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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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58 votes

Why can't we feel the Earth's revolution?

You do, but it's too small to really notice First, it's not correct to say that we don't feel Earth's rotation because it's rotating at a constant speed. Think about driving a car, or riding in an ...
ap55's user avatar
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54 votes
Accepted

How do scientists know that the distant parts of the universe obey the physical laws exactly as we observe around us?

We don't know in general but to the extent we can measure, the laws seem to be the same, even if conditions are not. For example radioactive decay: We know how fast various elements decay, and we can ...
James K's user avatar
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53 votes

If all stars rotate, why was there a theory developed that requires non-rotating stars?

In a similar way, we could ask... No beams can be exactly 1 meter long. No beams can be exactly straight. The material making up a beam cannot be truly isotropic. So why should we bother ...
James's user avatar
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47 votes

If all stars rotate, why was there a theory developed that requires non-rotating stars?

All models are approximations, we judge a model on how useful it is. Understanding the collapse of a non-rotating star to a black hole gives insight into the nature of gravitational collapse. Much of ...
James K's user avatar
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40 votes

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

You want nature to be frugal and efficient. You want all the energy of the sun to have a purpose. However what you want nature to be like has no bearing on what it is. The light from the sun is a ...
James K's user avatar
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38 votes

What would happen to a polished marble statue left in space for a million years?

There are three main space weathering processes that will affect the surface of the marble. Cosmic rays, high energy particle from the sun and beyond, will hit the surface. This can change the ...
James K's user avatar
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36 votes
Accepted

Why don't (or can't) stars be more than 325 or so times the mass of the sun? What limits their size?

There is basically an upper limit to the mass of a star because their luminosity is so great that the radiation pressure prevents the accretion of further mass. However, the upper limit depends on the ...
ProfRob's user avatar
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34 votes

Why is the Moon's orbit so complicated?

The moon is so big that the processes that circularize and reduce the equatorial inclination would take much longer. The moon is big because of how it formed: a huge collision in the early solar ...
James K's user avatar
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34 votes

Why are there no gamma-ray bursts detected in our galaxy?

All models of gamma-ray bursts involve extremely energetic phenomena: particular types of supernovae, the coalescence of binary compact objects, strong magnetar flares, or tidal disruption events. It ...
HDE 226868's user avatar
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34 votes

Why doesn't dark matter clump strongly in the center of galaxies, since it doesn't feel either radiation pressure or the Pauli exclusion effect?

The reason is the fact that dark matter is non collisional. The dark matter particles interact only gravitationally, they feel no pressure, right, but they also feel no drag! No drag, no friction, ...
Prallax's user avatar
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26 votes

How to be an astrophysicist?

Different education systems differ, however At school you would take maths and physics courses, at least covering calculus. As an undergraduate, taking (or majoring in) physics. Also probably doing ...
James K's user avatar
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26 votes

Is there anything currently 46 billion light years away from Earth that we can see?

The CMBR came from a sphere of matter with an extrapolated comoving radius of around 46 billion ly. That's the most distant thing we can see. The observable universe is sometimes defined to end there, ...
benrg's user avatar
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26 votes
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Why don't we just unshift the IR photos from Webb?

The infrared data cannot be unshifted to produce visually pleasing images simply by linearly scaling observed wavelengths because JWST does not work like the human eye nor like the camera in your ...
doubleunary's user avatar
25 votes
Accepted

Why does the sensitivity to GWs drops off inversely proportional to the distance?

EDIT I'm leaving the original, highly upvoted answer below, but I've had a fundamental rethink about this, prompted by questions from Keith McClary and a helpful clarification from a Physics SE ...
ProfRob's user avatar
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25 votes

Is Dark Matter possible if there is dynamical friction?

Dynamical friction is considered in the study of dark matter. The timescale for dynamical friction to cause a significant loss of kinetic energy is roughly $$\tau \sim \frac{10^{5}}{\sqrt3 \ln \Lambda}...
ProfRob's user avatar
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24 votes
Accepted

Why can't we feel the Earth's revolution?

