The discovery of the Neutron Star
The History of the Neutron Star
The neutron, an elementary particle with neutral charge in the nucleus of atoms, was predicted by Ernest Rutherford in 1920. Twelve years later, James Chadwick found experimental proof of the existence of neutrons and later received the Nobel Prize for his discovery. Only two years after this discovery, astrophysicists predicted the existence of neutron stars.
In 1934, after months of rigorous calculations regarding star death and of analysis of the properties of neutrons, Fritz Zwicky and Walter Baade published the paper Supernovae and Cosmic Rays. In this paper, they predict, “With all reserve we advance the view that supernovae represent the transitions from ordinary stars into neutron stars, which in their final stages consist of extremely closely packed neutrons.” They believed that these neutron stars were rapidly spinning, dense remnants of dead stars.
Ernest Rutherford Fritz Zwicky James Chadwick Walter Baade
The Discovery of the Pulsar
While scientists were initially skeptical of neutron stars, further research lead to more widespread conviction of their existence. Experimental proof, however, would not come until 33 years after Zwicky and Baade's first proposal.
In 1967, Jocelyn Bell was doing graduate research in radio astronomy at Cambridge University under advisor Anthony Hewish. She unexpectedly detected regular radio pulses from the sky with a period of about 1.3 seconds. They named these sources pulsars, short for pulsating radio star.
Jocelyn Bell with the original data she collected leading to the discovery of the pulsar.
At first, they believed these pulses were coming from an unnatural, man-made radio source. Eventually, however, they realized these radio pulses were coming from outer space. Bell and Hewish were astounded by the regularity of the objects pulses and jokingly referred to the signals as the first transmission received from an advanced alien civilization. Consequently, the first discovered pulsars were named LGM 1, LGM 2, LGM 3, and LGM 4, with LGM denoting Little Green Men.
Pulsars and Their Relation to Neutron Stars
As astronomers mapped out the locations of the pulsars Bell and Hewish had discovered, they realized that these pulsars were actually concentrated within the plane of our own galaxy, the Milky Way. Very few celestial objects could emit this much radiation. Using a process of elimination, astronomers ruled out many stellar objects.
Eventually, most scientists were convinced that these pulsars represented the first detection of actual neutron stars. In 1968, this hypothesis was confirmed with the discovery of pulsars at the center of the Crab Nebula and the Vela Supernova Remnant - two places in which neutron stars would be expected to be present as the remains of supernova explosions.
The discovery of pulsars represents another example of science progressing through observational confirmation of a theoretical hypothesis. Theory and observation meet and create new laws of science, even if they are almost 40 years apart!
Pulsar at the center of the Crab Nebula in radio Vela Pulsar in X-ray
How Do Neutron Stars Form?
Neutron stars result from the death of massive stars, which are around 8-15 times more massive than our Sun. When a massive star collapses and explodes in a tremendous supernova explosion, the dense inner core is all that remains.
When the dying star is collapsing, the core gets progressively denser. Eventually, the elementary particles in the core (protons and electrons) cannot withstand the force of gravity, and they fuse together to form neutron. Eventually, all of the material in the core is converted into neutrons and the origin of the object's name becomes obvious.
Properties of Neutron Stars
Neutron stars have four basic properties: a small diameter, high density, strong gravity, and strong magnetic field.
Neutron stars are only several kilometers in diameter, often no more than the size of a city. They are at least forty percent more massive than our Sun, however! Consequently, neutron stars are extremely dense, about 1014 (one hundred trillion) times denser than water. A teaspoonful of neutron star could weigh up to one billion tons.
The gravitational force of a neutron star is proportional to its mass and inversely proportional to its radius squared. Thus, neutron stars have very strong gravitational fields. All of the particles in a neutron star are pulled together as tightly as possible, leaving a very smooth, spherical surface. If you tried to visit a neutron star, you would immediately be squished into elementary particles. On the surface of a neutron star, the escape speed (the speed at which you would have to travel in order to escape the gravitational field of a neutron star) is 160,000 km/s, over half the speed of light. Compare this to the escape speed on earth, which is only 11.2 km/s.
Neutron star compared to Manhattan
The magnetic field on a neutron star is over one billion times stronger than the field on Earth. This strong magnetic field causes the pulsing effect in neutron stars.
