NGC 6543

We Are Stardust

The Evolution of Stars


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What are stars? How do they affect our existence?
The answers are startling.

Read on for a surprising story that touches all the sciences.


What is a Star?

We see many points of light on a clear, dark, moonless night. Some are bright, others barely visible. If we look carefully we can make out colours. The bright star Vega (overhead in the Northern Hemisphere summer) is white; Capella (overhead in the winter) is yellow; Regulus (of Leo) is blue, Arcturus (a springtime star) is orange; Antares (from the Greek "rival of Mars" - brightest star is Scorpius) is, of course, red.

At first glance it may seem a difficult thing to find out anything about the stars. But by using the gas laws and the laws of thermodynamics it is possible to find out about their physical conditions. Using atomic physics and the laws of radiation (like light) we can find out about their chemistry.

Although stars are very distant, we are lucky to have one very close to us: the Sun. The Sun is a typical star.

Light (travelling at 300,000 km per sec) goes seven times around the Earth in one second. From the Sun it takes just 8.3 minutes to reach us (from a distance of 149 million km). The nearest star after the Sun is so far away that light takes over 4 years to travel that distance. If this huge distance is expressed in kilometres, it would mean nothing to most people. We say that the nearest star is over four light years away. A light year is the distance light travels in one year.

I will leave it to the reader to calculate that distance in kilometres or miles!


The Chemistry of the Universe

The Universe is composed of 85% Hydrogen (H), 14% Helium (He) and 1% of everything else. That "everything else" includes the Carbon (C) of life, Oxygen (O), Silicon (Si) that makes up rocks, Iron (Fe), Uranium (U): in fact all the other 90 or so chemical elements in existence.

Clearly, the Earth is not typical of this composition. The Earth is mainly rock and metal. However, if we look at the stars, their chemistry is different. The Sun, as previously mentioned, is a typical star. It has a mass 300,000 times that of the Earth. Its composition is 85% H, 14% He, and 1% (everything else: C, Si, N, P, O, Na, Fe, Ni, K, etc).

The sun is a huge (109 Earth diameters) ball of glowing gas. It is surrounded by bits left over after its formation. The Earth and the other planets that orbit the Sun, and their moons compose a mass of over 460 times that of the Earth, or more graphically, about 0.1% the mass of the Sun. In other words, we can describe the Sun to be composed of a huge sphere of glowing gas made up mostly of Hydrogen and Helium with a scattering of more solid material going around it.

This is what a typical star is like. A star's chemical composition reflects that of the Universe in general.


Star Formation and Source of Energy

A star like the Sun forms from clouds of gas which float in space. We can observe these clouds in the spaces between the stars. They are called nebulas. A typical gas cloud (nebula) may have the mass of a million suns! These clouds float in the black coldness of interstellar space for millions of years.

Occasionally, a condensation may occur which increases the density of the cloud in a small area. When this happens, the gravity of this area of gas is slightly higher then nearby regions. Gravity is a force that pulls things together. Under the action of gravity, a ball of gas will eventually form. This gas then condenses under the pull of its own gravity.

A gas ball that is shrinking will heat up. If a gas is compressed, its temperature will rise. When a bicycle pump is used, air is being compressed and this heats it. The pump gets hot.

For hundreds of thousands of years, gravity will pull in the ball of gas while compression will heat it. Eventually the heat produced will cause the ball of gas to glow a dull red. We have a proto star ("proto" is Greek for "first"). The first appearance of energy will not stop the collapse of the object under the pull of its own gravity. So it carries on becoming smaller and hotter.

Eventually, the temperature in the centre reaches the value of about 20 million degrees. When this critical stage is reached, a change takes place. A change that is the key to our very existence.

At these high temperatures, the Hydrogen atoms in the centre of the ball of gas begin to fuse together. This is a Nuclear Reaction, much like those that occur within a Hydrogen bomb. The nuclei of Hydrogen combine to form nuclei of Helium. This reaction yields energy; lots of energy. The single Helium atom formed has a mass slightly less that the mass of the Hydrogen atoms that formed it. The missing mass is converted to energy as described by Einstein's equation,

E = mc2.

For example, let us consider the sun.

Every second, 400 million tonnes of Hydrogen disappear to be replaced by 396 million tonnes of Helium. The 4 million tonnes that vanish are converted into energy.

Einstein's equation says that mass, m, when multiplied by the speed of light squared (c2 - a large number multiplied by itself), gives the amount of energy. Our missing 4 million tonnes yields an unbelievably large amount of energy every second. Even though the sun is losing so much matter, it is so massive that it can keep it up for millions of years.

When a glowing ball of gas begins the nuclear reactions in its core, it becomes a star. The energy streaming out counteracts the force of gravity pulling in. The object is now stable. It is called a Main Sequence Star. Our sun is a Main Sequence star. Energy is created by the H to He reaction in its core and this balances gravity's tendency to pull the sun in on itself.


Stars of Different Mass

The Sun has a surface temperature of 6000 K. This makes it golden yellow. Its luminosity (how much light it gives out) is determined by its mass (300,000 Earths, or 1 Sun).

