Exotic helium isotopes are the unstable isotopes of helium. A subset of exotic light nuclei, these synthetic radioactive isotopes have larger atomic weights than helium's natural isotopes, helium-3 and helium-4.
Although all exotic helium isotopes decay with a half-life of less than one second, researchers have eagerly created exotic light isotopes though particle accelerator collisions to create unusual atomic nuclei for elements such as helium, lithium, and nitrogen. The bizarre nuclear structures of such isotopes may offer insight into the isolated properties of neutrons.
The most widely-studied exotic helium isotope, for example, is helium-8. This isotope is thought to consist of a normal helium-4 nucleus surrounded by four neutrons dubbed a "halo." Halo nuclei have become an area of intense research. Isotopes up to helium-10, with two protons and eight neutrons, have been confirmed. By comparison, the most common 4He isotope has only two neutrons.
Helium gas is used to fill the space between lenses in some telescopes because its extremely low index of refraction reduces the distorting effect of temperature variations in the gas filling the telescope (some telescopes use vacuum in the telescope instead).
Helium was first detected on August 18, 1868 as a bright yellow line with a wavelength of 587.49 nm in the spectrum of the chromosphere of the Sun, by French astronomer Pierre Janssen during a total solar eclipse in Guntur City (India).
Helium is the second most abundant element in the known Universe after hydrogen and constitutes 23% of all elemental matter measured by mass ('elemental matter' does not include dark matter or dark energy, which together may account for 96% of the Universe).
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