Dwarf star

A dwarf star refers to any star with an average or relatively low size, mass, and luminosity. The sun is a dwarf star, specifically a yellow dwarf main sequence star. Coined by Ejnar Hertzsprung in 1906, he used the term "dwarf star" to distinguish between prominent K- and M-type (spectral classes) stars that are brighter or dimmer than the sun. Those that are bigger and much brighter are referred to as "giant stars" and the small/fainter stars are called "dwarf stars."

As the majority of main sequence stars are dwarf stars, the phrases can sometimes be used interchangeably. Main sequence dwarf stars are yellow dwarfs, orange dwarfs, and red dwarfs; they reside within the luminosity class V. However, the word "dwarf" is also used to describe non-main sequence stars in the later stages of stellar evolution such as white dwarfs. "Dwarf" is also used for the class of substellar objects known as brown dwarfs.

Stars can be classified using a range of different parameters. The most common way of classifying stars is via their spectra (the element that they absorb) and their temperature. This separates stars into seven main types:

Stellar spectral classes with O being the hottest (surface temperature) and M the coolest.

This is known as the Harvard classification system, with blue or O being the hottest and red or M being the coolest, based on surface temperature. The Morgan-Keenan system separates each class assigning numbers with 0 being the hottest and 9 the coolest.

Another parameter used to distinguish between stars is luminosity, with the following ten classes:

Hertzsprung-Russell Diagram.

The Hertzsprung-Russell (H-R) diagram plots star color (spectral type or surface temperature) vs. luminosity (intrinsic brightness or absolute magnitude). The H-R diagram allows astronomers to plot the color, temperature, luminosity, spectral type, and evolutionary stage of each star. It shows three distinct types of stars:

• Main sequence stars fueled by nuclear fusion (converting hydrogen into helium). The hotter a main-sequence star is, the brighter it is (correlation from top left to bottom right). The most stable part of a star's existence, roughly 90% of observable stars are classed as main sequence stars.
• As stars age and the level of hydrogen in the core drops, they begin to die and become giants/supergiants (above the main sequence). The core contracts and the outer layers expand. Depending on their mass, these stars will explode and become a planetary nebula or supernova, and then become neutron stars, black holes, or shrink into white dwarfs.
• White dwarfs can be seen below the main sequence. These hot but faint shrinking stars will eventually cool and become black dwarfs.

Types of dwarf stars in the main sequence include the following:

• Yellow dwarf—also known as a G dwarf star, they are within the luminosity class V in the main sequence. The mass of a yellow dwarf equates to roughly 0.84 to 1.15 solar masses, and its surface temperature ranges from 5,300K to 6,000K.
• Orange dwarf—also known as a K-type main sequence star. Orange dwarfs have a mass between 0.5 and 0.8 solar masses and a surface temperature between 3,900K and 5,200K.
• Red dwarf—also known as an M-type main sequence star. They make up the lower end of the main sequence band with a mass up to 0.8 solar masses and a maximum temperature of 5,200K. Approximately three-quarters of the stars in the Milky Way are red dwarfs, although their low luminosity means they are not visible to the naked eye.

Later stage dwarf stars, include the following:

• Blue dwarf—the theoretical remnant of a red dwarf star when the hydrogen in their core is spent and their luminosity increases.
• White dwarf—star remnant made of electron-degenerate matter. White dwarfs have an estimated mass of 0.17 to 1.33 solar masses. Observed white dwarfs typically have a surface temperature between 8,000K and 40,000K.
• Black dwarf—theoretical end-stage of a white dwarf as it cools and no longer emits light.

Substellar objects, known as brown dwarfs, are neither a star or a planet. they are often referred to as "failed stars" as they form in a similar manner but were not massive enough to support hydrogen to helium fusion. the mass of a brown dwarf is roughly 0.08 that of the sun, or 13 to 80 times that of Jupiter.