What is Sun: Definition and 1000 Discussions

The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy mainly as visible light and infrared radiation. It is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometres (864,000 miles), or 109 times that of Earth. Its mass is about 330,000 times that of Earth; it accounts for about 99.86% of the total mass of the Solar System.
Roughly three quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon and iron.The Sun is a G-type main-sequence star (G2V) based on its spectral class. As such, it is informally and not completely accurately referred to as a yellow dwarf (its light is closer to white than yellow). It formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core. It is thought that almost all stars form by this process.
The Sun's core fuses about 600 million tons of hydrogen into helium every second, converting 4 million tons of matter into energy every second as a result. This energy, which can take between 10,000 and 170,000 years to escape the core, is the source of the Sun's light and heat. When hydrogen fusion in its core has diminished to the point at which the Sun is no longer in hydrostatic equilibrium, its core will undergo a marked increase in density and temperature while its outer layers expand, eventually transforming the Sun into a red giant. It is calculated that the Sun will become sufficiently large to engulf the current orbits of Mercury and Venus, and render Earth uninhabitable – but not for about five billion years. After this, it will shed its outer layers and become a dense type of cooling star known as a white dwarf, and no longer produce energy by fusion, but still glow and give off heat from its previous fusion.
The enormous effect of the Sun on Earth has been recognized since prehistoric times. The Sun was thought of by some cultures as a deity. The synodic rotation of Earth and its orbit around the Sun are the basis of solar calendars, one of which is the Gregorian calendar, the predominant calendar in use today.

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  1. H

    B Quasi-Moons

    A quasi-moon of Earth is a rock to that orbits both the Earth and Sun with the same solar orbital period as Earth.
  2. applied-physics

    I Sun shade on Earth ground, is it a straight line or a curved line?

    How did you find PF?: via Google search Hi, I wonder if the shade from the sun on the ground, on a sunny day, is it a straight line or a curved line?? and if it is curved, how curvature can be calculated ?? Let´s say I have a pole or building, and I can follow up the shade of its top on the...
  3. E

    A four foot diameter black hole destroys our Sun

    Homework Statement: I am a writer completing a science fiction novel involving a four foot diameter black hole (with approx 1.5 times the mass of Saturn) In my novel, the black hole is being drawn toward our much more massive sun. I assume the black hole would begin to consume plasma/energy...
  4. P

    Stefan-Boltzmann's Law. How to apply to the Sun

    r^2=(3.828*10^(26))/(4π*σ(5772)^4) It's diffrent from what I know...
  5. G

    What is the power per square meter of a sun on a planet?

    From the previous question I got the star is giving off around 4.01*10^27 W. I am unsure how to find the answer it asks for. Please help.
  6. Hak

    B How quickly does the Earth feel the effects of a halved Sun's mass?

    I have a doubt about gravitation. Suppose the mass of the Sun halves in an instant, after how long does the Earth ''notice'' it? That is, does the gravitational force also decrease instantaneously? Instinctively I would say yes, but I don't understand why it should be so. If, for example, we...
  7. Hak

    I Nuclear reactions in the Sun and other topics on stars

    I don't know if the Forum in which I am posting is right, if it is not please freely move my question to the Forum you think is most appropriate. You argue that nuclear reactions occur in the Sun, which produce the energy that spreads out into space in the form of light. A friend of yours...
  8. accalternata

    Calculating the central temperature of the Sun using the ideal gas law

    I derived the equation for P so I substituted that into this equation. I'm struggling with finding rho_c (central density) and rho. Am I supposed to use the average density for rho (can calculate this since I know the radius of Sun and mass)? That still leaves the problem with the central...
  9. K

    Engineering Calculate angle between spacecraft & Sun vectors

    Hi, so I have no clue how to solve this problem but I started off by rewriting the issue as a dot product to find the angle. So; cos(θ)= RSC⋅RSun / ∣RSC∣⋅∣RSun∣ Where Rsc = space crafts position vector. Rsun is the Suns position vector. ∣RSC∣ is the length of the spacecrafts position...
  10. S

    I Is the Sun's photosphere fluorescent?