Firstly the speeds are massively different (about 1000 mph (1610 kph) on the equator for Earth's rotation and 70,000 mph (112,654 kph) for the revolution), so the change is not large. Secondly, the ...
Steve Linton's user avatar
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23 votes

How can astronomers pinpoint the location of the source of a neutrino?

You correctly state that neutrinos do not interact too often. The physical parameter describing that is the effective cross-section. So what you observe in a detector is not the neutrino itself, but ...
B--rian's user avatar
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23 votes

Do these results mean that I have found this exoplanet?

It could be an exoplanet transit (but that doesn't mean it is). The star in question, TIC92352620, is an F8 main sequence star, which would be much larger than any plausible planet. If a planet ...
ProfRob's user avatar
  • 153k
21 votes
Accepted

How to be an astrophysicist?

The main way into astrophysics is - as James K outlines in his answer well - to study physics with an emphasis on astrophysics or astronomy. The entry into these academic careers are easier for those ...
planetmaker's user avatar
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20 votes
Accepted

If all stars rotate, why was there a theory developed that requires non-rotating stars?

Another consideration is that the physics that describe a rotating black hole was much harder to develop. The maths describing the Schwarzschild (uncharged, non-spinning) black hole was developed in ...
Ingolifs's user avatar
  • 4,155
20 votes

Gravitational Properties of Asteroids

Of course you would need to specify who the person is - an Olympic athlete? Let us assume so and then you can scale downwards accordingly. So an Olympic high jumper can jump hard enough to raise ...
ProfRob's user avatar
  • 153k
20 votes

Is there anything currently 46 billion light years away from Earth that we can see?

We don't see stars and galaxies at a proper distance of 46 Gly, because this distance corresponds to a light travel time of 13.7 billion years, or very shortly after the big bang. When we look into ...
James K's user avatar
  • 123k
18 votes

Is there a possibility that a white dwarf can turn into a neutron star or a black hole?

The answer is: to a neutron star - possibly; to a black hole, no. The process whereby a neutron star is formed is known as an accretion induced collapse and is being seriously debated, especially in ...
ProfRob's user avatar
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18 votes
Accepted

Problem regarding the absorption lines of the Sun

Possibly you are under the misapprehension that the number of photons is a conserved quantity? That isn't true, there are more photons at any given wavelength when you are deeper into the star because ...
ProfRob's user avatar
  • 153k
18 votes

Has anyone back-calculated previous close encounters between the Apophis asteroid and Earth?

The JPL Small Body Database lists Apophis close approaches dating back 100 years before discovery. Three fairly close ones were: 1907-04-13, 0.029 au 1949-04-14, 0.028 au 1990-04-14, 0.033 au While ...
Mike G's user avatar
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18 votes
Accepted

Why isn't most hydrogen in the universe molecular (diatomic), instead of atomic (monoatomic)?

Yes, the atomic hydrogen is probably mostly left over from the Big Bang. [Edited to add: Not sure how much that is true and how much present-day atomic hydrogen is the result of recombination.] And, ...
Peter Erwin's user avatar
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17 votes
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Have I nearly found the event horizon of a black hole?

This is the Newtonian model of gravity. It is a very good model, it is used for accurate calculating the motion of objects in the solar system to a very high degree of accuracy. However, for very ...
James K's user avatar
  • 123k
16 votes

Why doesn't the Earth's rotation throw us off the surface?

OK, the reason we don't get flung off the surface of the Earth is that the rotational forces are not large enough to do it. Keep in mind that Earth formed because material was pulled together by it's ...
StephenG - Help Ukraine's user avatar
16 votes
Accepted

How is the Hubble constant determined from gravitational waves?

If you measure the gravitational waveform from an inspiralling binary, you can at any point measure the amplitude, instantaneous frequency and the rate of change of frequency. The last two give you ...
ProfRob's user avatar
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