We Are Stardust

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 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 Scorpio) 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 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 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 I expressed this huge distance 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!

Stars are so far away that they look like points of light. The sun is so close (relatively) that it dominates the sky. How do stars form? What is the source of their energy? How do they die? For the answers read on ....

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 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 I've said before, 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 about 400 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 like the Sun forms from clouds of gas which float in space. We can observe these clouds in the spaces between the stars. A typical gas cloud may have the mass of a million suns! These clouds float in the black coldness of interstellar space for eons. 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, as any apple fan can tell you, pulls things in. A ball of gas will eventually form. This gas condenses under the pull of its own gravity.

A gas ball that is shrinking will heat up. If a gas is compressed, its temperature rises. Remember the bicycle pump! 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 huge value of 20 million degrees. When this critical stage is reached, a change takes place. A change that is the key to our very existence. For at these high temperatures, the Hydrogen in the centre of the object begin to fuse. Nuclear reactions occur 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 Helium formed has a mass slightly less that the mass of the Hydrogen that formed it. The missing mass is converted to energy as described by Einstein's equation,

For example, let's take the sun. Every second, 400 million tonnes of Hydrogen disappear to be replaced by 396 million tonnes of Helium. The 4 million tonnes that vanishes is converted into energy. Now, 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.

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 Main Sequence star (ie stable) for approximately 11,000 million years! The age of the Sun at present (and of the rest of the rubbish floating around it - including us) is about 5,000 million years. We can say therefore that the Sun is a "middle aged" star. Any star with the sun's mass will have the same temperature, luminosity and lifetime. But 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 survive longer 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 has 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 what stars are like for different masses:

From the table you can see a general rule for Main Sequence 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!

What happens to a star when its time on the Main Sequence is complete? The answer to that depends on the mass of the star. While I discuss this I'll divide stars into three groups: Small (masses up to 0.5 that of the Sun), Medium or Sunlike (0.5 to 1.5 that of the Sun), and Large (over 1.5 solar masses).

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 big'uns, they are bright enough to be visible from here.

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

Small stars simply stop producing energy in the cores. All the power that has kept gravity at bay for eons, stops. The small red star begins to collapse and heats up. It will be too small to ignite further nuclear reactions but its collapse will heat it to white heat. It becomes a tiny white star, a White Dwarf. It then spends further eons cooling down. Not a very interesting end. But not all stars are small. Our Sun isn't.

Medium size stars like our Sun will develop differently. When the core runs out of Hydrogen, it will collapse. This collapsing heats the core. When it reaches a temperature of 100 million K, new nuclear reaction will begin. These reactions involve Helium reacting to form Oxygen and Carbon. The new release of energy stops the core's collapse. However the new reactions give out more energy than the old ones. This causes the outer layers of the star to expand. The star gives out much more energy than before. But because the star has expanded the surface temperature drops. The star reddens but grows brighter: it is 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 Antares in Scorpio is a typical Red Giant. It is larger than the orbit of Mars and has a luminosity of 20,000 that of the Sun. If it was placed where the Sun is, the Earth would be inside it.

Red Giants convert Helium to other elements. Eventually, the Helium in the core runs out. What next? Well, as before, if the core stops producing energy, gravity will take over and the core continues the collapse began all those eons ago when the star first formed. The heat generated from this causes the star's outer layers to be pushed further from the star until the 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 and ends up as 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 don't mix much with the material from the core.

So, in the end, both small and medium stars end up as White Dwarfs, the latter via a period of being a Red Giant. Although Red Giants radically alter in their chemical composition, the new materials formed remain within the dense White Dwarf. White Dwarfs, incidentally, 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! This will be the ultimate fate of the Sun.

The real fireworks are reserved for the Large stars. 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 K 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, larger and cooler. This stage cannot last as long as previous stages. The Carbon runs out after, perhaps 40 million years. More collapse of the core, higher temperatures in the core, more nuclear reactions. The star is now very luminous (perhaps over 100,000 suns!). This continues for several stages. Each time the 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 until Iron is reached. Up to the element Iron, nuclear fusion produces energy. Unfortunately for this type of star (which by now is a Red Supergiant, shining with a luminosity of perhaps half a million Suns), Iron cannot react to produce energy. Any nuclear reaction involving Iron consumes energy. When the core is made up totally of Iron, gravity, which has been waiting patiently in the wings, takes over. The core collapses. This time any nuclear reactions that occur 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 it up so dramatically that the power produced blows the outer layers of the star in a huge explosion. This is a Supernova. The huge amounts of energy produced are enough to produce all the heavier elements 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 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 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. Since Helium is chemically inert and Hydrogen only forms a double molecule with itself, there was not a lot of chemistry in the early Universe. When stars formed, there were no heavy elements to form solid planets (and of course no Carbon for life). It was only after Large stars formed, produced the heavy elements, and spewed them out when they went Supernova, that subsequent stars 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 Supernovas 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.

So, 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 Kryss Katsiavriades