Questions tagged [stellar-astrophysics]

Questions about inducing chemistry, physics, and many more branches of science into astronomy.

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64
votes
2answers
11k views

Is it dark inside the Sun?

This might sound like a strange question, but something got me thinking about it recently. The opacity of plasma in stellar interiors can get quite high, making for shorter free-paths for photons. In ...
26
votes
1answer
884 views

Why are there no green stars?

There are red stars, and orange stars, and yellow stars, and blue stars, and they are all understandable save the fact that there is a 'gap': There are no green stars. Is this because of hydrogen's ...
21
votes
3answers
957 views

How does the evolution of a solar system not break the second law of thermodynamics?

Please forgive: I am a layman when it comes to physics and cosmology, and have tried finding an answer to this that I can understand, with no luck. As I understand it, the solar system evolved from a ...
20
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3answers
6k views

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

Can a planet, star or otherwise have a magnetic field that is stronger or have more range than its gravity?
16
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2answers
741 views

Open problems in astronomy that an amateur (with a PhD in some other field) would have a chance of solving?

What are some open problems in astronomy that an amateur would have a chance of solving? Suppose the amateur has a PhD in some other field, owns a basic telescope, a set of filters, diffraction ...
14
votes
1answer
913 views

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

Without quantum tunneling our Sun wouldn't be hot or massive enough to produce the energy it does at the moment. So what would have been the temperature or mass of our Sun without quantum tunneling of ...
13
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0answers
291 views

What was the absolute limit to the possible sizes of the first stars formed from “primordial material with no metals”?

This answer to Why don't or (can't) stars be more than 325 or so times the mass of the sun? What limits their size? includes the following: ...The upper limit you refer to is for compositions similar ...
12
votes
1answer
3k views

How exactly is the Initial Mass Function (IMF) calculated?

The Initial Mass Function (IMF) is the empirical function which describes the initial masses of a population of stars. My questions are, 1) What are the various IMF's which are used? 2) For each, ...
11
votes
2answers
1k views

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

Mass, size, temperature, luminosity, chemical composition, the initial abundance of the molecular cloud, distance, brightness, age, and evolutionary cycle can all be used to characterize a star. A ...
10
votes
1answer
240 views

Is any consensus forming on the solution to the “Lithium Problem”?

The "Lithium Problem" relates to the fact very-low-metallicity stars appear to have a Li/H ratio approximately one third of what would be expected. The ratio should be the same as the prediction from ...
9
votes
3answers
4k views

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? Why does this not speed up, i.e. one fusion event creates energy for two ...
9
votes
2answers
1k views

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

Are there likely to be stars for which the average density is greater than the density at the centre? Intuitively, I would say not, as density tends to decrease outwards, but ideally I would ...
7
votes
1answer
933 views

Why does lithium fuse at lower temperatures than hydrogen?

This is a basic question, but it's been bugging me. In the Wikipedia article for lithium burning, it states that: Stars, which by definition must achieve the high temperature (2.5 × 10^6 K) ...
7
votes
1answer
126 views

Why does the convective core in an intermediate to high mass star shrink?

The image below shows the evolution of the hydrogen mass fraction profile for a 5 solar mass star in the main sequence. I would expect that the size of the convective core stays roughly constant as ...
6
votes
2answers
1k views

Why we define Stellar motions with respect to sun?

We take observations from earth but to define radial velocity, transverse velocity and the proper motion of stars, why we consider them with respect to the sun? and then we do some corrections due to ...
6
votes
3answers
478 views

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

We know about events like two neutron star colliding and resulting in a black hole, also collision of black holes, and collision of galaxies. But we never see a satellite such as a moon colliding with ...
6
votes
1answer
444 views

Is there a star simulation software that can handle mass ejections and supernovae?

I use MESA right now to play around with stellar dynamics, but it can't do mass ejections and therefore, I can't think of a way to let the simulated stars explode as supernovae. What simulation ...
6
votes
1answer
91 views

Do star systems stripped from galaxies have different expected behavior?

