All Questions
2,115 questions
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Is this how we get the stellar parallax angle? [duplicate]
I know this question has been asked before, but for me, something is missing in the answers. I think I might have it figured out though. So the parallax is usually explained with an illustration ...
CommunityBot
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-1
votes
1
answer
58
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Does average length of sidereal day change if earths axis is tilted in different way
Can precession of axis of rotation of earth cause change in average sidereal day length?
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2
answers
110
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Why is the time from vernal and autumnal equinox different than from the autumnal to the vernal one?
In my lecture the professor stated that the time from vernal to autumnal equinox is about 186 days , while that from autumnal to vernal equinox is about 179 days.
I was wondering why are these time ...
- 41k
0
votes
0
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43
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How do I approximately calculate the sidereal time at an specific location in a specific time and month (no year)?
How do I approximately calculate the sidereal time at an specific location in a specific time and month ? Most formulas required to input a year too, and then they want you to deal with Julian day, ...
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0
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1
answer
92
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How can the distance to the Sun from the Earth be determined without relying on any prior baseline measurements?
I am asking if there is a way that a man could determine the distance from the earth to the sun without already having a baseline distance or dimension measurement to begin with, and without having ...
- 213k
2
votes
0
answers
57
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Would ocean tides on the moon of a gas giant really be kilometers high?
I was reading an article on exomoon habitability constrained by illumination and tidal heating. The article imagines an Earth-like exomoon around Jupiter-like host planet. That got me thinking about ...
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12
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2
answers
5k
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From how far away could Earth's telescopes detect Earth-like radio signals?
The Earth has been broadcasting human generated radio signals for about 100 years now. If a nearby civilization were broadcasting similar radio signals, could we detect them with our own radio ...
- 213k
4
votes
2
answers
341
views
How to measure the ratio of a planet's radius to a star?
I was reading a physics problem related to astronomy, and upon re-reading it, I realized that it could be really indicated to extrapolate some really interesting physics-related information.
One of ...
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1
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1
answer
77
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Have we independently verified Leavitt’s luminosity-period relationship?
After Henrietta Leavitt discovered the relationship between luminosity period amongst cephids in the Magellanic Clouds, how was she so sure that you can extend this relationship to other Cepheids ...
- 137k
7
votes
1
answer
1k
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What is the smallest known gravitationally bound system?
Recent discovery of a satellite orbiting Dinkinesh made me wonder how fragile that system must be. Based on the numbers and photos given by NASA, I estimate that binding energy of the two bodies is of ...
- 213k
7
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2
answers
186
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What conditions should an asteroid or comet impact need to meet in order to start an impact winter?
Is there a way to calculate if an impact from a given asteroid (say 5 miles in diameter, solid rock to which we know the mass) would start an impact winter? If yes, I would like to see a real example ...
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1
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1
answer
73
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Andromeda Galaxy and Hubble
When Hubble determined the distance to Andromeda he based his estimation on Cepheids. However, the result was less than half the current value. What was the cause of this error and could another ...
- 60.5k
1
vote
0
answers
40
views
Orbital obliquity of the Earth
Consider that we have all the orbital parameters that characterize the Earth. How would one calculate the orbital obliquity of Earth?
One could argue that since the rotation of Earth doesn't change ...
- 213k
40
votes
6
answers
14k
views
What is the evidence for a supermassive black hole at the center of Milky Way?
Black holes cannot be seen because they do not emit visible light or any electromagnetic radiation. Then how do astronomers infer their existence? I think it's now almost established in the scientific ...
- 24.6k
-4
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1
answer
84
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Are there laws of creation for biological life? [closed]
I am no scientist, but I do love science as well as the unknown. Are there laws of creation for biological life? Would it be safe to assume that our bodies are made up from things that came directly ...
- 213k
2
votes
2
answers
86
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Length measure in Astronomy
In cosmology, we have a couple of different length measures. Among them, comoving and proper distnlances seem to be good fit to real world use cases in astronomy.
So my quesion is which length measure ...
16
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8
answers
3k
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Why is there one fewer solar day per year than there are sidereal days?
solar day = time between solar noons
sidereal day = period of Earth's spin
Wikipedia says "relative to the stars, the Sun appears to move around Earth once per year. Therefore, there is one fewer ...