The amount of energy it gives out and its total mass mean that the Sun will remain a stable (also called Main Sequence) for approximately 11,000 million years. The current age of the Sun (and of the rest of the items floating around it - including the Earth and us) is about 5,000 million years. The Sun is "middle aged" . Any star with the Sun's mass will have the same temperature, luminosity and lifetime.

However, not all stars have the Sun's mass.

If a ball of gas had half the Sun's mass it would settle down with a surface temperature of 4500 K, and would be generating energy at a much lower rate than the Sun. Its lower temperature would give it an orange or red colour. Its luminosity would be perhaps 1/100th that of the Sun. However, it would remain stable for a longer period because it would be using up its Hydrogen at a lower rate. Its lifetime would be around 30,000 million years.

If another ball of gas had twice the Sun's mass, its surface temperature would be around 10,000 K so it would glow white or bluish. Its luminosity would be maybe 30 times that of the Sun. However this star would be using up its energy resources so quickly that it would remain stable for less than 1000 million years.

The table below shows the various properties of stars with different masses:

MASS
Sun=1
TEMPERATURE
degrees K
COLOUR
 
LUMINOSITY
Sun=1
LIFETIME
millions of years
0.2 3,000 red 0.00001 50,000
0.5 4,500 orange 0.01 30,000
1 6,000 yellow 1 11,000
2 10,000 white 30 1,000
5 15,000 blue 10,000 100

From the table you can see a general rule for Main Sequence (stable) stars. All their properties depend on the initial mass of the object that formed it. The less massive a star, the cooler it is, the redder its colour and the longer it survives. The more massive it is, the hotter it is, the bluer its colour and the shorter period it survives.

Interestingly, the Earth required 500 million years to cool down after the Sun formed. Life began around 1000 million years after the formation of our star. It probably will not be useful to look for life on planets around the more massive stars because they will not survive long enough for life to develop!


Stellar Evolution

What happens to a star when its time on the Main Sequence is complete? When it is no longer capable of producing energy by fusing Hydrogen to helium?

The answer to that depends on the mass of the star. For this discussion we can divide stars into three groups:

Our galaxy (all the stars we can see in the sky plus a lot more!) contains 100,000 million stars (twenty for every human being on this planet). Of these 60% are Small, 30% are Medium, and 10% are Large. Most stars are less massive than the Sun. These stars tend to be too faint to be visible. Most of the stars we see at night are the large and luminous ones. They are bright enough to be visible from our region of space.

We can now look at the interesting period of stars after their cores run out of the Hydrogen that gives them their energy.


Evolution of Small Stars

Small stars simply stop producing energy in the cores. All the outpouring of power that has kept gravity at bay for its stable lifetime stops.

When this happens, the small red star begins to collapse. This heats it up again. However, the star is too small to ignite further nuclear reactions. Its collapse will heat it to white heat. It becomes a very small white star called a White Dwarf. Another long period is spent cooling down.

This is not a very spectacular end for a star. But not all stars are small. Our Sun is not.


Evolution of Medium (Sunlike) Stars

Medium size stars like our Sun will develop differently.

When the core runs out of Hydrogen, it will collapse as with small stars. Again, this collapsing heats the core.

When the core reaches a temperature of about 100 million degrees, new nuclear reactions will begin. These reactions involve the Helium produced from Hydrogen while the star had been a stable Main Sequence object. Helium undergoes nuclear reactions and forms Oxygen and Carbon.

The new release of energy from this new set of reactions stops the collapse of the core. These new reactions give out more energy than the reaaction of Hydrogen to Helium. This extra energy causes the outer layers of the star to expand, sometimes by a great deal.

The star is now giving out much more energy than before when it was stable. Because the star has expanded the surface temperature drops. The star reddens and grows brighter: it is now a type of star called a Red Giant.

After about 5000 million years our own Sun will evolve into a Red Giant. When it does so, conditions on the Earth will no longer be suitable for life. The Sun will swell up so much that it will swallow up the inner planets, Mercury, Venus and Earth. The very Sun that keeps the Earth going will eventually destroy it.

With its new energy source, the Sun will have received an extension to its life. Although changed in properties, the Sun can continue shining for perhaps another 1000 million years as a Red Giant. However, Red Giants are not quite as stable as Main Sequence stars: they vary in radius, they alter their brightness.

The bright star Albebaran in Taurus is an example of a Red Giant. It has over 40 times the diameter of the Sun.

Red Giants convert Helium to other elements. Eventually, the Helium in the core runs out. As before, if the core stops producing energy, gravity will take over again and the core continues to collapse. The heat generated from this collapse causes the outer layers of the star to be pushed further from the star until its gravity cannot hold them.

At the end of its life, a Sunlike star will end up with a shell of hot gases moving away from the white hot core which gets left behind. This core is a White Dwarf. The outer material blends with the background interstellar gas. Its composition is the same as it has always been since the star's outer layers do not mix much with the material from the core. Objects like this can be observed and are called Planetary Nebulas (because they look like distant planets through a telescope).

