Chapter 21. Analyzing the Crab Pulsar Light Curve

21.1 Introduction

The Crab Nebula - Wreckage of an Exploded Star

The goals of this module: After completing this exercise, you should be able to:

1. Identify and explain a light curve.
2. Connect the light curve data of the Crab pulsar to the standard pulsar model.

In this module you will explore:

1. The history of the discovery of the first pulsar.
2. How the evidence, light curves, were used by astronomers to build a model.
3. How the 'reasonableness' of a conclusion sometimes doesn't matter - science isn't about what we expect, but what the evidence says!

Why you are doing it: The life cycle of stars much more massive than our Sun is fascinating, partly because they are the primary source of important heavy metals like titanium and gold. The more massive the star when it's born, the stronger the gravity, and the more crushed and dense the core of the star ends up when it dies. These ultra-dense cores are fascinating "laboratories" to study intense gravitational and magnetic fields that can't be reproduced on Earth.

21.2 Background

A Radio Recording of the Pulsar PSR 0329+54

Most stars, including our own sun, will end life as a white dwarf. (See the separate resource "Life and Death Beyond the Main Sequence.") However, in 1933 Fritz Zwicky and Walter Baade predicted that stars starting out with a lot more mass than our Sun could end up with their stellar cores collapsed into a sphere of neutrons. They called these remnants of dead stars "neutron stars." This is one of the most bizarre predictions in astronomy, but what was actually found turned out to be even weirder!

This prediction remained just a hypothesis until 1967, when Jocelyn Bell, part of Anthony Hewish's research group, found a number of sources that 'pulsed' in the long radio wavelengths of light. One in particular, the source in the center of the Crab Nebula, was flashing 30 times a second! Binary stars can vary in brightness if they eclipse each other from our perspective, but they would have to be inside each other to be changing brightness this quickly, something that is as physically impossible as it sounds. Stars can change their diameter, moving in and out, but this takes days, not seconds. If something even as small as a white dwarf were spinning this fast, it would tear itself apart.

The figure above shows the evidence for one of the first pulsars to be discovered. Its name means that it is a pulsar located at a right ascension of 03 hours 29 arcseconds and a declination of +54 degrees. This pulsar has a longer period than the Crab pulsar, at 0.714 seconds.

Astronomers reluctantly began to consider a neutron star as the explanation for these pulses, a specific observation not predicted by Zwicky and Baade.

21.3 Neutron Stars Pulse Across the Electromagnetic Spectrum

The Crab pulsar flashes strongly in the radio, visible, and X-ray, all at the same rate of 30 times a second! A movie that puts together visible images of the Crab pulsar is shown below.

Speed up the period of the flashes to the actual Crab pulsar period of 0.033 seconds.

21.4 The Crab Pulsar Light Curves

The Sun's Spectrum

Ordinary stars like the Sun shine because their surfaces are hot. Not surprisingly, our star gives off most of its light in the visible part of the electromagnetic spectrum, and less at the very short (X-ray) and very long wavelengths (radio). You may be familiar with the graph called a spectrum where intensity is on the vertical axis and wavelength (color) is on the horizontal axis.

The Crab pulsar, though, gives off light fairly evenly at many wavelengths. What’s particularly interesting about objects like the Crab pulsar is their flashing. To study the changes in light over time, a light curve is better than a spectrum. Add a light curve to this movie by clicking the Light Curve on.

Question Sequence

Question 21.2

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3

Try again.

Correct. A light curve helps us study the changes in light over time.

Incorrect. A light curve helps us study the changes in light over time.

Question 21.3

Which of the following best represents the light curve of an ordinary star that does not change?

14uvPW9VFVb6cpQOoTs2lxN42rl8q/bFiVUCxaUvhCXXQ6fKONk6YIpR/e0jgTQsVJFsbha8INJJtMZx7vFcuSF+MZM9wSs4IrU9buKlIAI1I5K+f3oivAy2n2XnInJMgZoiRVf9f27sU6y4r4+MW/hn3jyLYLdqx8ERZ93O8bOFNNy9fNB024PfcwNz4L4zOameIF1vXu9y/aX74FIpUlgw1XrG/dkdbCMBsHAKMv4qYmO+VKGDAUSNeeMEEtkxS/UezY+AS1Kcvlnowp3idLKO22U8F1j0rmWnGYa9U3g6XwFjEDUPr9TP0qmgld4gW3ql9RRGRt2jAeUP5g04zvpoljsDqt6qpIsjSLgPVhrF+hekwxOj/ht8edOMdvVEKkJDwzx3S+mpVETswG6JHCtUyIh/If0kce1aHQ==

3

Try again.

Correct. The intensity of light of the other object, presumably a star, doesn't change over a few seconds of time.

Incorrect. The intensity of light of the other object, presumably a star, doesn't change over a few seconds of time.

21.5 Connection to the Pulsar Model

The rapid spin of pulsars is also what astronomers expect from the collapsed core of a dead, massive star. This is because the remnant core not only contains the original mass in the core, but also the original spin. Just as an ice skater spins up when she pulls in her arms, a collapsing star also speeds up as its size shrinks. The magnetic field also intensifies for a pulsar for similar reasons.

This is a sketch of the pulsar model, a neutron star spinning rapidly with a strong magnetic field offset from the spin axis. When the 'beam' from the magnetic poles points in the Earth's direction, we see the flash of light and identify this as a pulsar. This implies that there are many other neutron stars that are not pulsars, since their jets do not happen to ever aim at the Earth.

Pulsars are Spinning Neutron Stars

Even though the idea that something as massive as a stellar remnant being as dense as the nucleus of an atom seems far-fetched and was not taken seriously by astronomers for decades, the evidence for a tiny, rapidly spinning object with a strong magnetic field eventually convinced astronomers to accept that neutron stars can explain pulsars.

21.6 Quick Check

Indepth Activity: The Nature of Black Holes