The science regarding the structure, formation and evolution of stars.

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52
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2answers
7k 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 ...
20
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2answers
3k 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?
18
votes
4answers
2k 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 ...
13
votes
7answers
2k 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 ...
11
votes
1answer
734 views

What proportion of a star's hydrogen is consumed in its life?

I've heard in a lecture that a star like the sun would burn 3% of its Hydrogen before expiring. I would have thought it would be much more.
9
votes
3answers
355 views

How to cut a stone on a White Dwarf?

I've heard that white dwarf stars are extremely dense and hard. So, if I had a piece of white dwarf matter, would it be possible to cut it (or otherwise) into a custom shape? How could one do that?
9
votes
2answers
1k views

What is the Sun's core made of?

The obvious answer is hydrogen and helium plasma but the nuclear fusion can also create heavier elements. Are these heavier elements a significant portion of the core? Do the heavier elements "sink" ...
9
votes
1answer
167 views

Could micro black holes obey the Eddington limit?

A stellar-mass black hole has recently been discovered in the Andromeda galaxy. One interesting part of the release is that this black hole shines close to its Eddington limit. Quasars are ...
8
votes
4answers
478 views

Could a Dyson sphere destroy a star?

Freeman Dyson proposed that the power needs of an advanced civilisation would eventually require the entire energy output of a star to be collected, so that the star would end up surrounded by a ...
8
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1answer
2k views

How can such a high exponent arise in this physics equation?

This site http://what-if.xkcd.com/14/ states that during a helium flash, "the reaction rate is proportional to the 40th power of the temperature". Taking for granted that this is true, how can ...
7
votes
1answer
109 views

What's the most accepted theory for Blue Stragglers These Days?

I'm working on a post against a recent creationism article about blue stragglers. From when I was in undergrad, the general explanation was that they were likely second generation stars within ...
6
votes
1answer
103 views

How are stellar and galactic ages determined?

Being a 4th year undergraduate physicist, you'd think I'd know this! But it's never really taught in any detail, just vague mentions of metallicities and, in galactic case, redshift. So how exactly ...
5
votes
1answer
1k views

Reason why stars do not produce elements heavier than iron

In his book "Origins", Neil deGrasse Tyson says that "if you seek to split iron nuclei apart, you must them with additional energy. On the other hand, if you combine iron atoms they will also absorb ...
5
votes
2answers
833 views

How to calculate the force of gravity that wants to collapse a star?

In astrophysics, the pressure created by the outflow of energy from the interior of the star counteracts the force of gravity that wants to collapse the star. If I want to calculate this force for a ...
5
votes
2answers
69 views

Why are stellar clusters useful for developing stellar evolution models?

I know that for globular clusters it is assumed that the stars developed around the same time thus their initial chemical compositions are roughly the same. My question is how does this tell us about ...
4
votes
4answers
722 views

What happens if a black hole sucks up a massive amount of oxygen and hydrogen?

Suppose a black hole were to suck up a massive amount of surrounding oxygen and hydrogen. Would this ignite inside the black hole and turn it back into a star?
4
votes
2answers
98 views

How do we know what happens to stars during their life cycle?

It is common knowledge than Sun-sized stars will eventually become red giants, and later they will get gradually smaller again until they cool down into a brown dwarf, and that bigger stars can ...
4
votes
1answer
1k views

If Betelgeuse were to go supernova

If Betelgeuse were to go supernova would the sky be twice as bright, or day time extended, depending on what time of year it happened in. Basically when it does supernova how bright and large will it ...
4
votes
1answer
177 views

PP Chain and CNO cycle relationship

At what temperature would the energy generation rates of the PP-Chain and CNO cycles be roughly equivalent? The dependences are so vastly different that I am wondering how and by what equations they ...
4
votes
2answers
426 views

The Opera Neutrino Experiment and the Supernova 1987

So this probably stems from my massive ignorance about post-Newtonian physics but the supernova results of 1987 which measured neutrinos arriving 3 hours before the light from the supernova have been ...
3
votes
2answers
468 views

How can a low-mass star increase its mass to 1.4 Msun?

In my astronomy class I learned that a only low-mass stars (< 0.5 Msun) will contract, and then become degenerate, until it is a white dwarf. However, we also learned about the Chandrasekhar limit, ...
3
votes
1answer
135 views

Dark Matter and Rotational Velocity Curves of Galaxies and Stellar Systems

The way dark matter is introduced to documentary watchers such as myself is to account for the difference between the rotational velocity of the stars in a galaxy compared to that of planets in ...
3
votes
2answers
855 views

How were the heavy elements from iron to uranium made? [duplicate]

Possible Duplicate: Age of the Earth and the star that preceded the Sun How were the heavy elements from iron to uranium made? References: ...
3
votes
1answer
179 views

Solar neutrino predictions

The solar neutrino problem has been "solved" by discovering that neutrinos have mass and they oscillate. So how accurate are now our predictions about the number and types of solar neutrinos that ...
3
votes
1answer
129 views

Looking for a resource that explains all colors and shapes of stars and galaxies

I've been looking at some of the astronomy pictures of the day from NASA. They all have content that is indicative of certain situations or events. Supernovas have the scattered lines look, new stars ...
3
votes
1answer
60 views

Sun energy production as a function of depth

I've heard a few times that the Sun's energy output per unit volume is less than that of a human, with the Sun's much higher power output being a result of its much greater size. In a blog post, ...
3
votes
1answer
22 views

Could asteroseismology have a significant potential to unravel the mechanisms of solar cycles that last thousands of years?

