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175 votes
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How can there be 1,000 stellar ancestors before our Sun?

The Sun is actually a THIRD generation star. What I mean by this is that there are chemical elements in the Sun that were made inside another star, but that star itself can only have made those ...
ProfRob's user avatar
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114 votes
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Would we have more than 8 minutes of light, if the Sun "went out"?

If nuclear fusion were to suddenly stop in the centre of the Sun, then the only clear signature we would have of this is the lack of detectable neutrinos received at Earth, starting about 8 minutes ...
ProfRob's user avatar
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86 votes
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How did Astronomers deduce that the Sun was not a ball of fire?

I think it's maybe not the case that there was a moment when the astronomy community conclusively rejected the ball-of-fire hypothesis; astronomers simply accumulated more and more evidence against it....
HDE 226868's user avatar
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76 votes
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Is it dark inside the Sun?

No, it's not. The radiation field in the interior of the Sun is very close to a blackbody spectrum. If you look in any particular direction the brightness (power per unit area) you see is $\sigma T^4$,...
ProfRob's user avatar
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74 votes
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Why can't we see Saturn's phases from earth?

Phases are just different perceived illuminations of an object at different illumination and observing angles. If the observer is, with respect to the object, located in a similar direction as the ...
Swike's user avatar
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71 votes

Sun constantly converts mass into energy, will this cause its gravity to decrease?

If the sun is constantly converting the mass into energy, then will its gravitational field go on decreasing? It's a very interesting question and the answer is yes! The solar constant indicates the ...
uhoh's user avatar
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70 votes

What happens to the 99.9 % of the sun-rays that do not fall on any planets or any other celestial body?

The light from the Sun spreads, at least initially, in a roughly isotropic fashion into the universe. As it gets further from the Sun, some of that light will interact with the interstellar medium (...
ProfRob's user avatar
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68 votes
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When will the Sun end all human life on Earth?

The Sun is gradually getting larger and brighter. In fact, as called2voyage pointed out, its brightness is increasing by 1% every 100 million years. You can see how the Sun will change in the future ...
Sir Cumference's user avatar
64 votes

Why is moon light not the same color as sunlight?

The light from the moon is light being reflected from the sun. This is at least one reason you should not expect the Moon to have the same color. Sunlight hitting an e.g. blue object would appear ...
StephenG - Help Ukraine's user avatar
63 votes

How were sundials and moondials possible 800 years ago?

As @JohnHoltz points out in a comment, planting a stick in the ground or in a wall and watching where the shadow falls is something very easy; sundials have been known since prehistoric times. I’m not ...
Pierre Paquette's user avatar
61 votes

Do our sun and moon have names?

What is a name? A name is a word, that is reasonably unique, that is used to identify a person or thing. When a child is born there is no word that identifies it, and so its parents have to choose a &...
James K's user avatar
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60 votes
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Why isn't the asteroid belt affected by Jupiter's gravitational field?

The asteroid belt is affected by Jupiter's gravity. There are stable orbits inside of Jupiter's orbit. Jupiter's Hill Sphere has a radius of 53 million km. If you are more than 53 million km from ...
James K's user avatar
  • 123k
60 votes

Is there enough matter orbiting the sun to make a second sun?

The vast majority of the stuff in the solar system other than the Sun itself is contained in one body, Jupiter. The total mass of the solar system is estimated to be about 1.0014 solar masses, or ...
David Hammen's user avatar
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57 votes

Why can't we see Saturn's phases from earth?

All the other answers here are complete, and more in-depth than anything I would write. However, if you prefer to look at things visually, here is a terrible not-to-scale 2 minute paint drawing. No ...
Azrantha's user avatar
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56 votes
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Why does the Sun have different day lengths, but not the gas giants?

It's a matter of how "day" is defined. Wikipedia's article on Jupiter cites this IAU/IAG paper for the length of a Jupiter day. In it, footnote (e) of table I has the following: The equations for ...
Mark's user avatar
  • 3,393
52 votes
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Is the sun's volume shrinking currently?

Is the sun shrinking currently? It's the other way around: The Sun is slowly growing hotter and thus is slowly expanding. The Sun accumulates ever more helium in its core as it ages. This growing ...
David Hammen's user avatar
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52 votes
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How much of the surface of other planets is lit by the sun?

OK, let start with some assumptions: spherical objects and no atmospheric effects. Here's the relevant geometry with the object on the left representing a planet and the object on the right ...
GrapefruitIsAwesome's user avatar
50 votes
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Is the Solar core hard?

The solar core can be considered soft in a relative sense (compared to other materials at the same density), but hard and incompressible in an absolute sense. The material behaves almost exactly like ...
ProfRob's user avatar
  • 153k
48 votes

When will the Sun end all human life on Earth?

At what point will the sun make earth uninhabitable for humans? It nearly happened on May 23, 1967. According to this history paper released yesterday, summarized nicely here on Space.com, at the ...
WBT's user avatar
  • 597
48 votes
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How did the ancient cultures determine that the year was actually a fraction of an extra day beyond 365 days?

This is not so hard to do. All you really need is a bit of care, a few sticks and a convenient place to observe the sunrise or sunset. So note that there is a cycle of the year and you want to know ...
James K's user avatar
  • 123k
47 votes
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Does the sun cross other spiral arms in its movement around the galaxy's center?

What is a spiral arm? The reason that the Sun, in principle (but see below), may cross spiral arms is that galactic spiral arms are not rigid entities consisting of some particular stars; rather they ...
pela's user avatar
  • 38.6k
47 votes
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Why is the summer solstice night shorter than the winter solstice day?

Why is the summer solstice night shorter than the winter solstice day? Aside: If you use your sunrise / sunset calculator you will see that day and night are not equal at the equinoxes. The reason ...
David Hammen's user avatar
  • 34.1k
46 votes

Why is the Sun's density less than the inner planets?

The sun isn't the same density all the way through. According to MSFC's solar interior page, the core density at the centre of the sun is a whopping 150,000 kg/m$^3$. Surrounding it the radiative ...
Andy's user avatar
  • 2,467
46 votes

Can a lunar eclipse happen at night and be followed by a solar eclipse the next day?

No. A lunar eclipse occurs when the moon is on the opposite side of the Earth as the sun. A solar eclipse occurs when the moon is on the same side of the Earth as the sun. And it takes the moon 14 ...
James K's user avatar
  • 123k
45 votes
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Do we know a star that is similar to the Sun when it would be a red giant?

Models for the future behaviour of the Sun do vary, mainly as a result of uncertainty of mass loss during the red giant (H shell burning) and asymptotic red giant (H+He shell burning) phases. A ...
ProfRob's user avatar
  • 153k
45 votes

How were sundials and moondials possible 800 years ago?

Those 2 wheels work like a perfect sundial. No they don't. They're flat. The timespan from one hour to the next on a flat sundial varies from season to season. At the same time that that temple was ...
David Hammen's user avatar
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45 votes
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Why is there a year 1 B.C., a year 0, and a year 1 A.D. in NASA’s Five Millennium Canon of Solar Eclipses?

This is all explained on the home page All dates are astronomical dates. These match CE (or AD) dates for positive values but differ by 1 for BCE dates. The Gregorian calendar is used for all dates ...
James K's user avatar
  • 123k
44 votes

Sun constantly converts mass into energy, will this cause its gravity to decrease?

I wanted to expand a bit on a point @uhoh made. (I would have made this a comment, but lack the reputation). The Sun is not converting mass to energy. The Sun is converting matter (not mass) into ...
S. Melted's user avatar
  • 541

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