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Interior Structure of Star Prior to Core Collapse
(c) R.J. Hall

Cosmic Fireworks

The late scientist and educator Dr. Carl Sagan was fond of saying “we are made of star stuff.” What did he mean by this? In this month’s column, I would like to address this by discussing “Cosmic Fireworks,” in honor of our nation’s birthday on Friday.

The lifecycles of stars

Stars, like people, experience different stages of their lives, from birth and infancy through middle age and finally towards an inevitable death. These lives are guided by a singular struggle between the competing forces of pressure trying to blow the star apart and gravity trying to squeeze it down to a pinpoint. This struggle is controlled by a finely-tuned balance called hydrostatic equilibrium. This balance needs to be maintained at every point within the interior of a stable star. Wherever pressure exceeds gravity, the star expands and cools until the pressure decreases. When gravity is dominant, it contracts and heats up until the pressure increases to counter its pull.

As one moves from the surface of the star inwards towards its core, the force of gravity grows stronger due to the weight of all of the gas above. In order to balance this force, the pressure needs to grow as well. For stars, the most efficient means of increasing this pressure is by increasing the temperature. This is why it is much hotter inside of stars than on their surface. Models of stellar interiors suggest that the temperature at the center of our Sun is about 15 million Kelvin compared to a surface temperature of about 6000 Kelvin.

The nature of how a star lives and inevitably dies is guided by a single parameter: its initial mass. A star like our Sun will spend about 9 billion years converting hydrogen nuclei into helium in its core via nuclear fusion, until the supply of available hydrogen is consumed. Right now, 4½ billion years after it formed, the Sun is in the midst of middle age. Less massive stars will take even longer to consume their fuel, while a more massive star will run through its available supply of hydrogen on much shorter timescales.

For this column, I want to focus on the lives of stars that start out with at least eight to ten times as much mass as our Sun. Because of their greater weight, they require higher internal pressures to counteract the force of gravity. This in turn is supplied by very high core temperatures, around 50 million Kelvin. At such extremely high temperatures, the rates for nuclear reactions are much greater than in a star like our Sun. As such, these massive stars consume their available supply of hydrogen on much shorter timescales. For stars as large as one hundred times the mass of our Sun, they can burn through their fuel in as little as a few million years.

What happens when a massive star runs out of hydrogen fuel in its core?

Old age is unkind to stars. They get bloated. Their surface temperatures drop precipitously until they are like burning embers in a campfire. They expel mass at a prodigious rate via powerful stellar winds and mass eruptions. These are just the symptoms observed in the visible outer layers of a dying star.

What occurs in its core is far more extreme. With the consumption of its hydrogen fuel, the star’s inner core becomes increasingly concentrated with helium nuclei. However, it isn’t hot enough yet for the helium to undergo nuclear fusion.

Without a steady source of energy being generated, the weight of the core begins to exceed the pressure that is holding it up and the core begins to contract. As it contracts, it gets hotter and hotter.

Meanwhile, in a shell surrounding this helium core, the remaining hydrogen fuel is still undergoing fusion into more helium, which falls onto this contracting core. This added weight results in even stronger gravity and faster contraction. Eventually, the core temperature will have risen above 100 million Kelvin and the helium undergoes stable fusion reactions into carbon nuclei.

As carbon starts to build up in the core, helium fusion continues in a shell surrounding this region. Outside of that shell, hydrogen fusion continues to generate more helium in another shell. Since the core temperature is not hot enough for the carbon to undergo fusion into heavier elements, it starts to contract and heat up. Eventually, when the temperature reaches 600 million Kelvin, the carbon begins to undergo fusion into neon and oxygen.

For these massive stars, each stage is completed in a shorter time interval. Where it takes several million years to completely consume the hydrogen in the core, it may take only a few hundred thousand years to use up the helium. Then it takes only a few hundred years for the carbon to be used up.

As the core consumes each in these series of fusion reactions, they continue to burn in a series of shells surrounding the core. After carbon is converted into neon and oxygen, these are subsequently consumed in reactions that form magnesium and silicon (at temperatures as high as 1.5 billion Kelvin). This process takes about one year to complete.

When the contracting core of silicon and magnesium has reached a temperature of about 3 billion Kelvin, these nuclei begin to undergo fusion reactions that completely consume the available fuel in a matter of days. The end result of these reactions is a core consisting primarily of iron.

Surrounding this iron core is a series of concentric shells, much like the interior of an onion. Within each shell, a different atomic species is undergoing fusion, from hydrogen in the outermost shell to silicon just outside the iron core. At this point, there can be no more energy extracted from nuclear fusion reactions in the core as the iron nuclei are the most tightly bound of all elements in the universe.

Old stars are a bunch of degenerates

When a star loses its ability to extract energy by fusing light nuclei into heavier nuclei, it is no longer able to maintain its internal pressure to stop gravity from crushing it. It does, however, get a brief respite from its ultimate fate thanks to a quantum mechanical phenomenon known as degeneracy pressure.

