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1500 questions
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Is Jupiter a dwarf planet?
I know the question seems silly, but as far as I've understood, is this the definition of a dwarf planet:
circling around a star, and not being a star itself
having a round shape (otherwise every comet or asteroid could be a (dwarf) planet)
not…
Dominique
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What does the velocity dispersion of a galaxy mean?
Also, what is the advantage of getting information about the velocity dispersion of a galaxy?
J. H. Azeez
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11
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6 answers
How is the Universe's Expansion Accelerating if the Hubble Constant is Decreasing?
I am just getting into the field of cosmology and was wondering what it means practically to say that the Hubble Constant is decreasing while the expansion rate of the Universe is accelerating. I am not sure how this phenomenon is possible. How do…
nosn
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Are there upper or lower mass limits for black holes?
Natarajan & Treister (2008) describes a practical upper limit for black hole masses at $\sim 10^{10} M_\odot$. This is all due to the black hole's interactions with nearby matter.
However, is there a theoretical upper mass limit for black holes in…
Sir Cumference
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The deduction of the Hill Sphere formula
This question may be a little lazy, but can anybody give me a proof of the Hill sphere formula? Acording to wikipedia, the formula for the radius, $r$, is
$$r\approx a(1-e)\left(\frac{m}{3M}\right)^{1/3}$$
where a body of mass $m$ is orbiting a much…
Carlos Vázquez Monzón
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Why do galaxies have arms?
A couple of years ago I started playing a game called Kerbal Space Program (KSP) and my understanding about how orbits actually work increased dramatically. Because of KSP I was looking at a picture of a galaxy with its spiral arms and something…
Hal Clark
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In km/h, what actually is the "speed" of Andromeda away from us: cosmologically?
Andromeda is about 2.5 million ly away.
Actually, in this universe, at what "speed" (in km/h) are two objects separating cosmologically - I mean strictly due to the "expansion of the universe" - if they are 2.5 million ly apart?
I do understand that…
Fattie
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How easy would be to see the Moon from Mars?
This question arises from "What planet is better than earth to infer solar system configuration?", where one of the answers pointed that it's easier to discover the Moon from Mars than discovering the Galilean satellites of Jupiter from Earth.
Then…
Pere
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Can an amateur astronomer bounce a laser off the moon?
In the TV show "Big Bang Theory" episode "The Lunar Excitation", the gang fires a laser from their rooftop, bounces it off mirrors on the moon, and measures the laser coming back on a computer.
Is this really possible?
I know scientists have…
Reactgular
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How do I know, mathematically rather than from observation, if a moon is full?
I know about the equations to describe the orbit of a moon around a planet. I know the moon's semi-major axis and eccentricity, and the same for its host world with the star they orbit.
Is there any equation that tells me how much of the moon is…
Kalcipher23
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2 answers
Do black holes die?
I know of stars actually dying (exploding as a supernova) but I have never heard of what happen to black holes. Do they continue to live forever, waiting there for more matter to absorb? Do they explode at some point ?
MaxiWheat
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How far away is the nearest compact star remnant likely to be?
Neutron stars and black holes are hard to detect when they are solitary, and there seems to be big uncertainties about how common they are. White dwarfs are much easier to detect and the nearest one is Sirius B only 2.6 parsec from here. Should we…
LocalFluff
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How was Earth's "quasi-satellite" 2016 HO3 "first spotted" and it's orbit determined?
Under a question I asked a few days ago Have there been any documented mini-moons since 2006 RH120? @Hobbes mentioned the recent news about 2016 HO3 - a near-Earth asteroid that stays near Earth because its orbit around the sun is coupled to Earth.…
uhoh
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What's the distinction between "planetary formation mechanisms" and "gravitational collapse"?
As I was curious about the exact identical comment to my title here: https://astronomy.stackexchange.com/a/16135/10102
And as it seems no one asked it yet, I'm simply interested in the difference of the both, so:
What's the distinction between…
Zaibis
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Is there anything special about our position in the galaxy thats necessary for life?
It's a fact that our planet's location from our parent star (among other factors) is of crucial importance to the existence of life. But can our position in the galaxy be considered a factor when considering potential planets to carry life?
In any…
Yoda
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