# Astronomy Stack Exchange 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

### 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 ...

### 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

### 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

### 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

### 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-α ...

observational-astronomy galaxy cosmology universe interstellar-medium

### 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

## 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

### 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

### 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

### 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

### 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?

atmosphere

### 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

### 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?

the-sun

## 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 ...

moon-phases