What Are Pulsars?

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  • 0:01 Pulsating Stars &…
  • 0:40 What Are Pulsars?
  • 2:18 Momentum, Mass, &…
  • 3:37 Pulses & Magnetic Fields
  • 5:04 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will define and describe pulsars. Importantly, you'll learn why the term is a bit of a misnomer, what acts as a pulsar and why, and what it pulses out.

Pulsating Stars and Mistaken Identity

Back in the 1960s, graduate student and now famed astrophysicist Jocelyn Bell discovered proof of intelligent life in the universe outside of our earth. Well, at least at first that's what some people thought. But soon she and the rest of Anthony Hewish's research team realized that actually wasn't true, tantalizing even though it may be.

What was discovered was a star that gave off a very regular radio pulse that was initially misinterpreted as being of intelligent origin outside of our planet. What this actually was and how it gives off such a pulse will be outlined now.

What Are Pulsars?

A pulsating star, shortened to pulsar, is a spinning neutron star that gives off extremely regular bursts of electromagnetic radiation. Electromagnetic radiation includes things such as, but not limited to, visible light, radio waves, X-rays, and gamma rays, all four of which have been detected as emanating from a pulsar.

The term 'pulsating star' is actually a bit of a misnomer itself, though. You'll learn very soon that a pulsar isn't an object that sits still and pulses like an emergency beacon sits still and pulses out a light for people to spot. A pulsar is a spinning object that sweeps beams of radiation across the sky like a lighthouse sweeps beams of light across the sky. When the beam sweeps past Earth, astronomers can detect that beam, and it appears as a pulse to us because it's only momentary, as a result of the spinning star.

The radio pulses that were detected by Bell were coming in at the regular interval of every 1.3373011 seconds. At that time, no one knew of any astronomical objects that could do that so quickly. This led the team to suspect it may be E.T. after all. But soon after, many such pulsating sources with periods ranging from 0.033 to 3.75 seconds were discovered, and the idea this was E.T. was thrown out. The question naturally arose then, what objects that are of natural astronomical origin could do such a thing?

Momentum, Mass, and Neutron Stars

Basically, stars spin like a basketball would spin on your finger. Our sun's rate of rotation is about 25 days. The only way an object can spin faster and faster without being ripped apart is by decreasing its mass distribution; what I mean is, it spins faster when it brings its mass inwards, closer, or makes it more compact (however you want to phrase it).

For example, an ice skater begins to spin on the ice with her arms outstretched. But she spins faster and faster as she brings her arms inwards since she becomes more compact and her mass distribution decreases. This concept is known as the conservation of angular momentum. The same thing that occurs with our ice skater has to occur with a stellar object. It spins faster as it compacts itself and its mass.

What you should gather from all of this is simply the following: In order to spin fast enough to give off signals as often as a pulsar and not be ripped apart, the object has to be very compact, very small. The only visible thing that can be this compact and this small is a neutron star, a celestial object that forms from the collapse of a once gigantic star's core to ultra-dense levels. Not even a white dwarf is compact enough to rotate as fast as a neutron star without spinning itself apart.

Pulses and Magnetic Fields

I hope that wasn't too hard to imagine. Just think back to our ice skater example if you ever get confused. Now we know which celestial objects can spin fast enough to give off a signal, a regular pulse, as it spins. But, what is causing the pulses to begin with?

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