Armagh Observatory: School Student Essays

Evolution of stars A brief look at Sakurai’s Object Tania Hendron, Coleraine High School, 13 February 2003

Sakurai’s Object (V4334 Sagittarii) was discovered on the 20^th of Febuary 1996 by a Japanese astronomer Yukio Sakurai. He photographed a new 12^th Magnitude star in Sagittarrius. He then reported this to the International Astronomical Union’s Central Bureau for Astronomical Telegrams. Initially astronomers thought it to be a nova (an explosion on the surface of a white dwarf). A nova usually brightens very quickly within a few days, then fades over the next several weeks. Sakuarai’s object continued to shine brightly on, the first indication that this was no ordinary star. Astronomers then studied the spectrum of the star. When it failed to show any hydrogen emissions from the star, the charateristic of a nova, they realised that this was a very interesting star. It was then discovered that Sakaurai’s object has a faint planetary nebula surrounding the star.

Today astronomers belive that this is a "born again" giant, a dying star that temporarily postponed its fate as a stellar ember by using its final bit of fuel to grow to super giant size one last time.

What is a planetary nebula? And what does it accually mean in the case of Sakurai’s object? To answer these questions you have to think about the structure of the star and how a star stays "alive". A star relies on the process of fusion to keep it ‘burning’. It is a process involving the transforming of mass into energy, in the form of heat. Light atoms such as those of hydrogen, are fused or joined to form a heavier atom like helium (the next lightest element).

A Planetary Nebula is a series of glowing shells of gas around the remains of the star. When a planetary nebula is formed, it means that the star can no longer support itself by the process of fusion in its center. This can be due to the hydrogen being used up. The forces of gravity causes the inner core of the star to heat up and condense and then it rapidly collapses. The high temperature that builds up results in the central regions driving the outer half of the star away in a brisk stellar wind, which usually lasts a few thousand years.

Then the remaining core is uncovered and the distant gases are heated and start to glow, while at the center a white dwarf star has been formed. The collapse of the star doesn’t

stop until the density is at a maxium level, where a cubic centimetre would weigh several hundred kilograms. The white dwarf that has formed continues to give off light because of the heat still trapped inside it, but over millions of years, it will eventually cool and fade away.

But why is Sakurai’s object so interesting? Are all other stars not the same?To fully comprehened the importance of Sakurai’s object you have to consider the life cycle of a normal star. This is best accomplished by a Hertzsprung-Russell Diagram.

 

A stars positon on the graph is determined by the total energy output (luminosity) and the temperature. A star will move across the diagram in a specific way. Normally this motion across the diagram would take billions of years. So the motion will not been seen in a human life time. This is one of the main reasons for Sakurai’s objects importance.

Take for example a star that is twice the size of our sun. It will start off somewhere along the main sequence. Where it does depends on its luminosity and its temperature. It will then remain there for the bulk of its life turning hydrogen into helium.You can see this in the diagram below left, the blue dot represents this phase. In time the core is completly turned from hydrogen into helium. The fusion continues around the core where there is an ample supply of hydrogen. The core then gives way to gravity and it compresses and heats up. This causes the outer layer to expand and its surface cools. This is when the star becomes a red giant, it has now moved into the red branch of the diagram. Meanwhile the core continues to shrink. When it reaches 100 million degrees, helium spontaneously converts into carbon and oxygen, the star will be on the horizontal branch. This ignition occurs quickly and fluffs up the star’s outer layers. The hydrogen shell then extinguishes. Helium fusion then continues on in the core, but it is only about 1/10 efficent as hydrogen, therefore the helium vanishes 10 times as quickly. Once more the star swells and brightens, becoming a red supergiant. It is now several hundred times larger than when it was in the main sequence and has moved back into the red giant branch. It will then begin to exhaust it’s remaining gas over the next few years of its life. As it now ages it will have a "helium shell flash" any time the helium is converted in to carbon and oxygen. Due to the helium shell flashes the star will lose between 30 and 80 percent of its mass. This continues until there is only the core left and then there is a final helium flash and the white dwarf and the planetary nebula are formed.

This should be the end of it for the star. That is for all except "born again" stars. If the star is changing from a red super giant to a white dwarf when the star has its final Helium flash it will swell up to be a red supergiant once more. It seems to be a useless attempt at former glory, unfortunately all it does is slow down the approach of its inevitable downfall.

Sakurai’s object was in fact a white dwarf when it went through this last helium burst. Usually a white dwarf is a slowly cooling dead star that has stopped generating energy by nuclear reactions, and collapsed under its own weight. Yet Sakurai’s object found enough helium to produce one final helium shell flash. Infact this seems to suggest that actually it is a highly evolved star that is going though another evolution!

Another interesting point is that when a helium flash occurs, the matter created in the core of the star is released. This allows astronomers the opportunity to see directly in to the star and gives a model for theorys to be based upon. We can also witness the evolution of a new outer layer (photosphere) and the birth of a new nebula. These areas of research are a lot closer to finding suitable answers thanks to the discovery of Sakurai’s object. What makes this object interesting is that we get to watch a process unfold that takes years and decades, not millenia! We can actually see what is going on at the surface of the star as it reaches a key point in evolution. Sakurai’s Object promises to be a mile stone of understanding the evolution of ancient stars.