Reading the chapter titles in a modern college astronomy textbook is muchlike reading the titles of science fiction stories. Astronomers regularly talk about subjects like black holes, neutron stars, pulsars, quasars, dark matter, novae, supernovae, and even more bizarre topics. Some of the ideas in astronomy push at the limits of what we know, or can know. Many ideas in astronomy are so strange that some astronomers have difficulty accepting them. Understanding these ideas in astronomy requires knowledge of all branches of mathematics, including some less well-known branches like tensor calculus and gauge theories.

Novae and Supernovae

When a star with a mass about the same as our Sun reaches the end of its life, its outer layers slough off, leaving behind a solid carbon core. This core,  known as a “white dwarf,” is very small, about the size of a small planet. Initially, the core is extremely hot with a temperature of over one million kelvin,  although it does eventually cool off. It is also very dense, packing half the mass of the Sun into a sphere the size of Earth or smaller.

Sometimes, the white dwarf can be part of a binary star system. The brightest star in our night sky, Sirius, is an example of this kind of system.

Sirius has a white dwarf companion called Sirius B. In a binary star system with a normal star and a white dwarf the companion to the white dwarf may also reach the end of its life cycle and become a “red giant.” When this happens, the red giant can swell up so much that its outer envelope can be pulled onto the surface of the white dwarf. This happens because the red giant swells up past the Roche limit. Outside the red giant’s Roche limit,  the gravity from the white dwarf is stronger than the gravity from the red giant.

When matter spills over it can collect on the surface of the small, dense white dwarf. The new material is compressed by the white dwarf’s intense gravity and is also heated from below. Compression and heating can raise the temperature of the new material until it passes 10,000,000 K, the temperature at which nuclear fusion begins. Normally, nuclear fusion is taking place deep in the interior of a star. In this case, however, nuclear fusion happens on the surface of the white dwarf. It can be likened to millions of hydrogen bombs going off all at once. This event is called a “nova” (from the Latin nova stellaris, “new star”).

Supernovae are thousands of times more luminous than novae. One kind of supernova results from the death of a high-mass star, several times more massive than the Sun. Through nuclear fusion, these stars have converted hydrogen into helium, then helium into carbon, and continued to fuse nuclei into heavier and heavier elements until the core of the star is made of iron. However, the appearance of iron in the core terminates the fusion process. When iron fuses with another nucleus, it absorbs energy instead of releasing energy. This turns off energy production in the core and it begins to collapse causing more iron fusion. As more iron fuses the collapse goes faster and faster, approaching the speed of light. The upper layers of the star are no longer supported by heat and pressure from the core, and they also come crashing down, and the star implodes.

At the time of implosion, the core temperature of the star is several billion kelvin. The intense radiation from this enormously hot material has so much energy it begins to tear the atoms of iron and other elements apart, converting the core into protons, electrons, and neutrons. In less than a second, the star undoes all the effects of the last 10 million years! The core is now so dense that the protons and electrons are forced to combine and also become neutrons. The core of the star becomes a solid, rigid sphere of neutrons. When the outer layer of the star crashes down onto this rigid sphere,  the whole thing bounces, sending all of the remaining layers of the star off into space in an enormous explosion called a supernova. All that remains of the original star is the core of neutrons.

Another type of supernova has a different cause. It is similar to an ordinary nova, and also occurs in a double star system with one white dwarf.

When the companion becomes a red giant, material can fall onto the surface of the white dwarf. In this case, the material does not explode but simply collects, increasing the mass of the white dwarf. However, if the resulting object exceeds 1.4 solar masses, it can collapse. The 1.4 solar mass limit is known as the Chandrasekhar limit, named after the Nobel prize winning physicist Subramanyan Chandrasekhar, who proposed the idea. Above 1.4 solar masses, the white dwarf cannot support itself and it collapses as the carbon atoms begin to fuse. The core is so dense and rigid that all of the

carbon fuses into heavier elements in a few moments. The star blows up and becomes as bright as a galaxy for a few days. Unlike an implosion-type supernova, the carbon detonation supernova leaves nothing behind.

