All Questions
1,007 questions
4
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views
Will Hawking radiation violate baryon number conservation around gravitating bodies other than black holes?
Numberous articles discussing a recent research paper suggest that even stars and planets will eventually radiate away their mass like hawking radiation. My question is will this violate baryon ...
- 317
0
votes
1
answer
48
views
Effect of Sun temperature on the thermosphere
Correct me if I’m wrong here.
The thermosphere is hot due to its absorption of moderately high energy UV radiation. (<200nm)
Cooler stars emit fewer high energy photons. So if the Earth orbited an ...
- 3,001
0
votes
2
answers
120
views
What’s the lightest you could make a "star" if you made it out of different materials?
What’s the lightest you could make a "star" if you made it out of different materials?
How large would the "star" be?
For example according to here https://astronomy.stackexchange....
- 3,001
0
votes
1
answer
78
views
What's the nearest star that could go Supernova in the near future?
https://en.wikipedia.org/wiki/IK_Pegasi B is the nearest supernova candidate, but that white dwarf that's part of the system won't go supernova for around 2 billion years.
What's the nearest star that ...
- 3,001
7
votes
2
answers
200
views
Why are black holes sometimes formed without supernovae?
I've heard that very massive stars can sometimes collapse into black holes without creating supernovae.
How does this happen?
(I suspect it's something to do with the relative lack of Urca process ...
- 3,001
2
votes
2
answers
759
views
Making sense of the Jeans mass
The Jeans mass, given by $M_J=\sqrt{\left(\frac{-5k_BT}{Gm}\right)^3\cdot\left(\frac{3}{4\pi\rho}\right)}$, is the threshold mass a dust cloud must have in order to begin gravitationally collapsing ...
- 1,880
0
votes
1
answer
147
views
What’s the minimum mass required for a star to burn helium?
What’s the minimum mass required for a star to burn helium? I’ve liked online but I’ve gotten inconsistent answers.
- 3,001
0
votes
1
answer
39
views
Difference between star formation rate and star formation history
When we speak about galaxies evolution, what is the difference between the star formation rate and the star formation history?
0
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2
answers
114
views
Time of collapse of stellar dust cloud
A stellar gas cloud collapses onto itself once it reaches Jean's mass, and the time it takes for said cloud to collapse is given by: $t_{coll}=\sqrt{\frac{3\pi}{32G\rho_0}}$, where $\rho_0$ is the ...
- 1,880
3
votes
1
answer
128
views
How many type II supernovae are there per stellar mass formed?
I have been searching for the number of type II (core-collapse) supernovae per unit of stellar mass formed.
It is my understanding that a star must have an initial mass of at least 8 times and no more ...
- 31
0
votes
3
answers
166
views
How line of sight is determined?
How do they calculate the line of sight of a galaxy or binary star system from an observer's point of view. the velocity of a star in a binary star system depends on the line of sight so how do they ...
1
vote
1
answer
192
views
Numerical Integration for White Dwarf Model [closed]
I'm creating a numerical integration model of a white dwarf. So I've started with calculating electron pressure vs number density across a wide range of values.
Using the equations of state in ...
- 11
1
vote
2
answers
146
views
Star with quadrupole in binary system violates Newton's 3th law?
Suppose that, in a binary system of two stars, the star A (and only the star A) has a non-zero quadrupole moment $Q_A$. Then, the star B feels the usual gravity force plus an additional force, ...
- 179
8
votes
2
answers
794
views
At what distance can I assume peculiar velocity is negligible compared to expansion velocity, to use Hubble's law?
I am working on a high school physics assignment and am trying to figure out a method to determine if the star is a giant or main sequence from its spectral and photometric data (from SDSS). I picked ...
0
votes
1
answer
37
views
SETI: Are there holes in the EM spectrum that are quiet enough to communicate at decently large distances?
So this is a variant of this other question.
I know stars are big, they radiate a lotta energy, they have spectral lines. But how dense are these spectral lines and is there a noise floor at ...
- 1,733
9
votes
2
answers
1k
views
Why don't less massive stars explode in form of supernovas?
I'm a high school student with a question about supernovas and the life cycle of stars. I understand that supernovas occur in massive stars when they run out of fuel and collapse, resulting in a ...
- 647
0
votes
0
answers
29
views
The role of gas pressure in the life cycle of stars
As a high school student, I am curious about the significance of gas pressure in the life cycle of a star. While I have read about the radiation pressure and the pressure caused by the energy released ...
- 647
3
votes
1
answer
163
views
Color temperature or effective temperature?