In the end, both small and medium stars end up as White Dwarfs, the latter after a period of being a Red Giant.

Although Red Giants radically alter in their chemical composition, the new materials formed remain within the dense core that becomes a White Dwarf.

Incidentally, White Dwarfs cannot be seen with the naked eye as they are too small and faint. They are typically the size of a small planet but have all the mass of a star. They are incredibly dense. A cubic centimetre may weigh several tonnes! The brightest star, Sirius has a companion orbiting it (called Sirius B) that is a White Dwarf.

The ultimate fate of the Sun will be to end up as a White Dwarf.


Evolution of Large Stars

The Large stars end their lives in the most spectacular way.

A large star will be stable as a Main Sequence object for a very short period, in geological terms. A star of five solar masses will remain on the Main Sequence only for about 500 million years, barely time for planets to form around it.

When, the Hydrogen in the core is exhausted, its huge gravity takes over and condenses the core. This causes the 100 million degree mark to be reached so that Helium reactions begin. The star evolves into a Red Giant, but more luminous and larger than the Red Giants formed from Medium stars.

Very soon (perhaps 150 million years), the Helium runs out in the core. The core collapses. This time the mass and gravity are great enough for further nuclear reactions to begin.

Carbon can be changed to Oxygen and Silicon. The Red Giant grows more luminous, it becomes larger and it cools because of the larger surface area. This stage cannot last as long as previous stages.

The Carbon in the core runs out after, perhaps, 40 million years. The core collapses again. Higher core temperatures are reached. More nuclear reactions occur, this time using Oxygen.

The star is now very luminous (perhaps over 100,000 suns!). It is also unstable, changing its radius and luminosity, sometimes on a daily basis.

This process continues for several more stages. Each time a nuclear fuel runs out in the core, the core collapses. This collapse raises the temperature which, in turn, triggers off more nuclear reactions. Each subsequent set of reactions lasts for a shorter period than the last.

This can continue through the elements until Iron is reached. By this stage the star is a Red Supergiant, shining with a luminosity of, perhaps, half a million Suns.

For atoms that are lighter than the element Iron, nuclear fusion of lighter atomic nucleii to form heavier ones produces energy. However, Iron nuclei cannot react to produce energy. Any nuclear reaction involving Iron consumes energy.

It is as if the star has been using its gravity to borrow an extension to its energy. Now, it must pay everything back.

When the core is made up totally of Iron, gravity finally takes over. The core collapses again. Any nuclear reactions that occur now consume energy so that gravity cannot be held at bay. The core collapses catastrophically. In an instant it shrinks uncontrollably to almost nothing. The huge amount of shrinking heats up the core so dramatically that the power produced blows the outer layers of the star away in a huge explosion.

This is a Supernova. The huge amounts of energy produced is enough to produce all the heavier elements beyond Ireon and scatter them into space.

In 1054, Chinese records talk of a guest star that appeared in the constellation of Taurus. It was so brilliant that for two months it was visible in the daytime. It remained visible for nearly two years and then it disappeared and was forgotten. During the 18th century, an astronomer scanning the sky with a telescope, noticed a small patch of cloud in the same spot. The cloud, when studied in detail, resembled an explosion. It is called the Crab Nebula because of its appearance in small telescopes.

In the centre of the Crab Nebula is an object only 10km across with the mass of about 1.3 Suns. It spins around at 33 times per second. The matter in this object is so condensed that all the protons and electrons in the atoms have combined to form neutrons. It is a Neutron Star.

It is also called a Pulsar, because it sends 33 pulses per second of light, radio waves, infra red, ultra violet and even X-rays. It is a remarkable object, the remnant of a star larger than the Sun that exploded 900 years ago. The space around has been enriched with the heavy elements produced by the explosion.


Supernova Effects

Supernova explosions are very destructive but they are vital for our existence. These explosions have two effects on the interstellar environment.

Firstly, when the universe began in the Big Bang, conditions were such that only Hydrogen and Helium atoms existed. Helium is chemically inert and Hydrogen only forms a double molecule with itself. There was not, therefore, a lot of potential for chemical reactions and the formation of complex molecules in the early Universe. Life requires complex molecules.

When the early stars formed, there were no heavy elements to form solid planets (and of course no Carbon for life). When Large stars were formed, they produced the heavy elements and spewed them out when they exploded as Supernovas. Subsequent stars forming out of the enriched interstellar gases had the right chemistry to form solid planets (like the Earth).

There is evidence that the material that makes up the Sun and planets is "third generation"; in other words, it is material that has been inside two previous stars before our Sun formed.

We are, indeed, stardust.

The second effect of these Supernova explosions is the blast itself. The blast wave smashes into the interstellar gas and compresses it slightly giving it the first push towards condensing into a star.

Large stars, even though they make up a minority of the stars, create the chemistry for planets and life, spread this material into the interstellar environment and give the material its first push to form new stars.

Stars have made us in the most fundamental way, but do they, as astrologers assert, influence our personalities? Another question, another answer ....

© 1997, 2004 KryssTal

This essay is dedicated to Joni Mitchell.


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