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I watch the discovery science channel a lot & enjoy the material about space, stars, planets, etc. In one episode the narrator states our Sun will one day become a white dwarf star. The size of our Sun will double and ultimately the Earth will be inside this enlarged Sun. This the end of all life on Earth. Then I read a recent Snapple bottle cap that stated our Sun is really white. I read it again and chuckled: is it already a White dwarf star at this point? I take it that the original color of the star happens to be white. How will we know when the Sun will become a White dwarf and increase in size the engulf the Earth? Basically what is the time frame in this context.

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    $\begingroup$ Our sun will become a red giant and then will engulf several of its planets. After that phase it will become a white dwarf. As far as the color of the light from the sun goes, it's white: askanastronomer.org/stars/faq/2015/11/06/… $\endgroup$ – Phil N DeBlanc May 9 '18 at 20:03
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    $\begingroup$ @PhilNDeBlanc If you expanded on that comment a little (e.g. explain the difference between a star like the Sun and a white dwarf ) it would make a viable answer. On Astronomy SE we'd prefer people to make proper answers (which they can get voted up for) than comments. $\endgroup$ – StephenG May 9 '18 at 20:08
  • $\begingroup$ @StephenG Thanks, but like the OP, I'm a Science Channel fan and an armchair astronomer. I'm far from an expert on the subject and didn't feel comfortable coming across as such. $\endgroup$ – Phil N DeBlanc May 9 '18 at 20:24
  • $\begingroup$ In terms of color, white dwarfs are likely blue due to their high surface temperature, at least shortly after they first form. As they cool they turn whiter then reder then darker over millions or billions of years. Our sun is actually white if viewed from space. Our atmosphere makes the sun appear yellow. $\endgroup$ – userLTK May 10 '18 at 3:44
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    $\begingroup$ @userLTK - Our atmosphere makes the Sun appear red or yellow when your eyes can bear looking at the Sun. When the Sun is roughly 20° high in the sky or higher, it's as white as white can be. You just can't see that because looking at the Sun at such elevations would almost immediately result in eye injury. $\endgroup$ – David Hammen May 10 '18 at 19:52
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First. In about 6 billion years the Sun will become much larger than it is now, maybe a factor of 20. It will be a red giant. Then a little while later it will get really big, as large as the earth's orbit and perhaps the Earth will be engulfed.

Second. Only after this does the Sun lose about half of its mass and the remainder is exposed as a small, hot, white dwarf star, about the same size as the Earth but half the mass of the Sun. It will have a density about 1 million times that of water and it is held up by a pressure that only arises because of the quantum mechanical nature of the electrons within it.

The Sun is not currently a white dwarf. It is a G-type dwarf star, supported by "normal" gas pressure and will remain reasonably stable, though will become about 50% more luminous over the next 5 billion years or so.

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The Sun is white, in the sense that you can hold up a white piece of paper to it and see no color, it's what our eyes have adapted to. And it is a dwarf, in the sense that it is a main-sequence star, and all such stars are called dwarfs. But it is definitely not a white dwarf, that's a much smaller (size of Earth) and much older star. The names are not always very clear!

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  • $\begingroup$ +1 just for the last sentence. Dwarf planet, white dwarf, red herring: They all have something in common. The names are not always very clear. $\endgroup$ – David Hammen May 10 '18 at 19:54
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There's a bit of confusion in the nomenclature here. This is very important: The nomenclature for labeling largish objects in space was developed before scientists knew anything about fusion. Stars on the main sequence fuse hydrogen to form helium. Except for extremely massive stars, every star on the main sequence is a so-called dwarf star. These main sequence stars are called dwarfs because they aren't anywhere near as large as are the giant stars.

Our Sun is on the main sequence (i.e., it's fusing hydrogen into helium) and will remain so for another five to seven billion years. Star-sized objects leave the main sequence when they've consumed all available hydrogen. Stars that aren't too small will eventually start fusing helium after they leave the main sequence.

Despite the common word "dwarf", white dwarfs form a category of objects that is very distinct from dwarf stars. White dwarfs are objects that not only have left the main sequence, but have also fused every last dreg of fusible material available to them. This necessarily makes white dwarfs very compact, and temporarily, extremely hot. Note well: "Temporarily" to an astronomer or astrophysicist means something very different than it does to everyone else. Once again, the nomenclature was developed over a hundred years ago, well before nuclear fusion was known to exist.

It would perhaps be nice if the International Astronomical Union would come up with a working definition of what constitutes a "star". However, I suspect they have had their fill with naming conventions given the huge controversy over their definition of what qualifies as a "planet".

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  • $\begingroup$ Aside: I fully agree with the IAU's definition of what constitutes a planet. The only disagreement I have is their choice of the term "dwarf planet". "Dwarf planet" sounds like it denotes a subcategory of "planet", but that is not the case. Dwarf planets are objects that are massive enough to have pulled themselves into a roundish shape, but not so massive so as to ever have a chance at clearing their neighborhood of lesser objects. $\endgroup$ – David Hammen May 10 '18 at 20:39

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