The science dealing with objects and phenomena located beyond Earth. In particular, this applies to observations and data. At its core, astronomy is the physically informed cataloging and classifying of the contents of the universe in order to better understand what is out there.

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Why is the sky of the moon always dark?

Why is the sky of the moon always dark compared to the sky of the earth, doesn't it have day and night like earth?
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33 views

Which technique is used to measure huge astronomical distances which are in terms of billions of light years? [duplicate]

I know what a "light year" means. However, I am very curious to know about the technique through which scientists are able to calculate the distance of various astronomical bodies from earth which are ...
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1answer
42 views

Redshift of supernova light curve

I am trying to understand how the width of a supernova light curve depends on the redshift of its component frequencies. Let us make the simple assumption that the light curve is Gaussian. The ...
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1answer
47 views

Where else is there fire?

Apart from on Earth, where else does fire occur in the universe? I'd hazard a guess that it is quite rare for oxidation to occur naturally, could someone elaborate on this?
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0answers
74 views

Using nuclear bombs to detect near earth orbit objects

This question is based on an article written some years ago by A. C. Clarke, in which he attempted to solve two problems with one solution. His idea was to remove the Earth's stockpile of nuclear ...
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38 views

How would I find equatorial velocity given an R of 10 KM and a P of 0.0002 sec? [closed]

Any help would be appreciated. I can't find anything relating to this problem online.
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8 views

Structure formation: growth of density perturbations in sub-horizon vs super-horizon scales

I've always had difficulty visualizing how sub-horizon scales work versus super-horizon scales. Inflation causes perturbations due to quantum fluctuations. These are under-densities and ...
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1answer
36 views

Calculating the area of region of the sky

I am given the right ascensions ($\alpha_1$ and $\alpha_2$) and declinations ($\delta_1$ and $\delta_2$) of a specific region of the sky. How can we find the area of this region? I know that there ...
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1answer
32 views

Photometric surveys vs. Spectroscopic surveys

I consistently read about certain astronomical surveys which are either described as "photometric surveys" or "redshift surveys". I'm still unclear as to how these two methods differ. Photometry ...
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43 views

What prevents Digital interferometry in an optical telescope array?

I understand it is common to combine an array of radio telescopes in to a single instrument using interferometry. This has the photon collecting area of the combined radio telescopes but an aperture ...
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1answer
266 views

How to tell if a star is in a galaxy?

An astronomer is studying a star that appears to be in a galaxy. How does the astronomer know the star is actually in the galaxy and not just on the same line of sight as the galaxy? I'm guessing ...
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3answers
63 views

What is meant by a “sodium line”?

What speed should a galaxy move with respect to us so that the sodium line at $589.0\ \mathrm{nm}$ is observed at $589.6\ \mathrm{nm}$? In the above question what's meant by a sodium line? The ...
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14 views

For a Plummer model mass distribution, what is the timescale of dissolution?

Given an initial system of masses distributed in a Plummer model close encounters cause stars to gain enough energy to leave the system. What is the timescale over which the whole cluster with N ...
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15 views

from right ascension and declination to angle from semi-major axis

I am working on a research project and having trouble converting from ascension and declination to angles with respect to the semi-major axis. The target coordinate system has its origin at the ...
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1answer
70 views

Experimental Data for Mass Distribution of a Galaxy

My goal here is not to discuss dark matter in general. I know there are many other observational clues that hint us towards Dark matter. My goal is simply to understand this argument here a little ...
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0answers
30 views

CMB parameter, what's the meaning of the matter power spectrum normalization sigma_8?

Most CMB experiments like WMAP and Planck include a certain cosmological parameter called $\sigma_8$. My understanding is that normalization of the matter power spectrum is not a theoretical ...
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1answer
57 views

What is the direction of our solar system in the Milky way galaxy and in the universe?

What is the direction of our solar system in the Milky way galaxy and in the whole universe? Are we heading toward another constellation, not including Andromeda?
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1answer
124 views

Difference between Gunn Peterson trough and the Lyman Alpha Forest? Cosmological implications?

I'm having difficulty understanding the full implications of the Lyman alpha forest and its use in cosmology. My understanding is this: we detect features in the Intergalactic Medium (IGM) by very ...
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3answers
134 views

Are some sources of electromagnetic radiation theoretically (or perhaps technically!) more challenging to detect than others?

In a previous question, I learned that in order to detect an object in space, what matters is how much electromagnetic radiation it is giving off, and what sources of EM radiation the sensor can pick ...
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1answer
66 views

What are the coordinates of the center of rotation of our galaxy, relative to ourselves?

So standing outside in the garden, can we physically point to the center of rotation of the galaxy? I understand that from our viewpoint on Earth, it would be a moving point, so allowing for Earth ...
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1answer
54 views

Does all the theoretical work of astrophysicists have to be confirmed by the observations of astronomers?

I am a chemist an I have some doubts about the work of astrophysicists. I know that astrophysicists do a lot of theoretical calculations based in other theoretical work and also based in real ...
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2answers
137 views

What made Kepler think that orbits are not circular which came to be elliptical?

Kepler formulated his laws in a sort of time where human began to believe in heliocentric universe and telescope was not yet invented/ discovered. So what made Kepler think that orbits aren't ...
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1answer
83 views

What is the distance to the closest star similar to our Sun? [closed]

My question was inspired by this nice question. But, I recall the phenomenon of strange objects appearing on our sky, and then moving away with a velocity that seems not yet attainable by manmade ...
4
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1answer
58 views

Desynchronised pixels in images of the Sun?

