Why do some stars twinkle in colour? The answer here
Monday, September 12, 2016, 5:07 PM - This week's Full Harvest Moon presents a challenging eclipse for skywatchers, and we look at why some of the brightest stars in the sky twinkle in colour. It's the Night Sky this Week!
A shadowy Harvest Moon
This week features the September Full Moon - the so-called Harvest Moon - and it will have a few special effects tacked onto it for our enjoyment.
Animation of the Moon's phases, starting on Monday night and leading up to Friday night's Harvest Moon. Credit: Stellarium
Firstly, by the accounting of retired NASA eclipse guru Fred Espenak, this particular Full Moon will also be a supermoon - that is, when the Moon is full, it is within 90 per cent of its closest distance to Earth for the month. This isn't the closest Full Moon of 2016. That doesn't occur until November 14. It's also not the closest Harvest Moon, since the September 27 Super Harvest Blood Moon was even closer, but this week's full moon does share one thing with the 2015 Harvest Moon.
Depending on where you are in the world, this will be something of a shadowy Harvest Moon, because the Moon will be experiencing an eclipse!
Back in August, there was talk of an "almost lunar eclipse" because the south pole of the Moon was going to just miss the northern limb of Earth's outer shadow. This month, the Moon will definitely pass into Earth's shadow, however, it will only be covered by the outer part of Earth's shadow - the faint, grey penumbra.
So, we won't see the glorious crimson of a total lunar eclipse, and even for those watching from the best locations - anywhere in eastern Europe, eastern Africa, most of Asia and western Australia - it will still present quite the challenge.
Who will and will not have a chance to see this particular eclipse, based on their location. Credit: NASA
Since a penumbral eclipse only reduces the amount of light the Moon receives by a small amount, it's very difficult to notice unless you're watching very closely. Some may not notice any difference at all.
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Bright stars twinkle in colour
When we look up into the night sky, most, if not all of the stars we see just look white, but every once in awhile, a particularly bright star will appear to change colours as it twinkles.
Stars appear white because our eyes are only capturing a very small amount of the light they emit, so there typically isn't enough light reaching our retinas to discern colour. The brighter a star is, and the longer we stare at it - especially through binoculars or a telescope, since those focus more of the star's light into our eye - the more colour we can pick out.
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When we look at the very brightest stars in the sky - such as Arcturus and Capella (shown above) - especially when they are low on the horizon, though, we often see something that's a bit strange. These objects appear to change colour rapidly, from blue to white to red and back.
This doesn't have anything to do with the stars themselves, however. This colour oscillation is due to the atmosphere.
Although the air may appear clear and steady, when the light from stars passes through the air, it encounters turbulent eddies, especially in the warmer months of the year. These eddies refract the starlight, and this is why all stars appear to twinkle. Even planets can appear to twinkle if the atmosphere is turbulent enough.
When the light from these brightest stars passes through the turbulent eddies, though (and for Arcturus and Capella, they are so bright because Arcturus is a relatively close red giant star and Capella is actually a close quadruple star system!), the brighter light is refracted more strongly and is split into different parts of the spectrum, from red to blue. These different wavelengths of light can be split in different directions by the chaotic motion of the air, so the different colours reach our eye at different times, and it can appear to oscillate between both colours.
This happens when the stars are close to the horizon because the light is passing through more atmosphere from that angle, and thus interact with more turbulence than when they are higher up in the sky.