# Tag Info

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 ...
• 117k
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### 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 $\sigma T^4$,...
• 117k

### 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 ...
• 5,170
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### Why are there no green stars?

Human color vision is based on three types of "cones" in the eye that respond differently to different wavelengths of light. Thus, not counting overall brightness, the human color space has two ...
• 7,684

### 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 ...
• 371
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### Is any consensus forming on the solution to the "Lithium Problem"?

There is no absolute consensus and nothing proven beyond doubt, but there are favourite explanations. The discrepancy between the predicted big bang nucleosynthetic abundance of Lithium 7 and the ...
• 117k

### 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 ...
• 117k
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 ...
• 32.4k

### 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 ...
• 259

### 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 ...
• 117k

### 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, ...
• 117k

### 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 ...
• 117k

### 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 ...
• 1,271

### 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 ...
• 117k

### 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 ...
• 117k

### 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 ...
• 3,919

### 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 ...
• 7,390

### 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 ...

### 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 "...
• 835

### 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 ...
• 10.5k
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### 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 deuterium ...
• 117k

### 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 ...
• 34.1k
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### Where does energy at the beginning of a star's lifecycle (before any nuclear reactions) come from?

This is basic thermodynamics. When you compress a gas, you inject energy into it. Think of the pump you use to inflate the tires on your bike. It takes some force to move the piston, right? That ...
• 17.5k
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### 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 ...
• 7,390
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### 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 ...
• 117k
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### 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 ...
• 117k
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### 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 ...
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### Which stellar properties can we describe as "first principles" in which we can derive the rest?

Rob Jeffries has covered pretty much everything in his answer, but I'll add that this is a question with a long history, enough that there is a famous answer to it called the Russell-Vogt or Vogt-...
• 4,744
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### What does the Sun "look like" below 100 MHz?

Below about 300 MHz you can only see the sun's corona as the frequency is too low to penetrate the coronal plasma from further below. In Wikipedia's Solar radio emission you can see images of the sun ...
• 2,318