Neutron Stars

At the top of the page is an artist's rendering of a lone neutron star. Below that picture is a recently discovered neutron star named XTE J1739-285. It is spinning on its axis 1,122 times every second, siphoning matter off of its compantion star, and sending powerful beams of x-rays into space. These stars is almost unimaginably small, with a diameters of 12-20 km, so dense that a teaspoonful would weigh billions of tons, and so hot that numbers struggle to describe its interior. The second subject of the Smallest Stars Chapter is the fascinating Neutron Star. I have chosen a very dark background color for this page, but not as dark as the next page will be.

Neutron stars, as their name indicates, are stars whose mass is crushed into such a tiny ball that outward anti-gravitational pressures such as thermonuclear fusion pressure and electron degeneracy are unable to hold the star against the relentless inward pressure of gravity. As described in the previous page, the Neutron Star has lost almost all of the properties of an atom except its mass. The electrons that normally occupy a distant point from the nucleus are now crushed beyond the limits of Quantum Mechanic's 6-dimensional space, and cease to exist, having finally merged into the very central-most nuclear particle ... the proton. The positive charge of the proton is neutralized by the negative charge of the electron, and there is no longer anything to prevent gravitational pressure from collapsing the star to an extremely small size.

At any mass in excess of the Chandrasekhar Limit of 1.4 solar masses, the degenerate electrons merge into protons. Normally, an atom is made of one part matter and 10^15 parts space. Our Sun has an average density the same as that of water ... 1.0 grams/cm^3. With no more pressure from degenerate electrons, the star collapses past the Earth-sized White Dwarf object with a density of 10^15 grams/cm^3. Neutrons become degenerate at a density of 10^14 grams/cm^3. This means simply that the very neutrons cannot be compressed any closer to each other than when they are all touching, and even this degeneracy is surpassed slightly because these collapsed Neutron Stars have densities in excess of 10^14 gm/cm^3. Instead of a diameter of 30 km, our Sun would actually collapse into a ball of only 20 km diameter. These conditions are so extreme, that the Neutron Star crystallizes! The theoretical structure of a Neutron Star is shown in the schematic below.

The possibility of a Neutron Star was first proposed in 1930 by Fritz Zwicky of the Mt. Wilson Observatory in California. He reasoned that the Type Ia Supernova from a White Dwarf in excess of 1.4 solar masses would most likely tear itself apart during the detonation. However, when a massive giant star collapses its core during a Type II Supernova, the Iron core being crushed by gravity is broken back to its fundamental particles, (dissociated protons, neutrons, and electrons), that quickly merge into neutrons, and the core suddenly reaches neutron degeneracy. The pressure of the degenerate neutrons prevents further collapse and a Neutron Star is born. Zwicky believed that the center of the Crab Nebula, a supernova witnessed by the Chinese in 1054, held a Neutron Star because it had no absorption lines (meaning it no longer was made of gas). However, it was not until 1967 when astronomers first really discovered Neutron Stars.

In 1967, British radio astronomer Anthony Hewish and graduate student Jocelyn Bell were turning their radio telescope toward flickering radio sources as they passed through the Sun's corona. They were trying to learn more about the properties of the corona when Jocelyn Bell picked up an unusual repeating signal from deeper space. She was recording faint pulses every 1.3 seconds. Though they appeared and disappeared from time to time, they maintained a perfect regularity of one pulse every 1.337011 seconds! Believing that they might be listening to a signal from intelligent life in space, they dubbed this radio source "Little Green Man #1." Indeed, a total of four different repeating sources were located that summer and appropriately named LGM 1-4. (These are not to be confused with the LGMs that adore Buzz Lightyear as their hero, and were saved by Mr. Potato Head and later adopted by Mrs. Potato Head in the Toy Story movie.)

Further analysis indicated that the radio sources were not intelligent life signals, but something almost as strange. Since only two physical processes can produce such repeating pulses (actual pulsations and rotation of a beam of energy), it was fairly simple to conclude that the radio source was a rapidly spinning Neutron Star whose radiation beam was flashing toward Earth like a beacon of a lighthouse. A White Dwarf spinning that fast would tear itself apart, so the repeater object must be significantly smaller and more dense to hold itself together. Zwicky's Neutron Star model fit perfectly with the observed radio source. Such a pulsating radio source became known as a "Pulsar," and the central star of the Crab Nebula was soon identified as a spinning Neutron Star ... a Pulsar with an astonishing pulsation period of 0.033 seconds!

What causes a Pulsar to pulse?

