City-size neutron stars may actually be bigger than we thought Scientists ponder how to get samples from Saturn’s weird moon Titan
Scientists have found neutron stars – the densest known objects in the universe aside from black holes – have a radius of between 13.25 and 14.25 kilometres (8.2 and 8.9 miles). Previously it was believed these stars, leftovers of huge supernova explosions, have a slightly smaller radius of up to 12 kilometres (7.5 miles). Within this relatively small radius – whichever one you look at – an amount of matter that equals
1.4 times the mass of the Sun is squeezed. The Sun, in comparison, has a radius of 695,508 kilometres (432,169 miles).
These estimates were made not by measuring neutron stars directly, but by looking at the so-called ‘neutron skin’, or an outer layer of neutrons, surrounding the nucleus of a lead atom. But what does lead have to do with the size of neutron stars? Lead is one of the densest materials that can be naturally found on Earth. Even though lead is nowhere near as dense as neutron stars, scientists assume that the physical principles governing the structure of lead atoms must be similar to those behind the structure of neutron stars.
“There is no experiment that we can carry out in the laboratory that can probe the structure of the neutron star,” said Jorge Piekarewicz, a nuclear physicist at Florida State University. “A neutron star is such an exotic object that we have not been able to recreate it in the lab, so anything that can be done in the lab to constrain or inform us about the properties of a neutron star is very helpful.” The researchers probed lead atoms with electron beams and found that the neutron skin of the lead isotope Pb-208 is 0.28 femtometres (0.28 trillionths of a millimetre) thick, about double the thickness predicted.
Atoms generally contain positively charged protons, negatively charged electrons and neutrons with no charge. Protons are squeezed inside the atom’s nucleus, while electrons make up its outer shell. With neutrons, it’s a bit more complex. Heavier atoms, such as lead, usually have more neutrons. Some of these neutrons are squeezed in the nucleus together with the protons, but not all of them fit. The rest are pushed out to the edge of the nucleus, forming what physicists call the neutron skin.
The most common isotope of lead, Pb-208, which is frequently used for experiments, contains 82 protons and 126 neutrons. Physicists know that the size of the skin correlates with the size of the atom. Piekarewicz believes the same can be assumed about the much bigger and denser neutron stars. “The dimensions of that skin, how it extends further, is something that correlates with the size of the neutron star,” said Piekarewicz, though he added that his estimates were rather preliminary. More work and advancements in technology would be needed for scientists to really get a proper grasp of neutron stars.
“There is no experiment that can probe the structure of the neutron star”
A sample-return mission to Saturn’s moon Titan could discover unexpected forms of life and bring back chemical compounds that cannot be found on Earth. A team of engineers from the NASA Glenn Research Center in Cleveland, Ohio, has recently received a $125,000 (£88,000) NASA Innovative Advanced Concepts (NIAC) grant to look into the feasibility of bringing a sample of material from this intriguing world to Earth.
“We expect landing on Titan to be relatively easy,” said Steven Oleson, head of the Compass
Lab at Glenn. “Titan has a thick atmosphere of nitrogen – 1.5 times the atmospheric pressure of Earth – which can slow the lander’s velocity with an aeroshell and a parachute for a soft landing, just like astronauts returning to Earth. With the landing out of the way, the next big challenge would be to get the sample up to space, on its way back to Earth. For that, the Glenn team envisions, the surface lakes of methane will come in handy.
“Producing rocket fuel on Titan wouldn’t require chemical processing – you just need a pipe and a pump,” said Oleson. “The methane is already in a liquid state, so it’s ready to go.” The Compass
Lab team will now investigate how to effectively produce liquid oxygen to enable the fuel to burn. Although the sample-return concept may never fly, Titan can expect a robotic visitor soon. Dragonfly is an eight-bladed rotorcraft scheduled to begin the eight-year journey to the Saturn system in 2027.