While the nuclei in the core undergo the above sequence of fusion reactions, the electrons are zipping about unencumbered by the furious conditions surrounding them. However, as the core continues to contract under its own weight, the available space for the electrons gets smaller and smaller.

A property of particles like electrons (as well as protons and neutrons) is that they don’t like to be squeezed together too tightly. Stated more formally, no two electrons can occupy the same quantum state. As the core contracts, the available quantum states get filled up and eventually the electrons build up this degeneracy pressure to prevent further contraction of the core.

But for these very massive stars, even this pressure cannot prevent the ultimate victory by gravity. As the mass of iron in the core exceeds 1.4x the mass of our Sun (known as the Chandrasekhar limit), these degenerate electrons get squeezed into the iron nuclei where they combine with the protons and convert them into neutrons.

One result of this reaction is the generation of a tremendous number of ghostlike particles called neutrinos. These particles interact with ordinary matter very weakly. There is a constant stream of billions of these neutrinos passing through our bodies every second which come from the core of our Sun. A number of neutrino observatories have been in operation since the 1960s around the world.

A second and more immediate result of this conversion of electrons and protons into neutrons is the removal of the degeneracy pressure holding up the core of the star. Without it, the core contracts in less than one second from about the size of the Earth (a few thousand kilometers in diameter) to the size of a small city (about a dozen kilometers in diameter).

At this point, this contracting ball of neutrons develops its own degeneracy pressure to counter the collapse (or if the core is massive enough it may collapse all the way into a black hole). The core “bounces” and generates a powerful shock wave that propagates through the remaining parts of the star, blasting it apart in a supernova explosion.

“Cosmic Fireworks!”

A galaxy like the Milky Way is expected to produce an average of one supernova every century. The last one observed to occur in our galaxy was in 1604 (Kepler’s supernova). In 1987, there was a supernova observed in our neighboring satellite galaxy called the Large Magellanic Cloud. This was about 160,000 light years distant and appeared as bright as the North Star. Special neutrino detectors buried deep under the Earth observed a total of 25 neutrinos emitted when the core of this star collapsed under its own weight.

Supernovae are the most powerful explosive events in the present-day universe. At maximum light, a single supernova can emit as much energy as an entire galaxy of billions of stars. The total energy output by a supernova explosion is comparable to the amount of energy our Sun will emit over its entire 10 billion year lifetime!

During a core-collapse supernova explosion like I’ve described above, there is an enormous amount of energy available to create many different types of atomic nuclei via the r-process (for rapid). Most atoms in the universe heavier than iron are created in these explosive events. They are expelled into space with all the matter that is blasted apart by the supernova.

Over the billions of years since the universe originated, these heavy elements have increased their abundances incrementally. Multiple generations of massive stars that live their brief lives and erupt in explosive deaths have seeded the universe with copper and uranium, silver and gold. When you look at a piece of gold jewelry that you own, realize that the atoms were first forged billions of years ago during the brief explosive moments when a massive star died.
This month, we will be treated to a close encounter in the evening sky between Saturn and Mars. On Wednesday, 09 July, the two will be separated by about 1 ½ lunar diameters as they approach the western horizon. Three days earlier, the thin crescent moon will be in their vicinity as well, making a magnificent apparition between the three worlds.

Also on the 9th, Jupiter reaches opposition. This means that it will rise at sunset and set at sunrise. It is closest to the Earth at this time and thus appears at its brightest for this year.

Venus presents a challenging sight in the early evening hours as it sets shortly after the Sun. You’ll need a clear western horizon in order to pick up it brilliant glow.

Looking beyond our solar system, the summer sky is dominated by the broad swath of the Milky Way Galaxy. After it gets dark, you should be able to trace the shape of our home system from the northeastern horizon down towards the southern horizon. As you gaze towards the south, you are actually looking towards the heart of the Milky Way. Its center lies within the constellation of Sagittarius, which is often referred to as the “teapot” for obvious reasons.

High above in the sky are three bright stars which mark the “summer triangle.” Vega, the brightest of the three is located in the constellation of Lyre, the Harp. Deneb is at the tail of Cygnus, the Swan and Altair is the eye of Aquila, the Eagle. These three are a welcome sight every summer, as they bring warm evenings and clear skies to enjoy the view.

This month, the Boise Astronomical Society will hold their annual pizza and ice cream social on Friday the 11th at 7 p.m. in Classroom #2 of the Discovery Center of Idaho. This will mark three years since I first attended one of their activities when I was relocating to Boise. Later this month, we will be holding a public star party at Ponderosa State Park in McCall, Idaho over the weekend of 25-27 July.

The Bruneau Dunes Observatory, which houses a 25” reflecting telescope (the largest in the state of Idaho) has programs for the public every Friday and Saturday night until October. For the weekend of July 4th, I will be the guest speaker. On Friday, my topic will be “Cosmic Fireworks: From Supernovae to Gamma-ray Bursts” (which was the inspiration for this month’s column). Saturday, I will present “Total Solar Eclipses: Standing in the Shadow of the Moon.” The talks begin at 9 p.m. and are followed with open observing using several telescopes including the 25”. The fee is $3/person.

About Irwin Horowitz

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