Life Cycle of Medium Sized Stars

Our sun is our star, like all stars it has a life cycle, with a ‘birth’ and in time a ‘death’. Our sun is a medium sized star, the size of a star affects how long it will exist and the possible future of the star once it runs out of fuel.

The creation, existence and fate of a star is a balancing act between the force of gravity and expansion due to heat from the process of nuclear fusion within the star. Gravity acts to pull the star in on itself and crush it, the heat acts to expand the gas cloud that makes up the star.

All stars are a super hot ball of Hydrogen, Helium, and as the star ages an increasing number of the lighter elements in the periodic table. Knowing what elements are present in a star, and the proportions can help astronomers age a star. Carbon for example is only formed in stars near the end of their life cycles. It is from the stars that the earth got all its elements, all the atoms that make up your body were formed in a star billions of years ago, you were literally made in a star!

Life Cycle for a Medium Sized Star

Nebula → Protostar → Star (Quiescent phase) → Red Giant → White Dwarf

Stars, figure 1

Nebula: A large very low density gas cloud containing mainly hydrogen. The density is so low that it is almost a vacuum. However, there is enough gas to produce a weak pull of gravity between the atoms and over billions of years this pull brings the hydrogen together. As this happens the gas cloud gets more dense and the rate of gravitation pull increases.

ProtoStar: At this stage the gas cloud is more dense and forms a definite point in space. At present the density and pressure is too low to allow nuclear fusion so it is still cold and does not glow. Gravity continues to pull the gas cloud closer and closer together, increasing the pressure.

Star: When the density and pressure of the gas cloud is high enough the hydrogen starts to fuse together to form helium. Nuclear fusion has begun, a point in the life cycle known as the stellar ignition. The star will continue in this steady state for many billions of years if it is small or medium in size. The heat from the fusion balances the gravity and the gas cloud stays the same size. Larger stars burn their hydrogen faster than smaller stars and so have a short life. This period is known as the quiescent phase, the quiet phase. Our star is in this phase and has been for 5 billion years, it has enough hydrogen to remain this way for another 5 billion years.

Stars, figure 2

Red Giant: When all the hydrogen in the core of the star has been fused into helium, the fusion of the hydrogen in the outer layers of the star will begin. This makes the outer atmosphere of the star heat up and expand, this expansion however then causes it to cool. This turns the light from the star red, hence the name a Red Giant. For our star this will make it expand to consume the inner planets including the Earth. The Giant is roughly 400 times larger than the original star.

White Dwarf: Once all the hydrogen fuel is used up the remaining gas cloud will cool and gravity will begin to make it collapse, There is a brief period just before this collapse begins when the outer atmosphere of the star is blow onto space scattering some of the elements that have been generated by fusion. The remaining matter will continue to collapse under gravity until it is a cold dense sphere of matter about the size of the Earth.

Life Cycleof Large Stars

A star that is much larger than our own star is formed in the same way from a nebula. The larger size of the star means that the temperatures and pressures in the core are much higher. This increases the rate of fusion and so results in a shorter life cycle. When the fuel in the core is exhausted these large stars undergo a sudden collapse as gravity pulls the star inwards. This creates a sudden and massive increase in pressure and temperature which results in a final burst of nuclear reactions and an explosion known as a Supernova. This sends energy and material out into space at over 30,000 km/s.

What remains of the star will now begin to collapse under gravity, and can form one of two stellar objects depending on the remaining mass of material, either a super dense Neutron Star or a Black Hole.

Neutron Star: This may have a mass twice that of our sun, but is compressed into an object only 20 -30 km across. At this density the atoms are broken down into just neutrons. The material is so dense that if your mobile was made of this material it would weigh the same as the whole of the UK.

Black Hole: In some stars the material continues to collapse under gravity into a singularity. This is so dense that it creates a gravitational field that is so strong nothing can escape its pull. Even light travelling at 3 x 108 m/s is too slow to escape its pull.

Observing the Universe

In the early years of observing the cosmos scientists had to rely completely upon the naked eye for their observations. As early as 3500 BC the Egyptians and the Babylonians were making systematic records of the stars and the planets.

Sometime between 700 - 780 AD Muhammad al-Fazari built the first Astrolabe to help record the position of stella objects with greater accuracy, in the next century the first modern observatory was built in Baghdad,during the Islamic Golden Age.

1608 saw the invention of the first refracting telescope by the Dutch astronomer Hans Lippershey, followed in 1616 by the design for a reflecting telescope by Niccola Zucchi, however it was not until 1668 that one was built by Sir Isaac Newton.

The move to looking at the universe in wavelengths other than visible light began with the construction of the first radio-telescope in 1930 by Karl Jansky. In the 1970’s the observation of the universe moved into orbit with the launch of an x-ray telescope. Many space based telescope now orbit the Earth, probably the most famous is the Hubble Space Telescope launched in 1990. Observations from the Earth’s surface are always hampered by the weather, the refraction and absorption of light by the atmosphere and increasingly by light pollution from artificial light sources. Placing telescopes in space avoids all these problems and results in data and images that are of a much high quality and resolution.

Hubble Space Telescope launched in April 1990, can observe the universe without the interference of the Earth’s atmosphere, its 40 year mission is to boldly see what no one has seen before.

Stars, figure 1

What is the likely fate of our sun in about 5 billion years time when it begins to run out of hydrogen in its core.
Your answer should include: fuse / hydrogen / Red Giant / cool / collapse / under / gravity / White Dwarf
Explanation: It will start to fuse the hydrogen in its outer atmosphere, changing it to a Red Giant, this will eventually cool and collapse under gravity to form a White Dwarf about the size of the Earth.
Explain why a large star has a shorter life cycle what happens to it once it has used the hydrogen in its core.
Your answer should include: Supernova / Neutron Star / Black Hole
Explanation: The much higher pressures and temperature in the core of a large star mean that the nuclear fusion of hydrogen to helium happens at a much faster rate, this means larger stars have a shorter life cycle. Once the hydrogen in the core is consumed it will undergo a sudden collapse resulting in a burst of nuclear reactions releasing large amounts of energy that cause the star to go Supernova. The remains will collapse under their own gravity to form either a Neutron Star or a singularity at the heart of a Black Hole.
Give two reasons to place optical telescopes in orbit.
Your answer should include: loss / light / refraction
Explanation: They are not subject to the loss of light and the refraction of light as it passes through the atmosphere, nor are they affected by weather or artificial light pollution.