The magnetic field of a neutron star is a dipole with north and south poles, just like Earth's magnetic field and, indeed, every magnetic field known in the universe. These magnetic poles are not aligned with the axis of spin of the neutron star. Charged particles are accelerated in the magnetic field of the neutron star above the magnetic poles. As they are accelerated through the magnetic field, the charged particles give off radiation. These particles create beams of radiation aligned with each of the magnetic poles.
Since the magnetic poles and spin axis of the neutron star are not aligned, the beams rotate with the neutron star and act like the beams of a lighthouse. We see this pulse of radio waves every time it passes by earth, similar to how we see the light beam from a lighthouse every time it passes by our eyes. Not surprisingly, this effect is called the lighthouse effect of pulsars. The number of times a neutron star pulses varies from once every few seconds to once every few thousandths of a seconds.
Every time we see a pulse from a pulsar, we are seeing radio waves from the beam of radiation emitted from the magnetic pole of a rotating neutron star. Each pulse represents a single pass of the beam.
A neutron star is not necessarily a pulsar, however. Imagine being exactly above the spin axis of a neutron star. The beam of radio waves would never pass by you, and you would never detect radio pulses. Consequently, we would not detect this neutron star as a pulsar.
Millisecond Pulsars
Optical image of a millisecond pulsar with data from the pulses
Second Digital Sky Survey
Millisecond pulsars are a specific type of pulsar. They rotate hundreds of times per second at speeds close to one tenth of the speed of light at their equator. Astronomers think that these pulsars are relatively old and attain their high rotational speeds by pulling mass off of a companion star in a binary orbit.
When new mass is added to the system, conservation of angular (rotational) momentum causes the system to speed up. Angular momentum is conserved, which means that, in a spinning system, when an object stays the same size but becomes more massive, it speeds up, and when an object of constant size becomes less massive, it slows down. By accreting mass, the millisecond pulsar can reach extremely high speeds.
Atomic clock
Neutron stars emit a lot of energy through their beams of radiation. As the neutron star loses energy, it slows down very gradually. In spite of this slowing, pulsars, especially millisecond pulsars, are incredibly accurate timekeepers. Their signals are received here on earth like the beat of a cosmic clock. If this signal is corrected for the slowing down effect with a computer, millisecond pulsars keep time as accurately as the best atomic clocks in the world.
Pulsar in the Crab Supernova Remnant
One well-known pulsar lies at the center of the Crab Nebula in the constellation Taurus. This nebula is the result of the supernova that occurred in 1054 A.D. The Chinese first discovered this supernova remnant, describing this phenomenon as a “guest star” in the night sky.
The Crab pulsar rotates 30 times every second. One pulse occurs every 33 milliseconds. The Crab pulsar is relatively young and still radiates an enormous amount of power and emits everything from radio waves to gamma rays. This pulsar generates approximately 100,000 times more radiation than our sun.
As the pulsar continues to give off radiation, it gradually spins slower and slower as more and more energy is blasted into space. The pulsar’s period lengthens by 15 millionths of a second each year.
Following the discovery of the pulsar in radio wavelengths, optical astronomers detected the pulsar in optical wavelengths, making it the first optical pulsar ever discovered.
Crab Pulsar in X-Ray (blue) Optical (red)
NASA / CXC / ASU / J. Hester et al. Images of pulses from the Crab Pulsar
Reading Quiz:
1. Who discovered the first pulsar?
a. Ernest Rutherford
b. James Chadwick
c. Jocelyn Bell
d. Karl Schwarzschild
2. What was the significance of the discovery of the pulsars in the Crab Nebula and the Vela Supernova Remnant?
a. Pulsars were discovered in places where neutron stars were predicted to exist
b. Astronomers realized that pulsars emitted all types of electromagnetic radiation, from gamma ray to radio
c. This event marked the discovery of the first millisecond pulsars
d. Astronomers discovered that pulsars could be optically viewed, as opposed to just measuring radio waves
3. Which of the following is NOT a basic property of neutron stars?
a. Strong gravitational field
b. Strong magnetic field
c. Extremely high density
d. Extremely large volume
4. When radio astronomers view pulsars, they are really viewing
a. Beams of radiation from a neutron star that are pointed at Earth
b. The spin axis of a neutron star
c. The intense gravitational field of a neutron star
d. The magnetic field of a neutron star
5. True/False: Neutron stars are always pulsars
a. True
b. False
6. Which of the following is NOT a characteristic of
millisecond pulsars?
a. They are relatively old.
b. They are smaller than normal pulsars.
c. They spin more rapidly than normal pulsars.
d. They make very accurate timekeepers.
