Helium was discovered in the gaseous atmosphere surrounding the Sun by the French astronomer Pierre Janssen (Janssen, Pierre), who detected a bright yellow line in the spectrum of the solar chromosphere during an eclipse in 1868; this line was initially assumed to represent the element sodium. That same year, the English astronomer Joseph Norman Lockyer (Lockyer, Sir Joseph Norman) observed a yellow line in the solar spectrum that did not correspond to the known D₁ and D₂ lines of sodium, and so he named it the D₃ line. Lockyer concluded that the D₃ line was caused by an element in the Sun that was unknown on Earth; he and the chemist Edward Frankland (Frankland, Sir Edward) used the Greek word for sun, hēlios, in naming the element. The British chemist Sir William Ramsay (Ramsay, Sir William) discovered the existence of helium on Earth in 1895. Ramsay obtained a sample of the uranium-bearing mineral cleveite, and upon investigating the gas produced by heating the sample, he found that a unique bright-yellow line in its spectrum matched that of the D₃ line observed in the spectrum of the Sun; the new element of helium was thus conclusively identified. In 1903 Ramsay and Frederick Soddy (Soddy, Frederick) further determined that helium is a product of the spontaneous disintegration of radioactive (radioactivity) substances.

Helium constitutes about 23 percent of the mass of the universe and is thus second in abundance to hydrogen in the cosmos. Helium is concentrated in stars (star), where it is synthesized from hydrogen by nuclear fusion. Although helium occurs in Earth's atmosphere only to the extent of 1 part in 200,000 (0.0005 percent), and small amounts occur in radioactive minerals, meteoric iron, and mineral springs, great volumes of helium are found as a component (up to 7.6 percent) in natural gases (natural gas) in the United States (especially in Texas, New Mexico, Kansas, Oklahoma, Arizona, and Utah). Smaller supplies have been discovered in Canada and South Africa and in the Sahara Desert. Ordinary air contains about 5 parts per million of helium, and Earth's crust is only about 8 parts per billion.

The nucleus of every helium atom contains two protons (proton), but as is the case with all elements, isotopes (isotope) of helium exist. The known isotopes of helium contain from one to six neutrons (neutron), so their mass numbers range from three to eight. Of these six isotopes, only those with mass numbers of three (helium-3, or ³He) and four (helium-4, or ⁴He) are stable; all the others are radioactive, decaying very rapidly into other substances. The helium that is present on Earth is not a primordial component but has been generated by radioactive decay. Alpha particles (alpha particle), ejected from the nuclei of heavier radioactive substances, are nuclei of the isotope helium-4. Helium does not accumulate in large quantities in the atmosphere because Earth's gravity is not sufficient to prevent its gradual escape into space. The trace of the isotope helium-3 on Earth is attributable to the negative beta decay of the rare hydrogen-3 isotope ( tritium). Helium-4 is by far the most plentiful of the stable isotopes: helium-4 atoms outnumber those of helium-3 about 700,000:1 in atmospheric helium and about 7,000,000:1 in certain helium-bearing minerals.

A liquid mixture of the two isotopes helium-3 and helium-4 separates at temperatures below about 0.8 K (−272.4 °C, or −458.2 °F) into two layers. One layer is practically pure helium-3; the other is mostly helium-4 but retains about 6 percent helium-3 even at the lowest temperatures achieved. The dissolution of helium-3 in helium-4 is accompanied by a cooling effect that has been used in the construction of cryostats (devices for production of very low temperatures) that can attain—and maintain for days—temperatures as low as 0.01 K (−273.14 °C, or −459.65 °F).

Helium gas (98.2 percent pure) is isolated from natural gas by liquefying the other components at low temperatures and under high pressures. Adsorption of other gases on cooled, activated charcoal yields 99.995 percent pure helium. Some helium is supplied from liquefaction of air on a large scale; the amount of helium obtainable from 1,000 tons (900 metric tons) of air is about 112 cubic feet (3.17 cubic metres), as measured at room temperature and at normal atmospheric pressure.

Helium is used as an inert-gas atmosphere for welding metals such as aluminum; in rocket propulsion (to pressurize fuel tanks, especially those for liquid hydrogen, because only helium is still a gas at liquid-hydrogen temperature); in meteorology (as a lifting gas for instrument-carrying balloons (balloon)); in cryogenics (as a coolant because liquid helium is the coldest substance); and in high-pressure breathing operations (mixed with oxygen, as in scuba diving and caisson work, especially because of its low solubility in the bloodstream). Meteorites (meteorite) and rocks have been analyzed for helium content as a means of dating.