MBR
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How can we tell that the Milky Way is a spiral galaxy?
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39 votes

There are several clues. The Milky Way is a flat disk The first one, and the simplest one, is that we live in a disk. As you can see on images like the ones from the 2MASS survey in the infrared range:...

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Why has Venus's atmosphere not been stripped away by solar wind?
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35 votes

Venus has a strong ionosphere that protects it against violent solar winds. So, even though Venus has no intrisic magnetic field, it has an effective, induced magnetic field due to the interaction ...

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Why is our solar system "tipped" about 63° with respect to the plane of our galaxy?
21 votes

What you could think at first, regarding the orientation of any planetary system, is that it should be roughly in the plane of the galaxy, simply by angular momentum conservation. But, when you take ...

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How could a hobbyist astronomer determine apparent magnitude of a star?
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19 votes

The easiest way to determine the magnitude of a given star is probably to use the Pogson relation. The idea is to determine the magnitude of a star knowing the magnitude of a reference star; it is ...

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How dense are Saturn's rings?
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18 votes

Elements of answer: It is not an easy question, as we lack of data to constrain strongly the density and mass of Saturn's rings. However, a first clue of their density is their optical depth (that ...

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Why is the Sun's atmosphere (the corona) so hot?
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18 votes

It is still an open question, even though it is clear that it is linked to the magnetic field of the Sun. The following hypothesis are the following: Heating waves mechanism: propagation of ...

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Why does the Moon stablize Earth's axis?
16 votes

One way to look at this problem is to consider angular momentum. The Earth spins around its axis, and has therefore some angular momentum by itself. The angular momentum is proportional to the mass of ...

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What is the evidence that galaxies rotate?
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15 votes

Rotation: One of the easiest way to show that galaxies rotates are Doppler observations of atomic or molecular lines. For example, if the observed galaxy is seen edge-on and rotates clockwise, the ...

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What is the current routine of modern astronomy?
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14 votes

What you say is not quite true: the search for exoplanets is clearly intensive, but it is far from the only things astronomers are looking at. Most of the time, in two words, the situation is: ...

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What is the difference between asteroids, comets and meteors?
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13 votes

The objects you are refering to are actually two different objects: asteroids and comets. Meteor and meteorite are other names for an asteroid, at a given time of its interaction with our planet. We'...

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What are the current observational constraints on the existence of Nemesis?
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12 votes

Few important points about WISE: it was able to detect anything with a temperature above 70-100 K, whereas the coolest brown dwarfs are in the 500-600 K range (the coolest was discovered by WISE ...

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What is the orbital path of the newly discovered star-less planet PSO J318.5-22?
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12 votes

There is no "orbital path" detected, that's why it is a "free-floating planet". There is no radial velocity mesured, but the informations given by its kinematic location show that it belongs to the ...

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How long does it take to produce a star? Why does it take that long?
11 votes

The time to "produce" a star actually depends on the mass of the star. Let's start with a solar type star. For a typical solar type star, the formation time is about 10 million years (you ...

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Hypothetical trans-Neptunian planets orbiting the Sun
11 votes

If such a planet exists, WISE should observe it. WISE is an infrared satellite that imaged the entire sky. In particular: it was able to detect anything with a temperature above 70-100 K, whereas the ...

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Why are some planets so close to their star?
11 votes

Big picture: The reason why there are planets so close to their host star is probably planetary migration. These planets are difficult to explain by in situ formation models (first of all there is ...

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What is there in the intergalactic space?
11 votes

You can find all kind of objects, like stars, planetary nebulæ etc. But it is mostly a hot, ionized void. How void? The density of the intergalactic medium is about 1 to 100 particles per cubic meter ...

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What is the formula to predict lunar and solar eclipses accurately?
10 votes

Calculation of solar eclipses can be done using Besselian elements. The basic idea is to compute the motion of the Moon's shadow on a plane that crosses the Earth's center. Then, the shadow cone of ...

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What study profiles could land me the job of astronomer?
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10 votes

What you need would be at least the following: Physics: astronomy is physics in the end, and you'll need strong basis in physics; Math: as for any good education in physics, you'll need a reasonable ...

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How do we know dark matter/dark energy exists?
10 votes

Dark matter and dark energy are two different things, accounting for different observations. Dark matter: Dark matter is needed to explain, among other things, the rotation curve of galaxies. One ...

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How does interstellar matter density vary?
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10 votes

The interstellar medium is a multiphase medium, and you can find (some references in this lecture and in this thesis manuscript (this one is in French, but numbers are international)): the hot ...

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What gases are needed to make a star?
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10 votes

It depends on the kind of star you want to make. Basically, what you really need is hydrodgen; as soon as enough hydrodgen is gathered somewhere and collapse, you end up with a star. But, if you ...

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Sources of Turbulence in the ISM
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9 votes

Turbulence sources: There are numerous sources of turbulence in the interstellar medium, at all scales: at large scales, there is the shear from galactic rotation. One way to sustain turbulence and ...

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Why do stars appear to twinkle?
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9 votes

Stars tend to twinkle for two main reasons: first, stars are very far away (the closest star is about 4 light-year from the Sun) and are therefore seen as point sources. Second, Earth has an ...

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Can protoplanetary disks form main-sequence stars?
8 votes

No, not really. Stars can form in circumstellar disks, that are, in general, disks surrounding forming stars, but not in protoplanetary disks. Protoplanetary disks are, by definition, flat, ...

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Official registry of lunar placenames?
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8 votes

The resource you cite, in addition to this full list of crater's names are the official, definitive lists of named craters. Note that you have the coordinates of each crater in the complete list; I ...

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Is there any evidence that the Gas Giant planets in our solar system are experiencing orbital migration?
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8 votes

There is no direct evidence of change of orbits of the gaseous planets of the Solar System. But one could wonder about the stability of the Solar System, and if such events could happen again. Theory:...

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Why do the planets in the Solar system stay in the same orbital plane?
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7 votes

Angular momentum conservation To put it in more mathematical terms, you can play with the energy and the angular momentum of a bunch of particles orbinting a central mass $M$, given by $$E = \sum_i ...

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How exactly is the Initial Mass Function (IMF) calculated?
7 votes

What is it? An IMF, $\Phi(m)$, is defined such as $\Phi(m){\rm d}m$ gives the fractions of stars with a mass between $m - {\rm d}m/2$ and $m + {\rm d}m/2$, and with a normalized distribution $$\int_{...

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What are some theories that explain the 11 year solar cycle?
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6 votes

It is linked to the solar dynamo and the dynamical behavior of the so-called $\alpha$-effect. One way to understand easily the solar dynamo and the the $\alpha$-effect with the hand is the following: ...

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Why does the Earth have a tilt of ~23°?
6 votes

The tilt of the rotation axis of the Earth is probably twofold. First, it is probably due to Earth formation history. Even though, by angular momentum conservation, planets should have a rotation axis ...

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