The Life Cycle of Stars

Sample Student Paper (names removed)

The Birth of a Star In order for a star to go through the

process of nuclear fusion, it needs fuel. These fuels are mostly Hydrogen and Helium, also accompanied by discrete amounts of Carbon, Nitrogen, and Oxygen. While a Star is using up one type of fuel, it must change it's size and pressure in order to use another fuel. Making these changes is a very gradual process which usually happens over the course of millions to billions of years. The Star is born in an immense cloud of gas and dust known as a Nebula.

NebulaThe word 'Nebula' in latin means 'cloud', which is perfectly suited for what it is. A Nebula is a cosmic cloud of dust and gas floating in space. All the elements needed for a Star are stored within the Nebula. They are Hydrogen, Helium, and small amounts of Oxygen, Carbon, and Nitrogen, which are called 'heavier' elements compared to Hydrogen and Helium. About 90% of the Nebula is Hydrogen, about 10% is Helium, and about 0.1% are 'heavier' elements as mentioned before. Nebulae are among the largest objects in space. In fact, many Nebulae are at least dozens, if not hundreds of light years wide. There are also five different types of Nebulae, Emission Nebulae, Reflection Nebulae, Dark Nebulae, Planetary Nebulae, and Supernova Nebulae. By the way, Planetary Nebulae have nothing to do with planets!

The Crab Nebula

The Eagle Nebula

Nebula to StarIn order for the new-born Star to go from a Nebula to a Star, it must reach a critical Mass of approximately 80 times Jupiter's, due to accretion of matter. Once this occurs, the internal pressure raises the core temperature high enough to begin nuclear fusion. Dust, gas, and other materials gradually wait in the Nebula for any gravitational disturbance to pass through or by the Nebula. Once this happens, it causes ripples and a process called accretion. Accretion is growth in size or extent. This means the Stars grow larger.

StarA Star is a bright globe of gas which produces heat and light by nuclear fusion. Stars consist mostly of Hydrogen and Helium. Surface temperatures of a Star vary from 2000o C to 30,000o C, depending upon the content and size of the Star. The hottest Stars tend to be a blue-white color while red Stars are typically the cooler Stars. The smallest Mass possible for a Star is 155950.3608kg because if it is any less, nuclear fusion can not occur, therefore causing the object to become a very dim object, or very large planet.

A Star named 'Electra'

Our Sun

Star to Red GiantAs a few billion years pass, a Star runs out of it's protons. The result is a core left with alphas. Even though the outer layers still contain Hydrogen, they aren't hot enough to fuse. Without fuel, the Star begins to cool and contract. The outer layers sink into the core due to Gravity, which causes them to heat up. The Star now has a source of energy. The core is now hotter than it was during it's normal life. The heat causes the Star to swell and expand. By the time the radiation reaches the surface of the Star, it has become weak. Weak radiation makes the Star red, as the Star grows huge. I think it's obvious where they got the name from.

A Red Giant is a bright, immense Star with a low mass nearing the end of it's evolution.

Red GiantRed Giants are very bright, red Stars (hence the name) that are not very hot. Red Giants literally blow up like a balloon because all of the Helium that it possesses. The size of a Red Giant can be 10 to 100 times the diameter of our Sun. Red Giants of, formerly, large Stars are sometimes called Super Giants and can be up to 1000 times the diameter of our Sun. They also have luminosities of often 1,000,000 times greater than that of our Sun.

Betelgeuse (a Red Giant in the constellation Orion)

Red Giant to White DwarfOnce a small Red Giant runs out of fuel, it begins to cool down and contract due to gravity. The inner parts of the Star contract which gives off heat and causes the outer parts to expand. The expansion of the outer parts causes them to slowly separate and form a Planetary Nebula. The inner parts of the star continue to contract until it reaches the size of Earth. Electrons begin to overlap due to the atoms being crushed together. Since two Electrons cannot take up the same space, they begin to repel. This forms a small sphere of matter, which is called a White-Dwarf star.

This picture depicts the Sun as a white dwarf star and the Sun now. The size difference is quite obvious.

White DwarfA White Dwarf is a very

small, hot Star and is the last stage of the life cycle for a

Star such as our Sun. White Dwarfs have a similar mass to our Sun,

except a White Dwarf is about 1% of the Sun's diameter. Approximately, a White Dwarf is the size of Earth. White Dwarfs have a surface temperature of 8,000o C or more, but being small and hot costs the White Dwarf a bright luminosity. White Dwarfs have the luminosity of 1% of our Sun's luminosity.

