Monday, July 23, 2012

Star Trek: Binary Stars and Pulsars

Star Trek the Next Generation (1987-1994) (Picture)
Season 1, Episode 23
We'll Always Have Paris

In search of the source of a time disturbance, Picard and the Enterprise follow a relay signal to a binary star system. There they find the source of the disturbance and the scientist responsible.

What are binary star systems? Properties of these systems can provide insight into the nature of space and time.

Binary Star Systems

Binary star systems are systems of two stars that orbit around a common center of mass. The brighter of the two stars is called the primary star, and the other is the companion star. These systems can be observed by monitoring the intensity of the radiation coming from the system at a distance. When one star eclipses the other the intensity drops because the intensity from both stars in no longer being observed. Sometimes binary star systems can also be observed by telescopes directly.

 The orbits of binary stars (Source)
In the Star Trek episode the companion star in the binary star system is supposedly a pulsar.

Pulsars

Pulsars are highly magnetized neutrons stars that rotate and emit beams of electromagnetic radiation. In order to understand more about pulsars, it is important to know what neutron stars are.

Neutron Stars

Neutron stars sometimes form during the collapse of massive stars. They collapse to a very small sphere, comprised almost completely of neutrons. Neutrons are subatomic particles comprised of two down quarks and an up quark. They are found in the nuclei of almost all atoms. In normal matter atoms take up a lot of space because the nuclei are surrounded by a lot of space and electrons. In this way, matter is mostly empty space and it is the electrons surrounding the nuclei which are responsible for your ability to sit on a chair without falling through or to stand on the earth.

A basic diagram of the inside of a neutron star (Source)

In neutron stars this empty space disappears. When the star can no longer burn nuclear fuel, there is no longer an outward pressure created from the emission of energy from the fusion process. This sparks the supernova and what remains falls inward under the force of gravity. What it left is an extremely dense sphere of neutrons.

Neutron stars tend to be approximately 20km in diameter and contain approximately 1.5 solar masses, which is 1.5 times the mass of our sun. In a neutron star, the matter is packed so densely that one teaspoon of neutron star matter would weigh one billion tons on earth!

So Pulsars Are...

Pulsars (pulsating stars) are neutron stars with very strong magnetic fields, which emit strong beams of radiation. These beams of radiation are emitted along the magnetic axis of the pulsar, which is not always the same as the rotational axis of the pulsar. The radiation beam is therefore seen once per rotation of the pulsar. The pulsars rotate quickly because they are formed from very massive stars, which have some angular momentum. By conservation of angular momentum, when the star collapses to a neutron star its angular velocity increases greatly. Thus pulsars rotate with very high frequency and the beams of radiation, which span the gamut from radio waves to gamma rays, are separated by a very short amount of time.

A diagram of a pulsar. Note that the magnetic axis is not the same as the axis of rotation. (Source)

With each pulse of radiation the neutron star loses a small amount of energy (relative to its total energy). In this manner it begins to slow down over time.

What do pulsars have to do with space and time?

Pulsars are extremely dense objects, meaning the gravitational fields are very strong near them. If you imagine space-time as being a grid that expands through the universe, points of high density, like pulsars (and black holes) would look like sharp dents in the fabric of the universe. These sharp dents therefore warp our perception of space and time at those points.

Therefore, since the scientists in Star Trek were interested in carrying out time experiments, it makes sense that they would choose to be located near pulsars. Near dense objects, our perception of space and time are different than how we experience them on earth. These two phenomena, which seem so separate in our lives under most conditions, are inextricably linked. By studying pulsars we can gain some insight into the relationship between the two.

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