James K
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How can many stars be formed from the remains of one supernova?
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35 votes

Stars don't "come from" a supernova. Stars come from the interstellar gas in the galaxy, particularly where it is more concentrated into nebulae. This gas is mostly hydrogen and helium, but ...

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Why are fewer stars seen next to the horizon?
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34 votes

When you look towards the horizon you are looking through a much greater thickness of air. The air does absorb some light. Dense air near surface absorbs more, and if you look towards the horizon ...

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Does this photo show the "Little Dipper" and "Big Dipper"?
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34 votes

As mentioned, these are the Pleiades, and the belt of Orion. These are visible in the South at this time of year. The Big and Little Dippers are in the North, so turn around. The best way to find ...

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Is it possible to do planet observation during the day?
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34 votes

Jupiter can be seen during the day. This image is by Philip Crude. Philip is an experienced astro-photographer. On his webpage http://www.billionplanetsquest.com/p/planets.html he gives details of ...

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Are there only $10^{83}$ atoms in the universe?
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33 votes

This is a reasonable estimate for the number of atoms in the observable universe. It might seem like a small number, compared with the number of atoms in a human only as a result of our brain's ...

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Why is the Moon's orbit so complicated?
33 votes

The moon is so big that the processes that circularize and reduce the equatorial inclination would take much longer. The moon is big because of how it formed: a huge collision in the early solar ...

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How did Ole Christensen Romer measure the speed of light?
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33 votes

Ole Rømer did not measure a change in the frequency of light. He measured an apparent change in the orbital period of Io, one of Jupiter's moons. The orbit of Io can be measured very accurately by ...

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How is it possible for a collision to be responsible for Uranus's axial tilt?
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30 votes

For objects on the scale of a planet, the state of matter doesn't really matter much. A colliding planet or planetesimal would not "just pass through" The amount and density of the gas ...

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The newly discovered comet NEOWISE has been given the name C/2020 F3. What is the meaning of the F3 and the C in this name?
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30 votes

C means "comet": it has a coma which means that volatiles are being released due to solar heating. Other possible letters are "A" for asteroid, "P" for (short) periodic ...

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Why are perihelion and the shortest day in the northern hemisphere different?
30 votes

We have "days" because the Earth is rotating. The shape of the orbit has little effect on the daily cycle of light and dark. The length of the day is determined by the tilt of Earth to the orbit of ...

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Why is the L3 Lagrangian point not perfectly stable? And why is the Earth-Sun L3 point a bit less than one A.U.?
29 votes

L1, L2 and L3 are saddle points in the effective potential of the gravitational field in a rotating frame of reference. That is if you combine gravity (of Earth and Sun) with the centrifugal force on ...

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Starting a fire in a cold planet that was full of flammable gas
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29 votes

The outer parts of Neptune are mostly hydrogen and helium. There are small amounts of other gases such as methane, ammonia and water vapour. However, there is no oxygen at all. If you took some of ...

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Can I sense a bright star pointing an eight foot antenna towards it?
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29 votes

Stars are too dim for amateur radio equipment. There are two possible radio sources that you can detect: the sun and Jupiter. Jupiter is particularly interesting as interactions between Io and its ...

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Does a parabolic trajectory really exist in nature?
28 votes

No, but nor do elliptical or hyperbolic trajectories. These are features of a model we have for gravity, not part of nature. The modelling process involves constructing a mathematical framework that ...

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Quantum Mechanics after the detection of Gravitational Waves
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27 votes

No more than the observation of light waves disproves quantum mechanics. Light has properties of both a particle and a wave. At low energies, the particle nature of light is hard to detect: radio ...

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Why nuclear fusion with hydrogen alone, why not with others?
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26 votes

For nuclear fusion to occur, there must be a certain temperature and pressure. Generally for heavier elements, higher temperatures and pressures are needed, because the nuclei have a greater electric ...

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Telescope on drones to escape light pollution?
26 votes

Yes, but not in the exact way you think of. To avoid light pollution it is better to go sideways. If you are in a light-polluted city, there would still be light pollution at an altitude of 400ft (...

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Is a perfectly circular orbit possible?
26 votes

"Perfectly" is a funny word. Perfect circles are a mathematical abstraction. Real objects are not "perfect". So supposing a "perfectly spherical planet" is to suppose something that does not and ...

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How many planets have we discovered that can support human life?
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26 votes

There is currently only one planet known to be capable of supporting human life, and you're on it. Several planets have been found in the region in which we expect water to be liquid on much of the ...

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How to be an astrophysicist?
25 votes

Different education systems differ, however At school you would take maths and physics courses, at least covering calculus. As an undergraduate, taking (or majoring in) physics. Also probably doing ...

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How do we distinguish old craters from new ones on the Moon?
25 votes

The "weathering" processes are very slow compared to those on Earth. They are caused by the impact of micro-meteorites and the effect of the solar wind and cosmic radiation on the surface. The ...

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What caused this mysterious stellar occultation on July 10, 2017 from something ~100 km away from 486958 Arrokoth?
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24 votes

There were three attempts to measure Arrokoth by occultation, and the June 3rd attempt didn't detect anything. The July 10th attempt had a tiny blip, that appeared to be in the "wrong place"...

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Looking for a detailed Milky Way map
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24 votes

It doesn't exist. It is actually rather difficult to see the shape of the milky way, because we are inside it. The more distant parts are obscured behind the nearer and very much of it is invisible. (...

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Can I make a black hole with one or two atoms?
24 votes

There are two answers: yes and no. Yes because every mass M has a Schwarzschild radius given by $\frac{2GM}{c^2}$ (where G is the gravitational constant (about $6.7\times10^{-11}$ and c is the speed ...

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Can you see something active in the sky apart from satellites? Can there be amateur time-domain astronomy?
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23 votes

If it moves or flashes it isn't astronomy, it is meteorology or technology. There are only a few exceptions to this: Meteors are an atmospheric phenomenon, and a meteor will appear to move rapidly ...

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Why does the closest approach of star S2 to Sgr A* not appear to be near the focus of its elliptical orbit?
23 votes

The orbital elements are on wikipedia: $$e=0.884\ a=0.125'',\ i=134^\circ,\, \Omega=228^\circ$$ (At an assumed distance of 8kpc, $0.125'' = 1000au$) It is the inclination that means that the black ...

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Was the Sun's gravitational lensing observed in other solar eclipses than the one in 1919?
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23 votes

Yes, observations of this kind are within the technical scope of amateur astronomers. Several groups succeeded in replicating the experiment during the 2017 eclipse that crossed the USA. For example ...

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Why do stars twinkle but the Sun doesn't (I'm asking this because the Sun is also a star)
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23 votes

Stars twinkle because they are effectively a point of light. This point of light can be distorted and magnified by movement of patches of varying density in the atmosphere. These act as lenses causing ...

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How can a supermassive black hole cause so much energy to enlighten its matter when its massive gravity prevents light to escape?
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23 votes

It is quite correct that a black hole has so much mass that light cannot escape from a region around the black hole. The edge of this region is called the event horizon. If you cross an event horizon ...

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Line of lights moving in a straight line, with a few following
22 votes

Given the date and timing, this could be most like the Starlink satellites in their "stacked" configuration. They are currently in a line, but they will later move to separate orbits. Dr Marco ...

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