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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If there were infinite many stars [duplicate]

If there were infinite many stars, is the sky then always full of light so is there than even night?
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22 views

Why dark matter do not gather together? [duplicate]

Stars and planets were gathered together by gravity, thus emit light. Why dark matter doesn't behave that way? Why they won't be gathered together by gravity and form stars?
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31 views

Is there any evidence of black holes existing? [duplicate]

Yes, I am aware of observations of "massive objects", for example near the center of the galaxy. But is there any evidence that the object in question is a black hole, with all its commonly attributed ...
20
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6answers
386 views

Given a photo of the Moon, taken from Earth, is it possible to calculate the position of the photographer's site?

Given a photo of the Moon, taken from Earth, is it possible to calculate the position (Earth longitude and latitude) of the photographer's site? I am thinking about photos taken with a normal camera ...
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1answer
72 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 ...
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0answers
48 views

Gravity effect of a star behind the sun during a solar eclipse [closed]

We know that stars blocked by the sun during a solar eclipse may still be observable as if they are not behind the sun but rather visible to the side unblocked by the sun (because the photons are ...
5
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1answer
560 views

Why are solar eclipses more common in the southern hemisphere?

I've seen the claim that solar eclipses are more common in the southern hemisphere than the northern hemisphere and would like to understand why and if that is the case? Does it relate more to the ...
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0answers
79 views

Matter and dark energy

So dark energy is the biggest thing in the universe, it keeps accelerating the expansion. But it's a constant so at some point there was a matter - dark energy equivalence. My question is how can you ...
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1answer
53 views

How long was the universe radiation dominated?

Using the Friedmann metric I've been trying to calculate how long the universe was radiation energy dominated. I've reduced the metric to: $c.dt$ = $a(t).dr$ where a is the scale factor. I can work ...
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1answer
56 views

Why aren't pictures of Pluto darker than those of other planets?

Just as the intensity of the light from a candle decreases with distance, I would expect the light from the sun to illuminate the distant planets less than the closer ones. However, the pictures of ...
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1answer
71 views

Here's a fun question, what would happen if Venus was thrown off in a highly elliptical orbit? [closed]

I'd like to know what would happen if Venus was flung into a highly eccentric orbit like Sedna (except maybe with its current perihelion) with an orbital period measured in thousands of years by a ...
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2answers
911 views

Is there a simple, accurate formula for calculating transit times from rise and set times?

I have access to "rise" and "set" times for astronomical objects, and want to determine the corresponding times of culmination. Is there a reliable and accurate way to do this? It isn't clear to me ...
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0answers
23 views

Do the arms of spiral galaxies differ based on the speed of which the galaxy is rotating?

If I were to take a circle with strings attached to it and spin it, the strings would behave differently based on how fast the circle is rotating. Do galaxies behave the same way?
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2answers
50 views

Sound speed vs Speed of sound

Are 'sound speed' and 'speed of sound' the same thing? If not, what is the difference? If they are, could you clarify how the speed of sound applies in the below description of gaseous clouds? ...
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2answers
116 views

Why doesn't the Cosmic Microwave Background heat my food like a Microwave?

I know the reason we have a CMB is because the photons don't react with any of the matter in the universe, or the mean distance between interactions for photons generated since decoupling is now the ...
5
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1answer
251 views

Are ultracold atoms only created by intelligent life?

Nature has particle accelerators that are far beyond our capacity, but occasionally I hear atomic physicists claim that they are able to make something that has never been formed in any natural ...
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0answers
29 views

Reduced Chi-Square - How to compare values?

I've got two models of velocity curves fitting a Low Surface Brightness (LSB) galaxy: one is MOND-like, the other ΛCDM. I've fit them both with Chi-Square minimization. I understand the winning ...
2
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2answers
322 views

How accurately could we theoretically see into far space?

Travels to different galaxies are strongly limited by the speed of light. Unless we find a way to travel through space with some wormholes, we will never reach planets in another galaxy. But what ...
3
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1answer
51 views

Have we observed sufficient extra-solar planetary systems to establish a planetary distribution pattern? [duplicate]

From Kepler And Extra-Solar Planetary Observations As of January 2015, Kepler and its follow-up observations had found 1,013 confirmed exoplanets in about 440 stellar systems, along with a further ...
3
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2answers
150 views

Why is “gravitational” red-shift neglected in galaxy and galaxy cluster scales?

The red-shift of the light of a star in a galaxy or that of a galaxy in a cluster of galaxies is generally interpreted as how fast the star or the galaxy is moving, i.e. it is interpreted in a purely ...
4
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1answer
44 views

How do space probes identify molecules?

How does a space probe identify molecules without actually obtaining the molecules? The common identification techniques I can think of are spectroscopy and magnetic resonance, but for both of them, ...
16
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3answers
2k views

Are there any exact data about Earth's orbit?

Wikipedia says that the Earth's orbit's axis is $a=149\ 598\ 000\ \mathrm{km}$ and its eccentricity is $e=0.016\ 7086$, but if we use these values to find distance at aphelion and perihelion we get ...
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2answers
66 views

How we are able to see stars in night, even if they are light years away from earth? [closed]

I am wondering, how we are able to see so much stars in the sky even though they are light years away from us. Even light take years to reach from those stars to earth? Then, how we are seeing those ...
4
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3answers
166 views

How did Cook and other astronomers time the 1769 Venus transit?

The 1769 transit of Venus was observed and coordinated by over one hundred astronomers around the world. How did they measure time so accurately, key to the observations having any scientific value? I ...
6
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0answers
93 views

Eclipse of the sun - Intriguing luminosity curve

I've just recorded the luminosity during the sun eclipse, here is the resulting curve : Only green curve is relevant, blue curve should be ignored (it's actually the temperature). The sky was ...
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1answer
142 views

What is the curve that describes the Daytime line in a Day and Night World Map?

