Secrets of neutron dance

YOU may remember that late last year I reported on the first “observed” collision of two neutron stars.

Now, the results from that observation are helping us understand more about the properties of these strange objects.

Neutron stars are the collapsed cores of stars that have exploded. They are approximately 20km in diameter, about the size of a small city and its suburbs.

They consist almost entirely of subatomic particles called neutrons, which have no electrical charge. The neutrons are produced by positively charged protons and negatively charged electrons being forced together under the extreme conditions, and they have no electrical charge. A teaspoonful of neutron star material typically has a mass of around 500 million tonnes!

To recap, the observation last year was made using gravitational-wave detectors. The physics behind this is that objects being accelerated in a gravitational field can produce gravitational waves. They are very weak and extremely difficult to detect, but violent events out there in the universe could produce waves detectable with highly advanced equipment, because of the tiny spatial distortions that they produce.

Human ingenuity has had a great result: we have built detectors and have now used them to observe these waves as they pass by us, even though the disturbance they produce is less than the diameter of an atom.

It turns out that the gravitational wave produced by the collision of the neutron stars contained some useful information about the interiors of these stars.

Certainly, we know that neutron stars are very dense, but what are they really like inside? It has been suggested that deep inside some neutron stars, the density of the matter is so great that the neutrons themselves are broken down into their building blocks, called quarks.

Whether or not this is likely is strongly connected with the diameter of the neutron star. Until recently, it was suggested that the diameter could be limited to about 22km, and this would suggest that deep inside the star, the density could be high enough for the neutrons to succumb to the extreme conditions and break up into quarks.

The effect of this would be to make the neutron star less rigid, and subject to being distorted due to tidal effects in a situation in which two of them are in orbit around each other. Essentially, the stars would be stretched along one axis so that they were no longer spherical. We know that this can also happen with normal stars in close orbit around each other, with the stars taking on a shape looking a little more like an Australian Rules football than a sphere.

It all sounds very exciting, but the results from the neutron star collision, on analysis by two independent researchers, have shown the maximum diameter of neutron stars could be more like 27km. That’s enough to mean the cores are not dense enough to produce quarks after all.

Last year’s results show the collision was between two undistorted, rigid objects, which would therefore not have had the “softer” quark material in their cores. So, perhaps there are no quarks in there after all, but at least we have added to our knowledge.

You can add to your knowledge, too, by participating in the ABC stargazing party on Wednesday night this week.

In collaboration with the Australian National University, the aim is to set a Guinness world record for the number of people simultaneously gazing at the moon through binoculars or telescopes across multiple venues at the same time. Details for each site can be found on the ABC website www.abc.net.au/ourfocus/ stargazing/ and it is important to register using the site www.eventbrite.com.au, which contains further details about bringing your telescope and telescopes available for purchase on the night.

At the time of writing this article, there are three registered venues in Tasmania. In the south, an event will be held at the Huonville High School, conducted by the Zayed Huon Energy Futures Team. In the north, events will take place at Legerwood Park, run by Ringarooma Pool and Pure Organics Tasmania, and on the oval at East Launceston Primary School, where I shall be located to join the fun.

Martin George is manager of the Launceston Planetarium (QVMAG).