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The birth of stars

Messier 100, A spiral galaxy
The spiral galaxy M100, a galaxy similar to our own.
(© David Malin, Anglo Australian Observatory).

If we look at the image of a distant galaxy, which is typically 120,000 light years in diameter, we do not see the light from individual stars but the diffuse glow from the combined light of 100,000 million stars. Not all stars have the same brightness and the hotter, bluer, younger stars are concentrated in spiral arms often crossed by streaks and patches of dark dust and gas. Amongst the spiral arms are bright, luminous, slightly pink blobs. These are the places where stars are currently being born.

The Sun and Solar System lie near a spiral arm close to the edge of our galaxy. The nearest place to us where stars are being born is only about 1,500 light years away, in the constellation of Orion. If you look at the sword of Orion with a pair of binoculars, on a clear winter's night, it is possible to see a faint fuzzy patch, which is called the great nebula in Orion. The word nebula (plural nebulae) is a Latin word meaning cloud, used by astronomers to describe any diffuse-looking object. A photograph of the Orion nebula taken with a telescope shows a swirling luminous patch of gas.

The Orion Nebula
The Sword of Orion and in the centre the Orion nebula.
(© David Malin, Anglo-Australian Observatory/Royal Observatory Edinburgh)

The luminous patch, which has been likened to a bite taken from an apple, is only a small part of a much bigger unseen gas and dust cloud about 100,000 to a million times as massive as the Sun and 60 to 150 light years in diameter. This molecular cloud is one of many that can be found throughout our galaxy. They are mostly made of hydrogen and helium with a sprinkling of heavier elements that have been made by previous generations of stars. They are called molecular clouds because inside them, sheltered from the radiation of nearby stars, it is cool enough to allow molecules of hydrogen to form as well as more complicated molecules. These include ethyl alcohol and others that comprise the basic building blocks of life.

Molecular clouds are continually stirred and mixed by the tidal forces they feel from other molecular clouds, as they orbit around the galaxy. They are massive enough for the force of their own gravity to make them collapse in on themselves and eventually form stars. We do not know exactly how this happens but it must be more or less along the following lines. At first, the density of the molecular cloud is so low that the force of gravity inducing the cloud to collapse is easily resisted by the turbulence caused by the tidal forces, as well as by the presence of magnetic fields that thread through them. However, eventually all, or more usually part of the cloud starts to collapse, perhaps triggered by the compression caused by a passing shock wave originating in a nearby supernova explosion. Once this happens the density in that part of the cloud will rise and the effect of gravity will increase. The cloud will fragment and eventually fragments containing one thousand times the mass of the Sun will again fragment into individual masses ranging from a few hundred times to a fraction of the mass of the Sun. These fragments will eventually form individual stars and possibly attendant planetary systems. Many of the fragments collapse in close association and will form double or multiple star systems. There are always many more small fragments than large ones.

The Eagle Nebula
Detail of the Eagle Nebula - a contracting cloud about half a light year across. The small dense blobs, about the size of our solar system, show where stars will soon be born.
(© Hubble Space Telescope).

As the fragments collapse they become smaller and denser. This means that they will rotate faster and faster, because they must conserve their initial angular momentum. This results in some of the matter forming a disk of material around the central condensation.

A second consequence of the increase in density and necessary conservation of energy of the collapsing fragment is that its temperature also increases. This is the same effect, only on a much larger scale, that is experienced when a bicycle tyre is energetically pumped and the end of the pump gets hot. This shows that the same laws of physics that apply to cycle pumps also apply to stars! Heating the material tends to slow its collapse because it creates a pressure that resists the inward squeeze of gravity. The raised temperature then evaporates the dust particles in the contracting cloud, which makes it more transparent, allowing radiation to escape. This then allows further contraction to take place.

Like all processes associated with the evolution of stars, the rate at which things happen depends on the mass of the star. The greater the mass, the faster the star is born from its gas cloud and the faster it lives out its life. A star the mass of the Sun will take about 10 million years to form from the initial gas cloud, while a star 11 times the mass of the Sun will take only about 100,000 years.

Eventually the central temperature and pressure within the newly born star are high enough to start the fusion of hydrogen to make helium, which releases energy and will power the star for most of the rest of its life.

As newly born stars burst into full brilliance their radiation heats and disperses the remaining dust and gas, which may amount to three quarters of the mass of the original parent cloud.

NGC3603, A young star cluster
A young cluster of stars disperses the remainder of the cloud out of which it was born.
(© European Southern Observatory/Very Large Telescope).

Star formation is an inefficient process and a bright young cluster may no longer have enough mass, within its member stars, to hold it together. The random motion of its individual stars will cause the cluster to disperse over a few million years. Our Sun has orbited our galaxy at least twenty times since it was born, long since losing track of its sister stars. Recent research suggests that in 150 million years the great nebula in Orion, which currently contains 700 young stars, may look like the Pleiades with only seven bright stars and no gas clouds.

A protoplanetary disk
A disk seen from the side, surrounding a young star in the Orion Nebula. The diameter of the disk is about the diameter of Pluto's orbit around the Sun.
(© Hubble Space Telescope).

The disks, made of gas and dust, that we see around many newly born stars also soon disappear; partly in jets of material that stretch up to a light year above and below the disk and partly into planets, which are so small they no longer block the light of their star. We do not know exactly how the disks of gas and dust are transformed into planets. The fact that all the planets in our solar system orbit the Sun in the same direction and lie in the same plane, suggests they have formed out of a disk. One recent idea is that during the time that the jets are seen above and below

The Triffid Nebula
Detail of the Trifid Nebula - the thin column pointing to the left is a jet of material emerging from a very young star hidden inside the cloud of gas and dust. The visible part of the jet is about one light year long.
(© Hubble Space Telescope).
the disk, magnetic fields linking the star and the inner edge of the disk are melting the dust into little droplets and ejecting them far from the Sun, where they eventually clump together to form the building blocks of future planets. Some of the meteorites that land on the Earth, called carbonaceous chondrites, are made of this material. They are dark in colour and contain grains about one millimetre across bound together in a carbon-rich matrix.

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