All Questions
1,007 questions
107
votes
8
answers
22k
views
Why does the Sun's (or other stars') nuclear reaction not use up all its "fuel" immediately?
The temperature and pressure everywhere inside the Sun reach the critical point to start nuclear reactions - there is no reason for it to take such a long time to complete the reaction process.
Just ...
- 1,196
104
votes
7
answers
20k
views
Why is there a scarcity of lithium?
One of the major impediments to the widespread adoption of electric cars is a shortage of lithium for the batteries. I read an article a while back that says that there is simply not enough lithium ...
- 2,192
78
votes
6
answers
10k
views
How can we see planets thousands of light years away but don't know if there are more planets in the solar system?
That is basically my question, it arose when I saw an article (here is the scientific paper, which should be free to read) saying two Caltech scientists might have found the 9th planet of the solar ...
- 1,768
65
votes
3
answers
13k
views
Why is the Sun almost perfectly spherical?
Relatively recent measurements indicate that the Sun is nearly the roundest object ever measured. If scaled to the size of a beach ball, it would be so round that the difference between the widest ...
user81619
63
votes
3
answers
9k
views
Do solar systems typically spin in the same direction as their galaxy?
Is the net angular momentum vector of our solar system pointing in roughly the same direction as the Milky Way galaxy's net angular momentum vector? If yes or no, is that common for most stars in the ...
- 6,065
58
votes
7
answers
10k
views
Photons from stars--how do they fill in such large angular distances?
It would seem that far-away stars are at such a distance that I should be able to take a step to the side and not have the star's photons hit my eye. How do stars release so many photons to fill in ...
- 1,672
57
votes
4
answers
13k
views
Why does a supernova explode?
This is really bugging me. When you look up some educational text about stars life, this is what you find out:
Gravity creates the temperature and pressure to start fusion reactions.
The fusion ...
- 3,107
51
votes
9
answers
8k
views
Why doesn't the nuclear fusion in a star make it explode?
I have a rather naive question. In stars such as the Sun, what prevents the whole thing exploding at once? Why is the nuclear fusion happening slowly? I can only assume that something about the fusion ...
- 756
49
votes
5
answers
12k
views
When a star becomes a black hole, does its gravitational field become stronger?
I've seen in a documentary that when a star collapses and becomes a black hole, it starts to eat the planets around.
But it has the same mass, so how does its gravitational field strength increase?
- 525
45
votes
2
answers
8k
views
Does a star need to be inside a galaxy?
Must a star belong to a galaxy, or could it be completely isolated?
In case it can be isolated (not belong to a galaxy), could it have a planet orbiting around it?
- 453
43
votes
2
answers
7k
views
Is our solar system really that odd?
I have been learning about the solar system from popular science shows. In these shows they suggest that, after having seeing around 2500 other solar systems, astronomers have concluded that our ...
- 1,430
40
votes
4
answers
26k
views
Why are stars white?
That is may be an easy question, but I am not a professional. The Sun is a star, and when I look at the Sun it is usually yellow. Why are stars in the sky at night white?
I suppose it could be due to ...
- 775
39
votes
4
answers
6k
views
Are neutrino stars theoretically possible?
Since neutrinos
have a small mass and
are affected by gravity,
wouldn't it be theoretically possible to have such a large quantity of them so close to each other, that they would form a kind of a ...
- 775
37
votes
6
answers
6k
views
How many times has the matter in our Solar System been recycled from previous stars?
I've got a basic understanding of these facts:
The Universe is a little over 13 billion years old. Our Galaxy is almost that old.
Our Solar System is roughly 4.6 billion years old.
The heavier ...
- 473
36
votes
2
answers
3k
views
Can a neutron star become a black hole via cooling?
How much does thermal expansion affect neutron stars? Would the loss of temperature cause a neutron star to be more densely packed and thus collapse into a black hole?
- 916
35
votes
6
answers
14k
views
Could there be a star orbiting around a planet?
I wonder if there ever could be a star (really small) which may orbit around a planet (really big)?
- 351
34
votes
3
answers
5k
views
Do supernovae push neighboring stars outward?
I know that a supernova can mess up the heliosphere of nearby stars, but I'm wondering if it could physically push neighboring stars off their trajectories.