7. The Crab pulsar was the first _____ pulsar to be
discovered.
a. Millisecond
b. Radio
c. Optical
d. Binary
8. Pulsars have been viewed at what wavelengths?
a. Radio only
b. Radio and optical
c. Radio, optical, x-ray, and gamma-ray
d. All wavelengths in the electromagnetic spectrum
Activity: Timeline
Materials: Paper, pencils, colors, internet access (iPad)
Procedure: Illustrate a timeline that identifies (date and event) the important events in the discovery of neutron star. Include other important astronomy events that occurred during this time span that interest you.
1. Review the events that led to the discovery of the neutron star (See “History” section in above lesson).
2. Draw a timeline featuring at least 5 events (with their date) in the history of neutron stars and pulsars.
3. Illustrate several of these events with drawings.
4. Research online some other events in the history of astronomy in the twentieth century (possible topics include black holes, quasars, x-ray astronomy, or gamma ray astronomy). Add some events regarding these topics to your timeline.
Here are a few other timelines you can review for inspiration:
Timeline Index
Challenging the Space Frontier
Activity: Understanding Neutron Stars
Materials: A pencil, a sheet of paper, and a calculator.
Procedure: Escape speed is the speed at which you would have to be traveling in order to escape the gravitational field of a celestial body. For instance, in order to escape the gravitational pull of Earth, the space shuttle must travel faster than 11,200 m/s.
Derived from the laws of motion discovered by Sir Isaac Newton, the equation used to calculate escape speed is
Where ve is escape speed in meters per second, G is the gravitational constant (6.67 x 10-11N*m3/kg*s2), M is the mass of the celestial object (i.e. Earth or the neutron star) in kilograms, and R is the radius of the celestial object in meters.
Notice that the mass of the object trying to escape the gravitational field is irrelevant in this equation. A 91.7 kg alien spaceship and a 6350 kg NASA shuttle would both have to travel over 11,200 m/s to escape Earth's gravity.
Calculations: Using a calculator and 3 significant digits, answer the following questions.
1. Calculate the escape speed of the moon, with a mass of 7.35 × 1022 kg and a radius of 1.74 x 106 m.
2. Calculate the escape speed of our sun, with a mass of 1.92 × 1030 kg and a radius of 6.95 x 108 m.
3. Assume that you are exploring a neutron star with a mass of 1.99 × 1030 kg (which is the mass of our sun) and a radius of 10,000 m. How fast do you have to go to escape the gravity of a neutron star?
Disclaimer: Because of the extreme gravitational conditions on a neutron star, you cannot calculate the escape speed using Newton’s equations. In Newton’s times, he was unaware of the existence of these extreme conditions and his classical mechanics only apply to non-extreme objects. We can now calculate the escape speed on a neutron star using Einstein's theory of general relativity. Nonetheless, the calculation above will give you a rough idea of the tremendous speed required to escape the surface of a neutron star.
Pair Activity: Types and examples of pulsars
Materials: Two flashlights, a dark room, and two volunteers.
Procedure: Give the two flashlights to the student. Have the student extend their arms straight out from their sides and slowly rotate at a constant rate. Have another student count the number of times one of the beams passes them for a minute; this is the frequency of the “pulsar.”
What is the frequency of the pulsar?
f= number of rotationstime in seconds
f=
Then have the student with the two flashlights mover her arms so that one beam is pointing up at the ceiling at an angle and the other is pointing down towards the floor at an angle. Position the student so that only the downwards-pointing beam ever hits the audience (it may be necessary to place the student on a stool). Have another student find the frequency again. Point out to the students how only one beam of the “pulsar” is visible to them.
Finally, have the student point both flashlights perpendicular to the floor - one pointed straight at the ceiling, the other pointed straight at the floor. Now, have the student rotate; the audience does not see any light though! This demonstrates how a pulsar is always a neutron star but a neutron star is NOT always a pulsar!