# Questions tagged [astrophysics]

Questions involving the physics of the universe, especially the nature of astronomical objects, energy fields, and/or regions, rather than their positions or motions in space.

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### If the earth lost 1/81th of its mass how would it orbit change? [closed]

If a planet becomes lighter, then I believe its orbit will change. For example, the moon is 1/81 of the mass of the earth. So, if the moon disappeared, then the earth-moon system would lose 1/81th of ...
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+50

### Force to fix $\sigma_{8}$ or $A_{s}$ for Forecasts

In the context of Forecasts with Fisher's formalism, I make vary cosmological parameters to compute the elements of the Fisher matrix. First, I generate with CAMB code a linear power spectrum. Then, ...
137 views

### Can a black hole with the density of water exist?

During an outreach event, the speaker mentioned the fact that black holes weren't necessarily dense objects and that black holes of any density could exist, even black holes with the density of water. ...
69 views

### light emission of quasars

I'm studying Astrophysics and I have some video's from a past course. I'm currently studying the reionization of the universe. At the current point in the class, the instructor is talking about ...
45 views

### What is a good way to start learning Astrophysics [duplicate]

I am a junior in high school and throughout my life I have always had a fascination with space. I use to watch "Space's Deepest secrets" and stuff like that but after a while I stopped learning much ...
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### Why is blue light extincted more strongly than red?

We know that blue light suffers extinction more strongly than red (examples include ISM, atmosphere etc.), but why? Is it a property of the shorter wavelength itself? Can someone provide a conceptual ...
53 views

### Simulation and its necessity in astronomical research

I was wondering if mainly everything in Astronomical and Astrophysical was dependent on Simulations then what are the educational requirements are necessary to qualify as the Simulation Expert. What ...
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### Can there be a three-moon system where only two are visible most of the time?

I am trying to build a moon system for a sci-fi work in collaboration with my boyfriend. He's the main writer and wants there to be three moons circling the planet in a way that most of the time, only ...
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### Saw 8 stars moving in same direction one by one [duplicate]

I saw 8 stars moving in the same direction one by one and then at the same point they faded or you can say disappear. The speed was not too fast but not too slow either. I am more curious because ...
57 views

### How is the Dating of Samples of Cosmic Objects Performed, Including Pre-Solar Dust?

In the new Space.com article 7 billion-year-old stardust is oldest material found on Earth I had read about some pre-solar grains dating back to 7 billion years. What techniques are used in ...
113 views

### Plotting Astronomical Data Using Python

I have been trying to create a graph which shows the distances between the stars and the earth, yet I could not obtain the desired graph as seen below (click to zoom in): My graph: As @uhoh ...
120 views

### the more concentrated the mass, the more powerful is gravity?

the more concentrated the mass, the more powerful is gravity? or does it depend exclusively on the total mass? example: 1 $\small\mathsf{cm^3}$ material A = 1000 kg, 20 $\small\mathsf{cm^3}$ material ...
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### 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 ...
23 views

### Doppler Shift Vs Gravity Assist for Frequency Shifts from Rotating Galaxies

Frequency Shift of Light Leaving Rotating Galaxies: Differences in Doppler Shift due to Radial Velocity Vs Frequency Shift due to Gravity Assist from a Rotating Galaxy A description of estimating ...
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### Can “rock stars” form from interstellar dust?

Imagine a large and massive dust cloud made of solid phase micrometeoroids, asteroids and larger planetoids, all material in it is in solid phase, and contains no hydrogen or other volatiles in gas ...
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### Dark matter hypothesis and visible light

Dark matter is hypothesized as the mass based on gravitational observations is much higher than that observed via measuring how bright a galaxy is. Why could it not be that multiple stars in the ...
72 views

### Is the turn-off point of the main sequence the same as the terminal-age main sequence?

The turn-off (TO) point is one tipping point on the Hertzsprung-Russell diagram (HRD) where low-mass stars start to evolve from the main sequence (MS) toward lower temperatures. If I understand it ...
44 views

### who use the GADGET-2 pack for simulation? can I do it with my PC? [closed]

I want to do hydrodynamic simulatin for universe at vary redshifts. How can I do it? or how can I find an simulation of universe for use my work?
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### Calculate rest flux density from emitted flux density derived from fits data of SDSS data tool

Say I have a spectrum of a galaxy at a redshift z, with flux density units of erg/s/cm^2/Angstrom. I'd like to convert flux density(in the same units) at z=0, i.e. at its rest it's rest emisison. How ...
2k views

### Gravitational Properties of Asteroids

How large would an asteroid have to be in order to hold a person so that the person could not escape?
66 views

### Rocket Equation from Newton’s 3rd Law, Force of Gravitation, and dp/dt [closed]

I am using the derivative of momentum (dp/dt) with Newton’s 3rd Law with the gravitational force of Earth.    F - F_{gravity} = \dfrac{\mathrm{d}p}{\mathrm{d}t} \\ F - \dfrac{G \, m_e \, m_r}{...
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### Where does Love's number come from in $\delta m = \frac{1}{2}k_2M_2\lbrace\frac{R_1}{r_\star}\rbrace^3$, the formula for the mass of a tidal bulge?

