall that the force of gravity, F, between two bodies is calculated as

\[ F = \frac{GM_1M_2}{R^2} \nonumber \]

where G is the gravitational constant, \(6.67 \times 10^{-11} \text{ Nm}^2/\text{kg}^2\), M₁ and M₂ are the masses of the two bodies, and R is their separation. Also, from Newton’s second law,

\[F = M \times a \nonumber\]

where a is the acceleration of a body with mass M.

In this case, the mass is you, and the other object is white dwarf. You are M₁ and the body you are standing on is M₂. The distance between you and the center of gravity of the body on which you stand is its radius, R. The force exerted on you is

\[F = M_1 \times a = GM_1M_2 / R^2 \nonumber\]

Solving for a, the acceleration of gravity on that world, we get

\[g = \frac{(G \times M)}{R^2} \nonumber\]

Note that we have replaced the general symbol for acceleration, a, with the symbol scientists use for the acceleration of gravity, g.

Say that a particular white dwarf has the mass of the Sun \(2 \times 10^{30} \text{ kg}\) and the radius of Earth, \(6.4 \times 10^{6} \text{ m}\). What is the acceleration of gravity at the surface of the white dwarf?

Solution

The acceleration of gravity at the surface of the white dwarf is

\[g(\text{white dwarf}) = \frac{(G \times M_{\text{Sun}})}{R_{\text{Earth}}^{2}} = \frac{(6.67 \times 10^{-11} \text{ m}^2/\text{kg s}^2 \times 2 \times 10^{30} \text{ kg})}{(6.4 \times 10^{6} \text{ m})^2} = 3.26 \times 10^{6} \text{ m/s}^2 \nonumber\]

Compare this to g on the surface of Earth, which is 9.8 m/s².

Exercise Gravity on a 2M_Sun White Dwarf

What is the acceleration of gravity at the surface if the white dwarf has the twice the mass of the Sun and is only half the radius of Earth?

Answer

\[g(\text{white dwarf}) = \frac{(G \times 2M_{\text{Sun}})}{(0.5R_{\text{Earth}})^{2}} = \frac{(6.67 \times 10^{-11} \text{ m}^2/\text{kg s}^2 \times 4 \times 10^{30} \text{ kg})}{(3.2 \times 10^{6})^2} = 2.61 \times 10^{7} \text{ m/s}^2 \nonumber\]