Red giant is a luminous giant star of low or intermediate mass. Its mass usually ranges between 0.3 to eight times that of our Sun. These stars are usually in a very late phase of stellar evolution. Red giants have radii tens to hundreds of times larger than that of the Sun. Their outer envelope is lower in temperature, about 5,000 K and below.
Formation
When a star initially forms from a collapsing molecular cloud in the interstellar medium, it primarily contains hydrogen and helium, with trace amounts of “metals”, i.e., any element heavier than helium. When the star exhausts the hydrogen fuel in its core, nuclear reactions can no longer continue, and thus the core begins contracting due to its own gravity. This causes the remaining hydrogen to undergo fusion in a shell around the core at a faster rate. The outer layers of the star then expand greatly. This begins the red giant phase of a star’s life.
Colour and naming
Since the expansion of the star greatly increases its surface area, red giants tend to be cooler and burn with an orange hue. Despite their name, they are closer to orange in reality. The M-type stars HD 208527, HD 220074 and K-giants including Pollux, Gamma Cephei and lota Draconis are some examples of red giants with planets.
Life around red giants
It has traditionally been suggested that life could not evolve on planets orbiting them. However, current research suggests that there would be a habitable zone at twice the distance from Earth to Sun for a billion years. At a distance of nine AU, such a habitable zone would only exist for 100 million years. As of June 2014, 50 giant planets have been discovered around the giant stars. These giants are much larger than those found around sun-sized stars.
- compiled by Madhurani Chavan