Helium-4

Helium-4 (4He) is a <a href="/facts/Stable_isotope/vgcdxgjA">stable isotope</a> of the element <a href="/facts/Helium/yRul93TG">helium</a>. It is by far the more abundant of the two naturally occurring <a href="/facts/Isotope/KD9B1y6o">isotopes</a> of helium, making up about 99.99986% of the helium on <a href="/facts/Earth/HLLmDfj0">Earth</a>. Its nucleus is identical to an <a href="/facts/Alpha_particle/dzNMs9WP">alpha particle</a>, and consists of two <a href="/facts/Proton/clCjepXP">protons</a> and two <a href="/facts/Neutron/oRCM1geb">neutrons</a>.
Helium-4 makes up about one quarter of the ordinary matter in the universe by mass, with almost all of the rest being <a href="/facts/Hydrogen/BoYHjl0J">hydrogen</a>. While <a href="/facts/Nuclear_fusion/hP0gLDqj">nuclear fusion</a> in <a href="/facts/Star/PFbd6ICx">stars</a> also produces helium-4, most of the helium-4 in the Sun and in the universe is thought to have been produced during the <a href="/facts/Big_Bang/I5T1vTnv">Big Bang</a>, known as "<a href="/facts/Big_Bang_nucleosynthesis/IvM4gbKH">primordial</a> helium". However, primordial helium-4 is largely absent from the Earth, having escaped during the high-temperature phase of Earth's formation. On Earth, most naturally occurring helium-4 is produced by the <a href="/facts/Alpha_decay/rCsHNIUE">alpha decay</a> of heavy elements in the Earth's crust, after the planet cooled and solidified.
When liquid helium-4 is cooled to below 2.17 <a href="/facts/Kelvin/E3trzC4C">K</a> (−270.98 °C), it becomes a <a href="/facts/Superfluid/QuePDujN">superfluid</a>, with properties very different from those of an ordinary liquid. For example, if <a href="/facts/Superfluid_helium-4/J0YToKcL">superfluid helium-4</a> is placed in an open vessel, a thin <a href="/facts/Rollin_film/NPsnga37">Rollin film</a> will climb the sides of the vessel, causing the liquid to escape. The total spin of the helium-4 nucleus is an integer (zero), making it a <a href="/facts/Boson/XtPFcjb5">boson</a>. The superfluid behavior is a manifestation of <a href="/facts/Bose%E2%80%93Einstein_condensation/76peCwfW">Bose–Einstein condensation</a>, which occurs only in collections of bosons. 
It is theorized that at 0.2 K and 50 atm, solid helium-4 may be a <a href="/facts/Superglass/lAmCrCH0">superglass</a> (an <a href="/facts/Amorphous_solid/YTIzmrR0">amorphous solid</a> exhibiting <a href="/facts/Superfluidity/QuePDujN">superfluidity</a>).

Helium-4 open-in-new

Helium-4