The Life Cycle of a Star A Really “Hot” Topic By Samantha Edgington

Objectives Describe how stars are formed. Explain the concept of equilibrium. Define the “death” of a star. Identify the different types of stars. Describe how the process of nuclear fusion works within a star.

What is a Nebula? A nebula can be defined as a thinly spread cloud of interstellar gas and dust. Some nebulae are the remains of a supernova explosion. –the death and collapse of a massive star can cause this explosion, this means nebulae can be the remains of old, dead stars. Most nebulae are gravity-induced condensations of gases where protostars are born.

How is a Protostar Formed? Inside a nebula, there are areas where gravity causes dust and gas to “clump” together. As these “clumps” gather more and more mass their gravitational pull increases, forcing more atoms together. This process is known as accretion, and the result is a protostar.

How does a Protostar Become a Star? In order to comprehend this process, the concept of equilibrium must be understood. Equilibrium is in essence a balance. In the case of star formation this balance exists between gravity and gas pressure.

Achieving Equilibrium First gravity pulls gas and dust inward towards the core of the prospective star. Inside the core, density and temperature increases as atomic collisions increase, causing a rise in gas pressure. Finally when gas pressure is equal to gravity, the protostar has reached equilibrium and is therefore reached a reasonably stable size.

The Birth Once the protostar has achieved equilibrium one of two things occur:  If there is not a sufficient mass, it becomes a brown dwarf which is a “star” that doesn’t radiate much heat and light.  In the event it does contain an appropriate amount of matter, nuclear fusion begins and light is emitted.

The Main Sequence A star is basically a huge ball of gas undergoing nuclear fusion. The main sequence phase is where stars spend the majority of their “life” by fusing hydrogen into helium. There are two types of main sequence stars:  a red giant which is a large bright star with a cool surface.  a red dwarf which are very cool, faint and small stars

The Ending of Main Sequence The star slowly shrinks over billions of years as the hydrogen is used by fusion. The star’s temperature, density, and pressure at the core continues to increase. Once the hydrogen is depleted, the helium is fused into carbon, when this occurs the star has reached “old age”.

The Death There are one of two ways a star can die depending on its size. If the star is of low mass, it expands its outer layers, creating nebulae and a white dwarf forms from the core. If it is of high mass, death occurs in a massive explosion known as a supernova, the remaining core then transforms into a neutron star or a black hole.

What is a White Dwarf? They form from the core of a dead red giants that were too small to fuse carbon. Since they do not undergo fusion, they have no energy source and gradually fade. When they radiate away all of their energy they will theoretically become a black dwarf. Since white dwarfs cannot be older than the universe (13.7 billion years) no black dwarfs are currently in existence.

What is a Supernova? They can form when the gravitational potential energy—created by a sudden gravitational collapse of a large red giant—heats and expels the star's outer layers, resulting in an explosion. Also, they can form when a white dwarf ignites carbon fusion, which results in a runaway nuclear fusion reaction and causes a supernova. Supernovae can be so immense that the energy produced can equal the energy the Sun creates over a time period of 10 billion years!

What is a Neutron Star? A neutron star is formed as a result of a massive star being compressed. The core material, known as neutron degenerate matter, mostly consists of neutrons with a few protons and electrons. The gravity is so intense that if an object were to reach the surface it would disperse all of its subatomic particles and merge with the star! The matter is so dense that a teaspoon would weigh billions of tonnes! Some people view neutron stars as giant atoms.

What is a Stellar Black Hole? If a collapsing star exceeds the maximum mass a neutron star can be it will develop into a stellar black hole. Black holes are extremely dense areas with a gravitational pull so powerful not even light can escape! What could be a reason why black holes are black?

Chart of the Stars Star TypeSolar MassTemperature (K)Color Red Giant , ,500orange - red Red Dwarf ,500 – 3,500red Blue Giant ~30,000blue - white Brown Dwarf ~1,000red Yellow Dwarf , ,000white - yellow White Dwarf< 1.44, ,000white Neutron Star ~1,000,000blue

The Sun’s Life Now that a basic overview of a star’s life cycle has been covered, the details of the most important star’s life can be revealed. Can you guess what type of star the Sun is?

The Sun in Main Sequence The Sun was born like any other star, out of a nebulae and into a protostar. It is now in its main sequence and can be classified as a yellow dwarf. The Star is about 4.57 billion years old, about halfway through its complete lifecycle.

The Sun’s Death Since the Sun is considerably smaller it will not explode in a supernova. Instead it is predicted that it will become a red giant in about ~6 billion years. When this occurs, all the water on earth will be vaporized and life will cease to exist. In the final phase the Sun will form a white dwarf and slowly fade out of existence.

Works Cited aspire.cosmicray.org/labs/star_life/starlife_proto.html outreach.jach.hawaii.edu wikipedia.org