In a star, the outward flow of energy generated by nuclear fusion in the core balances the inward pull of gravity. Without such fusion, a star would collapse into a very small volume. And indeed, the universe contains objects with a mass comparable to that of a star, but with a size comparable to the Earth. These are called white dwarfs.
White Dwarfs
The astronomer Friedrich Bessel, discoverer of stellar parallax, was highly skilled at recording the precise positions of stars. In 1844, he announced that the bright star Sirius was shifting its position over time due to the gravitational influence of an unseen companion mass. Apparently, both objects orbited their common center of mass with a period of about fifty years. However, the companion remained undetected until Alvin Clark discovered a small, faint object next to Sirius in 1862. Named Sirius B, this object is difficult to detect because it is lost in the glare of the much brighter star (Sirius A) in all but the best telescopes. By measuring the motion of both objects, astronomers found that Sirius B orbits about twice as far from the common center of mass as Sirius A. This indicates that it has about half the mass of Sirius A, yet is over a thousand times fainter! How is that possible?
In this Hubble Space Telescope image of Sirius, the white dwarf Sirius B is indicated by the arrow.
In 1915, Walter Adams obtained a spectrum of Sirius B. Recall that a spectrum allows us to discover not only the composition of a light source, but also its temperature. Adams found that Sirius B was nearly three times hotter than Sirius A. And since the distance to Sirius was already known from its parallax, comparison of the apparent brightness allowed astronomers to compute the size of Sirius B. Despite having the mass of a star, Sirius B is only about the size of Earth; it is a white dwarf – the first one to be discovered. Its small size accounts for its faintness. Since then, many other white dwarfs have been discovered.
Since white dwarfs have the mass of a star contained in a volume the size of Earth, their density is outrageously high – about 200,000 times higher than Earth’s average density. To put that in perspective, if you could scoop up one cup of material from a white dwarf, it would have more mass than a 747 airliner. This extreme density is due to the fact that the atoms of a white dwarf have essentially no space between them; they are as compressed as atomic matter can be. As a result, the surface gravity on a white dwarf is about 100,000 times greater than that of Earth.
Apparently, white dwarfs do not undergo any sort of fusion in their core. So, there is no energy production to heat and expand the gas into a large sphere. Thus, the atoms are compacted into a small volume. The only thing preventing a white dwarf from collapsing into an even smaller volume is electron degeneracy pressure – the fact that multiple electrons in the shells of atoms cannot occupy the same quantum state. That is, they cannot be in the same place at the same time with the same energy and quantum spin.
Most astronomers believe that white dwarfs are what remains of a “normal” star that has run out of hydrogen and helium fuel in its core and collapsed in on itself. There is evidence that at least some white dwarfs were once stars. Planetary nebulae (expanding spheroidal or bipolar clouds of hydrogen gas) often surround either a normal star or a white dwarf. These expanding clouds may have once been the outer layers of hydrogen that the central star has since shed into space. The Ring Nebula, as one example, has a white dwarf at its center. Could this be what remains of a star that has collapsed in on itself?
The Ring Nebula (M57). The small dot at the center is a white dwarf.
Of course, we have not directly observed this process. So, we have left the realm of observational science. Nonetheless, it is at least theoretically possible and consistent with observations. The resulting white dwarf is expected to be somewhat less massive than its progenitor star since some of the star’s mass is blown into space. However, we should also consider the possibility that some white dwarfs are part of the original creation.
White dwarfs come in a range of masses, between 0.17 and 1.33 times the mass of the sun. But most of them have a mass of around 0.6 solar masses. There is a theoretical upper limit for the mass of a white dwarf at about 1.44 solar masses. This is called the Chandrasekhar limit after its discoverer. Beyond that limit the inward force of gravity exceeds the outward force of electron degeneracy pressure, and electrons are forced into the nucleus of their atoms, combining with protons to form neutrons. The result is a neutron star.
Neutron Stars
Neutron stars were first predicted in 1934 by Walter Baade and Fritz Zwicky. These astronomers computed that a supernova (an exploding star) should create sufficient pressure to crush its own core into a highly dense state, exceeding the Chandrasekhar limit. The result is a small, extremely massive object made entirely of neutrons. Astronomers first detected a neutron star in 1967.
Whereas white dwarfs have the mass of a star compressed into the volume of the Earth, a neutron star has the mass of a star compressed into a volume only a few miles across. A typical neutron star is about twelve miles in diameter and is 1.5 solar masses. With a density of 1017 kg/m3, a single BB made of neutron star material would have more mass than the One World Trade Center.
Such high density is because atoms are mostly empty space. The distance between the nucleus of an atom and its innermost electrons is enormous in comparison to the size of the nucleus. Remove that space, and the result is a neutron star consisting of the densest form of definable material in the universe.
Under normal conditions, electrons will not fall into the nucleus to convert protons into neutrons because neutrons have more mass than protons and thus the reaction would require energy. But the gravity of a neutron star reverses the situation, making it energetically favorable for hydrogen and helium atoms to collapse into neutrons. Under the sort of normal conditions we experience on Earth, a neutron that is not bound to any protons will decay into a proton and electron with a half-life of about fifteen minutes. But in a neutron star, gravity prevents such a reaction from occurring. So, the neutrons are stable. A neutron star is essentially a single giant atomic nucleus with an atomic number of zero and an atomic mass of around 1057.
Most astronomers believe that neutron stars (like white dwarfs) are the collapsed remains of a star. But since neutron stars have greater mass than white dwarfs, their progenitor star must also be more massive. Neutron stars are thought to form when a massive star (at least eight solar masses) explodes as a supernova. The outer layers of the star are blown into space, forming a supernova remnant – a large, rapidly expanding nebula with a chaotic structure. The inner portions are crushed into a volume below the Chandrasekhar limit, forming a neutron star. And we have good observations confirming that this does indeed happen from time to time. One example concerns some observations from nearly a thousand years ago.
