Over time, the forces acting on the star become unbalanced. When the inward gravitational forces are less than the outward radiation pressure forces, the star swells and cools, thus turning red.
High mass stars become red supergiants , low mass stars become red giants. The forces become unbalanced when the hydrogen begins to run out. The star begins to fuse helium and then increasingly heavier elements to maintain fusion. When iron is formed in the core of the star, nuclear fusion stops and the star contracts under its gravity. When helium runs out, the star will not be dense enough to form other heavy elements like iron, thus the fusion process will stop, and the star will collapse on its core due to inward acting gravity.
This happens because there is no longer any fusion energy to stabilize gravity. Red giant may eventually become white dwarfs, a cool and extremely dense star, with its size being shrunk several times, to that of a planet even. Because of this change in temperature, the star begins to shine in the redder part of the spectrum, leading to the name red giant, though they are often more orange in appearance.
Red giants stars remain in this stage from a few thousand to 1 billion years. They eventually run out of helium in their cores and thus fusion stops.
This causes the star to shrink until a new helium shell reaches its core. When the helium ignites, the outer layers of the star are blown off in huge clouds of gas and dust known as planetary nebulae. These shells are much larger and fainter than their parent stars. Red giants evolve out of main-sequence stars that have masses in the range from around 0.
Stars initially form from collapsing molecular clouds in the interstellar medium. These clouds contain hydrogen and helium, with trace amounts of metals, and all of these elements are uniformly mixed throughout the star. The star reaches the main sequence when the core reaches a temperature high enough to begin fusing hydrogen and establishes hydrostatic equilibrium.
When the hydrogen supplies are exhausted, nuclear reactions can no longer continue and thus the core begins to contract due to its own gravity. This brings additional hydrogen into a zone where the temperature and pressure are sufficient to cause fusion to resume in a shell around the core.
The hydrogen-burning shell results in a situation that has been described as the mirror principle, when the core within the shell contracts, the layers of the star outside the shell must expand. The evolutionary path the star takes as it moves along the red-giant phase depends solely on its mass. For example, the Sun and stars of less than 2 solar masses, the core will become dense enough that electron degeneracy pressure will prevent it from collapsing further.
Stellar Evolution. Latest Gallery Images. Contact Us Privacy Policy Proud to be part of. All Rights Reserved. We saw in Radiation and Spectra that a red color corresponds to cooler temperature.
So the star becomes simultaneously more luminous and cooler. On the H—R diagram, the star therefore leaves the main-sequence band and moves upward brighter and to the right cooler surface temperature.
Over time, massive stars become red supergiants, and lower-mass stars like the Sun become red giants. Note that red giant stars do not actually look deep red; their colors are more like orange or orange-red.
Just how different are these red giants and supergiants from a main-sequence star? Relative to the Sun, this supergiant has a much larger radius, a much lower average density, a cooler surface, and a much hotter core. Red giants can become so large that if we were to replace the Sun with one of them, its outer atmosphere would extend to the orbit of Mars or even beyond Figure 3.
Figure 3. In the left image, we see it in ultraviolet with the Hubble Space Telescope, in the first direct image ever made of the surface of another star. As shown by the scale at the bottom, Betelgeuse has an extended atmosphere so large that, if it were at the center of our solar system, it would stretch past the orbit of Jupiter.
As we discussed earlier, astronomers can construct computer models of stars with different masses and compositions to see how stars change throughout their lives. Figure 4, which is based on theoretical calculations by University of Illinois astronomer Icko Iben, shows an H—R diagram with several tracks of evolution from the main sequence to the giant stage. Tracks are shown for stars with different masses from 0.
The red line is the initial or zero-age main sequence. The numbers along the tracks indicate the time, in years, required for each star to reach those points in their evolution after leaving the main sequence. Once again, you can see that the more massive a star is, the more quickly it goes through each stage in its life. Figure 4. Evolutionary Tracks of Stars of Different Masses: The solid black lines show the predicted evolution from the main sequence through the red giant or supergiant stage on the H—R diagram.
Each track is labeled with the mass of the star it is describing. The numbers show how many years each star takes to become a giant after leaving the main sequence. The red line is the zero-age main sequence.
Note that the most massive star in this diagram has a mass similar to that of Betelgeuse, and so its evolutionary track shows approximately the history of Betelgeuse.
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