How can we study astronomy in laboratories on Earth?
Instead of using telescopes to look at astronomical objects, a new field of astronomy called laboratory astrophysics tries to recreate these same astronomical objects in laboratories on Earth. The scientists who study the universe in this way actually create miniature stars in their labs that can help answer a lot of questions we have about stars. Perhaps the most important reason for studying astronomy in this way is that scientists can control many aspects of the experiment. For example, astronomers who watch the sky may be surprised when a star explodes in a supernova, but scientists creating stars in their labs can plan an event like this and be ready to take data.
What is fusion and how does it help us understand the stars?
Stars are made out of a material called plasma, which is a very hot and dense form of matter. Plasma can be made in a laboratory by using a variety of methods, but details of the experiment are different if a miniature star is to be created. The method used to create these stars is called inertial confinement fusion (ICF). Fusion is the process that powers the stars. Simply defined, it is the joining of two nuclei of light atoms in order to produce a larger nucleus and release energy. A lot of energy must be applied to the system in order for the light atoms to smash together. The term inertial confinement fusion refers to a way of containing the fusion reaction. In stars, the fusion reaction is contained because of the huge gravitational forces present in the body. On Earth, however, it is necessary to find a way to hold the plasma together long enough to sustain such an intense reaction. The ICF method uses the idea of inertia, in which the shock waves compress the fuel. Once a miniature star is formed in this way, it only exists for less than a second. But if the scientists can record all of the things happening to the plasma, then they have a way to study the properties of the stars!
How are these laboratory experiments powered?
There must be some type of energy applied to the fusion target in order for the reaction to begin. Intense lasers are used to provide the “kick” which starts the fusion. Very powerful laser beams are focused onto the surface of the round fusion target filled with fuel. The laser energy vaporizes the solid surface of the capsule, and shock waves travel inward and compress the target until the fuel becomes a plasma of about 100 million degrees Celsius. When the plasma is hot enough, the fuel begins to fuse, which gives off more energy. The scientists now have a tiny star to study!
What do the targets look like?
One type of ICF, called indirect drive fusion, uses a gold cylinder-shaped shell that is called a hohlraum. The fusion target is placed inside this shell, and in this method, the laser beams hit the outside of the hohlraum rather than directly on the surface of the target. As the hohlraum is heated by the laser beams, the inner surface emits x-rays which compress the fuel.
What kind of astronomical objects can scientists study with these experiments?
By changing details of these experiments, such as the size of the target or the amount of energy in the laser beams, different situations can be modeled. Scientists study the properties of supernovae, which are stars that suddenly explode at the end of their lifetime. They can also study the remnants of these supernovae in order to understand how nebulae are formed. Studying these astronomical events in the lab experiments gives a new outlook to the phenomena and can help shed light on details that are hard to see with telescopes. The great benefits of being able to ‘build-your-own supernova’ are obvious: instead of having to wait for decades until an appropriate astronomical event occurs, it is possible to recreate these explosions on a very small scale in order to investigate the actual astronomical process.
Astronomical events like nebula formation take thousands of years to develop. So how can we hope to study this type of event in a laboratory on Earth within our lifetimes?
Think of the miniature star created in the laboratory as a "scale model" of the real star in the sky. The miniature star is of course much smaller, and the experiments are designed so that events that happen over a large period of time in the sky are represented by much shorter periods in the Earth experiments. For example, in a certain experiment designed to study supernova remnants, a distance of 100 mm in the lab experiment corresponded to a distance of 0.03 light years in the sky, and a time of 1 nanosecond in the experiment corresponded to a time of 1 year in the sky.
REFERENCES FOR THIS PAGE:
B. Remington, R. Drake, H. Takabe, D. Arnett. “A review of astrophysics experiments on intense lasers.” Physics of Plasmas, Volume 7, Number 5, 2000.
NOVA Laser Tour (page no longer active). http://lasers.llnl.gov/lasers/target/novatour/
What is Fusion? (page no longer active) http://www.lanl.gov/ICF/ICFIntro.html