Near the beginning of our universe, when ordinary matter condensed out a sea of radiation, the universe contained just two kinds of atoms, hydrogen and helium. However, the universe now contains many more kinds of atoms. The elements of the book you are holding, the elements that you are made of, the iron in your blood, the nitrogen in your DNA, the carbon stored in your tissues, the oxygen you are breathing, none of these existed when the material universe formed. So where did these elements come from?

Amazingly, all of the elements in the periodic table, including those elements that make people, came from the explosions of stars. Except for the hydrogen, you are made of stardust!

Neutron Stars and Pulsars

When that implosion-type supernova blew up and scattered its outer layers into space, it left a remnant behind. The core of the star, where all of the protons and electrons were forced together into neutrons, is still there. It is called a “neutron star” although it is technically no longer a star. It is a stellar remnant. Neutron stars are extremely hot at first, but also extremely small. A neutron star is only a few kilometers across, the size of an asteroid in our solar system. Yet a neutron star has more mass than our sun. A teaspoon of matter from a neutron star would have the mass of a mountain! In addition to large mass and small size, they also have one other important characteristic: They spin very rapidly. You may have noticed an ice skater pulling her or his arms in and spinning faster as a result. A neutron star does the same thing. The star from which it was formed may have been rotating once a month or so, but by the time the core has collapsed to the size of an asteroid, it is spinning several times a minute or faster.

The neutron star also has a powerful magnetic field, which is captured from the star that exploded. It is because of the rapid spin and strong magnetic fields of neutron stars that we know they exist. In 1967, a graduate student named Jocelyn Bell detected radio waves coming in rapid, regular pulses. They were so regular that Bell and her advisor, Antony Hewish, first referred to them as LGMs (for “Little Green Men”!) because they thought at first that they might be artificial. It soon became apparent that they were far too powerful to be any sort of artificial beacon. These objects are now known as pulsars. Most emit radio waves and a few also emit pulses of light or radiation at even higher frequencies.

A few pulsars are associated with supernova remnants. When a star blows up, it scatters its outer layers back into space. These tatters are visible for a few hundred years after the explosion. The best known is the Crab nebula, which originated from a supernova known to have exploded in 1054 C.E.At the heart of the Crab nebula, right where a neutron star would be expected, is the Crab nebula pulsar. Astronomers now suggest that pulsars are spinning neutron stars. But what causes the pulses?

The axis of Earth’s magnetic field does not line up with its axis of rotation. As the particles trapped in Earth’s magnetic fields crash into the atmosphere above the north and south magnetic poles, auroras are created.

From space, it is sometimes possible to see one or the other of these areas of aurora flash on and off as Earth rotates every 24 hours. Astronomers think that a similar thing is happening with some neutron stars. If the magnetic field of the spinning neutron star is at an angle to the axis of rotation,  then two rotating beams of radiation might be emitted, one from each of the magnetic poles of the neutron star. Because of its resemblance to the rotating beacon in a lighthouse, this is called the “lighthouse” model of pulsars. As these beams sweep by Earth, we perceive a pulsar. So pulsars are

evidence that neutron stars exist and neutron stars are the explanation for pulsars.

Black Holes

Sometimes even more massive stars collapse and blow up. If the remaining core has a mass greater than 3.0 solar masses, then no force is strong enough to stop its collapse. The core passes the neutron star phase and simply keeps on collapsing. It collapses right out of our universe, leaving behind nothing but its gravity! This bizarre end point of stellar evolution is called a “black hole.” Black holes are just about the strangest objects in our universe. The principles of Newtonian mechanics do not apply in the space near black holes. To understand what is going on, astronomers must use a more modern theory of gravity known as Einstein’s General Theory of Relativity.

This theory deals with ideas like curved space and time dilation.

We can get a hint of what is going on around a black hole by thinking about two consequences of relativity: Nothing can travel faster than light, and gravity acts on everything, including light. Imagine going outside and throwing a baseball straight up. It will rise to a certain height and then begin to fall back. If you throw it harder, it will rise higher before falling back. Now imagine throwing the baseball so hard that it will rise infinitely high before falling back. The speed at which you would have to throw the baseball (or any other object) so that it rise to an infinite height is known as escape speed. For Earth, escape speed is about 11 km/s.

