UY Scuti, The Largest Known Star.
Our own Sun is a class G yellow dwarf star about half way through its ten billion year lifespan. It has a diameter about 109 times that of Earth, or about 865 thousand miles. UY Scuti, however, is a red supergiant star 9,500 light years from Earth, the 38th variable star in the constellation of Scutum. It is the largest star known to exist, with a diameter of 1.5 billion miles; 1,700 times wider than the Sun; thus a volume nearly 5 billion times that of the Sun. Placed in our own solar system, it would extend far beyond the orbit of Jupiter and possibly even engulf the orbit of Saturn.
UY Scuti was first cataloged in 1860 by German astronomers at the Bonn Observatory during the first sky survey of stars for the Bonner Durchmusterung Stellar Catalogue. It was named BD -12 5055, the 5,055th star between 12°S and 13°S counting from 0h right ascension. It is located a few degrees north of the A-type naked-eye star Gamma Scuti and northeast of the Eagle Nebula, a 9th magnitude star.
Despite its large size, UY Scuti is not classified as a hypergiant. More commonly, hypergiants are classed as 1a-0, 1a+, or even just 1ae based solely on the observed spectra, and red supergiants rarely receive these extra spectral classifications. A high luminosity and large size are insufficient for it to be defined as a hypergiant. That requires the detection of the spectral signatures of atmospheric instability and high mass loss. In the case of UY Scuti, its spectrum has the presence of spectral lines of carbon, water, and silicon oxide, but it does not show any spectral lines of oxygen, neon, and other heavier elements, indication of an insufficient mass loss rate. Furthermore, its location in the Hertzsprung–Russell diagram is below the oval region of hypergiants, making it only classified as a bright-red supergiant.
Based on current models of stellar evolution, UY Scuti has begun to fuse helium and continues to fuse hydrogen in a shell around the core. The location of UY Scuti deep within the Milky Way disc suggests that it is a metal-rich star. It should fuse lithium, carbon, oxygen, neon, and silicon in its core within the next million years. After this, its core will begin to produce iron, disrupting the balance of gravity and radiation in its core and resulting in a core collapse supernova, creating a strong stellar wind that will eject its outer layers and expose the core, before exploding as a type IIb, IIn, or type Ib/Ic supernova.