Sky Matters: Why do stars twinkle?

We are all too familiar with the Sun setting earlier, but less obvious is the fact that it also sets further below the horizon the deeper we go into November. 

And that means its rays don’t reflect off the Earth’s atmosphere as much as in summer, so the skies really are darker. 

The subsequent contrast between the stars and this inky black background makes the stars pop out. 

Our perception is one of the sky being filled with millions of points of light. 

Alas, not true — even on the darkest of nights, from the darkest of sites, the number of stars visible to the unaided eye rarely exceeds about 2,500.

In November, the most noticeable stars are probably the three found in the belt of Orion, the great mythological hunter that inspired the most prominent winter constellation that bears his name.

Simply look for three bright stars in a line towards the east horizon, rising around 11pm and visible until dawn. 

Orion also contains Betelgeuse (a massive star nearing the end of its life) and Rigel (also a massive star but at the beginning of its life) which are two of the brightest stars in the sky.

If you follow the line of Orion’s belt to the lower left you’ll come across the star that appears to be the brightest in both the northern and southern hemisphere — Sirius. 

In reality, Sirius is a fraction of the brightness of Betelgeuse or Rigel, but it’s much closer and so appears to us to be much brighter.

Look at the stars, even casually, and you’ll notice many of them twinkle. The closer they are to the horizon the greater the effect. 

Twinkling is the result of turbulence in the atmosphere, which itself is caused by differences in temperature as we move upwards away from the ground. 

That turbulence bends the light from the star and also splits it into its different colours. So not only does the star twinkle in brightness it twinkles in colour.

Astronomers have developed techniques to counter many of the effects of twinkling. They put their telescopes on top of high mountains. They choose places that are dry, since water vapour is an added contributor to twinkling.

They make telescopes with mirrors that can change shape in less than a thousandth of a second to counter the worst effects of twinkling.

And they even use “artificial stars” by firing powerful pencil-beam lasers into the sky. 

Incredible feats of engineering — and all to try and beat the atmosphere. 

But when you really really want to beat the atmosphere there’s only one solution — you have to go above it. From a height of even a few hundred kilometres a space telescope can collect light from near or distant objects without any interference from Earth’s atmosphere. 

This gives them the ability to take incredibly detailed images that simply are not possible from the ground. 

So if you ever wondered why the Hubble Space Telescope wasn’t the Hubble Ground Telescope, now you know!

As in October, the planet Jupiter (evening sky) and Venus (morning sky) are the brightest objects on view. 

On November 9 the crescent Moon and Venus will form a spectacular morning sight while on 24th and 25th Jupiter will appear close to the Moon after sunset.

On the night of the 17th, the annual Leonid meteor shower will peak. 

Every 33 years, with the return of comet Tempel-Tuttle which spews out the dust particles that are responsible for the Leonids, the meteor shower can be truly spectacular.

That next maximum is in 2034, but this year with the Moon being below the horizon throughout most of the night the skies will at least be as dark as can be expected and so you might see 15-20 per hour. 

And the best way to observe? Just look up!