    Is the Sun photosphere fluorescent ? I have seen some beautiful images with sunspots etc, filtered to particular spectral lines eg green O[III] and violet Ca for better contrast. I am wondering if those lines are Fraunhofer line absorbances or is there extra luminance from fluorescence at those...
  11. S

    B Sun's Mass Loss: 4 Billion kg/sec from Sunlight & Ejected Particles

    Found out the sun loses 4 billion kg per second worth of sunlight in e = mc². How much mass does the sun lose per second if we include all particles that the sun ejects such as ions and neutrinos? (and, neutrons?)
  12. L

    B How bright is the Sun compared to the sky?

    What is the brightness of the sun compared to the sky that it lights up? I have a solar filter that only sees the sun. I just wonder if it is needed to be so dark that I can't see the sky anymore. I have plenty of camera options left to shorten exposure even with a filter a fraction as dense...
  13. D

    B What Would Happen to the Elements in the Human Body If Teleported to the Sun?

    The human body is made up of mostly oxygen, hydrogen, nitrogen, carbon, calcium, and phosphorus. Less than 1% is sulfur, potassium, sodium, chlorine, and magnesium. If a human body was teleported to the surface of the sun, what would happen to these elements? Which ones would cease to exist...
  14. D

    Calculating distance of Mars from the Sun based on two elongations

    I called the point E1 the point where Earth was at Feb,11 1990 and E2 at 30, Dec 1991, S for Sun and M for Mars and r for the Mars-Sun distance. Since we got a whole sidereal period between both alongation, I assumed Mars was on the same point in space (wrt Sun). I think I got the triangles...
  15. S

    Opening a portal to the center of the Sun

    Whilst perusing a D&D forum, I stopped to answer a question someone put out which was "what would happen if we used the gate spell to open a portal to the middle of the sun?" I replied (this was a while ago) and whilst I'm reviewing it, I am troubled by the fact that this uses such high forces...
  16. G

    I Angle of Sun by date and zip code?

    I don't know the correct works search to find this info online. I have been doing word searches since about Dec 15, 2022 but not finding what I want.??????? Some where there much be a chart by date and zip code for sun angle. I built a sun dial in the yard to learn the angle of the sun on...
  17. James1238765

    I Modeling the Earth and Sun (2 body orbits) using general relativity?

    Modeling the time evolution of the sun and earth orbiting each other using ##F = \frac{GMm}{r^2}## is straightforward. However, it appears that modeling the time evolution of the same 2 body system using general relativity seems to be a hard/intractable problem? There was in depth discussion by...
  18. Nik_2213

    I Gravity of the Sun: Einstein's Calculation and Beyond

    IIRC, the Sun's gravity was calculated by Einstein et-al to provide a 'focus' about 550 AU out. Current value is ~542 AU. Aside from effects due solar oblateness, frame dragging etc, may I assume this value also applies to eg neutrinos, gravitational waves etc etc ??
  19. .Scott

    Summer Is Here: Taking Back the Sun from the South!

    Enjoy the season. And let the southern hemisphere be on notice: We're taking the sun back!
  20. Z

    I Gravitational Waves from Vanishing Sun: What Happens?

    For some time I was wondering, what would happen if the Sun just disappeared like someone hit the delete button in Universal Sandbox. Specifically, what kind of gravitational waves will be produced in the wake of such an event? Would the law of conservation of Mass-Energy be miraculously...
  21. D

    How Can Planetary Movements Affect Time and Illumination on an Inhabited Planet?

    Hey,I was wondering if anyone could point me to some reliable and vulgarized sources about complex planetary movements? For instance, let's imagine an inhabited earth-like planet having four moons of various sizes and two suns. How can I reastically establish the "time decorum" on the planet...
  22. G

    I What is the angle of the Sun on June 21 & Dec 21 at US Zip Code 37129?

