The Life Cycle of a Star

Stars are formed in a large cloud of dust and gas

known as a nebula. Nebulas are cold, allowing enough gravity to overcome the inner pressure and form clumps of matter. Nebulas cover billions of kilometers in space and are very dense. They are

made mostly from the two elements, hydrogen and helium.

STELLAR NEBULA

Protostar

When the nebula collapses, a protostar is formed. A protostar is the stage of a star’s life cycle in which it has fragmented from the nebula, but has not approached the state of the beginning of nuclear fusion. A protostar can last from 100,000 years to 10 million years. It is surrounded by a thick layer of gas and dust, blocking visible light.

Main-Sequence Star

During the main-sequence stage, internal gas pressure is trying to expand the star while gravitational forces are trying to squeeze it. Hydrogen fusion supplies the star with the support to prevent the star from collapsing. An star spends 90% of its life in the main-sequence stage. However, different stars age at different rates. The hotter the star, the shorter its life span will last.

Hydrogen from the core of the star is eventually used up. Therefore, without hydrogen fusion occurring in the core, it gradually contracts and leaves behind a helium core. However, hydrogen fusion continues in the star’s outer shell. The core grows hotter as it contracts due to the converting of gravitational energy into heat energy. Some of the heat energy is radiated outward toward the star’s outer layer, expanding it. The body of the star eventually grows to become thousands of times larger than during the main-sequence stage. The surface of the star has a reddish color due to the cooling of the surface. While the outer shell continues to expand, the core continues to contract until it has reached 100 million K to convert helium into carbon.

Red-Giant Star

Planetary NebulaPlanetary Nebulas are created by medium-mass stars. They occur during its collapse from a red giant to a white dwarf. The outer layer of the star is exploded and creates a cloud of gas. The heat from the remaining, central white dwarf heats the cloud and causes it to glow.

SUPERNOVA EXPLOSION

The death of massive mass stars end with a supernova explosion. A supernova explosion involves the star growing millions of times brighter. Supernova explosions are rare. The last explosion observed was during the time of the invention of telescopes. A supernova explosion occurs when a star no longer has fuel to balance the gravity acting on the star. The star collapses, causing the store to burst inward. A shock wave is sent outwards, destroying the star and its outer shell.

White Dwarf

Low-mass and medium-mass stars eventually collapse into a white dwarf star. White dwarfs are small with very great densities. One spoonful of its substance can weigh several tons. When a star is contracting into a white dwarf, its surface becomes very hot. In fact, sometimes the temperature can exceed 25,000 K.

Black Dwarf

A black dwarf is the final stage of low-mass and medium-mass stars. A black dwarf is a white dwarf that no longer emits heat or light. A black dwarf is formed after hundreds of billions of years. Since our universe is 13.7 billion years old, black dwarfs do not exist in our universe yet.

Neutron StarsNeutron stars develop from massive-mass stars and are remnants of a supernova explosion. During the explosion, the core of the star collapses so intensely that the electrons in the star are forced to combine with protons to produce neutrons. The density of neutron stars are so great that a sugar-cube sized piece of the star can outweigh the human race.

Black Hole

Black holes are the alternative path to massive mass stars. They are very hot and dense. In fact, their gravitational force is so strong that light can not be seen through its surface. Since light can not be seen through a black hole, it is invisible to the naked eye. Scientists found evidence of the existance of black holes by seeing objects being quickly pulled into an open area. Any object that moves too close to a black hole will be pulled in by gravity and never seen again.

Low Mass StarMedium Mass

Star Massive Mass Star

There are three different stars, the low mass star, medium mass star, and massive mass star. These three stars are different based on their mass and the path of their deaths. Though these stars may vary differently in size, they share one thing in common: They will eventually run out of fuel and collapse.

After billions of years of being in the main-sequence

Like all stars, a low mass star is formed in a stellar nebula. After a few million years, the nebula collapses and forms a protostar. When the core of theprotostar has reached about 10 million K, nuclear fusion begins. In the main-sequence stage, the inner gas pressure expands the star while the force of gravity squeezes it.state, they collapse and become a white dwarf. A black dwarf is formed when the white dwarf has cooled and no longer emits light or heat.

Low mass Star

The formation of a medium mass star begins in the nebula.

The nebula eventually collapses after millions of years, forming a protostar. When the core of the protostar reaches 10 million K, nuclear fusion begins, forming a new star.During the main sequence stage, the star is balancing itself between gravity and its inner gas pressure.When the star has burned between 10-20% of its hydrogen, the core will run out of fuel. A layer of hydrogen forms around it, and the temperature will go up. The core continues to be contracted by gravity, while transferring its energy to the hydrogen layer, inflating it.

Once the fuel is exhausted, the star casts off the outer layer, creating a cloud of gas. The remaining white dwarf gives it a glow.

Like low mass stars, medium mass stars also collapse into white dwarfs.

When the white dwarf has cooled, it becomes a black dwarf.

Medium Mass Star

or

Massive Mass Star

A massive star begins in a nebula. Gradually, the nebula shrinks, and after millions of years of

contraction, it becomes a protostar. The protostar officially becomes a star when the core has

reached around 10 million K. At this stage, which is known as the main-sequence stage, the star

balances gravitational forces with its inner gas pressure. Hydrogen fusion provides the outward

pressure needed to prevent gravity from causing the star to collapse. Once the hydrogen core in

the main sequence star is exhausted, it leaves behind a helium core. As the core contracts, it

converts gravitational energy into heat energy and gets radiated outward, expanding the outer layer.In the supernova stage, the star collapses and becomes millions of times brighter. After the

supernova stage, the star can collapse to form a neutron star. The star can also collapse

into something even smaller and denser, known as a black hole.

The Sun

Of the billions of stars that exist in our galaxy, the sun is the closest star to Earth. The sun is 1.4 million kilometers in diameter. It is not the largest star in the galaxy, although it may appear to be so because of its distance from Earth. It is a medium-mass star in its main-sequence stage of life. It is currently about 4.6 billion years old, approximately halfway through its life. The sun is composed of 75% hydrogen, 23% helium, and 2% of other heavier elements. The sun’s core is 27 billion F, and at its surface it is 10,000 F.

Created byVennis Hong

6th Period