Now imagine squeezing Earth into a smaller, denser sphere one-fourth the size of its present radius. It would have the same mass but a smaller radius. If you were still standing on this smaller sphere, you would be closer to the center of its mass, so gravity would be higher and the escape speed would be greater too. It would be twice as large, about 22 km/s. If Earth were squeezed down to 1/1000 of its present size, its escape speed would be 630 km/s. Squeeze Earth down to a radius of one centimeter, and its escape speed would be 300,000 km/s. But 300,000 km/s is the speed of light, the fastest speed allowed by the laws of physics. So at a size of one centimeter, nothing could escape Earth’s gravity. It would be a black hole.

Black holes are not cosmic vacuum cleaners. If the Sun, by some strange and impossible process, turned into a black hole, Earth and all the other planets would continue to travel along in their orbits as if nothing had happened. The mass would still be there and the gravity of the Sun out at the orbit of Earth would be unchanged. However, matter that does fall into a black hole does get compressed and heated to extremely high temperatures.

This is how we know black holes exist. The matter spiraling into a one solar mass black hole would be heated to the point that it emits X-rays.

Dark Matter

Our galaxy is rotating. It is not a solid disk—each individual star or star cluster orbits the center of mass of the entire galaxy. Out at the edge of the visible galaxy, stars should be orbiting as if they were outside all of the matter of the galaxy. However, stars close to the edge of the visible disk of the galaxy are moving faster than can be accounted for by just the visible matter in our galaxy. The conclusion is that there is a large quantity of mass in the galaxy,  as much as 90 percent of the mass of the galaxy, which cannot be seen. It does not emit or reflect any form of electromagnetic radiation, so it is called cold dark matter. We cannot see it, but we know it must be there because of the effects of its gravity on the part of the galaxy we can see.

Astronomers do not know what the dark matter is. It is unlikely that dark matter is composed of black holes because the massive stars needed to form a black hole are not that common. The best candidate for at least some of the dark matter is a hypothetical exotic subatomic particle. Astronomers have dubbed these particles “Weakly Interacting Massive Particles” (WIMPs).

Quasars

Out near the limits of what we are able to observe with telescopes lie enormously energetic objects called “quasars.” The term is a combination of the words “quasi stellar” objects. The name originated because when quasars were first detected, they looked like points of light, similar to stars, but they had spectra completely different from the spectra expected from stars. At first, these were thought to be nearby objects. Then they were discovered to have very large red shifts. Some astronomers thought that some sort of cosmic explosion might have occurred to fling these objects at speeds near the speed of light, but no evidence of such an explosion has ever been found.

Most astronomers now think that quasars are at  cosmological distances. However, if quasars are this far away—200 million light years or more—they must emit prodigious amounts of radiation. Astronomers have proposed and rejected many different explanations for the enormous energy output of quasars. The only mechanism that seems to fit the data is matter falling into a super-massive black hole. A black hole with 10⁸ or 10⁹ solar masses can account for the energy output of even the most energetic quasar.

A quasar emitting 10⁴⁰ watts of power can be nicely explained by a 10⁹-solar mass black hole consuming the equivalent of 10 stars per year in mass.

This means that quasars are not a distinctly different class of objects, but are simply the extreme end of a spectrum of energy-emitting galaxies, including our own galaxy, all powered by massive black holes at the center. It also suggests that quasars and other active galaxies can evolve into normal galaxies as the available matter gradually falls into the black hole, leaving the remaining stars and other material orbiting at safe distances.

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Reference

Chaisson, Eric, and Steve McMillan. Astronomy Today, 3rd ed. Upper Saddle River, NJ: Prentice Hall, 1993.

Giancoli, Douglas C. Physics, 3rd ed., Englewood Cliffs, NJ: Prentice Hall, 1991. Sagan, Carl. Cosmos. New York: Random House, 1980.