A Day and Night World Map shows which parts of the Earth are in daylight and which are in night at a given instant. At one side of the Daytime line they are in daylight and at the other side they are ...
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2answers
28 views

What type of observational constraints are there for reionization?

What type of observational evidence is used to constrain reionization? At what redshift?
2
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0answers
57 views

Bulge-Disk decomposition in galaxies

I'm starting a new project on dark matter astrophysics in which I have to extract from some data (I will write some below) the surface brightness, to later construct the mass density profiles of de ...
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2answers
259 views

How long would it take to see the nearest star die?

If you were in the general proximity to the nearest star to Earth (besides the Sun) and you saw it turn to a neutron star or black hole at the very end of it's star cycle, how much longer would it ...
0
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1answer
98 views

Gas Mass Calcuation in Galaxy Cluster

Rob Jeffries found this document for me that describes how the X-Ray emissions from galaxy clusters are used to calculate the mass of the cluster. I'm unable to follow the steps to calculating the gas ...
1
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1answer
91 views

Standardising shadow length on sundials

The sundial is fundamentally flawed in that the length of each hourly shadow changes with the seasons. If the base of the sundial was engineered to move cyclically on an anual basis however, the ...
3
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1answer
92 views

Are we looking at single stars, or solar systems?

When looking at the night sky, the stars we can see just look like single dots. Are they alone in space- each one a single object? Or, when we look at a star, are we actually looking at a distant ...
2
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2answers
102 views

What is meant by the velocity of a star?

I recently read somewhere that among other things like size, radius, distance from earth, luminosity, age, etc of a star, velocity was another variable. What is exactly meant by the velocity of a ...
1
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1answer
40 views

Earth precession

I got a bit confused while studying the tropical and sidereal years. I noticed that the Earth's precession is in the opposite direction than that of a top's precession, when their rotational direction ...
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0answers
28 views

Information Brought by Electromagnetic Waves

Is it possible for a Technologically Advanced Civilization to watch in "Real Time" events of the Earth History? Let me be more specific: Let us suppose a civilization who resides on a sphere in the ...
2
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0answers
40 views

SOHO solar observatory, comet numbers and composition

SOHO discovered its 3,000th comet, cementing its standing as the greatest comet finder of all time. Prior to the 1995 launch of the observatory, commonly known as SOHO, only a dozen or so comets ...
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3answers
5k views

How does the central peak in moon craters form?

The central peak in many of the moon's large craters are visible with a telescope and they seem a little odd to me. Can someone explain how they form.
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2answers
30 views

How to measure the spectra of the stars

I recently made a mini spectrometer using a CD. I have used that to measure the spectrum of the Sun, moon and various artificial light sources. My question is how can I use it or modify it to measure ...
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0answers
25 views

Size of meteor-made crater [duplicate]

I am reading "What if" piece https://what-if.xkcd.com/20/ and I am very interested in how can one ever estimate the size of a crater? How can you ever know, it must depend on energy of a falling body ...
0
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1answer
100 views

Can the Earth be seen to transit the sun from the outer planets?

Transit of Mercury as seen from Mars The Curiosity rover on the planet Mars observed the planet Mercury transiting the Sun, marking the first time a planetary transit has been observed from a ...
6
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1answer
124 views

Are there rogue planets between Sun and Proxima Centauri?

Would we be able to detect (via emitted radiation, or its gravity) a rogue planet between us and Proxima Centauri? How big would that planet need to be?
13
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4answers
10k views

What is the farthest-away star visible to the naked eye?

My girlfriend and I were watching Cosmos, and something Carl Sagan said got us wondering what the farthest-away visible star is. Obviously "visible to the naked eye" is a fuzzy concept that might have ...
28
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3answers
6k views

Why are there no stars visible in this photograph?

Pluto and Charon, photo taken by New Horizons on July 8, 2015 from a distance of 6 gigameters. it's hard for me to believe there were no stars behind the twin dwarf planets in the field of vision. ...
4
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4answers
6k views

Why is 1 AU the distance between the Sun and the Earth?

Why 1 AU is defined as the distance between the Sun and the Earth? (approximately if you like to be precise) An astronomical unit (abbreviated as AU, au, a.u., or ua) is a unit of length ...
0
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1answer
53 views

NED vs. SIMBAD: Which is more accurate?

I'm doing some research on M33. NED lists the luminosity as 6.27. SIMBAD gives it as 5.27. First, it looks odd to me that the difference is exactly 1. Second, I've seen other differences comparing ...
1
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1answer
93 views

SDSS spectra FITS format: what do the rows mean?

This might be in the FITS Header, but I couldn't find it. I am looking at the FITS files for SDSS 1D spectro images from DR7 using the SDSS Data Archive Server at das.sdss.org (there might be an ...
4
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1answer
456 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 ...
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2answers
126 views

What is the definition of $[\alpha/H]$?

The notation $[A/B]$ denotes \begin{equation} [A/B] = \text{log}_{10}\Big[\frac{\text{(number of A atoms/number of B atoms)}_{\ *}}{\text{(number of A atoms/number of B atoms)}_{\ \odot}}\Big] ...
3
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1answer
558 views

Where can I find the list of the planetary motion equations?

The planetary motion equations are written in the ellipse equation format, i.e. $$x^2/a + y^2/b = 1.$$ Can anyone please tell me where I can find the list of all the planetary motion equations, ...
18
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2answers
5k views

How many stars are in the Milky Way galaxy, and how can we determine this?

I have heard multiple estimates on the quantity of stars within our galaxy, anything from 100 to 400 billion of them. The estimates seem to be increasing for the time being. What are the main methods ...