Stellar Evolution and the Fate of the Universe
How Do Stars Release Energy?
The core of a star is an intense environment. The pressures are enormous, and the temperatures can be greater than 15 million Kelvin. These are the kind of conditions you need for nuclear fusion to take place. Once these conditions are reached in the core of a star, nuclear fusion converts hydrogen atoms into helium atoms through a multi-stage process.
To complete this process, two hydrogen atoms are merged together to form helium. This whole reaction is exothermic, and so it releases a tremendous amount of energy.
What Does the Final Destination of a Star Depend On?
The evolution of a star depends on its size.
Small and Medium Stars
- When fuel is exhausted (Helium), the star dies, expelling outer layers as a planetary nebula, while the core (rich in Carbon and Oxygen) shrinks, forming a white dwarf.
- This white dwarf will cool slowly to burn out into a black dwarf. The cooling time is very long.
Giant Stars (Red Supergiants)
- When fuel runs out, they suffer an intense gravitational collapse: The nuclear stove stops, and gravity acts.
- An instant nuclear fusion produces chemical elements heavier than iron (gold, silver, uranium, etc.).
- The energy released causes a big explosion of the star, named a supernova.
- Outer layers, with the chemical elements formed, are ejected, forming a remaining nebula.
- The core forms a heavy residue, which can be a neutron star (very dense) or, in cases of supermassive stars, a black hole may be formed.
What are Black Holes?
Black holes are regions in space that have a gravitational force that is so intense that no matter or radiation can escape.
A black hole is a place in space where gravity pulls so much that even light cannot get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying.
Theories Explaining the Future of the Universe
Big Crunch
This theory says that the universe will one day stop expanding. Then, as gravity pulls on the matter, the universe will begin to contract, falling inward until it has collapsed back into a super-hot, super-dense singularity.
Indefinite Expansion
This theory says that the universe will never stop growing. Star formation will cease, and the Universe will become less dense and cooler. When the temperature reaches absolute zero, the galaxies and the universe will continue expanding forever.
Big Rip
Dark energy will move galaxies away from each other. In addition, it will end up separating all objects joined by gravity.
Origin of the Moon
The theory that explains the origin of the Moon is called the giant-impact hypothesis. This theory says that the Moon was formed because of the debris left over from a collision between the Earth and an astronomical body of the size of Mars, approximately 4.5 billion years ago.
Evidence Supporting the Giant-Impact Hypothesis
- Earth’s spin and the Moon’s orbit have similar orientations.
- Moon samples indicate that the Moon’s surface was once molten.
- The Moon has a relatively small iron core.
- The stable-isotope ratios of lunar and terrestrial rock are identical, implying a common origin.
- There is evidence in other star systems of similar collisions, resulting in debris disks.
- Giant collisions are consistent with the leading theories of the formation of the Solar System.