Why Do Stars Twinkle?

The question, ‘why do stars twinkle?’, has long been pondered. Now, first-of-their-kind computer simulations show why stars twinkle – due to the churning in a star’s depth, the journal Nature Astronomy reported.

By closely observing the innate twinkling of stars, the simulations could one day be used to discover what goes on inside stars larger than our Sun.

At the moment, however, the effects are too small for current telescopes to pick up. But this could change with improved telescopes.

Matteo Cantiello, a research scientist at the Flatiron Institute’s Centre for Computational Astrophysics (CCA), said: “We’ll be able to see the signature of the core which will be quite interesting because it will be a way to probe the very inner regions of stars.”

The paper is titled ‘The photometric variability of massive stars due to gravity waves excited by core convection’.

Uncovering the answer to ‘why do stars twinkle?’ will provide greater insight into the formation of our Universe.

For example, understanding the core of stars will help astronomers learn how stars form and evolve, how galaxies assemble, and how heavy elements such as the oxygen we breathe are created.

“Motions in the cores of stars launch waves like those on the ocean,” said study lead author Evan Anders, a postdoctoral researcher at Northwestern University.

“When the waves arrive at the star’s surface, they make it twinkle in a way that astronomers may be able to observe. For the first time, we have developed computer models which allow us to determine how much a star should twinkle as a result of these waves.

“This work allows future space telescopes to probe the central regions where stars forge the elements we depend upon to live and breathe.”

The new simulations also widen a stellar mystery.

Astronomers have consistently observed an unexplained pulsing, or red noise, that causes fluctuations in the brightness of hot, massive stars.

Previously, it was argued that the convection in a star’s core causes it to twinkle and flicker.

However, the new simulations show that the twinkling of the star induced by core convection is too faint to match the observed red noise.

Therefore, the researchers argue that something else must be responsible.

A star’s convection is powered by the nuclear reactor at its core. Here, intense pressure squeezes hydrogen atoms together to form helium atoms and excess energy.

This energy generates heat, causing clumps of plasma to rise. This convection is turbulent like a pot of boiling water, generating waves like those found in Earth’s oceans.

The waves then ripple outward to the star’s surface where they compress and decompress the star’s plasma. This causes the star to twinkle – resulting in the brightening and the dimming of the star’s light.

By studying why a star twinkles, the team realised they may be able to identify what is happening inside the core.

However, the researcher’s realised that studying the wave generation and propagation in a computer is extremely difficult. This is because while a wave-generating flow in the star’s core lasts a few weeks, waves generated can stick around for thousands of years.

To combat this challenge, the team was inspired by the sound waves that make up music, realising that the convection-induced wave generation in the core is like a group of musicians in a concert hall.

The team found that they could calculate an unaltered song of the convection-induced waves and then apply a filter that replicated the star’s acoustic properties.

The method was tested using sound waves from real music, including ‘Jupiter’ from Gustav Holst’s orchestral suite, ‘The Planets’, and ‘Twinkle, Twinkle, Little Star.’

The team simulated how those sound waves would bounce around inside stars of different sizes.

After the approach was validated, the researchers simulated the convection-induced waves and resulting starlight fluctuations of stars whose masses are three, 15, and 40 times that of our Sun.

For all three sizes, core convection was the answer to the question ‘why do stars twinkle?’.

However, the flickering light found near the surface was not at the frequencies or intensities characteristic of the red noise astronomers had seen.

Convection may be responsible for the red noise, but it would likely be far nearer to the star’s surface and therefore less telling of what’s going on in the star’s deep interior.

Now, the team are improving their simulations to consider additional effects, such as the rapid spinning of a star around its axis. This is a common feature of stars bigger than our Sun.

They aim to find out if fast-spinning stars have a strong enough flickering induced by core convection to be picked up by current telescopes.

“It’s an interesting question we’re hoping to get an answer to,” Cantiello stated.

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