Why stars twinkle

It is the air

Pockets of air at slightly different temperatures keep bending a star's light.

To the eye, stars are so far away they are points of light (opens in a new tab). Earth's atmosphere, though, is clumpy: pockets of air at slightly different temperatures each bend the light a little, so they make many shifting images of one star, and as the wind moves the air the images keep changing. That is twinkling.

The effect is the same one that blurs telescope images. NASA's Astronomy Picture of the Day notes that adaptive optics (opens in a new tab), mirrors that change shape to cancel the air's distortion, have made great advances against it; space telescopes avoid it altogether.

Sources for this section Robert Nemiroff and Jerry Bonnell, Why Stars Twinkle, Astronomy Picture of the Day, NASA (25 July 2000) (opens in a new tab)

Colours, and why planets twinkle less

Twinkling can flash colours, and planets, being tiny discs rather than points, mostly hold steady.

Turbulent air bends different colours of light by different amounts (opens in a new tab), so a star can flicker in colour as well as brightness. A star is effectively a point source, a narrow bundle of light rays, and so is easily disturbed.

A planet is much closer and shows a tiny disc, a broader bundle of light, which on average is less affected (opens in a new tab). In APOD's example, a swinging camera turned bright Regulus into a rainbow-coloured trail while Mars beside it stayed steadily reddish.

How to see it Watch Sirius when it is low. Near the horizon (opens in a new tab), the Night Sky Network notes, its light mixes with the atmosphere's turbulence and flickers with colour; higher up, through less air, it is calmer.

Sources for this section Robert Nemiroff and Jerry Bonnell, Scintillating, Astronomy Picture of the Day, NASA (28 April 2011) (opens in a new tab) · ; NASA Night Sky Network, Sirius and Procyon (opens in a new tab)

Twinkling is not a star changing

Twinkling lasts a fraction of a second; a variable star changes over days, months or years.

Some stars really do change. Betelgeuse (opens in a new tab) swells and shrinks, brightening and dimming, on a cycle of about 420 days, and in 2019–2020 it faded by more than two-thirds over several months.

Twinkling is far quicker: the shifting images in APOD's sequence of Betelgeuse (opens in a new tab) change faster than the eye can follow. If a star looks different from one night or week to the next, it may be a variable; if it shimmers from second to second, it is the air.

Sources for this section NASA Science, Hubble Finds that Betelgeuse's Mysterious Dimming Is Due to a Traumatic Outburst (2020) (opens in a new tab) · ; Robert Nemiroff and Jerry Bonnell, Why Stars Twinkle, Astronomy Picture of the Day, NASA (25 July 2000) (opens in a new tab)