It's fun to imagine all the stars ...
- 1,053
32
votes
5
answers
15k
views
Can there be Electron and/or Proton Stars?
What happens to all of the electrons and protons in the material of a neutron star?
Could there ever be an electron star or a proton star?
- 1,672
32
votes
4
answers
1k
views
Metal-rich star formation
While discussing star formation on cosmological scales with some classmates, we mentioned the breakdown between the different stellar populations via metallicity:
Population III: $Z = [{\rm Fe/H}] \...
- 28.8k
30
votes
5
answers
15k
views
Why is the Sun called an "average star"?
This is a statement (presumably in mass, longevity, energy output) many people that I've met have heard in school, and it is known in pop culture. However, according to Wikipedia, about 75% of the ...
- 2,899
29
votes
3
answers
24k
views
Is Jupiter a failed star?
In my physics lessons, my teachers have always been keen to tell my class that Jupiter is considered a 'failed star' by scientists. Is this true?
In my own effort I wondered if maybe this could just ...
- 545
28
votes
5
answers
4k
views
Can elements heavier than iron be present in a star's core?
My understanding is that elements heavier than iron and nickel are not formed in a star but, can heavy elements such as lead and others be present/found in a star's core ?
I ask because the following ...
- 381
27
votes
2
answers
4k
views
Why can't hydrogen and helium fuse?
In the hearts of stars, hydrogen atoms fuse together to make helium. After the hydrogen in the core is depleted, the star changes state and conditions at the heart of the star make it possible for ...
- 467
26
votes
3
answers
4k
views
What is the smallest distance possible between two stars?
If two stars of any type were to form near each other, how closely can they form before something prevents them from being two distinct stars?
- 6,065
26
votes
4
answers
3k
views
How do the “hot Jupiter” planets get so close to their host star?
Many of the extrasolar planets to date are Jovian sized planets that orbit very very close to their parent star. Traditional planetary formation models say that it is extremely unlikely (if not ...
- 6,467
25
votes
9
answers
5k
views
Are the physical structures in our sun of comparable complexity to those in the human brain? [closed]
The writings of Rupert Sheldrake tend to provoke strong emotions, be they ridicule, curiosity, outrage, sympathy, disgust, or otherwise. While Physics SE is not an appropriate forum in which either to ...
- 361
25
votes
4
answers
5k
views
A star or a galaxy?
When we look into the beautiful sky in the night, exclaiming how beautiful these shining stars are. My question is how could we tell, whether any of these shining "point" is a star or a galaxy?
If ...
- 613
25
votes
4
answers
6k
views
Where will the Goldilocks zone be when the Sun becomes a red giant?
In about 5 billion years, when our Sun expands into a red giant making our planet uninhabitable, where will the new Goldilocks zone be? Could life form on a new planet in the Goldilocks zone? ...
- 251
24
votes
4
answers
24k
views
Explanation for negative specific heat capacities in stars?
I've just found out that a negative specific heat capacity is possible. But I have been trying to find an explanation for this with no success.
Negative heat capacity would mean that when a system ...
- 3,934
23
votes
5
answers
8k
views
Why does a star collapse under its own gravity when the gravity at its centre is zero?
The gravity at the centre of a star is zero as in the case of any uniform solid sphere with some mass. When a massive star dies, why does it give rise to a black hole at it's centre?
I know how to ...
- 677
23
votes
3
answers
4k
views
How does the Pauli exclusion principle create a force in degenerate matter?
My understanding is that when it comes to forming a white dwarf, it is the electron degeneracy pressure, due to the Pauli Exclusion Principle, preventing collapse in of the white dwarf.
If the ...
- 4,067
23
votes
2
answers
5k
views
Why is the cut off mass for massive stars 8 solar masses? Why can't it be 10-11 solar masses or so?
I know that stars having a mass greater than or equal to 8 solar masses are termed "massive stars". But why is the cut-off 8 solar masses?
- 425
23
votes
1
answer
2k
views
If a photon goes up, does it come down?
If light can be bent by gravity, does a mass as dense as a star pull any fraction of photons back towards itself?
- 451
22
votes
5
answers
117k
views
Can the "Milky Way" galaxy be seen by the naked eye in a clear sky?