4
votes
2
answers
331
views
How do we know when the earth completes an orbit?
Two bodies in space always orbit their center of mass. So the relative motion of the Sun and the Earth happen in the same line, save for the rotation of the Sun. So, how do we measure
The time taken ...
- 13k
2
votes
1
answer
52
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Is this already an established functional relationship or have I created hodgepodge?
Last winter I started toying with the galaxy gravitational rotation curve graphs. I started modifying the exponent of $r$ that in effect change the $1/r^2$ law and therefore correct the mismatch, ...
- 1,854
1
vote
1
answer
224
views
Does NFW profile work for any galaxy?
We use Navarro–Frenk–White (NFW) to calculate Dark Matter (DM) density. Can we use it for DM halo in any galaxy or is it used only for Milky Way (MW)?
- 6,987
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1
answer
37
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Bolzmann Distribution question
I am busy working through an Astronomy textbook. I am presently working in the formation of spectral lines section.
I'm struggling with the statistical weights of the Boltzmann Distribution ...
- 11.2k
5
votes
2
answers
1k
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What is the Metric of the Gravitational Field of the Sun?
What metric determines the "geometry" of the gravitational field generated by the mass of the sun? Is there a general metric that incorporates arbitrary mass and devolves into the ...
- 137k
0
votes
4
answers
394
views
Would Earth always face Sun the same way if Earth didn't rotate around it's axis?
Imagine for a second that Earth no longer rotates around its axis. It still revolves around the Sun.
However, would the Earth:
face the same way towards the Sun as it goes around it (permanent time ...
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2
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1
answer
252
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Basic: calculating a satellites height from its angular speed
A few days ago, while loooking at the night sky, I saw a satellite and I wondered if it would be possible to tell how far it is just by looking at it. The only possible naked-eye measurement that I ...
- 1,779
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1
answer
130
views
How can we conclusively say that there is no Blackhole in the Solar System?
With the Gravitational perturbations among Neptune, Uranus & pluto, Is there anyway to know conclusively that there is no rogue Blackhole roaming the solar system? The Event Horizon may be very ...
- 137k
5
votes
2
answers
4k
views
Why do some stars have a negative parallax?
I am constructing a Hertzsprung-Russell diagram for stars within some radius around Pleiades and have repeatedly come across stars that have negative parallaxes. For example, http://vizier.u-strasbg....
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2
answers
377
views
Star gazing from the bottom of a well
I have read that it is a myth that you can see stars in daylight if you stood at the bottom of a well, however, if you stood at the bottom of a well at night, or built a long non-reflective tube and ...
CommunityBot
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2
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Why can't I see whole galaxies with the naked eye?
I have read these questions:
At what distance could you see andromeda with the naked eye?
Do all the individual stars that we can see in the night sky belong to Milky Way?
I look at the night sky ...
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9
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1
answer
2k
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When was today's radar measurement of the Earth-Sun distance made and by who?
I have seen other similar questions and someone mentioned radar measurements made in the 1960's. Who did that and when and where? I am not interested in ancient measurements. Only the currently ...
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22
votes
3
answers
1k
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Detectability of interstellar messages
Recently a debate started whether it is a good idea to send more messages into space in the hope of having alien civilizations receive them. There are some predecessors, most notably the 1974 Arecibo ...
- 3,719
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1
answer
78
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Number of stars in Ambartsumian's Knot NGC 3561 Galaxy
Is there any information on what type of stars and how many are in Ambartsumian's Knot Galaxy? Is there an information source to refer to?
TIA
0
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1
answer
166
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What if dark matter/energy did not exist?
What if dark matter and dark energy did not exist and were only due to a misinterpretation of the red shift of light or a measurement bias?
What would be the implications/consequences?
- 137k
2
votes
1
answer
4k
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Calculating the eccentricity of an exoplanet
I'm wondering how to calculate the eccentricity of an exoplanet by its radial velocity vs. phase graph. To clarify my question I will take an exoplanet called WASP-14b 2 as an example (http://...
- 137k
-2
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1
answer
89
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Why do sun emit light of different frequencies? [duplicate]
The sun emits white light, which is a mixture of light from all frequencies. Instead, it should have emitted a beam of light of the same frequency, as the source is the same?
So, can you please ...