I am a high school student interested in astronomy and physics, and I am trying to understand the difference between color temperature and effective temperature of a star. As I have searched this ...
- 647
0
votes
2
answers
34
views
$y$-axis on HR diagram
I have an exercise where I have to determine the luminosity of a star with 10 000K. However, I'm confused about how to read off the $y$-axis on the graph when the scale is uneven. What is the function ...
- 63
1
vote
1
answer
22
views
Star equations of hydrostatic equilibrium for a mix of 2 fleebly interacting gases/fluids and interaction term
For a single matter species, the equations of hydrostatic equilibirum for a star are
\begin{eqnarray}
\nabla^2 \phi &=& 4\pi G \rho\\
\vec{\nabla} P + \rho \vec{\nabla}\phi &=&0
\end{...
- 182
1
vote
1
answer
256
views
Can I find the surface temperature of the Sun using a spectrometer?
So I have a high school physics project and I essentially have this experiment idea where I use spectroscopy to find the surface temperature of the sun. Now I'm essentially going to assume the Sun is ...
2
votes
2
answers
302
views
Can a massive star become a red giant more than once?
Massive stars may undergo multiple fusion processes as they near the end of their lifespans. Our sun will eventually start fusing helium in its inner core so that carbon is formed. As this occurs, the ...
- 405
3
votes
1
answer
61
views
Can the quark-gluon plasma of the very early universe be accurately characterized as a type of quark star?
The very early universe was dense and opaque. During the quark epoch, the entirety of the universe, up to every boundary, was a filled-in ball of QGP. Much like a star is a ball of ionized nuclei that ...
- 335
1
vote
1
answer
38
views
Should blue & white stars appear red and inflated while observed on the lower part of the night sky for the similar reason as the Sun during sunset?
Should blue and white stars appear red and inflated while observed on the lower part of the night sky for the fairly same reason as the sunlight during sunset? As these stars emit blue light ...
- 2,275
0
votes
1
answer
619
views
Is there a way to calculate/estimate the orbital period of an exoplanet from only one transit detection?
I understand that multiple transit detections of an exoplanet are almost always used to derive its period, but is there a way to do it with only one transit detection? As in, with only one available ...
2
votes
1
answer
62
views
Is this scientifically useful exoplanet transit data? [closed]
For a physics project, I am trying to derive properties of exoplanets through their transit light curve. I tried to get the data myself, on the known exoplanet Qatar-6b.
My Equipment:
8" Newt. ...
2
votes
1
answer
2k
views
Is the temperature of the hottest star's core known?
WR 102 is believed to be the hottest star in the observable universe, whose surface temperature is $210,000 ^\circ K$. But the related wikipedia entry does not say anything about the temperature of ...
user89243
1
vote
0
answers
54
views
In a non-rotating sphere of fluid heated from the centre, what does the flow look like?
Consider a perfect sphere of fluid which is not rotating, is under the influence of its own gravity only, and is heated from the centre. What does the flow of fluid in the body look like?
In the ...
- 11
1
vote
0
answers
22
views
Stellar classification: Luminosity class
So in the astrophysics textbook by Carrol and Ostlie, when the luminosity class is discussed, it has such a sentence "The ratio of the strengths of two closely spaced lines is often employed to ...
- 191
0
votes
0
answers
34
views
Finding chemical equilibrium with an application to the Sun
Consider the following reversible reaction: $\rm H_{2}{\rightleftharpoons}2H$. To find chemical equilibrium, we use the chemical equilibrium constant. There are two equations for the chemical ...
- 23
14
votes
3
answers
3k
views
Is it possible to tell the difference between a young star that is just "big" and an older red giant?
I read the Wikipedia page for one of the biggest known stars, UY Scuti, and was curious to see the age of the star isn't really known at all.
When a star's hydrogen fuel is exhausted, it starts ...
- 253
1
vote
1
answer
74
views
What would the nucleus of a water planet be like?
If we have a planet made out of water, would it be all solid? What about the nucleus? Would it depend on the size of the planet?
- 151
0
votes
1
answer
542
views
How long will Brown Dwarfs live?
Brown Dwarfs are a technical star because of their masses. If Red Dwarfs live for 10 trillion years, then how long could Brown Dwarfs live?
2
votes
1
answer
316
views
Does the Sun increase entropy?
The Sun generates heat via fusion. The heat from this reaction gets distributed around the solar system and beyond. This process of spewing heat and radiation all over the place doesn't immediately ...
- 319
-1
votes
2
answers
77
views
Star gets eaten and spit out?
