The word “spicule” describes a pointed or needlelike structure. It is used in several different contexts in the sciences. Spicules can be observed on a microscopic and a macroscopic level, everywhere from the ocean floor to the Sun. The type of spicule under discussion is usually clear from the context of the discussion.
In biology, spicules are used by many invertebrate animals to support themselves and strengthen their skeletal structure. They are composed of a variety of materials, depending on the organization making the spicule, and they can be found in a variety of locations. Sponges are a classic example of an organism which uses spicules. Upon magnification, the tiny crystalline structures can be seen arrayed in a variety of patterns. Symmetry is often exhibited, and different sponge species can have different arrays.
Nematodes also sometimes have spicules, depending on the species. Even vertebrates such as frogs may develop these structures. Examination under a microscope is usually necessary to identify a spicule, as these structures are typically very small when they are found on animals. The function of the spicules also varies, with some providing support, others providing traction, and others serving a function in the reproductive tract, for example.
On the other end of the scale is the solar spicule, a burst of plasma which can explode from the surface of the Sun at an extremely rapid rate. The spicule is usually accompanied with rapid fluctuation in the Sun's magnetic field, and it can be viewed with a high powered telescope or similar observing instrument, with some of the best images coming from devices in orbit. Solar spicules can be around 300 miles (500 kilometers) wide, illustrating how impressive they are, and how distant the Sun is from Earth, as people cannot see them with their naked eyes, even were they to risk permanent eye damage by staring directly at the Sun.
Several hypotheses have been developed to explain how and why solar spicules form, to learn more about the Sun and similar stars. As of 2009, there was no firm consensus among astronomers about these tremendous releases of energy, suggesting that more observation is needed, along with study from probes which could take readings to gather information about the conditions on and around the Sun. Such observations can be challenging to collect, because the Sun can easily damage or destroy probes which get too close to its extremely hot and highly charged surface.