All Questions
2,115 questions
12
votes
3
answers
689
views
In general relativity, how do we know when the coordinates we compute are physical observables?
(One of) the whole point of general relativity, is that the coordinates we mathematically use are just "labels", that can change and live on a curved surface. But at some point we have to ...
- 3,199
4
votes
6
answers
7k
views
EM waves: How do they travel for billions of km without damping
If a star is 1 billion light years away, it means that the light we see from the star is emmitted billions of years ago.
How does this light not undergo a frequency change or get damped inspite of ...
4
votes
1
answer
2k
views
Very large absorption lines in stellar spectrum
I was puzzled by the wide absorption lines in a stellar spectrum I found. The following is what I expect absorption lines to look like - thin, crisp lines:
However, I found this stellar spectrum, ...
0
votes
0
answers
19
views
Finding the Flux Ratios of Two Stars in the V-Band [closed]
I am hoping to verify my approach / answer as I study for a final exam! I am given the V magnitudes for star A = 7.0 and star B = 8.5.
$$ \frac{F_B}{F_A} = 10^{-0.4(V_B - V_A)} $$
$$ \frac{F_B}{F_A} = ...
- 363k
3
votes
1
answer
140
views
What are these circular objects on the JWST image?
While admiring this beautiful image (Spiderweb protocluster field) https://esawebb.org/news/weic2428/ captured by the James Webb Space Telescope (JWST), I noticed several (more than five) circular ...
- 13.2k
1
vote
3
answers
92
views
Do bigger telescopes see more objects primarily because they collect more light, or because they resolve things better?
I encountered this question in an exam recently (which I have simplified to focus on the core question). After describing two modern astronomy telescopes, one big and one small, the question asks why ...
- 1,233
0
votes
1
answer
184
views
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 ...
CommunityBot
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19
votes
6
answers
50k
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What are good books for graduates/undergraduates in Astrophysics?
There are no book recommendations for Astrophysics here. I will write my own answer, but I am also interested in what are others' views on the question (I will NOT mark my own answer as the best one).
2
votes
0
answers
25
views
Auroral Kilometric Radiation (AKR) Peak Frequency Calculation
Auroral Kilometric Radiation (AKR) occurs due to emission from charged particles moving in helical paths around planetary magnetic field lines at or around the electron gyro frequency and/or its lower ...
- 3,348
0
votes
0
answers
24
views
Astronomical Horizon VS Apparent/Visible Horizon VS Geometric Horizon
The definition I got for the astronomical horizon is this:
The astronomical horizon is the imaginary horizontal plane always at a 90-degree angle from the observer's zenith (the point directly above ...
0
votes
0
answers
28
views
Relativistic time effects or gravitational lensing in solar observations made by observatories and satellites?
As mentioned, I was wondering about whether there are any phenomena affecting the measurement made in Earths time-scale after a physics seminar at IISERTVM. Any discussions and answers from specific ...
- 1
2
votes
1
answer
331
views
How to calculate luminosity in g-band from absolute AB magnitude and luminosity distance?
How can I calculate the (non-bolometric) luminosity $L$ of a galaxy (or a star for that matter) over a given band from its AB apparent magnitude $m_{AB}$ over that band and its luminosity distance $...
CommunityBot
- 1
17
votes
4
answers
6k
views
What does the sky look like from the moon?
From pictures taken on the moon, it appears that there are no stars visible in the sky, but I do not know if this is an effect due to cameras.
What is the actual appearance of the sky on the moon?
...
8
votes
4
answers
15k
views
View of the Sun from Voyager 1
Today Voyager 1 is approximately 122 AU distance from the Sun. What does the sun look like from this distance? How much brighter does the sun appear to the naked eye compared to other stars in the ...
- 851
3
votes
1
answer
351
views
Would Jupiter seen from near the planet itself, looks like it does in pictures?
Jupiter picture processed by one of citizen scientists
The picture above is artificially processed by one of the citizen scientists working for NASA. It was composed from monochromatic pictures taken ...
- 137k
7
votes
3
answers
2k
views
How do we detect a black hole?
If light cannot escape from a black hole, how do we detect its presence? I mean there is nothing that can be faster than light so if light can not escape from the black hole there should be nothing ...
