The solar radius is one way that astronomers use to classify stars in the Milky Way galaxy, and is a fundamental unit of size based on the radius of Earth's Sun, or the distance from its center to its outer surface. At approximately 432,164 miles (695,501 kilometers) in radius, the Sun is an average-sized yellow star in what is known as the main sequence in the Milky Way galaxy that comprises the overwhelming bulk of all stars there. While stellar evolution dictates that most stars spend 90% of their life spans in the main sequence range of stars, a small number of neutron stars also exist that can be as little as 12 miles (appx. 19 kilometers) in radius, which would translate to 0.00003 solar radii. In the Milky Way as of 2011, the largest star is the red hypergiant called VY Canis Majoris, which is larger than the Sun by about 1,950 times. By substituting the Sun with VY Canis Majoris in Earth's Solar System, it would have such an immense solar radius that it would encompass a region of space reaching all the way out to the orbit of the planet Saturn.
Most main sequence stars with a solar radius close to that of the Sun exist in the disk region of the Milky Way as opposed to its heavily-concentrated central bulge of stars. Main sequence stars range from tiny red dwarfs to yellow stars like the Sun, and blue giants. Red dwarfs are usually half the size of the Sun or smaller, and are the most common type of star in the Milky Way galaxy overall. The Solar System's nearest neighbor Alpha Centauri is a double star in a locked orbit with Proxima Centauri, a red dwarf, and Alpha Centauri has a solar radius of 1.227, making it slightly larger than the Sun. Blue giant stars are the upper end for stars in the main sequence and are between 10 to 100 solar radii in size.
Stars located in the outer disk regions of the Milky Way are referred to as Population I stars and are usually fairly young, with high concentrations of heavier elements like iron. The Sun is about 25,000 light years from the center of the galaxy, which is estimated to have a radius of about 50,000 light years. Other giant stars as well as red hypergiants like VY Canis Majoris or blue supergiants like Rigel, estimated to be at least 62 to 78 solar radii in size, exist in Population II regions such as galactic or globular clusters, as well as in the central bulge of the Milky Way. Galactic clusters contain usually about 1,000 of these largest stars by solar radius, and globular clusters can contain up to 1,000,000 such stars.
Though size is an important method for measuring stars, other factors like solar luminosity and solar mass are also important and can be inconsistent even if two stars are the same size. A red giant star like Betelgeuse with a solar radius of 1,180 has a solar mass so light that its surface density is less than the atmosphere on Earth. In contrast, a small matchbox volume of matter from a typical white dwarf star would weigh more than one ton on Earth.
An important diagram for classifying stars by spectral class or temperature against absolute brightness is the Hertzsprung–Russell Diagram. The H-R diagram ranks stars by decreasing temperature in letters: O, B, A, F, G, K, and M. The Sun is classified as a G-type star in this range and stars in the F or K range are also considered to be among the most stable of stars, with possible life-supporting planets in orbit around them. F-type stars like Canopus and Procyon have a solar radius on average of 1.7, and K-type stars like Aldebaran have an average solar radius of 0.8.