    What is angle of Sun June 21 & Dec 21 at zip code 37129. I need lowest angle & highest angle and probably angle of sun about Feb 20. Google word search shows nothing. I am probably need using correct word search to find what I want. Several years ago I accidently found a calendar that showed...
  23. H

    I Bending of Light Along the Sun: Explaining Half w/ Equivalence Principle

    The equivalence principle tels us that we cannot distinguish between gravity and an accelleration. using that fact one can reconstruct the bending of light along the sun, but only half of it results from the equivalence principle together with the gravitational field around the sun. Is it...
  24. yohananregal

    How's it possible that 70% of Earth receives sunlight simultaneously

    How did you find PF?: Google Hello, I am new here. I am wondering, how is it possible for nearly 70% of the spherical Earth to be illuminated by the sun every July 8th? How can a sphere be illuminated beyond 50% with a single light source. How do we explain this?
  25. S

    Gravitational PE for a certain distance from the Sun

    hello guys, sims back again with another question.. I don't understand what is up with question (b) cuz like.. to get ##v_esc## we assume that at ##r_0=\inf## ##v=0## but now if I assume at ##r=1.5E8## that ##v=0##. And then find ##v_esc## from the following: ##\frac12*mv_{esc}^2 -...
  26. DhruvSorathiya

    I Could the Sun burn an image sensor of a star tracker in space?

    Hello, I have a question. If a star tracker in space faces the sun directly then could it be possible that the sun completely burns the image sensor? To be specific we are a student satellite team (Student satellite program IIT Bombay) developing a star tracker and we don't have an exact idea...
  27. Astronuc

    B Sun contains more oxygen, silicon and neon than previously thought

    Ekaterina Magg, Maria Bergemann and colleagues have published results of new calculations concerning the composition of the sun. https://phys.org/news/2022-05-solar-spectrum-decade-long-controversy-sun.html As part of the work on her Ph.D. in that group, Ekaterina Magg set out to calculate in...
  28. Philosophaie

    I What happens first -- Sun Red Giant or Andromeda collision?

    What happens first Sun Red Giant or Andromeda collision?
  29. Field physics

    Is the Sun Playing Hard to Get?

    Just something dumb but funny to think about. Redshift indicates the object is moving away. The Earth orbits the sun The sun appears to be red. So in conclusion, the sun is running away and the Earth is chasing the sun. Lol hopefully that lightens your evening.
  30. U

    What day/exact time does Sun start to move again? (1980 Winter Solstice)

    I found a website (link listed below) that gives date and time of the Winter Solstice of 1980. The date (12/21/1980). The time (UT 16:52). Being that the Sun stays in the same position for 3 days, what is the exact day & time it starts to move again? Anyone know where to find this information...
  31. T

    B Why does the Sun seem so bright?

    I would like to know why the sun seems so bright. I know why the sun is a great source of light but that’s not what I’m asking. Why do my eyes perceive the brightness. Is it based on the number of photons that enter my eyeball? The more photons the brighter I perceive the light to be...
  32. BadgerBadger92

    B Is the Earth falling into the sun from gravity?

    Is the Earth getting closer to the sun from gravity?
  33. R

    I Can we predict the path of the Sun from a single observation point along the transit?

    Hi. I have this idea that might be useful for celestial navigation. If you were on a random place on the Earth, and you had a way of measuring the altitude of the sun above the horizon, at a precise moment, could you then predict the complete path, such that you could determine the altitude at...
  34. E

    I Why Do Longer Wavelengths Go Slower in Glass?

    I reviewed the derivation / solution of that GRT problem and do not see any dependence on the wavelength of the light. Is that correct, or am I missing something? This makes me wonder about my understanding of how prisms work. How or why do longer wavelengths go slower in glass than shorter...
  35. B

    B How hot is the Sun in space? (James Webb related)

    I'm reading about the engineering of the James Webb and it surprised me that the temperature difference will be about 318 degrees from one side of the telescope compared to the other (85 C on one side and -233 on the other). I didn't expect space to be that hot about a million miles further...
  36. A

    Torque imposed by the Moon and Sun on the Earth given its precession

    From the givens: Approximate Earth as a sphere: ##I_e = \frac{2}{5}MR^2 = \frac{2}{5}(5.97x10^{24})(6.371x10^6)^2 = 9.69x10^{37} kg*m^2## ##\omega_e = 7.29x10^{-5} \frac{rad}{s}## To calculate the rate of precession of the disk the Earth precesses around (1 revolution every 26,000 years)...
  37. J

    Cesium-133, the Sun, Moon, stars, or.... T.P. Scott-1000?

