One Finite Planet

Our Sun: Big, White and a Yellow Dwarf?

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Our Sun is our very definition of white in colour, as can be seen in photos from space, and larger than most stars. Despite this, our Sun is a yellow dwarf! The page explores a few facts and myths about our star, the Sun.

Colour: White. Pure White: The Sun Defines White.

So why does the Sun not look white?

The Sun does look white when seen from space, as can be seen in pictures from the space station.

The Earth’s atmosphere can make the Sun appear to range from Red to Orange to Yellow from sunrise through the main part of the day, and back to Orange and to Bright Red, as the Sun rises, passes overhead, and and eventually sets.

Light from the Sun passes through more atmosphere to reach observers at Sunrise and Sunset.

Without clouds in the way, the ‘sky’ or atmosphere changes the colour the of the disk of the Sun from yellow to orange to red. This colour change occurs due to ‘Raleigh scattering’, which is the effect where a small amount of blue light is scattered in all directions by air, so not all of the blue light arrives directly from the Sun. Remove a small amount of blue from white, and the result is yellow. Remove more blue and a little green, and the result is orange. Remove all the blue and much of the green, you have red.

As the diagram here shows, sunlight passes through more atmosphere before being seen when the Sun is on the horizon. This means the effect on light of passing though air, will be increased at Sunrise and Sunset. The effect of ‘reducing the blue a little and the green even less’ (Raleigh Scattering), is exaggerated at Sunset and Sunrise. The effect at midday just makes the sun yellow, but at sunrise and sunset changes the colour even further to a deeper yellow, then orange and potentially even almost red.

Removing the blue from white produces yellow, so the scattering makes the Sun look yellow, and the rest of the sky blue. The blue sky is a result of light which would have been heading directly to the ground at all other locations, also being scattered, with some of that blue light from each point in the sky coming towards us. The result is our white light from the sun comes in two parts. Reduced, but still significant blue light does come directly, and additional blue light comes from the entire rest of the sky.

The Sun Defines White: How we know the Blue Sky and Yellow Sun add up to White.

We define white as the ‘sum of all colours’, but exactly what mix of all colours is ‘true white’? When we buy light bulbs, it can be seen that there are ‘different temperature’ light bulbs, lower temperature bulbs provide ‘softer’ yellower light, and higher temperature bulbs provide bluer, harsher light. Further, cameras have a ‘white balance’ setting, to compensate for different relative strengths of higher frequency blue light vs lower frequency yellow (red and green) light. Clearly there can be different ‘whites’ that provide light usable as ‘white’, without being what we could define as true ‘white’. All that is needed is for all frequencies to be present in approximately equal amounts. Different mixes of ‘white’ are more noticeable when directly compared, but our vision adjusts for small changes. If our vision could not adapt, as the Sun starts to set, and we get less blue light, all colours would appear to change. The same adaptability needed to see consistent colours with different light bulbs, is also needed to see a different times of the day.

Sunrise on the moon.

However, we define ‘true white’ as what we experience in the main daylight hours, which is the mix of all light provided by our Sun, from the ‘yellow disk’ and the blue sky. It is not a coincidence that the Sun is white, because what we evolved to see as ‘white’ is the mix of sunlight from our Sun. The exact mix we get from the blue sky and direct sunlight is what we have evolved to vision systems to see as ‘normal white light’. If the sun was actually yellow, we would in total be bathed in yellow light on Earth, not white light. However, if we evolved on a planet orbiting an actual yellow star, we would have evolved to see that light as ‘normal white’ and light from white stars as slightly blue. In fact if the sky was not blue, we would then see the disk of the Sun as white because, with only that direct light, our colour balance would adjust.

So however the sun shines, that is true white for us.

Just that Sun itself only shows as true white when not seen through air, as when seen from the moon (above), or from the space station (further above).

So why does the moon look white?

The moon reflects light from the Sun, and as the Sun is white, then it makes sense that the moon is white. Except, why is there no scattering to make the moon look yellow even though it is white? This is an interesting question, and there are two parts the answer:

  • The moon as we see it is yellow.
  • The moon looks white because the sky is not blue.