NASA published this wonderfull video of Sun from SDO: https://www.youtube.com/watch?v=GSVv40M2aks And it occurred interesting question: Sun is very big sphere. Its so big that it took light 4,6 ...
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3answers
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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 ...
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1answer
61 views

The ever increasing pull of a black hole

If something is caught in the pull of a black hole and keeps accelerating it can't keep accelerating with no limits or else it would accelerate beyond c. So is there a limit on how fast acceleration ...
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2answers
91 views

Harvesting hydrogen from a star

I've been thinking about what highly technologically advanced civilisations would do once their energy requirements become comparable to the total output of a star, and how such activity could be ...
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48 views

2D Sérsic equation [closed]

The 1D Sérsic equation for disk galaxies is $I(r) = I_0 \exp[{-(r/h)^{1/n}}]$ I'm testing some profile extraction and fitting routines on 2D models and I just can't get my head around transforming it ...
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20 views

In astronomy, what is a 'reflex orbit'?

In astronomy, what is a reflex orbit? The term is used in one of my books, but u don't find a definition for it. Googling it gives me articles about 'Oculocardiac reflex' which is not what I want. ...
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26 views

Luminosity, brightness and magnitdes in cosmology

I just started learning Cosmology and am confused with these terms. What I know so far: Luminosity $(L)$ is the energy output (inc. visible light, radiation etc.) per unit time of a star. It is an ...
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22 views

Can a gas be modelled as a low density blackbody, if we want to consider how detectable it will be in space?

The answer to this question taught me about the sort of parameters I need to consider if I want to consider how "detectable" an object in space is. I want to consider the detectability of a ...
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2answers
81 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 $x$ ...
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27 views

Will any massless particle/object in the vacuum of space always travel at the speed of light? [duplicate]

The question as stated above, is "Will any massless particle/object in the vacuum of space always travel at the speed of light?"
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1answer
68 views

Did light already experience death of the universe immediately after the Big Bang?

Let's suppose that I am on Point A with a light-beam launcher. My goal is to launch a beam of light to Point B, which is about 8,000 quadrillion light years away. My friend named Jack is at Point B ...
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3answers
103 views

How big would a solar sail need to be to be detected from the orbit of Pluto?

Suppose we made a solar sail out of a highly reflective material. How big would that solar sail have to be for the Hubble Space Telescope to detect it visually at the average distance of Pluto?
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33 views

Convert angular power spectrum to spatial power spectrum

If we have a signal projected on a sphere, one routinely decomposes this in spherical harmonics, in analogy to a Fourier decomposition in flat space. One can then make the decomposition: ...
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2answers
46 views

Seeing the shadow of a black hole

Why can't we see the shadow of a black hole cast on earth. Shouldn't the black hole block some light from a star or a galaxy far behind as they line up.
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How did Kepler infer three-dimensional positions from Tycho Brahe's data?

This has bugged me for some time. Tycho Brahe's data on planetary observations, presumably, consisted of the direction in which a planet was observed at a given date and time. What techniques did ...
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39 views

Getting signals from a probe light years away

Is it only because we don't have fast enough spacecrafts that we don't send probes to nearby stars? Do we have sufficient radio technology? If a probe like new horizons was as far as the nearest ...
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1answer
23 views

Calculating the size of exoplanets with moons

When astronomers detect an exoplanet using its transit and calculate its size to be, say twice the earths size, do they have any way of knowing that its actually not a slightly smaller planet with a ...
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0answers
34 views

Finding the temperature of Earth from temperature of Mars and its distance from Sun [closed]

I am to compute the temperature of planet--in fact I've already found out it's Earth--knowing only: the surface temperature of Mars (210 K) and its distance from Sun (1.524 AU) and of course ...
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2answers
120 views

In a random direction, am I more likely to find a dwarf or giant galaxy?

First a couple of disclaimers: My title explains the idea of my question, but I will pose it slightly differently to make it less subjective. This ends up being in the style of a homework exercise ...
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1answer
39 views

What is a geocentric altitude?

In NAO TN no.69, Yallop defines ARCV as geocentric difference in altitude between the centre of the Sun and the centre of the Moon for a given latitude and longitude, ignoring the effects of ...
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27 views

Origin of the shape of the planets [duplicate]

Looking at our 8 planets over the years has left me a huge unanswered question. I know these planets compared to space is like a grain of sand on our entire planet. All the planets have the basic ...
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2answers
135 views

What parameters control the amount of thermal energy an object must possess for it to be detectable in space?

What parameters control the amount of thermal energy a space object must possess for it to be detectable by a sensor also in space (i.e. one that does not have to deal with interference from a ...
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1answer
314 views

How are problems in astronomical spectroscopy solved?

Astronomers based on the ground telescopes, watch the stars and make predictions about them based on the spectrum of light. But when the light from those stars reach the telescopes through the ...
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1answer
89 views

Is it normal to see the moon and sun at 3:30 pm in the sky at the same time?

The question has been asked and answered about it being seen in the early morning, but it does not explain anything about 3:30 in the afternoon? could someone give me insight on this please!
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1answer
48 views

How have the duration of the martian day changed in the past?

There is a panspermia theory which claims that life might have begun on Mars and I currently read a post that the human circadian rhythm is closer to the martian day length (about 25h) than the day ...
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31 views

Why is the Great Dark Spot so shortliving unlike the Great Red Spot?

The Great Dark Spot is an anti-cyclone in Neptune. But unlike the Great Red Spot of Jupiter which lasts for more than hundred years, the Great Dark Spot exists for only one year or so. Why is it so??
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1answer
51 views

How do I calculate the right ascension of the points along the ecliptic at given elevations?

How do I calculate the right ascension of the point along the ecliptic nearest the sun at sunset and sunrise that is at a given altitude/elevation? For example, I'd like to compute the right ...