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A stellar mid-life crisis: why do some cluster stars die early?

A new study shows only 30% of stars in a globular cluster will reach old age and become planetary nebulae. NASA

A stellar mid-life crisis: why do some cluster stars die early?

It was written in the stars all along, but we’ve just found out: a whopping 70% of stars in a widely-studied cluster die before reaching old age which, for stars, is the most productive stage of their lives.

The discovery, made by an international team of astrophysicists led by our research group at Monash University was reported in the international journal Nature today.

The finding shines a new light on the life-cycle of cluster stars and their contribution to our universe. Now, computer models of stars will need to be improved, and future observations of star clusters – both local and extra-galactic – will need to take this new information into account.

The life and times of stars

The life-cycle of a star similar to our sun (left to right) from birth in a giant gas cloud to the old-age red giant phase and finally to a white dwarf surrounded by a nebula. ESO

Combining increasingly powerful telescopic information with the study of physics has led us to a robust understanding of how stars work. Since most of the information about the universe comes to us through starlight, the understanding of stars underpins our understanding of the universe.

We now know that the light from stars originates deep in their cores, where nuclear fusion reactions (“nuclear burning”) releases heat and light. As each nuclear “fuel” runs out, stars move on to their next phase of life.

Our own star, the sun, is currently in its youthful phase, burning hydrogen in its core. After its core hydrogen runs out it will expand 100-fold to become a red giant – possibly engulfing Earth. Luckily we still have 5 billion years to prepare for this!

Next the sun will settle down in its middle age and become a blue(ish) giant, burning helium in its core. After it runs out of helium it will again expand, becoming a red giant for the second time, before finally dying as a white dwarf.

A star’s life cycle.

The “old age” second giant phase is important because it is the stage at which a star like the Sun makes its greatest contribution to the universe. It does this in two key ways:

  1. It burns very, very brightly – sending out light that can be seen far across the universe.
  2. It “puffs off” all its outer layers of gas and dust, enriching and polluting the surrounding space. This gas and dust gets recycled, going into the formation of the next generation of stars, planets – and possibly even life.

Missing stars

In today’s Nature we report the discovery that, contrary to expectations, not all stars get to live to old age. Using the European Very Large Telescope (VLT) in Chile we measured the amount of sodium in a range of stars in the globular star cluster NGC 6752.

The NGC 6752 globular star cluster which contains about a million stars. Stars in different stages of life can be seen here - red giants and blue giants, for example. NASA

The results are striking – 100% of the sample of stars in the final phase of life have low levels of sodium. This compares with only 30% of stars having low sodium in all other stages of life.

The only explanation for this is that 70% of stars - the ones with high sodium content - appear to be failing to get to the final giant phase.

Our star, the sun. NASA

One direct consequence of so many stars failing to reach old age is that most of the bright stars that we expected to be present will just not exist.

This causes problems for the study of the lives of stars because globular clusters, which are huge spherical collections of about a million stars, are routinely used to check computer models of stars - they are used as stellar evolution “testbeds”.

Our finding means that this method of checking the models is flawed, if old-age stars are used in the tests.

Is a low-sodium diet key to old age?

Just why such a huge proportion of stars should be failing to reach old age is a mystery to be solved.

Although it has been shown that sodium is a good indicator to track which stars will or will not reach old age, astrophysicists do not think it can cause the phenomenon.

But sodium must be strongly aligned to whatever the cause is. It is expected that this will be a universal trend for sodium-rich stars in all globular clusters but other clusters will be investigated to make sure.

Due to this new discovery, computer models of stars – including those calculated by our group at the Monash Centre for Astrophysics – will need to be improved so they can follow this phenomenon.