Red Giants

Red Giants

What are Red giants and how are they formed?

After a few billion years the center of a star runs out of protons (nuclei of hydrogen atoms). What is left is a core or central region made of alphas (nuclei of helium atoms). The outer layers of the star still contain hydrogen, but they are not hot enough to fuse.

Because it has run out of fuel, the star begins to cool, and contract. The outer layers of the star fall inwards under gravity, and as they fall they heat up. A shell surrounding the central core becomes hot enough to fuse protons into alphas. So the star gains a new source of energy. The core of the star is now hotter than it was during its normal life and this heat causes the outer parts of the star to swell. The star becomes a giant. The radiation from the fusing shell has grown weak by the time it reaches the surface of the star. Weak radiation is red, so the star becomes a red giant.

Meanwhile inside the shell, the core of the star shrinks and heats up enough to fuse the helium nuclei together into even heavier ones. Among the commonest nuclei are carbon, nitrogen and oxygen. Heavier and heavier nuclei are created inside a red giant, the heaviest nearest the middle. At its center are iron nuclei.

These fusions release only a little more energy, so they keep the red giant burning for a little longer. But they do not produce as much energy as the fusion of protons. Iron nuclei cannot be used as fuel because they need to be given energy to make them fuse. So iron nuclei collect in the heart of a red giant star.

The Sun will run out of fuel and become a red giant in about 4 billion years. What do you think will happen to the Earth then?

As already mentioned, the star begins to swell as the core becomes even hotter than it was during its normal life. All planets closest the sun, including Earth, will burn up. This event will take billions of years to arrive so hopefully by then future humans will have figured out some way of escaping our solar system.

Layers of a Red Giant

Inside a Red Giant:

An international team of astronomers has for the first time figured out what goes on inside red giants. After examining what goes on below the surface of dozens of red giant stars, the scientists found that these objects have incredibly strong magnetic fields.

To peer inside these stars, they used a technique called asteroseismology, which uses waves on their surface to interpret the turbulence in the deeper layers. This is similar to medical ultrasounds which use sound waves to see within the human body. The scientists looked at two different types of wave: pressure waves from internal turbulence (akin to sound waves), and gravity waves which are driven by the buoyancy of the different layers. The different types can penetrate to different depths inside a star.

The findings will expand our understanding of the life and inner mechanisms of stars. The study focuses on a particular type of objects called red giant branch (RGB) stars.

RGB stars are objects in a late phase of stellar evolution. They form when a star of low to intermediate mass (from 0.3 to eight times the mass of the Sun) runs out of hydrogen to fuse in its core. When that happens, the star begins to contract under its own gravity. This free-fall contraction eventually generates enough pressure and heat to start fusion in a shell around the core. The sudden restart of fusion generates an outward force that makes the star swell. The newly formed red giant has a tenuous and inflated atmosphere, a dense helium core, and a radius even hundreds of times larger than the original star.

The dense core makes red giants perfect candidates for asteroseismology. The pressure waves do not bounce off the core, but they are actually transformed into gravity waves. These waves travel across the interior of the stars and cause the red giants to oscillate in different patterns. One of these patterns is called dipole mode and can be observed as one side of the star becomes brighter and the other becomes dimmer.

Red Giant Facts:

  • Stars that are about the size of our very own Sun end up as Red Giants at the end of their lives. As far as the fate of our Sun is concerned, it is not going to die anytime sooner than 5 billion years from now. So, we are pretty much safe unless of course a freak asteroid is sent hurling towards Earth for a catastrophic extinction-level impact before Sun’s life ends.
  • Stars that are 1/3rd to 8 times the size of sun end up as Red Giant Stars. Now stars are usually made up of a gas called hydrogen. At the core of a star, the every two atoms of hydrogen fuse together to form one atom of helium – a heavier gas than hydrogen. This fusion is called nuclear fusion reaction.
  • The problem with stars is simple, they fuse hydrogen into helium at their core (very center) and hence, helium has nowhere to go. After billions of years of fusion reaction, stars accumulate huge quantities of helium at their cores and gradually, hydrogen that made up a star is slowly burned out (we mean fused into helium).
  • During this whole lifespan, i.e. when hydrogen is fused into helium, an external pressure is created because of the fusion reaction. It is this external pressure that keeps stars from collapsing because of the immense gravitation at the core of the stars.
  • With no external pressure left, the stars now succumb to the gravitational pull of their cores. With strong gravity into play, the stars now star contracting in on their own core. So, the stars become tighter and smaller.
  • With the onset of the compression, the stars heat up gradually because more and more mass is getting packed in smaller and smaller space. The gradual build-up of temperature eventually reaches a point where the helium present in the stars can now start fusing together into carbon atoms and oxygen atoms.
  • This helium fusion can start with a sudden and explosive flash or it can start gradually and slowly. Exactly how the helium fusion starts is dependent on the mass of the star.
  • Again, fusion reaction releases energy. This massive amount of energy produced because of helium fusing into carbon is trapped inside and tries to go out. This leads to enormous pressure build-up which in turn starts pushing outwards. This leads the stars to expand.
  • Hydrogen is actually the fuel that keeps stars alive. When the hydrogen is gone, the stars have no hydrogen molecules to fuse. This is where the stars come to the end of their lives.