Is this photo "real"? Are the stars not super-imposed in the image?
Ian
21
votes
2
answers
7k
views
How many stars are in the Milky Way galaxy, and how can we determine this?
I have heard multiple estimates on the quantity of stars within our galaxy, anything from 100 to 400 billion of them. The estimates seem to be increasing for the time being. What are the main methods ...
- 3,439
21
votes
4
answers
5k
views
How can a star emit light if it is in Plasma state?
I understand that star is in Plasma state (all nucleus and electrons are not bound to each other and moving around freely)
Photon is emitted when an excited electron moves back to lower orbit.
So in a ...
- 247
20
votes
3
answers
7k
views
Why do main sequence stars get bigger and more luminous as they age?
As stars age, the concentration of hydrogen in the core decreases, which lowers the power output, causing an imbalance between outward radiation pressure and inward gravitational pressure. This causes ...
20
votes
3
answers
4k
views
Why is the sky dark at night? [duplicate]
The question is the well-known Olbers' paradox:
If there are so many/infinite stars, in every direction, why is it dark at night? Where goes all the light we don't see?
Discussing with a friend ...
- 475
20
votes
1
answer
2k
views
Can we know if an exoplanet has a magnetic field?
The recent discovery of the exoplanet Proxima b gave rise to several questions concerning its habitability and its potential to host life. A major concern is the huge amount of radiation coming from ...
- 1,264
20
votes
2
answers
1k
views
Why don't stars in globular clusters all orbit in the same plane?
Globular clusters like Omega Centauri certainly don't seem to be very coplanar at all.
In other words, why doesn't the explanation at Why are our planets in the solar system all on the same disc/...
- 3,662
19
votes
7
answers
8k
views
What is a simple argument to prove that the stars in the sky are further away from the Earth than the Moon?
How do we know, without using modern equipment, that the stars are further away than the moon in the night sky? Further, is there a simple and actionable argument to prove that this is indeed the case?...
- 2,402
19
votes
6
answers
9k
views
If gravity is not a force, what makes massive objects spheroid?
For most of my life, the explanation given for why celestial bodies like stars, planets, etc. are round is due to gravitational force. Simply put, if an object has enough mass, it will, in turn, have ...
- 413
19
votes
5
answers
3k
views
How is hydrostatic pressure overcome when a star is formed?
If stars are formed by the collapse of dust clouds under gravity, how is the pressure of the dust cloud overcome?
As more material gathers together, gravity will increase, but pressure will also ...
- 952
19
votes
4
answers
26k
views
Why can't Iron fusion occur in stars?
It is said that iron fusion is endothermic and star can't sustain this kind of fusion (not until it goes supernova). However star is constantly releasing energy from fusion of elements like Hydrogen ...
- 673
19
votes
4
answers
3k
views
Can the Sun / Earth have a dark matter core?
If dark matter interacts with ordinary matter at all, it should most likely occur where ordinary matter is densest. Hence we have papers about neutron stars possibly containing dark matter cores (...
- 22.1k
19
votes
1
answer
3k
views
Why is the Sun made of light elements only?
Since the entire solar system inherits its heavy elements from supernovae unrelated to our star, I fail to understand why, while capturing most of said system's matter, the sun only contain light ...
- 3,256
19
votes
4
answers
5k
views
How long will the Universe's hydrogen reserves last for?
I recently became really interested in learning about physics and cosmology, but I still know very little. Hopefully someone with more knowledge will be able to shed some light on my questions.
Here ...
- 193
18
votes
6
answers
6k
views
If space is a vacuum, how do stars form?
According to what I have read, stars are formed due to the accumulation of gas and dust, which collapses due to gravity and starts to form stars. But then, if space is a vacuum, what is that gas that ...
- 837
17
votes
7
answers
3k
views
What is the easiest way to stop a star?
On long enough cosmological time scales, hydrogen and helium nucleii will become scarce in the Universe. It seems to me that any advanced civilisations that might exist in that epoch would have the ...
- 34.9k
16
votes
8
answers
11k
views
How do stars from far away affect Earth?
I know that we obviously get light (or we wouldn't be able to see them), but are there any other ways that they affect Earth and maybe just our solar system in general?
- 341