What is this twinkling colored dot of light? If I told you it is extremely far away and can only be seen at night? You guessed it, those are stars, bright ones. These are actually some of the brightest stars in our night sky and you’re about to see them in a very different way…
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. 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. However the light emanating from relatively close, so ‘large’ objects (sun, moon, planets…), overwhelms 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, but 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.
Sirius, the brightest star in our night sky
In this video I wanted to showcase the stellar scintillation of some of brightest stars in the northern hemisphere in a ver 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. The colors would then be more obvious, but several technical problems arose. The more out-of-focus the star was, the less ‘concentrated’ it light was, so the more I would have to compensate by increasing the ISO and reducing the shutter. However I realized on the spot that increasing my shutter speed would actually worsen the frame, since the scintillations were very often extremely quick and ephemeral. I had to keep a shutter speed between 1/15’’ and 1/30’’ to keep colors and details while still taking advantage of the maximum of light I could gather. I would also need the longest focal length lens I had with the widest aperture, so I after some test I decided to use the Samyang 135mm f/2, at f/2. While trying to get a decent sized bead in the frame, I necessarily had to bump up my ISO to at least 16,000 to get proper light, but that eventually caused some noise issue. After numerous tests and adjustments, these scenes were the best I could get out of cameras, lenses and time-being. 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, and 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!
I hope you liked this video that shed some novel light (pun intended) on stellar scintillation, and that it will encourage you to go outside and spend some time stargazing, because the possibilities are endless!
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