My question refers to page 165 of this paper. Specifically, how does one derive the equation $\delta m = \frac{1}{2}k_2M_2\lbrace\frac{R_1}{r_\star}\rbrace^3$ for the mass of a tidal bulge? A full ...
98 views

### A question related to black holes

Not a physicist just a simple guy, don't know much about physics as well. Just like to read things. I was reading about black holes and a question popped in my mind. If I understand it correctly ...
48 views

### Where can I find the previous years' International Astronomy Olympiad question papers?

I have looked at the official site but the link for previous papers isn't working.
213 views

### How much volume of gases could be added to Mars atmosphere that the planet gravity can retain?

This article Mars atmosphere states that Today Mars atmosphere is very thin, but it was not always like this. Around 3.5 billion years ago, the red planet had a thick enough atmosphere for liquid ...
585 views

### Can I see Harvest moon in India?

I just saw articles which say the Harvest moon will be visible in the US. Apart from the US, in which other country can people see it? Can I see it from India?
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### By convention, when observing an exoplanet, is the observer's line of sight (on Earth) perpendicular to the plane of sky?

This is just a quick question. In this picture of the orbit of an exoplanet, is the observer's line of sight perpendicular to the plane of reference, which is in the +z direction on the diagram? In ...
191 views

### Does the universe curve in on itself?

I've read a little bit about the universe, but its not clear to me if there is a place in the universe beyond which there are no stars. For example, is there anywhere in the universe that one could ...
128 views

### Tallest cliffs, by falling duration

If you threw something over a certain cliff on a moon of Uranus, it would take over ten minutes to hit bottom. By this measure, local fall time rather than absolute height, what are the near ...
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### Is there a way to calculate how much damage black hole merger shockwaves inflict on nearby objects?

Black hole mergers lose respective rotational energy during a merger. I understand that the energy 'emitted' is scattered as gravitational 'energy' that can henceforth be detected by gravitational ...
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### Is it possible that Venus was once a rogue planet? [closed]

As Venus rotates in clockwise direction, while other planets rotate in anti clockwise direction: Is it possible that Venus was once a rogue planet?
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### Maxwell stress contribution to $\nabla \cdot \mathbf{P}$ in the Navier-Stokes equation for fluid in stars

I was reading through the following extract outlining how the Maxwell stress contributes to the $\nabla \cdot \mathbf{P}$ term of the Navier-Stokes equation for fluids in a star. Here $\mathbf{P}$ is ...
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### If all stars rotate, why was there a theory developed that requires non-rotating stars?

According to Penrose's research, a non-rotating star would end up, after gravitational collapse, as a perfectly spherical black hole. However, every star in the universe has some kind of angular ...
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### What is the meaning of ellipticity of a galaxy being more than 1?

I was recently doing a project on calculating ellipticities of galaxies using SDSS data. I downloaded the stokes parameter values (q and u) from data \$e = 1 - \frac{b}{a} = 1 - \frac{1 - \sqrt{Q^2 + ...
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### Could anyone recommend a good book about the basics of astrophysics? [closed]

I am sorry if a similar question has been asked before. I look at other questions but was not entirely satisfied. So, I am a Psychology student who would like to know more about the basics of ...
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### What decides the shape of a galaxy?

As I know, most galaxies are spiral, some elliptical, and a rare type is ring-shaped. What causes a galaxy to become a specific shape (i.e. ring vs spiral)?
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### What is the significance of minus sign in the expression of apparent magnitude?

The apparent magnitude of a distant object in the sky is measured as m=(-)2.5 log (I/I0) Where I is the intensity of the object whose apparent magnitude is to be measured and I0 is the reference ...
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### When and where are we exposed to most cosmic radiation?

I understand the mechanical dynamics of being faced away from the Sun at night, having a lower altitude, not being in the weak points at the poles in the atmosphere, being in a valley and others ...
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### Tilting cycle vs orbital cycle of earth

Common sense tells us a year is conveniently divided exactly into four seasons. I'm wondering whether that means the earth's tilting cycle is equal to its orbital cycle. If so, there must be a reason. ...
128 views

### What is the effect of this expanding universe on the energy density of it?

We know that our universe is expanding i.e. volume of the universe is getting larger with time with total energy being conserved. My question is whether the energy density is getting smaller with ...
111 views

### Does nucleosynthesis happen near a black hole or during its formation?

During its lifetime, a star can synthesize elements to iron because of the pressure and temperature. A neutron star is composed of iron nuclei 56 and up to krypton 118. According to that, a black ...
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### Fermi-LAT gamma ray spectrum too high [closed]

I am extracting gamma ray spectrum from Fermi-LAT data. My spectrum is a lot higher than the ones on previous studies of the same object. What could cause a spectrum to go too high?
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### Any good books to read for complete beginners? [closed]

Currently, I am a Ph.D. student(mathematics) but always loved the field of astronomy hence I would like to read something on the "side" in this field along with my work. So is there any good ...
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### How to convert magnitude to spectral luminosity? [duplicate]

In the Gaia catalogue I have the magnitude of several stars in the G band. I also have the distances to these stars (parallax is known). Additionally, I oftentimes have the effective temperature in ...
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### Cosmology : Formula for the bias of galaxies

first I have posted originally on the physics stackexchange but I have not had answers about my question, so I try my luck on this specific forum (if forums leaders want to delete the original post on ...
179 views

### What is the time dilation less a magnetic field?

Does time dilation drop off like a gravity or does resemble a magnetic field model? Could a smaller body with a magnetic field have more time dilation then a larger body without and to what degree?
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### How to deal with data that's billions of years old?

We look at galaxies that are billions of light years distant and draw conclusions based on the red shift of the light. How is the age of the light accounted for. We see the reality of a galaxy perhaps ...
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### Book recommendation

I'm looking for a modern astrophysics text. I've had Barbara Ryden's book Foundations of Astrophysics recommended to me and it sounds okay, but the only review of it I've seen slagged off the problem ...