Another way of stating this question: Does the fact that a star system (or comparable concentrated mass) exists in a galaxy affect its long-term evolution, in terms of expansion, contraction, ...
5
votes
1answer
74 views

Activity of M dwarf stars

Why are M dwarf stars the most active (in terms of starspots and faculae) of all stars?
4
votes
2answers
511 views

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

David Christian's Maps of Time has this to say about the period during which the first stars started to form from the diffuse clouds of hydrogen and helium that then made up the universe: Under the ...
4
votes
1answer
44 views

How to model and subtract nebular emission lines from stellar spectrum

I have optical stellar spectra from a star forming region with spatially varying nebular background. I want to subtract the nebular lines from the stellar spectra (which are varying in intensity from ...
4
votes
2answers
14k views

How do we find the exact temperature of a star?

This is a very basic question, but I am a little confused. As far as I know, the temperature of a star is analyzed based on the color of the light it emits. So, if a star is moving away from us, then ...
4
votes
3answers
506 views

If a white dwarf collides with a giant star, could it create a TZO?

Thorne–Żytkow objects (TZOs) form from collisions between neutron stars and main sequence or giant stars. Ultimately, the neutron star becomes the "core" of the giant star. However, could this also ...
4
votes
1answer
4k views

How can I convolve a template spectrum with a photometric filter response spectrum?

Suppose I have a template stellar population spectrum (say, from Bruzual & Charlot 2003) which runs from like 1000 Angstroms to 160,000 Angstroms and which has x-axis wavelength units of Angstroms ...
4
votes
1answer
2k views

Why is the Sun's brightness and radius increasing, but not its temperature?

On the Sun's article on Wikipedia, there is an image showing how the Sun's brightness, radius and temperature have changed over time: For the past (and next) few billion years, I see the luminosity ...
4
votes
1answer
264 views

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

It has been proposed before that we could use this technique to remove hydrogen from the Sun to lower it's rate of fusion and extend it's life, so it doesn't fry our planet. I am wondering how much ...
4
votes
1answer
526 views

Is the Sun slightly blue in the center? - Wavelength-dependent limb darkening of the Sun

I've slightly modified the title to try to attract some attention. If we call sunlight "white" and limb darkening is a result of seeing deeper at normal incidence and shallower at oblique incidence, ...
4
votes
1answer
2k views

death of a red dwarf star / minimum mass needed for a white dwarf?

OK, first, I know there's a variety of sizes and types of red dwarf stars and the universe is too young for any of them to have reached the end of their main sequence phase yet, so it's all ...
4
votes
0answers
41 views

How does SOHO know what's going on in the sun's magnetic feild?

SOHO can image the sun's magnetosphere. I don't have the foggiest idea how they manage it, because at most a satellite can have a single compass, and measure one point in the magnetic field. I thought ...
4
votes
0answers
49 views

Strength of core-envelope coupling in stars

For a star with a given Zero-Age Main Sequence (ZAMS) mass, as a function of metallicity how strongly is the star's core coupled to its envelope? I understand that the core-envelope boundary is only (...
3
votes
4answers
537 views

Why there are less massive stars than low mass stars?

What factor determines the selection of star mass? And how we know that the number of massive stars is less than the number of low mass stars?
3
votes
2answers
2k views

Definition of stellar core?

This is a basic question, but I may as well ask it. I had always thought that the core of a main-sequence star is defined as the part hot enough for nuclear fusion. Some dictionaries seem to agree ...
3
votes
2answers
92 views

How to understand exactly why gravity darkening happens on rotating stars?

Phys.org's TESS delivers new insights into an ultrahot world links to KELT-9 b's Asymmetric TESS Transit Caused by Rapid Stellar Rotation and Spin–Orbit Misalignment (readable in arXiv) The assymetric ...
3
votes
1answer
53 views

What is the difference between emission line and recombination line?

