stylistic:tone
Helium-4 (⁴He) is a stable isotope of the element helium. It is by far the more abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on Earth. Its nucleus is identical to an alpha particle, and consists of two protons and two neutrons. Alpha decay of heavy elements in the Earth's crust is the source of most naturally occurring helium-4 on Earth, produced after the planet cooled and solidified. While it is also produced by nuclear fusion in stars, most helium-4 in the Sun and in the universe is thought to have been produced by the Big Bang, and is referred to as "primordial helium". However, primordial helium-4 is largely absent from the Earth, having escaped during the high-temperature phase of Earth's formation. Helium-4 makes up about one quarter of the ordinary matter in the universe by mass, with almost all of the rest being hydrogen. When liquid helium-4 is cooled to below 2.17 K (−270.98 °C), it becomes a superfluid, with properties that are very unlike those of an ordinary liquid. For example, if superfluid helium-4 is kept in an open vessel, a thin film will climb up the sides of the vessel and overflow. In this state and situation, it is called a "Rollin film". This strange behavior is a result of the Clausius–Clapeyron relation and cannot be explained by the current model of classical mechanics, nor by nuclear or electrical models — it can only be understood as a quantum-mechanical phenomenon. The total spin of the helium-4 nucleus is an integer (zero), and therefore it is a boson. The superfluid behavior is now understood to be a manifestation of Bose–Einstein condensation, which occurs only with collections of bosons. It is theorized that at 0.2 K and 50 atm, solid helium-4 may be a superglass. Helium-4 also exists on the Moon and — as on Earth — it is the most abundant helium isotope.