A giant star is a star with substantially larger radius and luminosity than a main-sequence (or dwarf) star of the same surface temperature.
They lie above the main sequence (luminosity class V in the Yerkes spectral classification) on the Hertzsprung-Russell diagram and correspond to luminosity classes II and III. The terms giant and dwarf were coined for stars of quite different luminosity despite similar temperature or spectral type by Ejnar Hertzsprung about 1905.
Giant stars have radii up to a few hundred times the Sun and luminosities between 10 and a few thousand times that of the Sun. Stars still more luminous than giants are referred to as supergiants and hypergiants.
A hot, luminous main-sequence star may also be referred to as a giant, but any main sequence star is properly called a dwarf no matter how large and luminous it is.A star becomes a giant star after all the hydrogen available for fusion at its core has been depleted and, as a result, leaves the main sequence.
The behaviour of a post-main-sequence star depends largely on its mass. Main-sequence stars with masses above about 12 solar masses are already very luminous and they move horizontally across the HR diagram when they leave the main sequence, briefly becoming blue giants before they expand further into blue supergiants.
They start core-helium burning before the core becomes degenerate and develop smoothly into red supergiants without a strong increase in luminosity.
A star whose initial mass is less than approximately 0.25 solar masses will not become a giant star at all. For most of their lifetimes, such stars have their interior thoroughly mixed by convection and so they can continue fusing hydrogen for a time in excess of 1012 years, much longer than the current age of the Universe.
They steadily become hotter and more luminous throughout this time. Supergiants are among the most massive and most luminous stars with temperatures from about 3,500 Kelvin to over 20,000 Kelvin, masses from 8 to 12 times the Sun upwards, and luminosities from about 1,000 to over a million times the Sun.
They vary greatly in radius, usually from 30 to 500, or even in excess of 1,000 solar radii. They go on to successively ignite heavier elements, usually all the way to iron. Also because of their high masses they are destined to explode as supernovae.
A hypergiant is a star with an enormous luminosity showing signs of a very high rate of mass loss. Stars with an initial mass above about 25 solar masses quickly move away from the main sequence and increase somewhat in luminosity to become blue supergiants.
They cool and enlarge at approximately constant luminosity to become a red supergiant, then contract and increase in temperature as the outer layers are blown away. They explode as a supernova or completely shed their outer layers to become a Wolf-Rayet star.
Stars with an initial mass above about 40 solar masses are simply too luminous to develop a stable extended atmosphere and so they never cool sufficiently to become red supergiants. Hypergiants are difficult to study due to their rarity.
Because of their high masses, the lifetime of a hypergiant is very short in astronomical timescales: only a few million years compared to around 10 billion years for stars like the Sun. Hypergiants are only created in the largest and densest areas of star formation and because of their short lives, only a small number are known despite their extreme luminosity.
History of the Future
There are some habitables planets possible at Giant Stars. But it is very dangerous to approach a giant star, the temperatures will rise quickly above the ship’s hull / shield limits when you’re going too close, causing its total destruction.