Comparison between Earth and a White Dwarf

A White Dwarf in Space

Red Giant to SupernovaMost small Red Giants die as a White Dwarf, but the massive Red Giants die in a more spectacular way, a Supernova. After the fuel in a Red Giant is exhausted, the core becomes cooler and the internal pressure begins to decrease, causing contraction. For massive Red Giants, this is a disastrous event which leads to the collapsing of the Star. The outer layers start to gain heat as they fall. This heat ignites nuclear fusion in the outer layers which causes them to explode. This is called a Supernova. For a few days, this explosion is brighter than a whole galaxy!

This is Kepler's Supernova. This Supernova occurred in the Milky Way and is the most recent Supernova to be seen by the naked eye.

Supernova

A Supernova is an explosive death of a massive Star. At its height, a Supernova can be as bright as 100 million bright Stars. Supernovae have been accounted for creating heavier elements than Hydrogen and Helium. Over the years, two types of Supernovae have been found.

Type I: A Supernova which happens in a binary star system. It occurs by gas from one Star falling onto a White Dwarf, causing it to explode.

Type II: A Supernova which occurs when a humongous star, at least ten times more massive than our Sun, explodes because of malfunctioning internal nuclear reactions towards the end of it's life. These large explosions either create a Neutron Star or a Black Hole.

Kepler's Supernova.

A Supernova captured in real time in August of

2006 through a gamma telescope.

Supernova to Neutron StarAfter the spectacular

explosion of a Supernova, the outer layers are blasted off into space and form a Nebula. The core is shrunk by gravity and condensed into a sphere shape approximately the size of Manhattan. The Electromagnetic(EM) force keeps the electrons out of the nucleus. Since the EM force is the only thing that matters, the Star wouldn't be able to shrink to such a size simply because of the Mass and number of atoms. However, because of the Star's Mass, Gravity defeats the EM force. Since the EM force is now broken, the protons and electrons combine to make neutrons. All that is left are neutrons, and the Neutron Star is formed.

A Neutron Star is formed when a massive Star collapses, or in this case, explodes.

Neutron Star

Neutrons Stars are stars that are mostly made

up of Neutrons, hence the name,and are

produced by the massive explosion of a

Supernova. Neutron Stars are very dense,

therefore they have a larger Mass compared to

Stars such as our Sun. Typical Neutron Star have

a Mass of three times our Sun's, but a diameter

of only 20 km! If the Mass of a Neutron Star is

greater than 3 times that of our Sun's, then the

gravity will be so strong that it will shrink into

itself and become a Black Hole.

A Neutron Star named 'RS 9862-81-8-9325421-

0 A.'

The remains of a Supernova with a Neutron

Star in the middle.

Supernova to Black Hole

As we just learned, a Neutron Star is made from a Supernova explosion. Also, Neutron Stars can create Black Holes. If the Neutron Star becomes too big, the gravitational forces become too much for the pressure gradients and the collapse cannot be stopped. The Neutron Star continues to shrink until finally it becomes a Black Hole.

Black Holes

Little is known about Black Holes, not even their Density. Of the information that we do know about them, Black Holes are the remnants of the most massive Stars in the universe. The gravitational pull of a Black Hole is so great that nothing can escape from it, not even light, therefore they are black.Black Holes are believed to distort the space around them, and also suck in surrounding objects.

Cygnus X-1, a Black Hole which is located 6,070 light years away.

The youngest Black Hole hole known to man, SN1979C.

Work CitedStages of a Star- The Life Cycle of a Star [Internet]. cited 2012 Jan 3] .

Available from: http://www.telescope.org/pparc/res8.htmlNebulae- Nebulae [Internet]. cited 2012 Jan 3] . Available from:

http://www.seasky.org/celestial-objects/nebulae.htmlThe Birth of a Star- Tangen KT, Weiss MW. Life Cycle of a Star

[Internet]. Mohegan Lake(NY):Oracle ThinkQuest; cited 2013 Jan 3] . Available from: http://library.thinkquest.org/26220/stars/formation.html

Nebula to Star- Stellar Birth [Internet].[2006 Jan 7, cited 2013 Jan 3] . Available from: http://burro.astr.cwru.edu/stu/advanced/stars_birth.html

Star to Red Giant- Seagrave WK. Red Giant [Internet]. :Penny Press Ltd.; [2012, cited 2013 Jan 3] . Available from: http://www.historyoftheuniverse.com/starold.html