The cause of a Pulsar turned out to be much more simple to explain than one might have initially thought. In the "world" of a Neutron Star whose densities are so extreme, so too are the magnetic fields ... densities a trillion times greater than Earth or Sun, and 10,000 times that of a White Dwarf. Like most celestial objects, the magnetic poles are typically tilted relative to the rotational poles. (Remember the magnetic field alignments of Uranus and Neptune are offset and tilted relative to the spin axis ... why even Earth's magnetic north pole is not the same as the geographic north pole ... but what causes such tilting is unknown to me or anyone else to whom I have posed the question.) The spinning magnet creates electric currents that radiate their energy outward along the wobbling rotational axis. When the wobble happens to be in line with Earth, we see the flash of the beam of radiation, and witness a "pulse." The schematic seen below depicts a rotating neutron star whose immense and dense magnetic field is flashing a continuous and narrow beam of radiation into space (the artwork is from the Scientific American library.

Hundreds of these pulsars are now known, all repeating a various rates, and some even showing interpulses (meaning the north and south poles are flashing at Earth as it spins sideways). The most famous Crab Pulsar is shown in the following three images ... the first image being that of the Crab Nebula as photographed by the NOAO/AURA/NSF facility; the second image of the center of the nebula as photographed by the Hubble Space Telescope; and the third image of the Pulsar itself, also photographed by the Hubble Space Telescope.

Eventually, a Neutron Star will slow its rate of spin, and by the time it has slowed its pulsing rate to 4 seconds, it has lost its ability to generate a measurable magnetic field, and becomes invisible to every telescope except radio dishes, and even then it might not be able to radiate at all. These old, non -pulsing Neutron Stars must be in our Galaxy, and perhaps there might be millions of them, but they are just too small to find. However, the Hubble Space Telescope did find a non-pulsing Neutron Star in 1998, whose apparent visual magnitude is measured at 25, and its position appears to be only 400 light years away. The surface temperature is estimated to be 1 million K, and when you apply Wein's Law to this temperature, we find the star radiates most of its energy at a wavelength of only 3 nanometers ... in the X-Ray range of the Electromagnetic Spectrum. This non-rotating Neutron Star is revealed in the Hubble Space Telescope image below.

Pulsars are the source of intense study, and a major topic of research at the Aricebo facility in Puerto Rico, and the Very Large Array in New Mexico. Both of these radio telescope sites are featured in the movie "Contact" and Jodi Foster's search for signs of extraterrestrial intelligence in space. One of the challenges to the SETI (Search for Extraterrestrial Intelligence) team members is to filter out the signals from Pulsars, and listen to potential radio signals from some alien civilization. As Jodi Foster was doing in the movie, so do the SETI team members ... listen for signals, find one, and then discern whether it is a new Pulsar or a signal from life elsewhere. To date, only Pulsar signals have been received.

Final Neutron Star comments

For unknown reasons, Pulsars seem to be unbound to any companion star. In most instances, when a Supernova occurs in a binary system, the companion star is not affected. We would expect to find many Pulsars orbiting a less-evolved companion, but the searches have not produced much. It is theorized that the collapsing star, during the Supernova event, may be slightly off-center from the explosion, which gives the resulting Pulsar a huge kick. The kick may be enough to shoot it out of its shell of expanding gas, and perhaps loft it clear out of the disk of the Galaxy. The image at the top of this page shows such a fast-moving Neutron star ... speeding along at hundreds of kilometers per second.

One thing that I find interesting is the velocity you much achieve just to escape the gravitational attraction of a typical Neutron Star. Going back to Newton's formula for Escape Velocity ... (Ve = (2gm/r)^-2 ... you will find that a typical Neutron Star with a mass of 2 Suns, and radius of 10 km would have an Escape Velocity of {(2 x (6.67x10^-11)(4x10^30kg)/(10,000m)}^-2 = (5.336x10^16)^-2 = 230,997,836m/sec = 230,998 km/sec. Remember that light travels at 300,000 km/sec ... so to get a spaceship to successfully launch from a Neutron Star, you would need to go at a speed almost as fast as light itself!

Here are a few additional sites that might interest you, if you want to learn more about Neutron Stars.

Link to magnetars

An excellent neutron stars site

good link to Neutron Stars

virtual trips to neutron stars and black holes

Is is possible for a star to defeat not only electron degeneracy, but also neutron degeneracy? To find out, please move to the next page where we will take a close look at the strangest stars in the Universe ... Black Holes. If this cannot grab your attention, then go back to the Smallest Stars Introduction, Star Introduction, or the Syllabus ... of just take a nap and hurry back here.

 


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