Achernar the elliptical Star

When you start to investigate some of our stellar neighbours you get a variation of characteristics presented to you. I think that Achernar is probably one of the most surprisingly unusual stars we have this close to the sun.

It is 139 light years from us although this figure seems to be quite elastic as other references put it at 144 lys from us. It is a hot class of star B3 Vpe main sequence still fusing hydrogen into helium as our own sun is, but has a luminosity of 2900 to 5400 time the sun.

It has a temperature ranging from 10,000 to 30,000 K but sources give an average of between 14,500 to 19,000K.  So that means it is in the colour range of Blue-white.

Its shape is unusual as it is not spherical as our Sum. It is an oblate ellipse. Actually, if you were to take some notice of the photographs below it is even more irregular in shape that this description made by other commentators which say that it is the least spherical star in the Milky Way galaxy studied to date. Its proportions are roughly 12:7.7 as a ratio of height to girth.

It has a estimated mass per unit volume of some 6 to 8 times the Sun and 7 times the Diameter of the sun assuming this is about its minor axis.

Even though it is rotating at 225km/sec it still takes 2.2 earth days to make one revolution.

Achernar

Achernar

This Photo show the star emitting gas from the end we will arbitrarily call the both pole.

Since the star is rotating at such a fast speed it is loosing a great deal of mass particularly from its equator. But because the star is loosing mass it is probably spinning at an ever increasing rate of spin.

Let’s assume that the star was spherical at one time and a large body collated with it glancing the star in such a way as to transfer energy to the star to rotate at a new faster speed. At the time this happened the star may have lost some mass, but at the faster rotation it continued to loose mass because the centrifugal force for many of the heavier gas particles was not enough to be counteracted by the gravitational pull of the star.

This put the star into a cycle of loosing mass and spinning faster. The theory of the conservation of angular motion says that as a particle becomes closer to the centre of mass it will spin faster around the mass. If we consider each atom of helium produced by the fusion process to be in an atmosphere around the star, each will move away from the star by centrifugal forces. as this happens the star becomes thinner at the equator and as a result the energy in the mass of the remaining particles in the atmosphere have to remain at the same energy level as a result start to move more quickly. With a spin period of 2.2 days and a speed of 225Km/sec it is easy to calculate the stars girth. With this it is again possible to work out the forces involved in the escaping gas from the star.

If this process were to continue the star will become quite an unusual type of star.

Colour saturation showing dark areas

Colour saturation showing dark areas

This Photo of Achernar shows the north pole at the bottom of the frame and was taken 10 days later than the first at the same time of night. This means that this photo is looking at the reverse side of the star from the one above and the one below.

You now can see some of the very hot gas around the star that has escaped the star’s atmosphere.

In addition you are able to see darker areas on the stars surface. these are not cooler areas but areas that are not producing visible light or light the camera can see. they may be  highly magnetic areas similar to our own sun.

Also it is easy to see this star as even more irregular that a simple oblate ellipse.

Hot and cold part of the star

Hot and cold part of the star

Finally, this photo was manipulated by reducing the brightness and increasing the contrast. If more contrast were used you would see a much darker area in the equatorial areas of the star. This agrees with stella commentators who suggest that these areas are cooler that the poles. This photo again shows the north pole at the top of the photo.

Article by David Holland

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