When an electron jumps from higher level to lower level a photon is emitted. This is seen as an emission line. But what exactly is a recombination line? I found them similar. Can you please tell me ...
3
votes
1answer
86 views

Are there ways to determine a star's spectral type without the spectrum itself?

Because of the lack of available data online, I am trying to find other ways to determine an exoplanet's host star's spectral type.
3
votes
1answer
68 views

Does the luminosity of a star on its main sequence increase or remain approximately constant?

From my understanding, the luminosity of a main sequence star should increase because, in its core, hydrogen is being burnt into helium which means that the number density of hydrogen decreases and ...
3
votes
2answers
110 views

Are there examples of the effect of shock waves on chemical evolution in of interstellar medium or circumstellar medium?

Are there any examples of calculations or observations of suspected effects of shock waves on chemical evolution in interstellar medium or circumstellar medium?
3
votes
1answer
82 views

What is a “differential chemical abundance”?

Probably this is a extremely basic question, but after a lot of time searching about it without results (in all the sites/publications i've found they only use this term but there is no specific ...
3
votes
1answer
46 views

How does gyrochronology deal with differential rotation and axial tilt?

K2 seems to become an even better mission than what a continuation of Kepler would have been, at least to some astronomers. By what means is differential star rotation being dealt with? The rotational ...
3
votes
1answer
162 views

Is it possible for stars not to rotate?

I'm just curious about that the existence of non-rotating stars. Is it physically possible for a star not to rotate at all? Does magnetic braking eventually stop the stellar rotation?
3
votes
1answer
172 views

Why does the adiabatic exponent decrease at the ionization zones?

The context is the ionization zones in stellar atmospheres or interiors, the Sun, for instance. The adiabatic exponent is the heat capacity ratio: $$\gamma = \frac{c_P}{c_V} = \frac{C_P}{C_V}$$ And,...
3
votes
3answers
196 views

Recommendation for learning about stellar astrophysics

I would like to know which are the best books to learn about stellar astrophysics at (just) graduate level. I have a basic formation in general astrophysics but I'm interested in learning about ...
3
votes
1answer
843 views

Why is $g$ tied to the oscillator strength $f$ in $\log{gf}_{\odot}$

The $\log{gf}$ value of an element in a star, where $f$ is the oscillator strength, or probability of transition in an atom, is a scientifically pursued quantity with relation to the model atmosphere ...
3
votes
1answer
61 views

How does the field decay of a magnetar power the emission of high-energy electromagnetic radiation?

In Space.com's Dr. Pulsar and Mr. Magnetar? 2 Star Types May Turn into Each Other (and in the linked video) "Tom Prince, a physics professor at the California Institute of Technology and a senior ...
3
votes
1answer
84 views

Why is the cloud around Eta Carinae shaped the way it is?

According to (german) wikipedia, Eta Carinae has a cloud shaped like two cones with the points at the poles of the star as well as a disk around the equator. These matter clouds are mostly or only ...
3
votes
1answer
204 views

What's an order-of-magnitude main sequence star look like?

I'm looking for a very rough, order-of-magnitude approach to the main sequence. What I mean is, I have a spherically-symmetric hydrogen distribution. I'm looking to get a rough approximation for ...
2
votes
2answers
205 views

Old star vs New star

We know that with the passage of time, hydrogen inside (near or at the core) a star is converted to helium and other heavy elements via fusion. We can then deduce that the old sun contains heavier ...
2
votes
1answer
54 views

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

When black holes forms out of a supernova do they have a very high velocity ?? Or do they remains at the position of parent star ??
2
votes
1answer
270 views

Mechanics of Supernovae

Where does the energy from a supernova come from? My understanding is that the iron core collapses into a large ball of neutrons - is that effectively a star-sized nuclear explosion? What is the ...
2
votes
1answer
985 views

Why are main sequence stars more massive than the Sun less dense? e.g. Vega, Spica etc

Why do main sequence stars more massive than the Sun have lower densities? e.g. Vega, Spica etc.