If you take a photo of the moon with the ‘white balance’ setting on a camera set for a sunny day, you will see that in the photo, the moon is in fact yellow. On a moonlit night, our eyes are adjusted for a more yellow white balance. In fact even our home lighting, and our computer and phone screens, now adjust for a more yellow white balance. If the sky around the moon appeared blue, we would see the moon as yellow at night. However, although there is still blue scattering increasing the blue of the sky at night, it is not bright enough for our eyes colour vision, so we see no blue sky at night, and as the colour we do see comes from the moon, so the moon is to us, white.

Why exactly is the sky blue?

Consider a clear light bulb. The light travels directly from the element to our eyes, so we see the element. No matter where we are in the room, the light comes directly from the element to our eyes. Even though light is coming through the entire globe, the globe does not appear lit, because the light travels directly through. Now consider a frosted light bulb. Light from the element hits the bulb and scatters. Because every part of the bulb scatters light in all directions, there is light travelling from every part of the bulb to every part of the room, so where ever we are the bulb looks white. The sky is like that frosted light bulb, but slightly frosted for blue light. Every part of the sky is hit by yellow light, but the light travels straight through so like clear light bulb, the sky does not appear lit by the yellow. Every part of the sky is also hit by blue light, but because that light is scattered, like a frosted light bulb, every part of the sky becomes a light source, at least for the blue light.

Size: Average, but much larger than most.

Larger than 75% of stars and brighter than 90% of stars.

How can the Sun be larger than most stars, but of ‘average size’? It depends on the spread of sizes.

Imagine a group of 20 adult male humans, with 15 of the group being the average American male height of 173cm or approximately 5’10”, 3 of the group being 185 cm (just of 6′) and 2 tall basketballers who are 210cm (6′ 10″) and two super giants as tall as the tallest man ever at 272 cm (8’11”). Despite those of 185cm being taller than 3/4 of the group, they are only average height for the group because of the presence of a couple of super tall people and a couple of super giants. This is what is it like for the the Sun.

Our star is a mid side ‘yellow dwarf’, the next size up from ‘red dwarf’.

Scientists think that 20 out of the 30 stars near Earth are red dwarfs. The closest star to the sun, Proxima Centauri, is a red dwarf. Red dwarfs include the smallest of the stars, weighing between 7.5% and 50% the mass of the sun.

space.com

The term yellow dwarf is a misnomer, because G-type stars actually range in color from white, for more luminous types like the Sun, to only very slightly yellow for the less massive and luminous G-type main-sequence stars.[9] The Sun is in fact white, but it can often appear yellow, orange or red through Earth‘s atmosphere due to atmospheric Rayleigh scattering, especially at sunrise and sunset.[10][11][12] In addition, although the term “dwarf” is used to contrast yellow main-sequence stars with giant stars, yellow dwarfs like the Sun outshine 90% of the stars in the Milky Way (which are largely much dimmer orange dwarfsred dwarfs, and white dwarfs, the last being a stellar remnant).

Wikipedia

So as a white yellow dwarf, the Sun is brighter than 9/10 of all stars, and bigger than around 3/4 of all stars. With 2/3 of stars being red dwarfs, all less than half the size of our Sun, and at least half of all main sequence stars (yellow dwarf) also being smaller than the Sun. However, despite being small in number, there are giants stars so huge that they really push up that average size, so even though the Sun is large compared to most stars, it is still only average size, and is just a tiny fraction of the size of the very rare extreme giants.

Why Yellow Dwarf?

As noted, the term ‘yellow dwarf’ is quite a misnomer, with most yellow dwarfs being white, and even the yellow ones being more the colour of a ‘warm’ light bulb, together with these stars being larger and brighter than the majority of other stars.

The term is old, and when it started, we just did not know better.

Why dwarf? Before better telescopes, we only observed ‘main cycle’ G-type yellow dwarfs like the Sun, and the few Giants that are even bigger. The bigger the type of star, the further away you can see it, so the largest stars will always be overrepresented in what we can see. The most common stars, red dwarfs, are not normally visible with the naked eye, and even the closest star other than the Sun, Proxima Centauri, can’t even be seen from Earth by the naked eye. When the only stars you can see are the biggest and brightest stars, and mostly relatively rare ‘giants’, then comparatively, stars that you can just barely see without a telescope, seemed like ‘dwarfs’.