- 4,537
1
vote
0
answers
44
views
CMB map construction
On wiki, it has the individual band maps of CMB, which I understand has emission from the galactic plane as shown below.
But the mostly used map is the smooth one like this. How to go from the maps ...
- 191
0
votes
3
answers
410
views
What is the theoretical limit for image sensors
Telescopes are principaly limited by a diffraction-limit.
Does this limit affect the image sensor?
The Rubin Observatory LSST Camera, the biggest camera in the world with 3.2 gigapixels is having a ...
CommunityBot
- 1
16
votes
5
answers
3k
views
How is it possible for astronomers to see something 13B light years away?
In a NPR News story from a few years back:
"A gamma-ray burst from about 13
billion light years away has become
the most distant object in the known
universe."
I'm a layman when it comes to ...
- 4,246
6
votes
3
answers
706
views
How did Jocelyn Bell aim her radio telescope (phased array)?
In 1967 Jocelyn Bell discovered pulsars using the Cambridge University radio telescope - a two hectare field of posts and wires.
The pulsar was found to be in the constellation Vulpecula. How did ...
27
votes
3
answers
3k
views
Why are the northern lights so much less visible to the naked eye than to smartphone cameras?
The northern lights have been visible where I live recently, but I've found them to be practically impossible to see with the naked eye. Phone cameras, however, show them quite brilliantly. How is ...
- 545
4
votes
2
answers
851
views
Good sources to get velocity/position vectors for all planets and moons in the Solar System for building an orbital simulator?
I’m building an $N$-body simulator, and I have everything ready to begin simulating. But my issue is is that I have no idea how to get all the starting positions and velocities for the celestial ...
CommunityBot
- 1
19
votes
1
answer
2k
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I'm trying to replicate Rømer's experiment but can't seem to get even close to the correct value for the speed of light
I am trying to replicate Rømer's experiment where he determined the speed of light by observing and measuring the eclipse timings of Io by Jupiter. I'm using Stellarium for this experiment and no ...
- 518
5
votes
3
answers
2k
views
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 ...
- 12.9k
8
votes
1
answer
8k
views
How is "little $h$" measured in cosmology? The dimensionless parameter from the Hubble constant, $H_0$
Hubble's law has been well-know for close to a century now. It is written as
$v = H_0 d$
where the Hubble constant $H_0$ is the constant of proportionality between recession speed $v$ and distance $...
1
vote
0
answers
43
views
Evidence for Dark Energy that *distinguishes* between a CDM model with a non-homogeneous universe from the standard $\Lambda$CDM Model
Has any evidence been found that distinguishes between a CDM model with a non-homogeneous universe and the standard $\Lambda$CDM Model ?
- 60.5k
25
votes
2
answers
3k
views
Is it possible to know where the Sun is just by looking at the Moon?
A nine year old child asked me if it is possible to know where the Sun is located just by looking at the Moon from the Earth.
I said I don't know, that I only know that the lunar surface reflects the ...
- 95
1
vote
1
answer
193
views
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 ...
CommunityBot
- 1
5
votes
2
answers
3k
views
Is there a simple yet accurate formula for where on Earth the Sun and Moon are directly overhead?
I'm trying to improve a site that shows the region of the Earth currently under daylight, and I
need a formula that, given the current time, tells where
(latitude/longitude) the sun and moon are ...
2
votes
2
answers
126
views
Does the length of days change substantially during the year, from more than 24 hours to less?
I just read an article in the 'Science' journal online about Monarch butterfly migration. (Mysterious monarch migrations may be triggered by the angle of the Sun , by Elizabeth Pennisi , Dec. 18, ...
- 12.9k
3
votes
2
answers
810
views
Wondering about ancient methods of estimating the relative planetary distances
Regarding https://physics.stackexchange.com/a/156155/349202 and estimating with geometry the relative distance of Venus to the sun prior to the Venus transit of 1769, I follow the logic of @viktor-...
- 41k
7
votes
1
answer
2k
views
Ephemeris calculations: Light time correction of the moon
I am currently trying to calculate apparent positions from raw JPL data.
I've got it pretty much figured out, but there is one thing that's bugging me: Has the light time correction of the moon to be ...
CommunityBot
- 1
2
votes
3
answers
115
views
What is the reason that the unstable Lagrange point $L_2$ is used for the JWST instead of the stable $L_4$ or $L_5$ points?