Could asteroseismology have a significant potential to unravel the mechanisms of solar cycles that last thousands of years? And what about solar cycles 1 billion years ago and 2 billion years ago ...
3
votes
3answers
207 views

Is it possible to calculate the radius of star?

Suppose I know the Luminosity $L$, temperature $T$ and Mass $M$ of star. Assuming the star is very heavy so that we can treat it to be radiation-dominated star. This would imply that pressure inside ...
3
votes
1answer
92 views

What causes the differential rotation of the Sun?

I have read the explanation in Wikipedia, but the problem is that because I'm a beginner in physics, I don't quite understand what more than half of the words mean! Here is the explanation from ...
3
votes
1answer
107 views

Mass loss in Red Giants via dusty-winds and chromosphere activity

I'm reading some literature on mass loss in the RGB/AGB branches and so far I'm getting a lot of information regarding mass loss via dusty-winds/pulsations but almost no explanation of mass loss by ...
3
votes
1answer
192 views

Predicting stellar evolution and life cycle of a star

Is there a way to model/predict the evolution and life cycle of a star based on certain initial conditions? That is, whether it will become a red giant, brown dwarf, etc. I'm basically looking for ...
2
votes
2answers
61 views

Measuring Rotation of Sun

If the sun had a uniform surface (i.e., if there were no sunspots to look at), is there a practical way to measure its rotation? In other words, if some external force flipped the sun's spin ...
2
votes
1answer
207 views

How to calculate the radius of a main sequence star based on mass?

What would I need in addition to the mass to figure out the radius of a main sequence star?
2
votes
2answers
154 views

Harvesting hydrogen from a star

I've been thinking about what highly technologically advanced civilisations would do once their energy requirements become comparable to the total output of a star, and how such activity could be ...
2
votes
2answers
64 views

What would happen if I gathered stellar sized masses of iron?

Lets say I had a bag that when turned upside would start pouring out iron shavings and never ever stop. Viola, there's my infinite source of iron. Now, lets say I just continued to dump this iron ...
2
votes
2answers
109 views

Why does the core of a low mass star contract after reaching electron degeneracy?

I am learning about the lives and deaths of stars with a solar mass of 0.4-2. What I understand is that once the star stops hydrogen fusion after using up all its hydrogen, the star leaves the main ...
2
votes
2answers
552 views

Are there formulae for calculating stellar luminosity and effective temperature as a function of age?

Is there a manageable formula or set of formulas or simple algorithms that approximate stellar luminosity and effective temperature (or radius) as a function of stellar age? I'm aware that accurate ...
2
votes
3answers
154 views

Are there more photons than nuclei within the Sun?

Are there more photons than nuclei within the Sun? Is there a good way to estimate what the ratio would be?
2
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0answers
33 views

How much energy does proton - carbon 12 fusion produce?

Page 25 of this document from the California Institute of Technology says that proton-carbon 12 fusion releases 7.54 MeV, while Wikipedia says it releases 1.95 MeV. Which one is correct?
2
votes
1answer
238 views

Properties of a star made from water? [closed]

What would the properties of a star formed from a sphere a water/ice be? One large (massive) enough to collapse into an ordinary, mostly plasma star. I understand that it would have particularly high ...
1
vote
2answers
114 views

Density of the Sun

Being either on the surface or somewhere inside; where is the density of the gases of the Sun equal to the density of the ground we stand on here on earth?
1
vote
2answers
47 views

Can solar flares affect gravity?

Can a large eruption of solar flares on our sun change the effects of its gravity ever so slightly, and is it possible to measure these changes with the equipment we have today.
1
vote
2answers
121 views

Habitable zones around other stars

I have a question about measuring the boundaries of habitable zones on other planets. Is it okay to assume that, if Sun's habitable zone starts at a distance $R_0$ and its luminosity is $L_0$, we can ...
1
vote
2answers
83 views

Good books or resources for Stellar Astrophysics at graduate level?

I wanted to know if there is a well established resource in the field of Stellar Astrophysics which is not very outdated. I have seen plenty of books at my university library but I do not know if ...
1
vote
2answers
428 views

How to calculate radius of a sphere of Ideal gas?

The problem I am faced with is approximating the mass of the sun, and I thought that a fancy approach would be to approximate the sun as being made up entirely of hydrogen atoms, finding the resulting ...
1
vote
1answer
62 views

Mass exchange between stars

I'm curious, are there specific conditions for a transfer of mass between two stars as they pass near each other? Its for my final project in computing, I'm writing a gravity simulation and thought ...
1
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1answer
639 views

Is there a simple formula for calculating semimajor axis bounds of “habitable zone”?

I understand that an accurate determination of the bounds of the "habitable zone" for a given stellar system depends on a large number of factors, including many beyond characteristics of the parent ...
1
vote
1answer
64 views

How does fusion work in the Sun if neutrons have more mass than protons?

According to my textbook, the next result of the fusion reactions in the Sun is: 4H -> He + neutrinos + gamma photons However, if hydrogen atoms are basically a proton and helium atoms are 2 protons ...
1
vote
1answer
23 views

Why do we ignore the Flux - Luminosity relation when deriving the Eddington Luminosity?

I was just doing the derivation for the Eddington Luminosity by considering the momentum equation, $$ -\frac{Gm}{4\pi r^4} = \frac{dP}{dm} $$ and the radiative transport pressure $$ \frac{dT}{dr} = ...
1
vote
0answers
64 views

What is the metric at the center of a star? [duplicate]

If there is only one star in the universe then is the metric at the center of the star flat?