- 97.8k
1
vote
1
answer
39
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Seyfert Galaxies: How does this statistical deduction about the age of their nucleus make sense?
As per this book, An Introduction to Active Galactic Nuclei by Bradley Peterson:
The nuclear emission must last more than $10^8$ years, because Seyfert galaxies constitute about 1 in 100 spiral ...
- 137k
2
votes
1
answer
412
views
Where can I find values for apparent brightness of stars?
I'm in high school and doing a project. I want to calculate the distance to stars using their luminosity and apparent brightness, from the equation $b=\frac{L}{4 \pi d^2}$. I have found values for ...
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3
votes
1
answer
117
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What happens to objects along spiral galaxy arms over long periods of time?
Observations of spiral galaxies reveal that objects within the same arm of a spiral galaxy move at around the same speeds, regardless of their distance from the center of the galaxy. Conversely, the ...
- 137k
5
votes
0
answers
76
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Why are there fewer brown dwarfs than stars?
Lighter stars are more common than heavier stars, you'd think that this would make brown dwarfs more common than red dwarfs and yet they are less common by factors of a few.
(e.g. see Table 5 of ...
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7
votes
3
answers
4k
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Explain relationship between angular diameter distance and luminosity distance, Etherington Theorem
I have a question relating to the Etherington Theorem.
The luminosity distance is defined by the equation for flux, i.e.
$F=\frac{L}{4\pi D_L^2}$
where flux is in units energy per unit time (...
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1
vote
2
answers
610
views
Factors in predictions for Tolman's Surface Brightness Test?
http://en.wikipedia.org/wiki/Tolman_surface_brightness_test
It says:
In a simple (static and flat) universe, the light received from an object drops inversely with the square of its distance, but ...
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2
votes
2
answers
269
views
Likelihood of MACHOs being the best candidate for dark matter
Massive compact halo objects ("MACHOs") include a wide variety of hardly detectable bodies such as brown / white / black dwarfs and black holes, to name a few. If we take into account the ...
- 137k
1
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1
answer
71
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In the future, can advanced telescopes be used to observe different laws of physics at the time of the big bang?
I've heard that because light takes time to travel from one place to another, we see objects in distant galaxies as they were when they released the light. new and advanced telescopes are able to see ...
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3
votes
1
answer
90
views
Modelling a fully relativistic White Dwarf Star
I am trying to model a fully relativistic white dwarf using different central densities. Using the below equations:
$\frac{dP}{dm} = -\frac{Gm}{4\pi r^4}$
$\frac{dr}{dm} = -\frac{1}{4\pi\rho r^2}$
and ...
- 137k
2
votes
1
answer
62
views
How close would a evaporating primodial black hole be to be detected? [closed]
The power output of a black holes hawking radiation is inversely proportional to the square of it's mass. According to here, in it's final second of existence, it'll emit over 2E22 joules of energy, ...
- 16.6k
1
vote
0
answers
104
views
Why does the Moon - which is grey - appear white to us even at daytime?
The color of the Moon as viewed more closely, is grey. But when we see it in the sky it appears white to us. Why?
Although this question has some answers here: Why is the surface of the Moon white? (...
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vote
3
answers
113
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Why do we See Black Holes?
The image of things falling into a Black Hole stays frozen on the event horizon forever. So how is it that we don't see the image of a dying star where a Black Hole formed?
We see the black, with an ...
- 213k
1
vote
2
answers
152
views
How can photons be emitted from a star, travel millions/billions of years in a "straight line", and be seen by two adjacent observers?
If photons are emitted by a star and travel millions/billions of years in a "straight" line, they can be seen by adjacent observers. This makes it seem like there must be an uncountable ...
- 213k
0
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1
answer
78
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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 ...
- 213k
0
votes
2
answers
747
views
Planet in which satellite (moon) and star (sun) appear together once a year
With a hypothetical system, where the moon would be always on the opposite side of the planet than the sun, in a way that the moon would only be visible at night on the planet.
I don't know if this ...
- 213k
4
votes
2
answers
391
views
Underestimation of peak wavelength by Wien's law for stellar spectra
I plotted some simulated stellar emission spectrums from PHOENIX which also each came with their respective effective temperatures. With the effective temperatures, I used Wien's law to estimate the ...
- 213k