Scientists Recreate Cause Of Fusion Inside Massive Stars

Solar disk from the NASA Earth Observatory. NASA

The energy of stars comes from nuclear fusion. At their cores, under incredible pressure and temperature, lighter elements like hydrogen and helium are fused into heavier ones. These conditions are extreme and it's only now that researchers are getting close to replicating them.

As reported in Nature Physics, American researchers have reproduced what’s happening inside massive stars (those that are 10-40 times the mass of the Sun) by compressing hydrogen to 1,000 times the density of lead and a temperature of 50 million°C (about 90 million°F). In these conditions, hydrogen is turned into an extremely dense and hot plasma. This allowed for nucleosynthesis, the formation of different elements from the fusion of simpler ones.

"Ordinarily, these kinds of nuclear astrophysics experiments are performed on accelerator experiments in the laboratory, which become particularly challenging at the low energies often relevant for nucleosynthesis," lead author Dan Casey, from the Lawrence Livermore National Laboratory, said in a statement.

From the experiment, the researchers were able to measure a very important quantity, called the nuclear reaction cross-section. This is the probability that the atoms in the plasma will actually undergo fusion reactions. In previous experiments, estimating the cross-section has been challenging. The measurements required corrections as they were plagued by several background sources of noise.

To achieve stellar-like plasma, researchers have also used particle accelerators. In that setup, the plasma was achieved by collisions of atoms moving at almost the speed of light. This experiment was conducted at the National Ignition Facility, where a gas-filled capsule was imploded using lasers bringing it to the plasma state.

"One of the most important findings is that we reproduced prior measurements made on accelerators in radically different conditions. This really establishes a new tool in the nuclear astrophysics field for studying various processes and reactions that may be difficult to access any other ways," Casey explained.

"Perhaps most importantly, this work lays the groundwork for potential experimental tests of phenomena that can only be found in the extreme plasma conditions of stellar interiors. One example is of plasma electron screening, a process that is important in nucleosynthesis but has not been observed experimentally."

Now that the team has established an appropriate technique, it will now plan more experiments at the National Ignition Facility.

The Sun (left) and the NIF plasma (right), obviously not to scale. LLNL

 

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