Dwarf: Definition & Type

Type of Dwarf

A star is a giant ball of extremely hot gas (plasma), sustained by nuclear fusion which turns hydrogen into helium. All this energy produces heat, light and bigger chemical elements. A star emits electromagnetic radiation that moves away from the star as light. The large mass of a star holds it together. Old stars change helium into other elements like carbon and oxygen.

Yellow dwarfs - main-sequence stars like our own Sun, Alpha Centauri A, Tau Ceti, etc, typically about 80 - 100% of the size of the Sun, and actually more white than yellow. They are also known as G V stars for their spectral type G and luminosity class V.

White stars - bright, main-sequence  stars  with  masses from 1.4 to 2.1 times the mass of the Sun and surface temperatures between 7,600°C and 10,000°C, such as Sirius A and Vega.

Red giants - luminous giant stars of low or intermediate mass (usually between 0.5 and 10 solar masses) in a late phase of stellar evolution, such as Aldeberan and Arcturus. When a main-sequence  star has fused all its hydrogen into helium, it then starts to burn its helium to produce carbon and oxygen, and

expands to many times its previous volume to become a red giant. After a relatively short time (in the region of two hundred million years), the red giant puffs out its outer layers in a gas cloud called a nebula and collapses in on itself to form a white dwarf. The largest red giants are known as red supergiants, and are the largest stars in the universe in terms of volume (well-known examples are Antares and Betelgeuse).

White dwarfs - small, dense, burnt-out husks of stars, no longer undergoing fusion reactions, and representing the final evolutionary state of most of the stars in our galaxy. When a red giant has used up its helium to produce carbon and oxygen, and has insufficient mass to generate the core temperatures required to fuse carbon, it sheds its outer layers to form a planetary nebula, and leaves behind an inert mass of carbon and oxygen. A white dwarf is typically only the size of the Earth, but 200,000 times more dense.

Black dwarfs - hypothetical stellar remnants created when a white dwarf becomes cool and dark after about ten billion years of life. Black dwarfs are very hard to detect, and very few would exist yet anyway in a universe only 13.7 billion years old.


Blue giants - bright, giant stars, between 10 and 100 times the size of the Sun, and between 10 and 1,000 times its luminosity. Because of their mass and hotness, they are relatively short-lived and quickly exhaust their hydrogen fuel, ending as red supergiants or neutron stars. The biggest and most luminous stars are referred to as blue supergiants and hypergiants. The best known blue supergiant is Rigel, the brightest star in the constellation of Orion, which has a mass about 20 times that of the Sun and a luminosity more than 60,000 times greater. The biggest and brightest ever found is 10 million times as bright as the Sun.

Brown dwarfs - "failed stars", which form from clouds of interstellar gas, as other stars do, but never reach sufficient mass, density and internal heat to start the nuclear fusion process (i.e. less than 8% of the mass of our Sun). Although they may glow dimly when newly formed (and are therefore in fact red not brown), they start to cool soon after and so are very difficult to spot. They may actually be among the most common type of stars.