All Questions
2,115 questions
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Measured Value of Hubble's Constant [duplicate]
As we know, Hubble's Constant is not exactly a constant, but its value varies with time. However, we also speak of measuring its present-day value. The measurements by cosmic distance ladder would ...
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2
votes
2
answers
185
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Why can we see all the way back to 600 million years after the Big Bang?
I apologise that what I'm about to ask has probably in some format been discussed before on the site. Like many recent questions here, this is motivated by the James Webb telescope: a relative of mine ...
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The expectation value of intersecting particles
If I have a light beam (a straight line)
it goes through a box of dust, which has travelling length $l$, dust molecule cross section $\sigma$, dust number density $n$.
how to calculate the expectation ...
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43
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Is our solar system really that odd?
I have been learning about the solar system from popular science shows. In these shows they suggest that, after having seeing around 2500 other solar systems, astronomers have concluded that our ...
- 1,430
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Difference between Earth's surface area seen from ISS and the theoretical area if light refraction is neglected
What is the difference between Earth's surface area seen from ISS and the theoretical area if light refraction is neglected? I watched a video telling that it is not possible to know why a great ...
- 2,275
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1
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Einstein's derivation of stellar aberration formula: replacing the direction of the ray with a difference in angles justified?
I was going through the translation of Einstein's 1905 paper "On the Electrodynamics of Moving Bodies"(See https://einsteinpapers.press.princeton.edu/vol2-trans/175). In deriving the stellar ...
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7
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What is a simple argument to prove that the stars in the sky are further away from the Earth than the Moon?
How do we know, without using modern equipment, that the stars are further away than the moon in the night sky? Further, is there a simple and actionable argument to prove that this is indeed the case?...
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Geocentrism and annual tide variation
I have read that in the before common era, Seleucus of Seleucia have studied tides and probably used annual variation as an evidence of heliocentric. However, tides are known to be affected by ...
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How is the trajectory of a star found relative to the Sun?
So i know we can get radial velocity by measuring blue shift and then we can use the distance to the star and its proper motion to get its tangential velocity. In the case of Bernards star, its ...
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Advantage of a single large telescope over many small ones for optical/infrared spectroscopy?
In a thought experiment where we would like to do optical/infrared spectroscopy on distant galaxies, and in which we would like to increase as much as possible the resolution of the spectroscopy (...
- 1,227
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76
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What math predicted the eclipse of Betelgeuse?
It's amazing that scientists can predict something as small as last weeks eclipse of Betelgeuse. The general relativity aspects are quite specialized. Can someone point me to the math involved in ...
- 4,857
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Why are stars white?
That is may be an easy question, but I am not a professional. The Sun is a star, and when I look at the Sun it is usually yellow. Why are stars in the sky at night white?
I suppose it could be due to ...
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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 ...
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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 ...
78
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How can we see planets thousands of light years away but don't know if there are more planets in the solar system?
That is basically my question, it arose when I saw an article (here is the scientific paper, which should be free to read) saying two Caltech scientists might have found the 9th planet of the solar ...
- 1,768
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3
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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 ...
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Does the Earth's magnetic field lag behind the Earth as Earth orbits the sun?
I imagine that when the earth orbits the sun, the earth's magnetic field is also subject to the sun's gravity, since photons and light are subject to gravity. As a result, the magnetic field does not ...
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1
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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 ...
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31
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5
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8k
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Did the new image of black hole confirm the general theory of relativity? (M87)
How can we do it just by looking at the image. But I heard in news saying "Einstein was right! black hole image confirms GTR. The image is so less detailed that I can't even make some pretty good ...
user225650
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5
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Can anything be seen from the center of the Boötes void? How dark would it be?
Let's say I was at the very center of the enormous Boötes void, way out in deep, deep space. What could I see with the naked eye? I assume I could see no individual stars, but could I resolve any ...
- 3,576
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2
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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 ...
- 1,880
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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?
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Proof/explanation that Sun is bigger and more distant from Earth than Moon is, Middle Age technology allowed only
If I was living in the Middle Ages, how can someone prove or at least explain to me in a simple way that the Sun is bigger and more far from Earth than the Moon?
Can a parallax be used for it?
$$\text{...
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130
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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 ...
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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 ...
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7
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Finding how much time it takes for a complete Earth revolution around the Sun
Long story short, my brother made a joke about how stupid it is to celebrate the Earth making one "trip" around the Sun: New Year's Eve.
So I got curious and was wondering: how could the first ...
- 320
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2
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Observing an event horizon while approaching one
A thought crossed my mind that I realized was hard to conceptualize so I decided to simplify the question by putting it in terms of event horizons.
If I am an observer approaching black hole "A&...
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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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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 ...
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225
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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)?
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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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2
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152
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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 ...
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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 ...
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What is the formula to calculate the parallax angle?
The images were rendered in Blender. The two images were rendered 20 meters apart, side by side. The focal length is 50 mm if that matters. What is the formula to calculate the parallax angle θ as ...
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1
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Why does the light from stars / satellites tremble? [duplicate]
Clear Sky.
1 ---> Light from stars visible.
2 ---> Light from satellites visible.
3 ---> Light from airplanes (passing by) visible.
Apart from the change in color, the light from 1 and ...
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Tilting of Earth rotational axis
In which way do the Earth's rotational axis tilted 23.5°.
see in the image we have been told that the axis of rotation of Earth is tilted 23.5° from the perpendicular to path.
But can anyone say in ...
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1
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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, ...
- 3,001
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3
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How big would my telescope have to be if I wanted to see the Mars rover from my backyard?
I imagine that with a big enough telescope, I would be able to zoom in and see the Mars rover in enough detail to make out the details (like the wheels, cameras, etc.). How large would the telescope ...
- 1,315
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2
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1k
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The Resolving Power of a spectrometer
I can't understand one thing in the definition of the resolving power of a spectrometer:
Let the resolving power be defined as: $R=\frac{\lambda}{\Delta \lambda}$ where $\Delta \lambda$ is the ...
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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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1
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How to calculate change in absolute magnitude due to a change in stellar radius?
Suppose that the radius of a star increases by some factor, how does this affect the absolute magntiude of the star?
I know that $M_1 - M_2 = \Delta M = 2.5 \log \frac{L_1}{L_2}$, so if I knew the ...
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What is a simple way to estimate $σ_{RV}$ assuming photon noise in radial velocity measurements?
Hello there i have been trying to figure out what is wanted in the question "Simple way to estimate σ_RV assuming photon noise" for a presentation. But i can not seems to find much, but i ...
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3
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If you decreased the mass of a planet, but kept the mass of the atmosphere the same, would the air density decrease?
So, I know that atmospheric pressure is a result of both the atmospheric mass and the force of gravity acting on it. If you were to decrease a planet's gravity, but keep the atmospheric mass the same, ...
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Why can we see the moon when it is between the Earth and the Sun? [duplicate]
A rather stupid question, why can we see the moon when it is between the Earth and the Sun?
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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, ...
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104
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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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Why was M87 targeted for the Event Horizon Telescope instead of Sagittarius A*?
The first image of a black hole has been released today, April 10th, 2019. The team targeted the black hole at the center of the M87 galaxy.
Why didn't the team target Sagittarius A* at the center of ...
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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 ...
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3
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2k
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Schwarzschild Radius of the Universe
According to the Wiki on the $R_s$, the $R_s$ of the observable universe is 13.7BLY.
https://en.wikipedia.org/wiki/Schwarzschild_radius
(The observable universe's mass has a Schwarzschild radius of ...
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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?
