What Is A Red Giant?
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A red giant is a star in a late stage of its evolution that has exhausted hydrogen in its core. The star expands dramatically (often to tens of times its original size), cools to a reddish color, and becomes much more luminous. Aldebaran and Arcturus are well-known red giants visible to the naked eye.
A red giant is one of the most dramatic phases in a star's evolution. After spending billions of years steadily fusing hydrogen on the main sequence, a star expands enormously and cools as it begins burning helium and heavier elements. Many of the brightest stars in the night sky are red giants, including Aldebaran, Arcturus, and Pollux. Understanding red giants helps explain the life cycle of most stars in the universe.
What causes a red giant phase?
Hydrogen exhaustion in the star's core. According to NASA, stars spend most of their lives on the main sequence, fusing hydrogen into helium. When core hydrogen runs out, the core contracts and heats up, while hydrogen fusion shifts to a shell around the core. The increased energy causes the outer layers to expand dramatically. The expanded surface is much farther from the energy source, so it cools and turns red. This is the red giant phase.
How big can red giants get?
Enormous. A typical red giant can be 10 to 100 times the diameter of the original star. Aldebaran, the brightest star in Taurus, has a diameter about 45 times the Sun's. Arcturus in Bootes is about 25 times the Sun's diameter. If the Sun became a red giant of typical size, it would expand past Mercury's orbit. Despite the enormous expansion, mass stays roughly the same, so the density of the outer layers becomes incredibly low (less than air at sea level).
Are red giants the same as red supergiants?
No, they're different. Red giants form from stars roughly between 0.5 and 8 solar masses, the most common range. Red supergiants form from much more massive stars (typically 10+ solar masses) and reach even more extreme sizes. Betelgeuse in Orion is a famous red supergiant; placed at the Sun's position, it would extend past Jupiter's orbit. Red supergiants are short-lived and end in supernova explosions, unlike normal red giants.
What happens after the red giant phase?
Most red giants become white dwarfs. After spending time as a red giant, the star eventually runs out of helium to fuse. The outer layers are gently expelled into space, forming a planetary nebula, while the dense core remains as a white dwarf. This transition takes hundreds of thousands to millions of years. Stars more massive than about 8 solar masses follow a different path leading to supernova and a neutron star or black hole.
A red giant is a star in a late stage of its evolution after exhausting hydrogen in its core. The star expands dramatically (often 10 to 100 times its original size), cools to a reddish color, and becomes more luminous. Aldebaran, Arcturus, and Pollux are well-known red giants visible to the naked eye. Most red giants eventually become white dwarfs after expelling their outer layers as planetary nebulae. Red supergiants are a separate, more massive category.
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