I understand how the Lagrange points work in the two-body problem. I can do that math. I understand that for L1, L2, or L3, if the object is perturbed to deviate away from that point, the ...
- 3,484
0
votes
1
answer
580
views
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 ...
CommunityBot
- 1
1
vote
1
answer
67
views
Problem understanding the bending of space-time curve as gravitational force! [duplicate]
I got the point of considering gravitational force as curvature on space-time fabric for bigger objects like stars, planets, blackholes. But my doubt is over the objects like us, what keeps us on this ...
- 3,484
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 ...
- 137k
2
votes
1
answer
339
views
Issue with the roots of the Equation of Time
Regarding the famous derivation of the Equation of Time available here that i'm trying to rewrite a modern version with much more detailed steps:
http://info.ifpan.edu.pl/firststep/aw-works/fsII/mul/...
- 9,970
9
votes
2
answers
2k
views
Has any astronomer ever observed that after a specific star going supernova it became a Black Hole?
Do we have any observational direct verification and historic record of a star after going supernova was turned into a Black hole?
- 137k
1
vote
1
answer
904
views
How is red colour from from rainbow hotter than blue (W. Hershel) if it has lower frequency than the latter?
We know that blue light has a smaller wavelength and higher frequency than red light, which is a consequence of higher energy in the former, then how is it true that, when scattering light from the ...
- 1
14
votes
3
answers
7k
views
What are the demographics of stars visible to the naked eye?
Of the stars that can be seen with the naked eye, what are the distributions of each type? For example, how many are main sequence? how many are Super Red giants/white dwarfs/neutron stars...etc. ...
- 137k
12
votes
2
answers
4k
views
Of the 9096 visible stars -- 90% are how close?
I was on a beach on a tropical island one night and, of course, the night sky was magnificent. It got me thinking:
I want to point to a star and say with $90$% certainty that it is probably $x$ ...
- 137k
3
votes
2
answers
3k
views
Map of the gravitational strength of the solar system
I am looking for a map or plot of the gravitational strength in the solar system. In an ideal world there should be something like google earth to move around in the solar system, zoom in and out and ...
CommunityBot
- 1
4
votes
1
answer
100
views
Observational evidence for superfluidity in neutron stars
What is the earliest evidence for the existence of superfluidity in neutron stars?
I'm about to present the subject in a seminar and I'm under the impression that at the moment there is no ...
0
votes
1
answer
36
views
AStronomic nova or supernova (Ia), what peak intensity and wavelength to expect in the UV, extinction coefficient of ozone layer?
see https://www.nasa.gov/centers-and-facilities/marshall/nasa-global-astronomers-await-rare-nova-explosion/. T Coronae Borealis is expected to go nova. Distance 2630 light-years. Its magnitude is ...
- 137k
0
votes
0
answers
33
views
Kepler's measurements in solar system
I am interested in knowing that does Kepler measured distances in solar system or only relative distances to a astronomical unit which he doesn't know how long is it in our meters.
- 213k
2
votes
2
answers
185
views
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 ...
- 597
1
vote
1
answer
2k
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What is the diameter size of the umbra shadow cone of the Earth when the Moon passes through it on a lunar eclipse?
I am sure this varies given the distance from moon to earth varies, but a range would be sufficient. I am trying to explain to a flat earther how there is not a lunar eclipse every full moon.
My ...
CommunityBot
- 1
0
votes
3
answers
4k
views
How should I interpret a Chi-Squared Result?
I've got a Model A with a reduced chi-square of 1.28. I've got a Model B with a reduced chi-square of 0.70. Which is a better model? The model closest to 1 or the model closest to zero?
(Yes, I ...
- 13.2k
1
vote
0
answers
37
views
Do magnetar stars relatively produce stronger gravitational waves than neutron stars? [closed]
According to Reissner-Nordstrom solution to Einstein's gravitational field equations in astronomy the electric or magnetic or both, field of a charged body of mass generates its own gravitational ...
- 4,376
0
votes
0
answers
33
views
Converting Multiple FITS files into one dataset?
I am doing a project involving data from LASCO C2 on the SOHO spacecraft. I tried to download the data from the SOHO science archive and it comes out as multiple FITS files. Is there a way to convert ...
- 213k