Stars in the Milky Way galaxy are classified by their solar radius, with the Sun being a medium-sized yellow star in the main sequence. Neutron stars can have a radius of just 12 miles, while the largest star is VY Canis Majoris, which is 1,950 times larger than the Sun. Most main-sequence stars are in the Milky Way’s disk region, with red dwarfs being the most common type of star. Stars in the outer disk are referred to as Population I stars and are usually young with high concentrations of heavier elements. The Hertzsprung-Russell diagram is used to classify stars by spectral class or temperature versus absolute luminosity.
The solar radius is one way astronomers use to classify the stars in the Milky Way galaxy, and it’s a fundamental unit of measurement based on the radius of Earth’s sun, or the distance from its center to its outer surface. At about 432,164 miles (695,501 kilometers) in radius, the Sun is a medium-sized yellow star in what is known as the main sequence in the Milky Way galaxy which includes the vast majority of all stars there. While stellar evolution dictates that most stars spend 90% of their lifetimes in the main-sequence range of stars, there are also a small number of neutron stars that may have a radius of as little as 12 miles (approximately 19 kilometers), 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 about 1,950 times larger than the Sun. Replacing the Sun with VY Canis Majoris in Earth’s Solar System, it would have a solar radius so immense that it would encompass a region of space reaching up to the orbit of the planet Saturn.
Most of the main-sequence stars with a solar radius close to that of the Sun exist in the Milky Way’s disk region in contrast 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 closest 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. The blue giant stars are the upper edge of stars in the main sequence and are between 10 and 100 solar radii in size.
Stars located in regions of the Milky Way’s outer disk are referred to as Population I stars and are usually quite young, with high concentrations of heavier elements such as 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 and red hypergiants such as VY Canis Majoris or blue supergiants such as Rigel, estimated to be at least 62-78 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 usually contain about 1,000 of these largest stars per solar radius, and globular clusters can contain up to 1,000,000 such stars.
While size is an important method of measuring stars, other factors such as 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 such a light solar mass that its surface density is less than that of Earth’s atmosphere. In contrast, a small volume of matter from a matchbox of a typical white dwarf star would weigh more than a ton on Earth.
An important diagram for classifying stars by spectral class or temperature versus absolute luminosity is the Hertzsprung-Russell diagram. The HR diagram classifies stars by decreasing temperature into the 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 stars, with possible life-sustaining planets orbiting them. F-type stars such as Canopus and Procyon have an average solar radius of 1.7, and K-type stars such as Aldebaran have an average solar radius of 0.8.
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