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70

The bottleneck in Solar fusion is getting two hydrogen nuclei, i.e. two protons, to fuse together. Protons collide all the time in the Sun's core, but there is no bound state of two protons because there aren't any neutrons to hold them together. Protons can only fuse if one of them undergoes beta plus decay to become a neutron at the moment of the ...


67

This is an answer that I made, as suggested by John Rennie, by cutting and pasting his answer and dmckee's and adding a little more material. There are four factors involved: Velocity distribution of the nuclei Small geometrical cross-section for head-on collisions of nuclei Quantum-mechanical tunneling probability For the p-p reaction, a weak-force ...


55

Yes. For example, on October 8th 1970 Earth was in the Sun's radiofrequency shadow with respect to quasar 3C 279. In other words, quasar 3C 279 was occluded by the sun. Observation from just before and after the occulation permitted measurement of the bending of radiowaves as a test of general relativity. The sun would also block other frequencies of ...


41

Clean dry air lets sunlight through; dirty moist air scatters it. Aerosols (small air borne particulate contamination) are more prominent near areas of dense population - due to power plants, cars, fires, ... These particles form nucleation sites for moisture - and these small water drops become very effective scatterers of sunlight. The humidity is high ...


40

The sun does rotate. We can see the rotation of the sun by the doppler shift of the light we get from the sun. . (Image from this page.) Since we know the characteristic spectrum of light from a hot body of a given temperature, we can use the same effect to determine if other stars rotate as well. Note that this only gives the spread in velocities along ...


33

You probably know that the mass of the Higgs boson is around $125$ GeV, which means the energy it takes to create a Higgs boson is around $125$ GeV and therefore that the temperature at which significant numbers of Higgs bosons will be created will be given by $kT = 125$ GeV. One GeV is $1.602 \times 10^{-10}$J, so the corresponding temperature is around ...


32

Briefly: Because the moon's orbit "wobbles" up and down, so it isn't always in the plane of the earth's orbit around the sun. There's a 2D plane you can form from the ellipse of the earth's orbit and the sun. This plane is known as the ecliptic. The moon's orbit is not exactly in the ecliptic at all times; see this (slightly overcomplicated) picture from ...


31

Yes, the sun and nearly all other stars do rotate. One can see the rotation of the sun by looking at the motion of sunspots on its surface. Over time, the sunspots will move across the sun's surface - proof of its rotation. Furthermore, the rate of the sun's rotation is not constant throughout the sun; it is higher near the equator and slower near the ...


31

The Sun isn't "made of fire". It's made mostly of hydrogen and helium. Its heat and light come from nuclear fusion, a very different process that doesn't require oxygen. Ordinary fire is a chemical reaction; fusion merges hydrogen nuclei into helium, and produces much more energy. (Other nuclear reactions are possible.) As for rockets, they carry both ...


30

The conditions at the core of the Sun are very different from those in a thermonuclear bomb. The first thermonuclear bomb used deuterium as the secondary. The Sun has to create deuterium before getting to this stage. It's the creation of deuterium that's the bottleneck in the fusion that occurs inside the Sun. Later bombs used lithium deuteride, which is ...


26

This is from the Physics FAQ article that I wrote 15 years ago: If shorter wavelengths are scattered most strongly, then there is a puzzle as to why the sky does not appear violet, the colour with the shortest visible wavelength. The spectrum of light emission from the sun is not constant at all wavelengths, and additionally is absorbed by the high ...


25

The premise that the sun has the same conditions all throughout is incorrect. For the most part the conditions (Temperature and Pressure) necessary for nuclear fusion to occur are only found within a small region in the core. For example, when hydrogen fusion occurs and creates helium, since that helium is heavier it tend to coalesce as the core. In ...


20

According to Opacity of an Ionized Gas, "light from regions [of the sun] where the pressure is greater than 0.01 atm. is cut off completely, so that all we see comes from a spherical shell of rarefied gas". There is no real surface of the Sun. Instead, the density and pressure of gas/plasma progressively increase from an infinitesimal value far from the ...


19

This is a really rough calculation that doesn't take into account the realistic direction of the bow shock, or calculation of the drag force. I just take the net momentum flow in the solar wind and direct it so as to produce the maximum decceleration and see what happens. Apparently the solar wind pressure is of the order of a nanoPascal. As I write this ...


