The Life Cycle of Stars and the Characteristics of the Universe

The Life Cycle of a Small Star

A nebula is a cosmic cloud of gas and dust. It is composed of mostly hydrogen. Nebulae are known as the building blocks of the universe.

A protostar is formed when a region of condesning matter in a nebula heats up and starts to glow. If enough matter is in the protostar, it will reach temperatures as high as 15 million degrees centigrade. At this temperature hydrogen can begin to fuse to become helium.

The protostar releases energy, which stops it from contracting. It begins to shine; now it's a main sequence star. It shines steadily until the hydrogen fully becomes helium. It takes about 10 billion years for the hydrogen to fuse to become helium in a star with one solar mass. The bigger the star is, the less time this will take. The helium core continues to contract with reactions forming around the core.

When the core is hot enough for the helium to fuse to become carbon, the outer layers expand and cool. The star shines less brightly and is now known as a red giant.

When the helium core runs out, the outer layers drift away from the core. The gas surrounding the core is called a planetary nebula.

The remaining core becomes a white dwarf. The core is crushed to high density by gravity. One teaspoon of the matter within a white dwarf weighs as much as an elephant (5.5 tons)!

The white dwarf eventually cools and dims. Once it stops shining, it is a black dwarf. This could take tens or even hundreds of billions of years to happen. There are no known black dwarfs yet because our universe's oldest stars are only 10 to 20 billion years old.

The Life Cycle of a Massive Star

A nebula is a cosmic cloud of gas and dust. It is composed of mostly hydrogen. Nebulae are known as the building blocks of the universe.

A protostar is formed when a region of condesning matter in a nebula heats up and starts to glow. If enough matter is in the protostar, it will reach temperatures as high as 15 million degrees centigrade. At this temperature hydrogen can begin to fuse to become helium.

The protostar releases energy, which stops it from contracting. It begins to shine; now it's a main sequence star. It shines steadily until the hydrogen fully becomes helium. This takes millions of years, which is considerly less time compared to small or medium sized stars which take billions of years.

Since the hydrogen became helium the star is now a red supergiant. Supergiants start out with helium cores that are surrounded by cooling, expanding gas. Energy is continued to be released, which causes the core to become iron. In the million years that follow, different elements form around the iron core.

The core takes less than a second to collapse. The resulting explosion is called a supernova. The supernova causes shockwaves that blow the outer layers of the star off.

If the surviving core is between 1.5 and 3 solar masses, then it will become a neutron star. A neutron star is compact, but extremely heavy. The heavier they are, they smaller they become.

If the surviving core is more than 3 solar masses, the core will collapse on itself and form a black hole.

The Hertzsprung-Russell diagram

Small Stars are stars that weigh 1.5 to 3 solar masses. They have a longer life than massive stars (billions of years compared to the massive stars' life of millions of years).

Stage 4

Stage 2

Stage 6

Stage 5

Stage 7

Stage 1

Stage 3

This diagram displays the luminosity and the temperature of the stars. There are clusters of certain stars at a time of their cycle and it interprets how the temperature and the shine interact with one another.

Black Dwarf:

Protostar:

White Dwarf:

Planetary Nebula:

Main Sequence Star:

Red Giant:

Nebula:

A massive star is a star that weighs about 10 solar masses. Most massive stars are 3 times the weight of the sun. Some weigh as much as 50 times the weight of the sun.

Stage 5

Stage 2

Stage 1

Stage 3

Stage 4

Neutron Star

Stage 6

Protostar:

Main Sequence Star:

Nebula:

Red Supergiant:

Supernova:

Black Hole