How many stars can we see? Unfortunately there is no definitive answer. Most sources suggest that from any one location and on a clear night you can see about 2 000 stars with the naked eye—perhaps as many as 3 000 if deep in the countryside and away from ambient light. With a good pair of binoculars the figure leaps to 50 000 and with a 2” telescope you should be able to see as many as 300 000. Of course, no one has ever counted them! It is easy to see a star. Just look up at the sky on a clear night. You’ll see masses of them—some bright points of light, others tiny faint smudges.
In reality there are billions and billions of stars. The heavens above appear so crowded that they seem to merge into blobs of light. But the reality is that outer space is virtually empty. Most stars are many trillions of kilometres away from each other. Nobody knows how many stars there really are, nor where space ends.
The sun is our nearest star at a mere 150 million kilometres distant. The second nearest star is Proxima Centauri, but he is 4.28 light years away—which is 40 491 924 million kilometres!
How a star born
Our present universe started with “the Big Bang” 13.8 billion years ago—according to modern perceived wisdom. What happened before that is the subject of even more fantastic speculation. So let’s call it the more recent “Big Bang”. This created primordial clouds of hydrogen and helium—ten hydrogen atoms for every one helium atom. Then the first stars formed out of these primordial clouds hundreds of millions of years later.
With no heavier elements to cool the gas clouds, these protostars grew massive, rapidly burned through their fuel and exploded in supernovae. During various burning stages of evolution of those early stars, before and after they exploded, their intense heat fused the hydrogen and helium atoms into heavier elements — the first metals — which in turn enabled the formation of long-lived, low-mass stars. Even today over 90% of all atoms in the universe are still hydrogen.
A protostar needs sufficient mass to ignite the fusion process—more than 0.08 of the mass of our Sun (a solar mass). Below that such bodies fail to get off the starting block, and ignominiously survive as brown dwarfs.
Under the force of gravity materials in a nebula coalesce into a protostar and are compressed, thus driving up the core temperature. Hydrogen atoms naturally repel each other and require extremely high temperatures before giving in to fusion—at over 167 million °C, to be precise. The hydrogen atoms are fused together, reaching speeds of over 1 000 kilometres per second, to form helium and energy. And a star is born.
About 90% of all new stars proceed onto the “Main Sequence” (as explained in the section on the Hertzsprung-Russell diagram). They remain here for billions of years in a stable state where the crushing gravitational forces are balanced by the outward pressure of fusion and the discharge of energy.
Mass is the single most important factor in determining the lifecycle of a star. The bigger they are the faster they live and the bigger they explode in a supernova. Nor is this a rare event. In fact stars explode somewhere in the universe every day. We just don’t see them because they are so far away. Easier for us to detect are supernovae in the Milky Way Galaxy. Astronomers estimate there are perhaps just three such events per century.
At birth all stars have one thing in common—their initial composition: hydrogen almost 90%, helium almost 10%, with minute traces of lithium and beryllium. These are the first elements that ever existed and the first four members of the periodic table.
The mass fraction of the sun, for example, is currently hydrogen 0.74, helium 0.25, and other 0.01. Mass fraction is the total mass of an element as a factor of the total mass of the star. In other words our sun is still 98.66%, hydrogen and helium, and 1.34% metallicity (the rest of the elements present). It will take most of the next five billion years to change from this current stable state.
Astronomers use the blanket term, “Metallicity”, to refer to all elements other than hydrogen and helium, as they are formed at the later stages of fusion. Super massive stars are able to take the process as far as iron. Heavier elements than iron are not created in a star. Only when a star goes supernova can heavier elements be created, albeit this requires total destruction of the star.
When a star is fusing elements into iron at its core, it is still giving off insane amounts of energy. The helium, hydrogen, carbon, oxygen, and silicon are still there in the star in different shells. Hydrogen is at the surface, still fusing to helium; a little further down, helium is fusing to carbon and oxygen; further down still we have silicon fusing to form the iron core. As the atomic nucleus gets heavier there is less energy the star is able to generate through nuclear fusion. It is simply unable to get the hydrogen fusion process to collide with the iron nucleus deep within the core. So essentially the process eventually peters out.
Stars that are rich in carbon, nitrogen, and oxygen, for example, are referred to as “metal rich” stars. On the other hand, metal-poor stars (ie those of much lower metallicity) are the oldest type of star, sometimes referred to as cosmic relics from the dawn of time.
The metal-poor stars are the oldest bodies in the universe simply for the fact that they originated in the Big Bang when metals did not exist. It was only when some of these stars became supernovae that heavier elements including metals were created. In turn these materials were incorporated when new stars formed. It follows that metal-rich stars are a lot younger.
Astronomers can use this fact in spectroscopy as a means to date a star. Spectroscopy identifies elements present in the light emitted by a star. Although this technique will only measure the composition of the surface of a star, astronomers can still make an educated guess as to the metal atoms forming inside. Stars with very small amounts of elements such as oxygen, iron, and gold, are likely very old. On the other hand if there is a large abundance of various elements then they must be formed from the remnants of previous explosions, making them part of a much newer generation.
Spin and Motion
All stars spin. Their motion derives from the way in which they are born: that is, by the accretion of materials gathering into a spinning motion under the force of gravity. And indeed they will rotate for their entire lives. When a massive star goes supernova, and is spinning fast enough, it will produce Gamma Ray Bursts (GBRs) from its poles. GBRs are very dangerous. One star, WR104, is expected to go supernova with GBRs within the next 100 000 years—and its pole is pointing right towards Earth. It could constitute a mass extinction event.
Researchers have found that metallicity affects the rate of spin. As the fusion process in a star creates the heavier elements below lithium, it is said to be turning metallic. Metal-rich stars spin down more effectively than metal-poor stars, and this is especially true for low-mass stars such as the sun, which incidentally rotates once every 25 days at about 7 000 kilometres an hour.
The fastest spinner is VFTS 102 located about 163 000 light years away in the Large Magellanic Cloud’s Tarantula Nebula. It is spinning so fast that a point on its equator is moving at about two million kilometres an hour, almost enough to pull the star apart.
Proper motion of a star is its apparent motion across the sky. This generally involves taking photographs several years apart and measuring the movement of the image of the star with respect to the background universe. Sometimes it can take decades before astronomers are happy with an accurate measurement.
Of course, not all stars conveniently move across our skies. Their progress may be directly head on, so to speak, with the star coming towards us or moving away from us. Astronomers call this “Radial Velocity”. How do astronomers measure this? Radial velocities of a star are measured using the Doppler Shift of the star’s spectrum—using a spectrograph. A star moving towards Earth will present a blue shift in its colour, if moving away it will appear as a red shift. If moving across out of line of sight, of course, there will be no spectral shift. Note that radial velocity is independent of distance.
True motions of a given star can be calculated by complex formula to take account of both its proper motion combined with its doppler shift, and its distance from Earth.
Finally, just to say that most stars wobble because they orbit a “common” centre of gravity—what astronomers call a “barycentre”.
For more in-depth discussion please refer to the following sections: The sun; Fusion; Nebulae; and the Hertzsprung-Russell Diagram.
By Nigel Benetton, science fiction author of Red Moon Burning and The Wild Sands of Rotar.
Last updated: Wednesday, 24th February 2021