In the year A.D. 1054, Chinese astronomers recorded a supernova in our galaxy. The explosion was bright enough that it was visible even during the day for three weeks. The supernova remained visible in the night sky for two years after the initial event. Today, when we point telescopes to the position of the supernova as recorded by the ancient astronomers, we see an expanding cloud of hydrogen gas – a supernova remnant known as the Crab Nebula. And at its exact center is a neutron star – apparently the crushed core of the progenitor star. Pictures of the Crab Nebula taken decades apart show that it is still expanding today. The expansion rate is consistent with explosion having occurred in A.D. 1054.[1]
Two images of the Crab Nebula taken 40 years apart show that it is still expanding into space.
Furthermore, when a stellar core collapses below the Chandrasekhar limit, the process of converting atoms into neutrons releases enormous amounts of particles called neutrinos which stream away at nearly the speed of light. When supernova 1987A exploded in a nearby galaxy, astronomers detected neutrinos from the event. This is exactly what we would expect if supernovae do result in neutron stars. However, as with white dwarfs, we should again consider the distinct possibility that some neutron stars are part of the original creation. We will examine some evidence of this shortly.
When the inner regions of a star are compressed into a neutron star during a supernova, several things happen as a result. Conservation of angular momentum causes the core to rotate much faster than the original star was rotating. This is the same effect we see in a skater who spins much faster as she pulls her arms and legs in. So, neutron stars are expected to be spinning very rapidly, completing a rotation in a matter of seconds or less. Furthermore, the magnetic field of the progenitor star is likewise concentrated into a very small space, which results in extremely high magnetic field strengths. Neutron stars have the strongest magnetic fields of anything in the known universe.
Pulsars
Since the magnetic field of a star is generally not exactly aligned with its rotation axis, the same will be true for any neutron star that results from the explosion of such a star. Thus, the magnetic poles of a neutron star wobble as it rotates. Emitted radiation is channeled along these poles and into space. Any observer in the path of the radiation beams would detect a bright flash every time the star rotates. It is similar to the way a lighthouse is seen as the light beams sweep past a person’s field of view. These bright flashes were first detected by radio in the neutron star at the center of the Crab Nebula in 1967. Every 1.337 seconds, astronomers detected a radio pulse coming from the center of the nebula.
This strange object was initially given the label LGM-1. The label is a humorous acronym standing for “little green men” because some astronomers initially thought these pulses might be radio transmissions from an alien civilization. After all, nothing like this phenomenon had ever been detected previously. But when other objects were discovered, each with their own unique pulsation period, astronomers realized that this was a natural phenomenon and not an artificial one. By 1968, these objects were rechristened “pulsars” – a contraction of “pulsating radio source.”
So, a pulsar is a neutron star for which our solar system happens to be in the path of one of the magnetic poles as the object rotates. If Earth is not in the path of the radiation beams, we do not detect a pulsar. So, all pulsars are neutron stars, but not all neutron stars are pulsars – at least from our point of view on Earth. If we could somehow travel out into deep space it is likely that every neutron star would be a pulsar as observed from some particular location. Pulsars typically have a period on the order of one second, but some are hundreds of times faster. The latter are called millisecond pulsars and have periods of less than ten milliseconds. Many astronomers believe that millisecond pulsars have been sped up from “normal” pulsars as material accreted on their surface.
Pulsar Planet Problems
The first planets discovered outside our solar system orbit a pulsar. The millisecond pulsar PSR B1257+12 has three orbiting planets. These were discovered in 1992 by monitoring the precise timing of the pulses as detected from Earth. The gravity of these three planets induces a wobble on the pulsar as they orbit, which affects the timing of the pulses. Based on the irregularities in its pulsations, astronomers were able to deduce the mass and orbital period for each of the three planets.
The discovery is interesting for several reasons. First, secular astronomers had not expected to find planets orbiting a pulsar. Secular formation scenarios had predicted that planets should form around sunlike stars, and with masses and distances similar to the planets in our solar system. Consequently, it was equally embarrassing that the first planet to be discovered orbiting a normal star was also contrary to secular expectations; a hot Jupiter-mass planet with an orbital period of only 4.2 days was discovered orbiting the star 51 Pegasi in 1995. Such discoveries not only demonstrate the power of science but also the weakness of secular origins stories.
Second, this paved the way for further research into extrasolar planets. Although normal stars do not emit such precise radio pulses as pulsars, a similar method can be used on them to discover orbiting planets. By measuring the Doppler shift in a star’s spectrum, slight gravitational perturbations from any orbiting planets can be detected. This was how the planet orbiting 51 Pegasi was discovered.
The reason why secular astronomers did not expect to find planets orbiting pulsars is because such planets should have been ejected when the progenitor star exploded. During a supernova, more than half the mass of the star is ejected into space. Thus, any planets orbiting the star would now be traveling faster than the escape velocity of the reduced central mass. Secular astronomers therefore assume that any pulsar planets must have formed or been captured sometime after the supernova. But the conditions surrounding a pulsar are not conducive to planet formation even by secular assumptions. Perhaps PSR B1257+12 and its planets were part of the original creation.
Neutron stars are comprised of the densest material known. But, like white dwarfs, there is a mass limit beyond which a neutron star cannot avoid further collapse. In the next entry we will explore what happens to stellar cores that exceed 2.1 solar masses.
[1] When I have taught astronomy classes at The Master’s University, one of the activities we do for the lab is to measure the expansion of the Crab Nebula in images taken at different years. We then estimate how long ago the gas was expelled from the location of the neutron star based on its present expansion speed. Inevitably, we get a date very close to A.D. 1054.