Ask anyone to name the most important molecule and the answer will undoubtedly be DNA -- the molecule of life. Most people are also familiar with the general structure of DNA (Figure 1). They know that it is a double helix. In other words, it looks a bit like a twisted ladder, in which the sides are composed of sugars and phosphates, while each rung is formed by a pair of two bases. The base adenine is always attached to thymine, and guanine to cytosine. The molecules of many proteins also contain a helical substructure known as the alpha helix (Figure 2). But helices appear not only on the tiny scales of molecules.
Amazingly, some astrophysical jets -- streams of charged particles collimated and accelerated over astronomical distances -- also exhibit a helical structure. One such jet emanates from the Vela pulsar. The Vela pulsar resulted from the collapse of the core of a massive star more than 10,000 years ago. The collapsed core formed a neutron star, an extremely compact and highly magnetized object with roughly the mass of the Sun, but with a radius of only about six miles. The pulsar spins around its axis, making more than 11 complete rotations every second. Acting like a high-voltage generator, it powers a jet that is more than half a light-year long, along which charged particles race at about 70 percent of the speed of light. The intriguing feature is that the jet appears to be whipping around like a loose fire hose, creating the helical structure (Figure 3).
Figure 1. A schematic structure of the double helix of DNA. Credit: Pam Jeffreys.
Figure 2. A schematic of the alpha helix, that characterizes the structure of many proteins. Credit: Pam Jeffreys.
Figure 3. The Vela Pulsar's jet. Credit: G. Pavlov, M. Teter, O. Kargaltsev, D. Sanwal (PSU), CXC, NASA.
The Vela pulsar is not the only astronomical object exhibiting a helical jet. Evidence for such jets has been observed in other pulsars and in active galactic nuclei. The latter represent supermassive black holes at the centers of galaxies. The black holes accrete mass from their vicinity, and the energy that is being released in the accretion disks that surround the black holes powers the spectacular jets. It is believed that the helical structure is one of the important pieces of evidence pointing to the major role played by magnetic fields in the formation of these jets. In fact, elementary electrodynamics shows that charged particles move along a helical path in a uniform magnetic field.
Helices thus provide one more manifestation of the remarkable fact that geometrical shapes that can be described by simple mathematics lie at the core of phenomena ranging from molecular to cosmic scales.