Why does Sirius twinkle?

Jan 17, 2012 By Adrian West, Universe Today
Orion and Sirius Credit Adrian West

At this time of year, after dark we in the northern hemisphere are able to see the mighty constellation of Orion rise high in the sky with a very bright companion in a nearby constellation: Sirius – The Dog Star.

Sirius is the brightest star in the sky and can easily be found in the faint of Canis Major to the left and below Orion. Its name comes from ancient Greek meaning “glowing” or “scorcher.”

Sirius (α CMa) is the alpha star in this trusty hound and is roughly 8.5 light years away from Earth, making it one of the closest stars to us. It has a tiny companion star making it a binary system composed of “Sirius A” the main component (which is a white main sequence star) and “Sirius B,” a white dwarf star. As seen with the naked eye, Sirius can be seen to twinkle many different colors low in the winter evening sky.

So why does Sirius twinkle?

Sirius. Image credit: Hubble

It’s not just Sirius that twinkles; all stars twinkle. Light travels many light years from stars and right at the end of its journey, it hits Earth’s atmosphere, which consists of nitrogen, oxygen and other gasses.

Earth’s atmosphere is constantly swirling around, and wind and air currents etc distort light travelling through it. This causes the light to slightly bend or shimmer and the light from distant stars twinkle. An extreme, more down-to-Earth example of this would be heat rising off of a road or a desert causing objects behind it to distort, shimmer and change colour.

Sirius appears to twinkle or shimmer more than other stars for some very simple reasons. It is very bright, which can amplify atmospheric effects and it is also very low down in the atmosphere for those in the . We are actually looking at it through a very dense part of the atmosphere which can be turbulent and contain many different particles and dust. The lower towards the horizon an observer is looking, the thicker the atmosphere. The higher an observer is looking, the thinner the atmosphere. This is also the cause of colourful sunrise and sunsets.

(Addition due to the questions in the comment section: planets don’t usually twinkle because they are closer and therefore bigger — they are disks of light instead of faraway points of light. The larger disks of light usually aren’t distorted; however if you are looking through especially turbulent areas of our atmosphere, and even sometimes when looking at planets that are low in the thicker parts of the atmosphere, they will twinkle. Phil Plait, the Bad Astronomer explains it very well on his website.)

This optical illusion is a big pain for astronomers and some very large telescopes such as those in Chile and Hawaii use special equipment and techniques to reduce the effects of the atmosphere.

One of most famous telescope of them all, the Hubble Space Telescope doesn’t get affected at all by our as it is in space, making the from crystal clear.

Twinkle, twinkle little star, now we know what you are (and why you are twinkling!)

Explore further: New mass map of a distant galaxy cluster is the most precise yet

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User comments : 10

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chuckscherl
1.7 / 5 (3) Jan 17, 2012
Sirius is a triplet star system
yyz
3.7 / 5 (3) Jan 17, 2012
"Sirius is a triplet star system"

Really? How so?

I see only two stars in images of the Sirian system: http://en.wikiped...-ray.jpg

(Note that the white dwarf Sirius B is the brightest star in that Chandra x-ray image.)
chuckscherl
4 / 5 (1) Jan 17, 2012
yyz
5 / 5 (2) Jan 17, 2012
Thanks for the link, chuck.

One problem I see with their analysis of perturbations in the Sirius AB system is reliance on observations made from before 1940 (they cite two 1932 studies and a 1983 paper which likely used some data taken pre-1940).

Back in the day, double star measurements were made by photographic plate or visual micrometry. While trying under normal circumstances, the unequal brightness of Sirius A & B makes these types of measurements incredibly difficult and, alas, prone to error. So I'm not too sure of the 6-year perturbation that these authors claim exists in the orbit of Sirius B.

Since this paper was published (1995), deep near-IR searches (as suggested by the authors) and detailed radial velocity studies (that could pick out even substellar planetary-mass companions) have repeatedly failed to turn up this hypothetical Sirius C.

IIRC, wide-field (over several degrees) searches for a distant, very long period companion have been made, also without success.
Au-Pu
1 / 5 (3) Jan 17, 2012
The allegedly thicker atmosphere near the horizon is a)not true.
The atmosphere is not thicker. But the light must travel through more atmosphere than it would when directly overhead and b) this is not the cause of colourful sunrises and sunsets. The cause of this is the gravitation of the planet bending the light, with the longer red end of the spectrum bending more than the shorter wavelengths.
Not a well written report.
Eoprime
not rated yet Jan 18, 2012
...The cause of this is the gravitation of the planet bending the light, with the longer red end of the spectrum bending more than the shorter wavelengths.


Source?
yyz
5 / 5 (1) Jan 18, 2012
"The cause of this is the gravitation of the planet bending the light, with the longer red end of the spectrum bending more than the shorter wavelengths."

So an observer on the moon would also experience colorful sunrises and sunsets? On Mercury?
TheGhostofOtto1923
5 / 5 (2) Jan 18, 2012
"Sirius is a triplet star system"

Really? How so?
"Some binary stars are so close to each other that the only way astronomers can tell if the star is single or double is through spectral analysis. These are known as spectroscopic binaries. Still others are detectable when the secondary star's orbit carries it in front of, and then behind, the primary, causing the system's total light output to fluctuate. Theses are called eclipsing binaries."
Source?
His butt-
Xbw
1 / 5 (1) Jan 18, 2012
@chuckscherl
That is a rather old source of information bud. Props for finding it but at the rate of scientific advancement, especially in the field of astronomy, I wouldn't credit it.

Ghost is correct. It is most likely a system of eclipsing binaries.
laserfloyd
not rated yet Jan 26, 2012
The allegedly thicker atmosphere near the horizon is a)not true.
The atmosphere is not thicker. But the light must travel through more atmosphere than it would when directly overhead and b) this is not the cause of colourful sunrises and sunsets. The cause of this is the gravitation of the planet bending the light, with the longer red end of the spectrum bending more than the shorter wavelengths.
Not a well written report.


It's thicker in relation to the observer. An object on horizon passes through much more atmosphere; i.e. it is thicker. Directly overhead it passes through less atmosphere; i.e. it is thinner.

There isn't more gravity at the horizon... We'd have bigger problems if we had THAT much gravity. Or, smaller problems I should say. :)