Massive systems held together by gravitational attraction, consisting of stellar associations, star systems, star clusters and types of interstellar clouds, stellar remnants, gasses, cosmic dust and an important but not yet completely understood component called dark matter.

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How many more galaxies are out there in the Universe (beyond the observable radius)?

Let's say that the number of large galaxies in the observable universe is $n$ (approximated to 350 billion). If the universe is homogenous and isotropic, what are the estimations for the total number ...
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3answers
2k views

Solar system, visible stars and deep sky objects

Since I've seen that galaxies are often called "deep sky objects", as opposed to individual stars, does this mean that all visible stars in the night sky actually only belong to The Milky Way Galaxy? ...
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2answers
2k views

Mass of a galaxy via luminosity

Is there a general formula for calculating the mass of a galaxy, or even a nebula from the luminosity? Or, is there a way of calculating the total mass of a galaxy from its energy output? Is there a ...
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1answer
145 views

Stellar Viscosity in Galaxies

Is there such as thing as the viscosity of stars in a galaxy, along the lines of gravitational attraction between stars changing the dynamics. If so, how is that put in terms of the Virial Theorem?
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435 views

What frame(s) of reference are used to measure the rotation of the Sun around the galaxy ?

I can find various speeds and estimated durations listed at numerous places but none specifically describe the frame of reference. Possible options as example of kind of answer I expect. Local ...
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2answers
234 views

How can we differentiate between matter and antimatter? [duplicate]

For instance if there was a galaxy, assume it to be made up of antimatter (isolated from other "normal" galaxies), how would we, or rather, would we be able to distinguish if it was made up of matter ...
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1answer
103 views

How are stellar and galactic ages determined?

Being a 4th year undergraduate physicist, you'd think I'd know this! But it's never really taught in any detail, just vague mentions of metallicities and, in galactic case, redshift. So how exactly ...
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2answers
2k views

Galaxy Spectra: Emission and Absorption Lines

Spectra from galaxies include both absorption and emission lines. I do understand how both types of spectral lines are produced but I am not quite sure where each type is coming from when we observe a ...
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2answers
162 views

How Galaxy is formed?

Given the distance among stars (the most massive objective in the space) is so huge, the difference of order of magnitude is about 7. And also, since gravity is such a weak force, how is it likely for ...
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406 views

Is most of the matter in the observable universe within galaxies?

Do we know, either through observations or through theory driven computer simulations, the location of the majority of the visible matter in the observable universe? That is, is it located within ...
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254 views

About the hump on galaxy rotation curves

The past days I have been studying the rotation curves of disk galaxies and I am currently trying to understand how we can extract information about the dark matter of a galaxy by looking its rotation ...
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1answer
194 views

Experimental Data for Mass Distribution of a Galaxy

My goal here is not to discuss dark matter in general. I know there are many other observational clues that hint us towards Dark matter. My goal is simply to understand this argument here a little ...
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1answer
125 views

Why does Omega Centauri have a distinct chemical signature from the rest of the Milky Way?

In answering a question about the orbital path of Omega Centauri, I learned that it has a distinct chemical signature from the rest of the Milky Way. Basically, it is very rich in s-process elements, ...
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1answer
495 views

How to tell if a star is in a galaxy?

An astronomer is studying a star that appears to be in a galaxy. How does the astronomer know the star is actually in the galaxy and not just on the same line of sight as the galaxy? I'm guessing ...
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1answer
139 views

Do the stars in a galaxy have a thermal kinetic energy distribution?

I think, there is practically everything given to that: many point-like masses, able to exchange energy pseudo-randomly, and far long enough time to reach a thermodynamical equilibrium. Of course, ...
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What is the frequency of occurrence of stellar classifications off the HR main-sequence?

An alternative version of this question would be: "if was to pick a star from the $10^{11}$ or so in our galaxy at random, what are the probabilities of it being various kinds of star?" (and I do mean ...
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3answers
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Why the red-shift of distant galaxies is considered to be the effect of expanding spacetime?

Why it's not explained just by Doppler redshift caused by faster movement of those galaxies billions of years ago when that light was emitted? Would the speeds of the galaxies necessary for Doppler ...
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2answers
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NED velocity to redshift conversion?

I've done some search with the Nasa Extragalactic Database (NED) and I have a very basic question about the velocity/redshift conversion. For example, for the first object of this page, we have ...
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1answer
526 views

Does conformal gravity explain the Bullet cluster lensing effects?

Conformal gravity is an "alternative" theory of gravity, where instead of using the Einstein-Hilbert action composed of the Ricci scalar, the square of the conformal Weyl tensor is used. It was ...
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7answers
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Is there a black hole in the centre of the Milky Way?

Is it true that the whole galaxy is actually revolving, and powered by a black hole? Has it been proven, and if it is true, how can our solar systems actually keep up the momentum to withstand the ...
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4answers
624 views

Are galactic stars spiraling inwards?

Are the stars in our galaxy spiraling inwards towards the center, or are they in a permanent orbit? And if we are heading towards the center then what is the rate of this process? I started ...
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5answers
353 views

Is dark matter around the Milky Way spread in a spiral shape (or, in a different shape)?

Dark matter doesn't interact with electromagnetic radiation, but it, at least, participates in gravitational interactions as known from the discovery of dark matter. But does dark matter exist in a ...
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1answer
2k views

How far apart are galaxies on average? If galaxies were the size of peas, how many would be in a cubic meter?

The actual number: How far apart are galaxies on average? An attempt to visualize such a thing: If galaxies were the size of peas, how many would be in a cubic meter?
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3answers
135 views

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 ...
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6answers
377 views

Why are distant galaxies not actually tiny bits of matter?

