Twinkle twinkle little star…
Have you ever wondered why stars twinkle, some more than others, and some not? Well here is your chance to find out in a very visual way.
Why stars twinkle in the night
The light emitted by objects at a very long distance from Earth (at least several light years) travels to us almost without being altered. However as photons are about to end their journey, they face a major obstacle: Earth’s atmosphere. The latter is made of cool gases but is very turbulent with zones of different densities and masses. This differences in pressure, temperature and density make our atmosphere a real hurdle for the light to arrive ‘in one piece’. As photons hit these different layers, they are being diffracted and scattered. This goes for all sources light coming from space.
Orion and its multiple bright stars showing beautiful colors as seen from the Aurora Borealis observatory.
However the light emanating from relatively close, so ‘large’ objects (sun, moon, planets…), overpower the hurdle without any problem. It’s a whole different story for the light coming from far-away objects like stars, creating pin-point beams. Since the beam is smaller with fewer photons, we will tend to notice their diffraction and scattering a lot more. Causing the light to rapidly and temporarily shift color and brightness.
We call it stellar scintillation. It has actually been observed and studied since the dawn of human kind. Recent research has found that stellar scintillation is not only a change in apparent brightness and color of a star, but also position. It has been shown that the fluctuations actually depend on the absolute magnitude of the object (apparent magnitude according to its distance to us) and its elevation in the night sky. Close to the horizon, the light of a star would have to cross more atmosphere, thus different layers, and subsequently twinkles more. Conversely a star nearing the zenith tends to twinkle less.
I was truly amazed
In this video I wanted to showcase the stellar scintillation of some of brightest stars in the northern hemisphere in a very different way. There exist some real-time videos of Vega lying around on the internet, but these are usually taken when stars are in focus. In order to increase the apparent area of a star, I needed to manually open up the aperture to its maximum at get an out-of-focus frame. Out of these very technical shots, I was amazed at what I was seeing. I had never seen in so much detail the frozen ‘facets’ of twinkling stars, the fleeting evidence of light diffraction by our atmosphere.
The successive marble-like dots changed in brightness and colors, although it was very difficult to verify the change in position (maybe too small to see). Very hot stars like Vega or Sirius emit blue light, because their whole emission spectrum is ‘dragged’ towards the shorter wavelengths of visible (blue, violet) and invisible (UV). You can still see other colors and even some occasional ‘rainbows’ that remind you that our atmosphere actually act like a prism.
Cooler stars like Betelgeuse or the group of stars Capella emit yellower or redder light, and their scintillation will have an overall red tint. Betelgeuse would be the most luminous star in the night sky if we were able to see all its radiations. But take a close look at its fluctuations: from sometimes invisible to extremely bright red, green or yellow, taken with exactly the same settings as the others. Then you can see its area is considerably smaller than the other 3 stars showcased. It’s the one-of-its-kind star that has the biggest fluctuations! Sirius is the brightest star in all our night sky, and its fluctuations seemed fewer than the other stars (however low in elevation it was when I took the shot: 25°), maybe because of its ‘close’ distance to Earth. However it was the easiest star to shoot, because it emits so much light!