Lighthouses in Space

When was the last time you jumped on a trampoline?

For me, it was the same time I had the unfortunate luck of going through the fabric of the trampoline and landing hard on the ground.

I ask because the fabric of space-time can be imaged to be like the surface of a trampoline. If you stand in the centre, the fabric of the trampoline bends towards you and anything you drop will rotate down until it hits you.

It’s similar for planets, stars, asteroids, anything.

Even you and the chair you’re sitting on, the pen you might have just put down. The cup of tea you forgot about that’s now cold. All of those things have mass and therefore they have gravity.

They bend the fabric of space-time it’s just in such a miniscule way that it’s completely unnoticeable to anything we can measure with. Your gravity is overwhelmed by Earths and we can’t measure it.

But we can measure objects that are big enough.

Think of black holes, the densest objects in the Universe, and take one step back. You have yourself a neutron star.

There is a distinct line where massive stars, after they go supernova, will either be a neutron star or a black hole and that line is 2.5 and 5 solar masses respectively.

There is a gap there, and no one knows what happens to objects in between that range. Are objects with 3 times the solar mass of our sun black holes or neutron stars? We don’t know. In 2019, the group of scientist at the National Science Foundation’s Laser Interferometer Gravitational-Wave Observatory (LIGO) discovered an object with 2.6 solar masses merging with a black hole of 23 solar masses.

But whether or not with 2.6 solar mass object was a neutron star or a black hole isn’t known.

For now, let’s keep our heads under the 2.5 solar mass limit and focus on neutron stars.

When a massive star reaches the end of its life it explodes in a conflagration and disperses raw elements out into the Universe. It’s thought that the remnants of these explosions are what first formed planets, and everything else, including us. Carl Sagan’s famous quote ‘We are made of star stuff’ is entirely true.

If the remnants of the supernova are below 2.5 solar masses, there isn’t enough energy for a black hole to form. Instead the leftovers congeal and spin, condensing to around about 20 kilometers in diameter.

On the scale of stars, neutron stars are tiny. The diameter of the Sun, for instance, is almost 1.4 million kilometers.

That means that it would take approximately 70 000 neutron stars to fit across the diameter of the sun. Perhaps a better way to visualise it is this; Long Island is 190km in length. That’s almost ten neutron stars.

The difference is the density of matter. Within that 1.4 million km diameter of the sun is 1 solar mass.

Within the 20km diameter of a typical neutron star is on average 1.4 solar masses.

That’s our sun, plus almost half, condensed into a point one tenth the length of Long Island.

If you put that on a trampoline you notice.

The often quoted line about neutron stars is that one teaspoon of the stuff will weigh about a billion tonnes. They are incredible object and they are incredible useful for astronomers.

There is a type of neutron star known as a pulsar. As the name suggests, these objects spin and pulse as they eject material from their poles at near the speed of light. As this material accelerates, the interaction with the magnetosphere of the pulsar creates a gamma ray burst. Eventually, these bursts will slow the rate of spin of the star, as energy is transferred and lost. These are visible as ‘pulses’ to us on Earth in the same way that a lighthouse creates a regular ‘pulse’ of light.

Due to the spin of the neutron star, and the fact that the material is ejected from the poles, these pulsars will appear to flick between on and off as they rotate, with one pole facing us, then away, then the other pole facing us, then away, and so on.

These pulses are so predictable that they were used on the famous image of the Voyager record.

Fourteen pulsars are used to denote the location of our solar system in the bottom left corner of the record. Because pulsars are incredibly reliable, the period of their electromagnetic emissions is mapped as well as their location, allowing our solar system to be located within space and time.

You may feel insignificant sitting on a trampoline, barely making an impression in the grand scheme of things. When compared to objects like neutron stars, we are tiny, but we aren’t insignificant. And if you feel disconnected by the time you finish this article, like the Universe is just too big, remember what Sagan said.

We are made of star stuff and that connects us all.

Further Reading/References:

Black hole or neutron star? | Penn State University (psu.edu)

Neutron Stars and Pulsars – Introduction (nasa.gov)

Voyager – The Golden Record Cover (nasa.gov)

Neutron Stars: Definition & Facts | Space

Stringent constraints on neutron-star radii from multimessenger observations and nuclear theory | Nature Astronomy