Distant galaxies are said to be moving away from the Milky Way (and us) at speeds approaching the speed of light. Since Special Relativity tells us that any object moving away from us at a velocity of ...
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2answers
266 views

Does the recent re-count of stars in elliptical gallaxies affect our understanding of the universal mass balance?

I've seen several popular reports of a new count of low-mass stars in elliptical galaxies (here's one). Edit: Pursuant to several correct comments I've changed the title to agree with the actual ...
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4answers
113 views

Why doesn't the light from galaxies appear stretched? [duplicate]

Maybe it's my ignorance of astrophysics/cosmology, but I have been wondering this: Why do galaxies not appear stretched when we observe them? Assuming a galaxy that we observe is 100,000 light years ...
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2answers
624 views

What is the area of the sky that is covered by the Hubble Ultra Deep Field image?

I have found two different numbers for the area of the sky covered by the Hubble Ultra Deep Field (HUDF). According to this, the image is roughly 2.4 arcminutes wide. The image is also attached to the ...
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1answer
3k views

Most accurate ways to find the average distance between stars in Milky way galaxy

I've already posted here on quora. But, I'm not totally sure if it's the most reasonable method. Would anyone care to elaborate on how to find the average distance between stars in a given galaxy ...
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1answer
820 views

Why are there not many detectable supernovas?

Astronomers estimate that there are between 200 billion to 400 billion stars contained within the Milky Way and Andromeda Galaxy probably has 1 trillion stars. There may be around 500 billion galaxies ...
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1answer
146 views

According to the initial mass function, should there be more brown dwarfs than red dwarfs?

According to the IMF and the stellar mass distribution, stars become more abundant the less massive they are. And while objects must have a mass > 0.075 solar mass to become a star, brown dwarfs with ...
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2answers
191 views

Why is the gas halo of the Milky Way so hot?

I have read on the webpage of NASA that there is a massive hot gas halo around our galaxy. Its temperature is between 100,000 and 1 million Kelvins or more. I do not understand why is it so hot. The ...
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1answer
156 views

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? ...
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1answer
469 views

How and why will the Milky way collide with the Andromeda?

Hubble's law says that the universe is expanding.How come the milky way and the andromeda are on a collision course?How will they end up colliding with each other?
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362 views

Does the dark matter halo rotate with the galaxy?

If the dark matter halo is stationary related to the arms of the galaxy then tidal effects should slow the galaxy rotation. If it rotates with the normal matter in the galaxy then shouldn't it ...
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1answer
279 views

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 ...
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0answers
282 views

Relation between isophotal radius and virial radius in spiral galaxies?

Is there any (proposed) relation between the B-band isophotal radius of a spiral galaxy and its virial radius (r_200)? If you know of such a relation, please post a reference paper.
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5answers
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Why the center of our galaxy doesn't absorb us?

Depending on the theories, the center of our galaxy is a super massive black hole, this is easy to accept as a truth, but what I couldn't simply devour is how the solar system is orbiting around it ...
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4answers
420 views

Explaining lightyear to non technical people

I need to explain the concept of a light-year to a non technical audience. Actually a presentation about planets and galaxies. It is quite difficult (for them) to comprehend the speed of light itself ...
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3answers
183 views

Faster than light galaxies/clusters?

A few years ago in an astronomy course, we calculated some (transverse?) velocity of a moving object and got super luminal results. The answer was apparent and not physical velocity of the object. ...
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2answers
551 views

Do green stars exists?

I asked a university lecturer why we don't observe green stars, and he said the blackbody curve averages at that frequency such that the cones in our eyes don't recognise it. I have a hunch that ...
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2answers
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At what distance could you see andromeda with the naked eye?

We've all seen the telescope photographs of andromeda galaxy: I'm wondering if it were possible to travel close enough to the andromeda galaxy could you achieve a same perspective with the naked ...
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2answers
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What does a galaxy orbit?

Moons orbit planets, planets orbit stars, and stars orbit the center of a galaxy. So, my question is what does a galaxy orbit? The center of universe? (I know that the universe has no center)
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3answers
677 views

What does this stellar mass distribution mean?

According to this pie above and for the "Red Dwarfs" part, which of these is correct : 1) 41% of the stellar mass of a galaxy is in stars with masses < $0.25$ $M_{\odot}$ or 2) 41% of the ...
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3answers
149 views

What is the name of this galaxy in HCG7?

What is the name of the predominantly blue spiral galaxy at center-top of this image of Hickson Compact Group 7? I've found a few articles mentioning the group, but they never seem to list the names ...
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2answers
3k views

Density of stars near the center of the Milky Way

At night, I can look up and see the Milky Way across the sky. My question is, supposing our solar system was, instead of way out on an 'arm' of the galaxy, if we were near the galactic center. Would ...
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3answers
73 views

Can the Hanbury-Brown and Twiss effect be used to measure the size of composite objects like galaxies?

I know that the Hanbury-Brown and Twiss effect can be used to measure the size of stars. Can it also be used to measure the size of galaxies?
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1answer
199 views

Population I and II stars

I have been thinking about the formation of the galaxy. I can easily understand that old, low-metallicity stars are in the halo, but I'm missing something when it comes to the disk. If ...
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2answers
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Distribution of dark matter in galactic halos

Often dark matter around galaxies is referred to as a 'halo'. I've seen the galactic rotation curves, but I'm having trouble visualizing how the dark matter is distributed for a typical rotating ...
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2answers
249 views

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 ...