Stardome educator, David Britten, breaks down how science educators can help explain the (almost!) unexplainable. 

The subject of black holes has a fascination that attracts children and adults alike. Add the towering figure of Albert Einstein and you have an irresistible topic. But this doesn’t necessarily make it an easy topic.

The job of science teachers is to explain the natural world to students. Educators at Stardome explain the universe beyond Earth, and our place in the cosmos. This involves communicating concepts and scales that are well beyond the reach of normal human experience, and often challenge everyday understanding and common sense. The world of atoms, nuclear particles and quantum physics doesn’t mirror anything in our daily lives. The world of alien planets and moons, colliding galaxies and exploding stars leads us to distances, sizes, forces and time scales that defy easy understanding.

The recent announcement of gravitational waves detected from merging neutron stars presents a number of challenges on how to make this understandable.

Starting with the Big Idea provides the ‘wow factor’ that gets students’ attention (and teachers & parents!). 

For the first time, astronomers both saw and heard an event in the distant cosmos.

What? How can you ‘hear’ something from space? The next step is to break the Big Idea into digestible chunks.

How do stars work?

Stars like our Sun are gigantic balls of hot gas that squeeze the centre so much that they change hydrogen into helium. The heat from this nuclear cauldron pushes outward, balancing the force of gravity pulling that star inward. Big stars are hotter, so can make heavier elements than our Sun can, i.e. carbon, nitrogen, oxygen.

When stars much bigger than the Sun run out of fuel, and gravity overwhelms the heat at the core, they collapse into a black hole, producing an explosion called a supernova in the process.

What is a black hole?

A black hole is an object so dense that its gravity prevents anything escaping its surface, including light (the fastest thing in the universe). Black holes from collapsed stars are tens of times the Sun’s mass. Supermassive black holes at the centre of galaxies are from a few million up to a trillion solar masses.

 What are gravitational waves?

Einstein showed that the universe comprises space and time, like a fabric, and that gravity warps that fabric. Moving objects create vibrations in space-time, but these are miniscule even for massive stars or whole galaxies travelling through the cosmos.

But when two black holes collide and merge, they produce vibrations that can be detected from millions of light-years away. These gravitational waves – like wobbles travelling across a bouncy castle – allow astronomers to ‘hear’ events in the universe.

How is a neutron star different?

A neutron star is produced when a star dies that is larger than the Sun but not large enough to produce a black hole. The gravity of a neutron star is so strong that the electrons in all its atoms are squeezed into the nucleus, reacting with the protons to become neutrons. With no room for electrons (only neutrons), a teaspoon of material would weigh a billion tonnes. Neutron stars have powerful magnetic fields and produce very bright beams of light, rotating like a very fast lighthouse.

Why is this event so important?

Because black holes are completely dark, no light escapes when two black holes merge. But when two neutron stars merge, the explosion is almost as bright as an entire galaxy.

For the first time, this event allowed observations of both gravitational waves and the light spectrum – from radio and microwaves through infrared to ultraviolet, x-rays and gamma rays – from the same source. Astronomers were able to both listen to and see the event.

We also need to show the relevance of what we’re explaining. In this example, the ground-breaking nature of the event will mean a lot to those with a good grounding in physics and cosmology. For children, it may simply be the notion of being able to hear the universe as well as see it. At present, it isn’t possible to find the exact source of black hole mergers, which can only be detected with gravitational waves. However, because we could see the event, the location of this neutron star merger was found within hours of the gravitational waves being detected.

The first discovery from these observations was the presence of heavy elements such as platinum and gold in the debris flung out from the collision. The collision ejected about 10,000 Earth masses of material! This shows that the elements heavier than zirconium are mostly or wholly produced during the extreme conditions of neutron star merger events. The silver, iodine, caesium, platinum, gold, uranium etc. have travelled around the galaxy before coalescing with the primordial material in the solar nebula when the Sun and the Solar System began forming.

What about the collision of a black hole with a neutron star?

The merger of a black hole and a neutron star has not yet been observed. This will be a much longer event than the mergers of two black holes or two neutron stars. The black hole will tear the neutron star apart, shredding it as it spirals ever faster into the black hole. This will be a spectacular catastrophe of two cosmic titans, which may provide the first glimpse of the inside of a neutron star and how its extreme magnetic field is generated.