115 votes
Accepted

Would we have more than 8 minutes of light, if the Sun "went out"?

If nuclear fusion were to suddenly stop in the centre of the Sun, then the only clear signature we would have of this is the lack of detectable neutrinos received at Earth, starting about 8 minutes ...
ProfRob's user avatar
  • 149k
75 votes
Accepted

Is it dark inside the Sun?

No, it's not. The radiation field in the interior of the Sun is very close to a blackbody spectrum. If you look in any particular direction the brightness (power per unit area) you see is $\sigma T^4$,...
ProfRob's user avatar
  • 149k
37 votes

Can a magnetic field of an object be stronger than its gravity?

It depends on what object it's acting on. There are many objects, including stars, that have magnetic fields where Lorentz forces on charged particles like electrons and protons are stronger than the ...
Ken G's user avatar
  • 5,320
22 votes

Why doesn't the fusion process of the sun speed up?

Am I correct in saying that the fusion process of the sun is constant, i.e. X amount of fusion happens per day, more or less? Yes, at least over human timescales. You could reasonably expect the ...
Neil Slater's user avatar
18 votes

What would the Sun be like if nuclear reactions could not proceed via quantum tunneling?

Short answer: Without tunnelling, stars like the Sun would never reach nuclear fusion temperatures; stars less massive than around $5M_{\odot}$ would become "hydrogen white dwarfs" supported by ...
ProfRob's user avatar
  • 149k
18 votes

Is it dark inside the Sun?

Coming from a different direction as @Rob's, Opacity and Thermal Radiation are orthogonal properties of a material. The photon flux at the interior of the sun is very high, so it is definitely not ...
MooseBoys's user avatar
  • 289
17 votes
Accepted

Can a magnetic field of an object be stronger than its gravity?

Let's look at the proper magnetic force (as opposed to the Lorentz force on a moving, charged object described in @KenG's answer) on a specimen $S$ of magnetized material with mass $M_S$ as a way to ...
uhoh's user avatar
  • 31.3k
16 votes
Accepted

What actually are line-driven winds?

The word "line" refers to a spectral line, i.e. an emission or absorption feature in a spectrum. In this case it's absorption. Line-driven vs. continuum-driven winds Stellar winds are driven ...
pela's user avatar
  • 37.6k
16 votes

Is the Sun hotter today, in terms of absolute temperature (i.e., NOT total luminosity), than it was in the distant past?

From the Wikipedia article on the sun, it is about 150 K: The Sun is gradually becoming hotter in its core, hotter at the surface, larger in radius, and more luminous during its time on the main ...
planetmaker's user avatar
  • 18.9k
14 votes

Which stellar properties can we describe as "first principles" in which we can derive the rest?

Starting from a protostar, one would hope to be able to predict everything about its future development if we knew its initial mass, chemical composition and angular momentum. Mass is fundamental ...
ProfRob's user avatar
  • 149k
13 votes

Why doesn't the fusion process of the sun speed up?

No, the fusion rate of the Sun is not absolutely constant in time. The Sun is gradually becoming more luminous and that luminosity is provided for almost exclusively by fusion in the core. However, ...
ProfRob's user avatar
  • 149k
13 votes
Accepted

Would stars have formed in the Universe if atomic hydrogen couldn't make molecular hydrogen?

The role of H$_2$ is to allow primordial gas to cool down sufficiently to allow the collapse to start and then to hold the gas at a relatively low temperature as it gets much more dense. The formation ...
ProfRob's user avatar
  • 149k
12 votes

Are there stars with an average density greater than the central density?

$$\frac{dP}{dr} = - \rho g,$$ is the equation of hydrostatic equilibrium, where $\rho$ and $g$ are the local density and gravity, $P$ is pressure and $r$ is the radial coordinate. This can be ...
ProfRob's user avatar
  • 149k
12 votes

Can a small black hole orbit a large star?

A black hole of a given mass will probably have arisen from the collapse/supernova of a much more massive star. In particular, stars with an initial mass of less than around 15-20 solar masses are ...
ProfRob's user avatar
  • 149k
12 votes

What does this tweeted Astronomy Plot of the Week mean? What does it represent?

