Physics Stack Exchange
Physics Stack Exchange

Questions tagged [galaxies]

Galaxies are gravitationally bound systems of stars, interstellar gas and dark matter, often hosting a central supermassive black hole. For questions about the structure, composition, dynamics, classification, etc. of galaxies. This includes small systems of interacting galaxies (i.e. merger, or galaxy + satellite system), but for galaxy groups and clusters use [galaxy-clusters], and for questions specific to our own Milky Way galaxy use [milky-way].

Galaxies are massive systems held together by gravitational attraction, consisting of , star systems, star clusters and the ISM (), stellar remnants, cosmic dust and an important but not yet completely understood component called . Many galaxies, perhaps all, host a central super-massive .

For questions about the structure, composition, dynamics, classification, etc. of galaxies. This includes small systems of interacting galaxies (i.e. merger, or galaxy + satellite system), but for galaxy groups and clusters use , and for questions specific to our own Milky Way galaxy use .

658 questions
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What is the name of our Galaxy?

I was told that the name 'Milky Way' first came about when people observered the milky looking stretch of sky; that is, before telescopes came about and we realised it wasn't a milky cloud, but rather a collection of stars, our galaxy. So strictly…
Jonathan.
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How to find the Andromeda galaxy without using a go-to telescope?

In other words, what is the proper technique (star-hopping or other?) in order to find and properly point a telescope to this target? Would a star atlas or other tool/reference help? Can I use the R.A. and Dec. coordinates to find such deep-space…
octy
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What happen when two galaxies collide?

As the title suggests, I know collision of galaxies happens. Questions are Why do they collide (obviously because of gravity, but in what conditions)? Aftermath of the collision? If there is a super massive blackhole in the center of the galaxies,…
user
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How much gas is in elliptical galaxies?

It is widely known that elliptical galaxies have relatively little or no measurable gas, but how is this determined? What is the amount of gas? Is there a known ratio of stellar mass to gas for ellipticals? How does this vary between normal…
Gigi Giles
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Galaxies seen from Earth

If we observe two galaxies from the Earth that are diametrically opposed and each 10,000 light-years from the Earth, will the separation distance between the galaxies be 20,000 light-years? Really, the question is: if the galaxies instead were…
jormansandoval
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AGN accretion disk vs. torus

The torus is the donut of dust encircling the Active Galactic nucleus. The accretion disk is inside the torus. Is there a boundary between the two? At what point does a torus become an accretion disk? What are their differences?
Gigi Giles
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Galaxies that are newer than our own one

Can we see any galaxies/stars that are newer than our own galaxy? As light takes (c) amount of times to reach us - so relatively speaking, light which would have left the newer galaxy (and far enough from us) still would not have reached us.…
Arun
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What keeps galaxies united like a solar system?

Blackholes may be really strong but they act in a very short range. For example if the sun was a black having the same mass, it will be dark but we will still be revolving around it. It wont engulf us. Also I hear that the outer stars in a galaxy…
TheTechGuy
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Mass/light ratio of galaxies

I'm really stuck. I need to figure out the mass/light ratio of a galaxy in solar units. I know its mass is 5.7 x 1010 solar masses. I know its absolute magnitude (-17.3) and distance (29 Mpc). I'm feeling dumb here because I think this ought to be…
Gigi Giles
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How are galaxy filaments formed? And do they have any analogues in stellar formation?

In physical cosmology, galaxy filaments, also called supercluster complexes or great walls, are, so far, the largest known cosmic structures in the universe. They are massive, thread-like structures with a typical length of 50 to 80…
InquilineKea
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How do we explain the phase change between high and low star formation rates?

During a recent talk I was at, someone, who models galaxy characteristics from dust amounts and spectral energy distributions (SEDs), quoted a fairly prompt change from 'high' to 'low' stellar formation rates. How do we explain this prompt phase…
Nic
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The average velocity of galaxies in a galaxy cluster

On slide 24 of these lecture slides (Its title is 'worked example, what Zwicky did'), the author says that $\langle\langle v^{2} \rangle\rangle = 3 \langle\langle{v^{2}}_{s}\rangle\rangle$. Can someone please explain why this is the case? The…
SD11
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Can you explain what is Heliocentric velocity

In this photo, What is the meaning of heliocentric velocity ? Is the velocity at which the Sun's motion was corrected or before the Sun's motion was corrected?
teacher
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Highly inclined galaxies

I found in the literature that galaxies with an inclination angle relative to the line of sight greater than 60 degrees are considered highly inclined. Does anyone know why this particular angle is considered? Why not 50 deg or 70 deg?
Lekha
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View from inside clashing galaxies

The below image from Hubble show two clashing galaxies. Is there a simulation (or whatever) as how the sky would look withing those galaxies? For example, would the night sky be noticeably brighter than ours?
blackened
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