The sun looks yellow because of Raleigh scattering. But why are other white stars described as yellow? Because when we observer stars on Earth, even the white ones can look a little yellow for another reason. Yellow is white, with some of the highest frequencies either reduced or ‘red shifted’, or both. Because highest frequencies are the most absorbed, even white things when viewed at sufficient distance can appear slightly yellow. Plus stars further away are ‘red shifted‘ because they are moving away from us.

A rare “only child”: Most Stars are in pairs or larger groups.

Imagine a sky inhabited by two suns? One reason this sometimes appears in science fiction, is that it is far more common that solar systems with a single star.

More than four-fifths of the single points of light we observe in the night sky are actually two or more stars orbiting together. The most common of the multiple star systems of stars [like ours] are binary stars, systems of only two stars together. These pairs come in an array of configurations that help scientists to classify stars, and could have impacts on the development of life. Some people even think that the sun is part of a binary system, [with the ‘twin’ having been displaced].

space.com

The latest data reveals that older data suggesting most stars are binaries was not correct, as newer data that includes the huge number of red dwarfs and even smaller stars reveals half of less of these smaller stars to be binaries. But for stars the size of the sun, the original data holds true: the significant majority of main sequence stars like the Sun are binary systems.

The Sun completely dominates the solar system: We can’t change it.

Sun – Planets Size comparison.

Even the mighty Jupiter is tiny in comparison with the Sun.

The Earth is hardly noticeable!

Even though some people had questioned if humans can be sufficiently significant to have affects that result in adjustment to Earth’s climate, it seems proven we have been able to achieve that. But to achieve a change of temperature on the Sun is an entirely different level. It seems far more logical to fix things on Earth, than count on being able to change the Sun.

Solar Lifespan 10 Billion years.

Our Sun has an estimated lifespan of 10 billion years. The entire time, the Sun gets hotter and hotter, with the increased energy levels making the Sun expand. Eventually the Sun will become a red giant, and expand so far that the orbits of Mercury, Venus, and possibly even Earth will be enveloped by the Sun itself. Clearly, life on Earth would not be viable at this time, but in fact, on Earth, the Sun will allow life is already almost at end.

Span of The Sun Supporting Life on Earth: 99% over.

The Sun Keeps Getting Hotter.

As with any main sequence yellow dwarf star, the Sun is continually getting hotter.

As the Sun continues to get hotter and hotter, eventually it will get too hot for life on Earth. When life on Earth began, the Sun was heating our area of the solar system to only approximately -94oC and only a lot of greenhouse gas kept us warm enough not be be an ice planet. Things have already warmed in our area of space, to around -18oC, which means we now need far less greenhouse gas to keep us warm. So things have warmed 78o in 4.5 billion years, and in one more billion it would already be getting hot with no CO2. The problem for us is, if there is no CO2, the plants all die. In fact there is around 25 million years left where we can keep the minimum level of CO2 need for plants, and still stay cool enough to prevent a heat spiral such as happened on Venus, and that would see the Earth reach temperatures of over 100oC. 25 million divided by the current age of the earth, 4.5 billion years, gives, 0.6%.

Forget the, ‘another 5 billion years’ before the Sun ‘explodes’, life has had over 99% of its time its quota already. Fortunately, for us humans, 25 million years is a long time to find a solution, but 99% over should sound a warning as to how fragile life on Earth is during the time that remains.

The ‘Goldilocks Zone’ keeps moving.

As the Sun, or any star, keeps getting hotter, the ‘Goldilocks Zone’ keeps moving further away. The pace the zone moves depends on the size of the star, with larger stars increasing in temperature more rapidly.

To be in zone where temperatures support life requires being in the zone where some combination of reflectivity and greenhouse gasses will result in the narrow window of ideal temperatures. Staying in the zone, as a star increases in temperature, requires either reducing greenhouses gas in response to temperature changes , or other measuring to reduce heat absorption, or moving further from the star.

Updates

  • 2021 April 6: Reformat