19

It just happens to be a coincidence. The current popular theory for how the Moon formed was a glancing impact on the Earth, late in the planet buiding process, by a Mars sized object. This caused the break up of the impactor and debris from both the impactor and the proto-Earth was flung into orbit to later coallesce into the Moon. So the Moon's size just ...


18

The reason being closer to a heat source makes you warmer is the inverse square law. Think of it this way: If you have a $1~\mathrm{m}^2$ piece of material facing the Sun and located at Mercury's orbit, it will be quite hot. What does the shadow of this square look like at Earth's orbit (about $2.5$ times further away than Mercury)? Well, it will be $2.5$ ...


18

When you're trying to understand the mechanics of a system it's usually convenient to choose coordinates that reflect the symmetry of the system. The solar system is roughly centrally symmetric because the Sun is by far the largest mass in it, and the coordinates that reflect this symmetry are polar coordinates with the Sun at the centre. For example in ...


17

Yes. A current example of how this is a concern in current science is in NASA's Kepler mission. Kepler is a space telescope designed to look at a specific part of the sky and consistently measure the amount of light coming from the stars in its field of view. When planets orbit in front of their host star they will create a shadow that decreases the ...


16

This is not possible. The lowest possible mass for a main sequence star (sustaining H-1 fusion; it's the regular kind of star) is around 80 Jupiter masses. Just below this, objects are referred to as Brown Dwarfs, which are technically not stars. Whereas the highest possible mass for a terrestrial planet is about 5-10 Earth masses (as per here). Above this ...


15

First, it must be said that the picture you provided in your question is extreme. The concept of light bending is true, but the amount that the light bends is nowhere near as large as the picture shows it. The quantification of how much light bends when transferring from one medium to another is called the "index of refraction," and air's index of ...


14

No, there is not a solar eclipse whenever we see a new moon. The reason we do not have a solar eclipse at every new moon is mostly due to the angle of Earth's axis (and by extension, the Moon's orbital plane) to the Earth-Sun line. See the diagram below (as requested) for a visual explanation. In the picture, the Sun is to the left. The upper image shows the ...


14

The Moon's orbit is inclined with respect to the Earth's orbit. In other words, if you imagine a Sun, Earth, and Moon model sitting on a tabletop, the Sun would sit approximately still and the Earth might slide around the desktop, while the Moon would orbit the Earth, hopping up off the table, and sinking back down into it. (I used to do this demonstration ...


13

It is much more. During its core-burning phase, the Sun will burn about 10-15% of its hydrogen supply. The gas is about 70% hydrogen by mass, so that translates into about $0.07$ to $0.1\,M_\odot$ of hydrogen burned. The exhausted core then contracts while hydrogen is burned in a shell that gradually moves outwards (in terms of mass). Helium ignition starts ...


13

@dmckee guessed correctly. From An excerpt from an address delivered before Section A of the American Association for the Advancement of Science, on August 23, 1882, by Prof. Win. Harkness, Chairman of the Section, and Vice President of the Association: (ref) He was destitute of what would now be regarded as the commonest instruments. The invention of ...


13

Great question. The electric field creates such a strong force that it would be very hard to move large amounts of just one type of charge. So astrophysical systems do generally eject equal numbers of protons and electrons. In particular, the solar wind is electrically neutral. So these cosmic rays are created in very nearly equal numbers, but by the ...


12

At the radius of the earth, the solar irradiance is approximately $1.412\;\mathrm{kW/m^2}$, giving a total power hitting the foil sheet (assuming normal incidence) of $\sim7.06\;\mathrm{MW}$. The average human in America is around $1.7\;\mathrm{m}$ tall, and somewhere around $0.5\;\mathrm{m}$ wide, making his cross sectional area around $0.85\;\mathrm{m^2}$. ...


12

Placing the solar filter before sunlight hits the instrument is the correct way of doing it. You could put the filter after the instrument, provided you don't mind being blind - the concentrated energy from the Sun heats up the filter, which sooner or later melts (if it's plastic) or cracks or explodes (if it's glass), your eye(s) receive a full dose of that ...


12

Solar wind is neutral overall else the Sun will become globally very strongly charged and we don't see that happening. It comprises Electrons/Protons and other particles.


11

Although the corona's temperature is very high, its density is extremely small, in the order of ~10^-11 kg/m^3. Therefore coronal refraction would be negligeable.



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