How does material get swallowed and torn apart by a black hole and have its light vanish. But somehow the same material escapes years later. After its light couldn't even escape?
They watched it for ...
5
votes
1
answer
35
views
Why is the common envelope ejected in some accretor-donor systems?
As an example, let us consider a binary system of a neutron star and an evolved star (e.g. red giant) that has expanded, filled its roche lobe, and started the mass transfer onto the neutron star.
...
- 95
3
votes
1
answer
338
views
Why are certain stars not moving in this timelapse video?
In this video, certain stars are not moving in the bottom left & top right corners. I thought only the pole star didn't move.
https://www.youtube.com/watch?v=QC8iQqtG0hg&ab_channel=...
3
votes
1
answer
216
views
Why is a star unstable if it's adiabatic exponents are less than 4/3?
In "Introduction to Stellar Structure" by Walter J. Maciel at page 76 it is said that for a partially ionized non degenerate hydrogen gas, the star is unstable if the adiabatic exponents are ...
1
vote
1
answer
22
views
Star convection gradient dependencies
In star convection you can work with radiative and adiabatic temperature gradients, that for example the radiative one can be defined as,
$$\nabla_{rad}=\frac{3\kappa L P}{16 \pi a c G m T^4}$$
This ...
- 529
0
votes
0
answers
41
views
Can starlight be described as a light beam?
I would like to study starlight propagating in the universe. Can I model the light as a beam with a radius $r$ and a unit wave vector $\hat {\mathbf k}$ with a frequency distribution $f(k)$? Also, the ...
- 105
2
votes
2
answers
83
views
What do these negative mass values obtained from the Lane-Emdem equation entail?
Starting from the hydrostatic equations for a star:
$$\frac{dM}{dr}=4\pi r^2\rho $$
$$\frac{dp}{dr}=-\frac{GM\rho}{r^2} $$
It is possible to obtain the following expression using the dimensionless ...
- 3,558
0
votes
0
answers
70
views
How is Starlight Reflected by Mirrors?
The chosen star is on the Z-axis of Mirror-A (Red). The Star image appears on the
face of this Mirror at the Origin of the CoSys.
Starlight Rays (Yellow lines) remain parallel, and do not diverge with ...
6
votes
1
answer
331
views
Why is lithium burned at lower temperatures than hydrogen inside stars?
The destruction of lithium inside stars through the reaction
$$ ^{7}_{3}{\rm Li} + {\rm p} \rightarrow 2\ ^{4}_{2}{\rm He}$$
takes place at just $\sim 3\times 10^6$ K.
This is much lower than the ...
- 137k
1
vote
0
answers
32
views
How can I calculate evolutionary timescales of low mass stars?
How can I calculate how long a star of a given mass will spend on an evolutionary branch before evolving off it?
I'm thinking about the evolution of low mass stars from the subgiant branch to the red ...
- 31
1
vote
1
answer
334
views
What is the average prevalence/density of stars in space?
By average density $D$, I mean that, if you look at a huge volume $V$, you would expect to see $V\cdot D$ stars in that volume.
Just a rough answer is fine. I'm sure the prevalence of stars changes ...
- 1,092
4
votes
2
answers
127
views
At what average rate do supernovas happen?
By average rate $R$, I mean I want an answer with units $$[R]=\frac{\text{supernovas}}{\text{length}^3 \cdot \text{time}}$$. That is to say, if I consider a huge volume $V$ and a long timeframe $T$, I ...
- 1,092
1
vote
0
answers
20
views
Comparison of temperature between white dwarfs and giants using the Saha equation
“ Consider the comparison of Fe I and Fe II lines in the spectrum of the sun and in the spectrum of a star with a very similar Teff but much lower gravity—for example, the supergiant β Draconis. From ...
- 327
0
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1
answer
126
views
Where can I find datasets for all confirmed exoplanets, terrestrial exoplanets, and Super-Earth exoplanets?
"The NASA Exoplanet Archive" (https://exoplanetarchive.ipac.caltech.edu/) implies that there are 5,197 confirmed exoplanets. The "5,197 Confirmed Planets 11/02/2022" link leads to ...
1
vote
0
answers
34
views
How small would iron stars with a mass of 0.50 solar mass be?
I am wondering how small a 0.50 solar mass iron star would be. Would they be the same size as a white dwarf of that mass which would be about the size of the Earth. Or would they be much smaller and ...
- 1,554
1
vote
1
answer
121
views
How could have Boson Stars formed in the universe at all?
Boson particles have been predicted by some to have been able to form stars known as Boson Stars. I am curious as to how these stars could have formed given that Boson particles are not truly affected ...
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