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CLEA - Radio Astronomy of Pulsars

How do we measure the distance to a pulsar? How can we know the age of a pulsar?


CLEA: Radio Astronomy of Pulsars. In this experiment, you will use a CLEA program to study the radio waves received from a rapidly rotating neutron star (i.e. a pulsar). You will look at the pattern of radio pulses received from the pulsar and determine the period of rotation. You will also look at the time delay of radio pulses at different frequencies in order to calculate the distance to the pulsar.



A neutron star rotates very rapidly about its axis of rotation. Its axis of rotation also rotates (i.e. precesses) like that of a spinning and precessing top. Due to its emission of radio waves along its axis of rotation and due to the precession of its axis, we receive periodic pulses of radio waves from the neutron star. A rapidly spinning neutron star that emits radio waves is called a pulsar.

Light travels at 3 x 108 m/s in vacuum (absence of matter). However, in any other medium, such as water, air, or glass, light travels more slowly.

The speed of light in a medium (such as glass) depends on the frequency of the light. For example, red light and violet light travel at different speeds in glass. This variation of speed for the different colors is the reason that a glass prism (or water, for example) separates white light into the visible colors (of the rainbow).

Higher frequency light has more energy and travels faster in a medium.

Though its density is very small, space is not a perfect vacuum. The interstellar medium, consisting mostly of hydrogen, slows light a very, very small amount. Though small, it is noticeable over very large distances and you can detect this when looking at the pulsar. The pulse emitted at a higher radio frequency (800 MHz) reaches us tenths of a second before the pulse emitted at a lower radio frequency (600 MHz). If we understand the interstellar medium well enough, then we can use the time delay between these two pulses (which left the pulsar at the same instant) to calculate the distance to the pulsar.

The lab handout uses an example of two sprinters. If the sprinters start at the same instant (the starting gun) and if you know their speeds, then you can measure the time delay between the sprinters at the finish line and use it to calculate their distance traveled.


We will use the CLEA program called Radio Astronomy of Pulsars. You should have already installed it.

1. Download two files: (1) the handout and (2) a spreadsheet for doing the calculations.

2. Watch the following instructional videos that will help you understand how to use the software.

CLEA: Radio Astronomy of Pulsars -- Part 1. This part shows you how to measure the period of a precessing pulsar.


CLEA: Radio Astronomy of Pulsars -- Part 2. This part shows you how to determine the distance to a pulsar.


3. Make the measurements described in the handout and enter your data in the spreadsheet. The green boxes are where you type your data and the purple box is a calculation.

High Point University       Last modified:   6/20/10 3:13 PM