94
votes
Accepted
Why doesn't the nuclear fusion in a star make it explode?
The fusion that occurs in the core of the Sun occurs in nothing like the conditions you might be thinking of in a bomb, or a fusion reactor. In particular, it occurs at much lower temperatures and at ...
- 136k
78
votes
Accepted
Does a star need to be inside a galaxy?
No, stars do not need to be inside a galaxy. It is estimated that about 10% of stars do not belong to a galaxy [1]. While most intergalactic stars formed inside a galaxy and were ejected by ...
- 4,110
66
votes
Accepted
If space is a vacuum, how do stars form?
Space is not a full vacuum. It's mostly a vacuum, and it's a better vacuum than the best vacuums that can be achieved in a laboratory, but there's still matter in it. See interstellar medium.
In all ...
- 22.1k
55
votes
Accepted
Are the physical structures in our sun of comparable complexity to those in the human brain?
The structure of the interior of the sun has been extensively studied by the technique of helioseismology and the findings are consistent with a model in which that structure is relatively simple.
...
- 97.8k
42
votes
Accepted
Do supernovae push neighboring stars outward?
Consider a star of mass $M$ and radius $R$ at a distance $r$ from the supernova. For a back-of-the-envelope estimate, consider how much momentum would be transferred to the star by the supernova. From ...
- 55k
42
votes
Accepted
Shouldn't some stars behave as black holes?
The true answer lies in General Relativity, but we can make a simple Newtonian argument.
From the outside, a uniform sphere attracts test masses exactly as if all of its mass was concentrated in the ...
- 4,776
38
votes
Accepted
A star or a galaxy?
With the naked eye, virtually every point you see is a star.* That's because there are very few galaxies that are visible with the naked eye.
With telescopes, for many galaxies you'll be able to ...
- 22.1k
36
votes
What is a simple argument to prove that the stars in the sky are further away from the Earth than the Moon?
The fact of parallax in the observed position of the moon was known in ancient times. This makes possible a calculation of the distance of the moon in terms of Earth radii. No parallax was then ...
- 11.8k
36
votes
Accepted
Can Jupiter turn into a star if it radiated off enough heat?
The smallest objects (given an elemental abundance mixture appropriate to a giant planet like Jupiter) that can attain hot enough interiors to ignite a sustained thermonuclear reaction are about 13 ...
- 136k
36
votes
Why is pressure in the outermost layer of a star lower than at its center?
Considering the only force keeping the star together is gravitational in nature,
Gravity is also what's keeping our oceans stuck to the Earth's surface. Where's the pressure at its highest, near sea ...
- 890
35
votes
Accepted
Why can't hydrogen and helium fuse?
Hydrogen and helium can briefly bind together to make lithium-5, but this is an extremely unstable nuclide which falls apart instantly (with a half-life of ${\sim}4\times 10^{-22}\:\rm s$) and which ...
- 135k
34
votes
What is a simple argument to prove that the stars in the sky are further away from the Earth than the Moon?
Lunar occultations. Just missed the moon block Mars last month. Not sure when Mars will block a star, thereby proving the stars are further than the moon.
- 39.5k
34
votes
Accepted
What is the origin of the Sun light?
The light from the Sun comes from the photosphere; a relatively thin layer, a few hundred km thick.
The photosphere of the Sun is in radiative equilibrium, getting neither hotter or colder on average.
...
- 136k
31
votes
Why do black holes warp spacetime so much more than stars that have the same mass?
They don't. The gravitational field is the same outside all spherical objects with a given mass. But a black hole is much much smaller than a star with the same mass, so you have access to regions ...
- 28.6k
31
votes
Accepted
What part of the photons emitted from a star are from black body radiation and what part originate from fusion reactions?
Fusion reactions produce high energy gamma radiation. None of those photons reach the surface of the star directly. Over timescales of $10^4-10^5$ years, they scatter around as their energy propagates ...
- 6,495
27
votes
Accepted
Why is the Sun made of light elements only?
The Sun didn’t “select out” heavy elements from the cloud. The planets selected out light elements because they don’t have enough gravity to hold on to their hydrogen and helium.
Source: http://...
- 52.2k
27
votes
If gravity is not a force, what makes massive objects spheroid?
The basic idea is still the same: the gravitational influence of the parts on each other tends to pull them together. The only difference is that now you don't have a force pulling them together, but ...
- 23k
26
votes
Do supernovae push neighboring stars outward?
Probably not. Supernovae are powerful, but space is really big. ;)
Supernova energies are often measured in foe; one foe is $10^{44}$ joules. According to Wikipedia, a big supernova can release around ...
- 13k
24
votes
Where will the Goldilocks zone be when the Sun becomes a red giant?
The habitable zone is typically defined as the range of orbits where liquid water can persist on a planet. This is somewhat ill-defined, since climate modelling of planets with various properties (...
- 33.4k
21
votes
Accepted
How does a spectral line tell us about the magnetic field of a star?
One way is through the Zeeman effect. The presence of a magnetic field in the region where the absorption line originates can split the energy levels of an atom into multiple components. Transitions ...
- 136k
21
votes
Accepted
Why do main sequence stars get bigger and more luminous as they age?
Why does the luminosity increase?
As core hydrogen burning proceeds, the number of mass units per particle in the core increases. i.e. 4 protons plus 4 electrons become 1 helium nucleus plus 2 ...
- 136k
20
votes
Accepted
Can the Sun / Earth have a dark matter core?
The easiest way for dark matter to become trapped inside another object is if it interacts and loses some kinetic energy. Otherwise it would just gain kinetic energy as it fell into a gravitational ...
- 136k
20
votes
What is the origin of the Sun light?
The origin of sunlight is the hot plasma at and near its surface. It can be reasonably well described by Planck's black body radiation.
Check out https://en.m.wikipedia.org/wiki/Sunlight and sources ...
- 26.6k
18
votes
Will a neutron star always collapse into a black hole in the future?
Terminology note: the Chandrasekhar limit $M_C \approx 1.4 M_\text{sun}$ is for electron-degenerate matter. The analogous limit for neutron-degenerate matter, $M_\text{TOV} \sim 2.5 M_\text{sun}$, is ...
rob♦
- 94.2k
18
votes
Accepted
Is it possible to tell the difference between a young star that is just "big" and an older red giant?
There are four classes of red giant, with different properties.
Three are produced by low(ish) mass stars (1-8 solar masses) and in order of evolutionary phase:
There are red giants that are burning ...
- 136k
17
votes
Accepted
Why is pressure in the outermost layer of a star lower than at its center?
There is a lot of mass above the centre, pushing down on it. On the surface there is no mass above. Hence you get high pressure at the centre and zero at the surface.
- 33.4k
16
votes
Accepted
Plasma and Stars
Are there stars not made of plasma?
.....
Plasma is an electrically neutral medium of unbound positive and negative particles (i.e. the overall charge of a plasma is roughly zero). It is important ...
- 235k
16
votes
Could Alpha Centauri be made of anti-matter?
An antimatter star would release an antimatter solar wind (antiprotons and positrons mainly). As this antimatter solar wind collides with the ordinary-matter solar wind from the ordinary-matter stars ...
- 35.8k
16
votes
Why doesn't the nuclear fusion in a star make it explode?
If fusion were to proceed faster, the core would get hotter, it would expand and become less dense, and with less density, fusion would slow down.
The main sequence in stars like the Sun does ...
- 11.8k
Only top scored, non community-wiki answers of a minimum length are eligible