Star Spy: Colour rings
RINGS OF COLOUR
The subtle colours in the rings of light around the South Celestial Pole could be seen in Fraser’s star trail image last week.
Stars have many similarities with each other, and also some significant differences.
They vary enormously in size, also in composition, the length of their lives, their density, their temperature, whether they are alone or have companion stars and/or planets.
Each star shines in a particular colour of light, which can change over its lifetime when the dynamic processes within it change as the star moves through the various stages of its life.
The two pointers to the Southern Cross are a good example, and one that we can easily spot in Fraser’s star trail photos.
Star trails give us a strong sample of light from many stars, and they can reveal important information about the stars themselves.
Alpha Centauri, the nearest star system to the Sun, the brighter and more distant of the two pointers, shines with white light, as does our Sun.
To most eyes gazing at Beta Centauri, the pointer appearing closer to the Cross (Crux) in the night sky, it looks a lot like Alpha Centauri, just not as bright. We would probably say that it is white.
However, our eyes only see the light that strikes them in an instant. They can’t gather light like a camera does when the shutter is left open.
While we can often see the orangey tinge of a red star, we generally have more trouble distinguishing blue stars from white ones using just our eyes.
In Fraser’s time-lapse images the colours become quite distinct. See the white trail of Alpha Centauri in the above image, next to the blue trail of Beta Centauri.
The reason for the difference is temperature. We humans tend to turn red when we are hot as more blood comes closer to the skin surface to rid itself of excess heat, and blue when we are cold as our blood moves away from the surface to preserve our internal heat.
That is a very specific physiological device employed by our bodies. However, in physics, the temperature / colour relationship is just the opposite.
In stars, for example, the really hot stars are blue, the really cool ones are red, and in between – white. (BTW cool stars are still hot objects!)
The light we see from Alpha Centauri is radiated from its two bright component stars, A and B.
Both are very similar to our Sun, the A star being a little brighter, and B a little dimmer.
The surface of the Sun is around 5500 degrees C. The average surface temperature of Alpha Centauri A and B is a slightly cooler 5250C.
All three are in the white range for colour.
There is less certainty about the components of Beta Centauri, also called Hadar or Agena, but it is also a multiple star system and is in the category of blue-white supergiants.
Its overall surface temperature is calculated to be in the neighbourhood of 24,000 degrees C – much hotter, and therefore bluer, than Alpha Centauri, as you can clearly see above.
OUR SUMMER SKY IS COMING
If you are awake around midnight or 1am, bundle up and go outside looking toward the East. Fraser took this photo just two nights ago, in the wee hours of the morning.
It is our summer sky, risen beyond the Church of the Good Shepherd in Lake Tekapo.
You will recognise some of the objects, looking from left to right, Matariki (Plaiedes or Seven Sisters), the up-side-down head of Taurus the Bull, Jupiter, and Orion the Hunter (also up-side-down) with The Pot or Saucepan in his middle.
Jupiter is already rising around 11pm. As Earth circles the Sun moving into our position for southern summer, this entire summer panorama will move north into our early night sky. Keep a watch out for it if you are up late.