A star is born

Powerful telescopes are shedding light on the mysterious process of starbirth. And these extraordinary new observations are beginning to give us a better understanding of our own origins, too, writes David Whitehouse

To many scientists, it is the stars, more than atoms and more than galaxies, that are the building blocks of the universe. We once thought that we understood the process of starbirth, but new observations show that we do not. The closer we scrutinise it, the more mysterious it seems.

As an example of how preconceptions about starbirth must be discarded, consider the mighty Eagle nebula. The dramatic pictures of it taken by the Hubble Space Telescope in 1995 became iconic, portraying it as a fertile region bursting forth with new generations of hot young stars. Yet observations made in the past year or so, also by Hubble, show the opposite to be the case. The Eagle nebula is essentially dead, its star formation petered out millions of years ago. All is not what it seems.

And, more recently, we discovered one of the first generation of stars to be formed after the Big Bang. This oldest star lacked the heavy elements that are made in subsequent generations of stars. Formed from the purest elements, it is telling us new things about how stars used to form.

We must understand the birth of stars because it has a bearing on almost everything in space; from the origin of life to the formation of planets, to the birth and evolution of galaxies. We are poised to look afresh at how stars such as our Sun are born because new thinking and the prospect of more powerful telescopes are throwing up new questions about their beginnings.

The birth of stars takes place in a wilderness of gas and dust that stretches through the vast uncharted regions between the stars. The Milky Way, our own galaxy, is filled with such natal clouds. The gas is mostly hydrogen, and the dust has the constituency of smoke. In such a cloud, little happens for countless ages. From a human perspective, the cloud is sluggish, but in its own timescale it has been active, fragmenting and contracting, preparing to give birth.

It is the initial stages of starbirth that are the least understood – the fragmentation and collapse of parts of a gas and dust cloud into protostellar lumps.

Seen from the outside, these clouds seem dark and gloomy. But on the inside they are arcades of gas brilliantly illuminated by hot newborn stars, and riven by long ribbons of gas forming darkened channels that lead nowhere. If you were inside the cloud, you would see the first stages of a new star as a tiny, far-off glow, as imperceptible as the Moon through thick cloud, and far smaller.

New thinking about this first stage in a star's life suggests that it is not the sedate, inward procession that it was once thought to be. Protostellar collapse may be quite violent, especially in large clouds that spawn great numbers of stars. The latest research indicates that the nature of the cloud itself plays a far more important role than was realised. It seems that it is the cloud's structure that determines the size of the fragments that will become individual stars and sometimes multiple star systems.

New observations with radio telescopes, many of which are yet to be published, have surveyed these placental clouds that are on the verge of giving birth. They show that the clouds are not smooth and uniform. They have structure on all sorts of scale in what scientists call a fractal pattern. It is satisfying to some that the same fractal behaviour that can describe the shapes of leaves on trees, snowflakes and shorelines also describes the birth pattern of the stars.

We now know that the old theory of stately stellar collapse, with material falling starward and the gradual ignition of a newborn star, is wrong. It has been replaced by chaotic infall of material with high-energy radiation coming from superheated gas. We now realise that powerful and contorted magnetic fields play an important role, as magnetic energy is pulled by infalling matter into ever smaller regions. And we see jets. The Hubble Space Telescope has seen many powerful jets of gas blown out from the new star in opposite directions, extending light-years into space.

New three-dimensional computer simulations show how a disc of dust and rubble forms around a young star. The portion of the disc that touches the star becomes superheated. Soon, however, the disc breaks up and forms clumps that grow ever larger as they sweep up debris, nascent planets. Each protoplanet is bathed by intense radiation, flares from their parent star a hundred to a thousand times more powerful than those produced by our own well-behaved, grown-up Sun.

Now, things happen rapidly. After only 10 million years, the star has switched on its primary energy source – hydrogen burning in a nuclear furnace – the fusion phase that will last for 10 billion years. This is the power that makes the light that warms its planets and possibly triggers the start of life on any suitable nearby planet. Then the young star blasts away all that remains of its placental gas cloud and the newborn stars leave home to wander the vastness of the Milky Way. Somewhere, out there in our galaxy, are the siblings of our Sun. Where they are we will never know because, since they were born, the spiral arms of our Milky Way have spun around almost 20 times.

Whereas we once thought we understood how stars are born, we now find ourselves poised for a revolution in our understanding of what really goes on. So much of starbirth is mysterious, but with the successor to the Hubble Space Telescope, a giant called the Webb Telescope due to be launched in a decade, or, much sooner, the Alma array of radio dishes in Chile that will make microwave observations, we could begin to see the real picture at last.

This is important, because understanding how the stars are born can help us to understand how we are here, as we are made of material that was once inside a star. The calcium in your bones and the iron in your blood were not around after the Big Bang, they were manufactured inside a star long before our Sun was born.

Inside mature stars, heavy atoms are assembled. When some of those stars explode, these heavy elements are scattered into space allowing subsequent generations of stars to use them to form planets and, possibly, life. More than anything else, it is the stars that shape the universe – they are the key to the ecology of space.

Dr David Whitehouse is the science editor of BBC News Online