TLDR; Its a diagram showing all the physics and modeling choices that go into different models (the colored boxes in the middle) to compute an SED. Longer answer: First a SED is a spectral energy ...
Rob's user avatar
  • 2,035
12 votes

Is Barnard's star an M4 red dwarf or an M0? Why is it called an M4.0V?

It's M4V, not M0V. In principle, spectral classes can be further subdivided, particularly in the M class, because there are significant differences between an M4 and an M5 spectrum for instance. ...
ProfRob's user avatar
  • 149k
11 votes
Accepted

Why does lithium fuse at lower temperatures than hydrogen?

The slowest reaction rate in the pp chain determines how quickly hydrogen can "burn" in the core of a sun-like star. That rate-determining step is actually the fusion of two protons to form ...
ProfRob's user avatar
  • 149k
11 votes

Would we have more than 8 minutes of light, if the Sun "went out"?

The "common theory" you're reading is not about the processes that produce light in stars, it's just intended as a demonstration of the speed of light through space. When it talks about the Sun "...
Barmar's user avatar
  • 975
11 votes

How much more life could the Sun acquire via star lifting?

TL;DR: the main sequence lifespan of the sun can be increased by a factor of 12.2. Perhaps the most complete astrophysical analysis of stellar engineering for extending Earth habitability is Martin ...
Anders Sandberg's user avatar
11 votes

Can the energy transport by radiation occur in the convection region of a star?

Radiative energy transport continues. The point is that the radiative flux, which is proportional to $dT/dr$ can be overtaken when the temperature gradient achieves the adiabatic value and convection ...
ProfRob's user avatar
  • 149k
10 votes

Can a small black hole orbit a large star?

This is a great series of questions! Such a low mass black hole (BH) could have originated from a few possibilities: 1) a result of stellar evolution (the resulting black hole mass depends ...
Daddy Kropotkin's user avatar
10 votes
Accepted

Does the luminosity of a star have the form of a Planck curve?

Radiant intensity depends on both the the (effective) temperature and emitting area of the star. If the spectrum can be represented as a blackbody, then the radiant intensity is proportional to $R^2 T^...
ProfRob's user avatar
  • 149k
9 votes

Why do neutron stars collide instead of just revolving around each other like planets revolve around the Sun?

Neutron stars are one of the possible end products of the evolution of stars greater than around 8 solar masses. If you start out with a close binary pair of these fairly massive stars -- not ...
Mark Olson's user avatar
  • 7,620
9 votes

Can a magnetic field of an object be stronger than its gravity?

It isn't impossible, but the short answer is "no". A gravitational field will accelerate all matter and energy equally while a magnetic field will only accelerate moving electric charges (other ...
anonymous's user avatar
8 votes

Old star vs New star

Let me try to add some numbers to Steve's answer. The Sun's luminosity is about $L_{\odot}=4\times10^{26}\text{ J/s}$. Now, if we assume that the majority of that energy comes from nuclear fusion, we ...
HDE 226868's user avatar
  • 36.3k
7 votes
Accepted

Why we define Stellar motions with respect to sun?

The Earth is a moving (actually, accelerating) platform from which we make our observations. If you want to describe the motion of a distant celestial body, then it does not make much sense to provide ...
ProfRob's user avatar
  • 149k
7 votes
Accepted

When black holes forms out of a supernova do they have a very high velocity?

There is no specific answer to this -- anything from "just sits there" to flys away at high speed is possible. It all depends on the symmetry of the supernova (SN) explosion. Extensive modelling ...
Mark Olson's user avatar
  • 7,620
7 votes
Accepted

Activity of M dwarf stars

The magnetic activity is driven by a dynamo that relies on a combination of rotation and convection. M dwarfs have large convection zones or are fully convective beyond spectral type M4. They also ...
ProfRob's user avatar
  • 149k
7 votes
Accepted

What is the significance of minus sign in the expression of apparent magnitude?

So if I read your question correctly, you're asking why as stars get dimmer, they are given higher magnitudes? The reason is purely historical. The ancient greeks assigned stars with 6 brightness ...
Ingolifs's user avatar
  • 4,157
7 votes

What is the difference between emission line and recombination line?

A recombination line is a special case of an emission line. Emission lines An emission line is any spectral feature that rises above the continuum — i.e. the average amplitude of the spectrum (in ...
pela's user avatar
  • 37.6k

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