Community Digest

Top new questions this week:

If the Sun disappeared, could some planets form a new orbital system?

If the sun were to suddenly disappear, the planets would continue to travel tangentially to their former orbits. (This I know from this answer to a somewhat related question here.) In such a scenario, ...

orbit gravity space-time rogue-planet  
asked by Psychonaut 17 votes
answered by Anders Sandberg 31 votes

Why do M dwarfs emit more X-ray radiation than larger, more luminous stars?

I have read that "The relative amount of harmful radiation (to life as we know it) that stars emit can be 80 to 500 times more intense for M dwarfs relative to our Sun". Source This seems ...

star astrophysics stellar-astrophysics radiation  
asked by Astroturf 6 votes
answered by ProfRob 4 votes

What exactly was the Moon's "Evection Resonance"?

Ward et. al define the evection resonance of the Moon with the Earth and Sun as: occurring when the lunar perigee precession period equals one year. Does this simply mean that the angle formed by ...

the-moon orbital-mechanics orbital-resonance  
asked by Connor Garcia 5 votes
answered by James K 8 votes

Fate of a planet orbiting a Lagrange point behind a star?

Let's say there is a planet orbiting at L5 behind a star, which is in turn orbiting a more massive star, like so: What is the ultimate fate of this planet? I'm predicting that it will spiral into the ...

star orbit planet binary-star lagrange-point  
asked by slowerthanstopped 5 votes
answered by James K 6 votes

Are red dwarfs really 30-100 times our Sun's density?

This article by K. E. Edgeworth claims red dwarfs tend to be around 30-100 times our Sun's density. However, this seems a bit high. With smaller stars, mass tends to be proportional to radius but ...

star mass density red-dwarf  
asked by Xi-K 5 votes
answered by Schwern 8 votes

Significantly blueshifted Lyman-$\alpha$

Lyman-$\alpha$ line in galaxies is notably known to show a double peaked profile, mainly due to its scattering in a moving medium, see e.g., the very recent Matthee et al. 2021, The X-SHOOTER Lyman-╬▒ ...

observational-astronomy galaxy cosmology universe interstellar-medium  
asked by Tony P. 4 votes

How to place an observer in a cosmological simulation

Suppose I have a cosmological simulation with points in xyz space at a certain redshift in a cube like this: And I have these for many cubes of different redshifts say from z=0 to z=2. Now I want to ...

cosmology software visualization sloan-dss  
asked by kauii8 4 votes

Greatest hits from previous weeks:

What will happen when landing on Jupiter?

Jupiter is a gas giant, so landing on it will not be like landing on Earth, our Moon or Mars etc., as it does not have a solid surface like these. If we have a hypothetical spaceship or probe landing ...

jupiter gas-giants surface core gas  
asked by Farhan 11 votes
answered by ProfRob 17 votes

What is the name of our Solar System?

what is our Solar System called rather than "the Solar System"? I've found "Sol System" and "Monmatia", too, but is there more to it?

solar-system naming  
asked by user7518 14 votes
answered by Peter Erwin 12 votes

How can there be 1,000 stellar ancestors before our Sun?

I've heard from a few sources* recently that the Sun is a 1,000th generation star, meaning it had a thousand stars that came before it based on its heavy-element content. I understand that earlier ...

the-sun stellar-evolution  
asked by Benjam 85 votes
answered by ProfRob 142 votes

How did Eratosthenes know that the sun is far away?

The famous measurements and calculations done by Eratosthenes around 300 BC are very widely known. He concluded correctly that the circumference of the Earth is about $252\,000$ times the length of an ...

distances history  
asked by Michael Hardy 29 votes
answered by James K 55 votes

Why do objects burn when they enter earth's atmosphere?

Why do all objects burn when they enter our atmosphere? is this because of our atmosphere composition? and does this happen on other planets as well?

asked by A.K 4 votes
answered by stevenvh 8 votes

Why not take a picture of a closer black hole?

There are closer galaxies than Messier 87 for sure, even ours! It sparked my curiosity that they went with one 53 million light years away. Is there a reason for this?

black-hole supermassive-black-hole event-horizon-telescope  
asked by Morgan 67 votes
answered by Ingolifs 83 votes

How much gold is there in our sun?

XKCD 1944 claims that there is "more gold in the sun than water in the oceans". Is this really true?

asked by Jakub Šturc 137 votes
answered by Peter Taylor 26 votes

Can you answer these questions?

Visual binary star with circular apparent orbit

Does anyone know a good example of visual (or interferometric) binary star apparent orbit of which looks circular due to projection though in fact is markedly elliptical? I need that for teaching.

orbit binary-star orbital-elements  
asked by Leos Ondra 1 vote

Moon phase angle

When I look at a visible moon in daylight its phase is shown at an angle. However textbook moon phase diagrams only show the phase vertically. What is it that determines the angle? The reason I ask ...

asked by Jen 3 votes

North Stars of each Solar system planet?

What are the North Stars of each individual body points too? E.g. Earth points to Polaris. Mars points to Deneb/Alpha Cygni. What about Sun, Mercury, Venus, Moon, Jupiter, Saturn, Uranus, Neptune, ...

planet solar-system celestial-mechanics navigation  
asked by Majoris 1 vote
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