    Early in the pandemic there was, at least in the U.S., a concern about toilet paper shortages. It all seems so quaint now but there were actually runs on t.p. in supermarkets and people were hoarding it. There are of course other ways to clean up afterwards. I reckon that most people in the...
  38. ohwilleke

    I How Big of an Object Orbiting Closer to the Sun Than Earth Could Be Overlooked?

    Science involves not just knowing what you know, but accurately estimating how sure you are of what you know and having a decent understanding of what you don't know. There are two known true planets closer to the Sun than Earth (Mercury and Venus, neither of which has a significant moon), both...
  39. S

    B Process of a sun changing into a white dwarf

    This is part of the text from the link: I want to ask several things: 1. From the second paragraph: "As soon as the sun begins to burn more hydrogen, it would be considered a “red giant”. " But from first paragraph: "But when the star has nothing left in the core to burn, gravitational...
  40. X

    I Is the Sun a low-mass star or a medium-mass star?

    In many text, the Sun is referred to as a low-mass star, yet it is also referred to as a medium-mass (or intermediate-mass) star in some other text. Which one is "correct"? What is the range of low-mass vs medium-mass? Is it really just low-mass vs high-mass and that medium-mass is just the...
  41. wnvl2

    I Absolute meaning of spatial deviaton angle of light around the sun

    Einstein first calculated the bending of light rays that are touching the sun as 1.75 arc-sec. For the calculation I refer e.g. to https://www.mathpages.com/rr/s8-09/8-09.htm I know that spatial angles in general relativity don’t have an intrinsic value (are not invariant). They are dependent...
  42. L

    Stargazing Does an Analemma visualize the Earth's actual orbit around the sun?

    An Analemma is the shape that arises from taking a picture of the sun everyday at the same time of the day. This shape also have depth to it since Earth's orbit is elliptical, showing what looks like an wobbly orbit. Isn't this the "real" orbit of Earth around the sun?
  43. D

    I Orbital distance increase during the white dwarf phase of the Sun

    Some estimates through calculating the sun mass loss and increase in mass loss say that the white dwarf phase of the Sun will have roughly about 50% the current mass of the Sun (not sure about it). Whatever the actual mass loss is going to be, assuming that the 50% mass loss is true, where would...
  44. T

    A Radius of the Sun: Can We Accurately Predict It Through Theoretical Models?

    Basically, I'm wondering if there have been any attempts to calculate/model what the radius of the sun should be based on gravitational, thermal, and electromagnetic pressures. If there has, where can I find the calculation/model, and how closely does it match the actual radius of the sun...
  45. S

    I Granules On The Sun: Photosphere Or Convection Zone?

    Are the granules that we see on the Sun actually in the photosphere? Or are they actually just the top of the convection zone, with the photosphere beginning just above them? Or is the photosphere simply defined as the top edge of the convection zone where the granules are present, having...
  46. E

    B Why does the Sun need sunlight?

    A photon is different from an electron. At least by the fact that it is neutral. But, flying into the silicon lattice, it knocks out an electron and takes its place. That is, it becomes an electron. How does such a magical transformation happen? I may be wrong, but I think that a photon becomes...
  47. H

    The Sun sets.... so why does it feel hotter overnight?

    I've always wondered why. Would the reason be that the air cools but the amount of moisture stays the same... which makes sweating less effective... which means no cooling effect even though the air is technically cooler.
  48. O

    Deflection of light passing the Sun

    The book says the actual measured deflection from some experiments is about ##9\times 10^{-6}## radians, and this is double the value that a naive computation returns because of general relativity effects. So I guess my goal is to compute a number that looks like half that. If suggests to do it...
  49. T

    B Seeing a half Moon - where is the Sun?

    A little off subject, but a few years ago I went out fishing just as the sun was rising. Perfect morning, not a cloud in the sky. The moon at this stage, was still clearly visible and both the moon and the entire sun were sharing the same sky. Which is pretty cool but what to this day makes me...
  50. W

    I Sun Disappearance: Impact on Earth's Path

    Hi. I'm new and hoping for some discussion. I've been looking at some physics at home on my own while Covid-19 restrictions keep me off work. I'm not registered as a student anywhere and so don't have any chance to chat with other students or university staff. Hence, I'd be grateful for some...