A Bubbling Star.

We know that eventually the Sun will become a red giant. What then will the Sun look like?

In about 4.5 to 5 billion years the sun will swell and become a red giant star; this happens as the star runs out of hydrogen to fuse for energy and turns to material that’s harder to fuse. This means the balance between gravity and the expansive force caused by this nuclear fusion starts to become unbalanced and the sun will swell. It will then go through a phase of expansion and contraction where it expels its outer layers leaving behind a planetary nebula.

The Sun is very active: images show it to be a swirling, seething place;

solar granulation

The Sun is made from seven different layers;

At the centre is the core, this is where nuclear fusion occurs.

Next there is the radiative zone which “radiates” the energy created in the core by the emission and reabsorption of photons.

Then comes the convection zone; light particles (and all other particles) can take 170,000 years to travel through this layer. The behaviour of photons here is called the “random walk” where the photon collides with other photons: this happens as it is such a dense region.


Next is the photosphere this is the first of the three parts that make up the Sun’s atmosphere: this is where photons are finally emitted and give the sun its brightness. Oddly enough this area is opaque to light, meaning we can’t see trough it. If we could we would be able to view the thermonuclear core directly!

The transfer of energy from the convection zone below appears in the form of granules (see the photo above). As the hotter gas rises up, the cooler gas descends only to be re-heated by the convection layer and the process repeats itself. Sometimes disturbances in the magnetic field will produce sunspots, which occur within the photoshphere.

The chromosphere is next; the temperature ranges from 4400K at the base to 25 000K at its outer edge; no-one knows why the temperature rises so dramatically as it goes away from the surface of the Sun, it’s possible magnetism may be involved but it remains a puzzle.

Finally we have the corona. This outer layer is very dim – a million times dimmer than the photosphere and is the hottest region of the Sun at 10^6 K. Because the Corona extends several million kilometres into space, there is a lot of room for molecules to move. It is this movement that is the source of the solar winds. The high temperature of the Corona can force ions to move as fast as a million kilometres per hour. We can only see the corona during total solar eclipses when the disc of the sun is totally obscured.

It seems that when the Sun does begin to enter its final phases of existence it will still be quite an active place; Astronomers using ESO’s Very Large Telescope have for the first time directly observed granulation patterns on the surface of a star outside the Solar System — the ageing red giant π1 Gruis. It is located 530 light-years from Earth in the constellation of Grus (The Crane), π1 Gruis is a cool red giant. It has about the same mass as our Sun, but is 700 times larger and several thousand times as bright.

The image below from the PIONIER instrument reveals the convective cells that make up the surface of this huge star, which has 700 times the diameter of the Sun. Each cell covers more than a quarter of the star’s diameter and measures about 120 million kilometres across.

The surface of the red giant star π1 Gruis from PIONIER on the

Just one of these granules would extend from the Sun past Venus. The surfaces (photospheres) of many giant stars are obscured by dust, which hinders observations. However, in the case of π1 Gruis, although dust is present it is far from the star so it does not have a significant effect on the new infrared observations.

The Sun’s photosphere, in comparison, contains about two million convective cells, with typical diameters of just 1500 kilometres. The vast size differences in the convective cells of these two stars can be explained in part by their varying surface gravities. π1 Gruis is just 1.5 times the mass of the Sun but much larger, resulting in a much lower surface gravity and just a few, extremely large, granules.

While stars more massive than eight solar masses end their lives in dramatic supernovae explosions, less massive stars like this one – and our Sun – gradually expel their outer layers, resulting in beautiful planetary nebulae. Previous studies of π1 Gruis found a shell of material 0.9 light-years away from the central star, thought to have been ejected around 20 000 years ago. This relatively short period in a star’s life lasts just a few tens of thousands of years – compared to the overall lifetime of several billion – and these observations reveal a new method for probing this fleeting red giant phase.

So it seems there will be a lot to observe on the Sun even as it enters its old age.

PIONIER or the Precision Integrated-Optics Near-infrared Imaging ExpeRiment is an instrument on the Interferometer of the Very Large Telescope. Interferometry is the process of collecting beams of light and combining them together to extract more information and greater resolution from the object being observed. PIONIER can collect up to six beams of light making it incredibly sensitive.

Julien Milli, ESO astronomer at Paranal gives this musical analogy: “the object represents the complete song, and each baseline represents one of the notes that make up the piece. The more baselines we have, the more notes we have, and the more complete our version of the song is.”

PIONIER instrument

Another highlight of PIONIER is its spectral coverage. Adding to the information obtained. For example “…this helps us to characterise the warm dust around a star, providing relevant insights on the formation process,” says Julien Milli.

By using two or more light beams, an interference pattern can be formed with these beams. Because the wavelength of the visible light is very short, small changes in the differences in the optical paths (distance travelled) between the two beams can be detected (as these differences will produce noticeable changes in the interference pattern) meaning more information can be extracted from the light. (This technique is also used in Radio Astronomy.)

How does PIONIER work? It is an interferometer, so once the light reaches the instrument, it is sent across an optical circuit, smaller than a credit card, which brings the light waves from up to four different telescopes together in a very precise way so that they create an interference pattern. An interference pattern consists of fringes, i.e. alternative dark and bright stripes with a given contrast between them, so the final result is not a conventional image.

There’s a lot of engineering goes into Astronomy!