# Tag Info

94

This whole question is a mistaken premise. There are spherical (or at least nearly spherical) galaxies! They fall into two basic categories - those elliptical galaxies that are pseudo-spherical in shape and the much smaller, so-called "dwarf spheroidal galaxies" that are found associated with our own Galaxy and other large galaxies in the "Local Group". Of ...

89

There are plenty of satellite galaxies orbiting larger galaxies. The question is how long are you willing to wait for an orbit? The Milky Way has a mass $M$ of something like $6\times10^{11}$ solar masses, or $10^{42}\ \mathrm{kg}$. The small Magellanic Cloud is at a distance $R$ of $2\times10^5$ light years, or $2\times10^{21}\ \mathrm{m}$. A test mass ...

61

There is no alignment between the Sun or the Solar System's net angular momentum and the "spin axis" of the Galaxy. Think for a moment about whether the line of the ecliptic (which marks the "equatorial line" of the Solar System) and the Milky Way (which roughly marks the plane of the Galaxy) are lined up? If this were so, then you would always see the ...

34

Short answer: A spiral galaxy is, in fact, spherical-like. To understand how, let us as a starting point look at Wikipedia's sketch of the structure of a spiral galaxy: A spiral galaxy consists of a disk embedded in a spheroidal halo. The galaxy rotates around an axis through the centre, parallel to the GNP$\leftrightarrow$GSP axis in the image. The ...

32

There are two separate questions there. The easiest one to answer is how we measure the velocity of the Earth, Milky Way etc, because we measure it relative to the cosmic microwave background (or CMB). If you measure the CMB in all directions and find it's the same in all directions then you are stationary in comoving coordinates. However if you find the ...

28

Not quite like in the photo above, which shows more than what the naked eye can see, but yes, absolutely! Our galaxy (well, the chunk of it visible from these parts) is a naked-eye object. The fact that your question even exists shows how much time is now spent by people under light-polluted skies. It will not be visible from the city, however. You need to ...

27

They do! There's an entire class of galaxy, called a 'satellite galaxy' which is defined entirely based on them orbiting a larger galaxy (which would be called a 'central galaxy'). Our own milky-way is known to have many orbiting satellite galaxies, or at least 'dwarf-galaxies'. If dwarf-galaxies aren't enough, the milky-way itself is gravitationally ...

26

The material (gas and stars) in the outer part of a galaxy move with roughly the same velocity as the inner part (for example, see this paper), which means that the inner portions do indeed have a faster angular speed; this is sometimes referred to as the "winding problem." One important feature of spiral arms is that they are bright more because they have ...

25

Why shouldn't the orbits of stars be Keplerian? The answer is simple. Keplerian orbits are predicated on a single central point mass. That assumption fails to some extent even in a solar system. It fails massively in a galaxy. A galaxy is not a point mass.

25

Assumption #1 is quite correct - there is a very large ("supermassive") black hole in the center of our galaxy. Assumption #2, however, is false. Black holes are no better at drawing in distant objects than any other thing in space with the same mass would be. If you collapsed the Sun into a black hole right now, the Earth's orbit would not change. That is ...

24

A typical giant galaxy, such as the one you've provided a picture of, has a radius of something like $10\;\rm kpc$ (kiloparsec - $1\;\rm pc \approx 3.2\;ly$). A supermassive black hole hosted in such a galaxy has a mass of something like $10^6-10^9\;\rm M_\odot$ (solar mass, $1\;\rm M_\odot \approx 2\times10^{30}\; kg$). The monstrous billion solar mass ...

24

The estimates I've read are similar to yours: 200 to 400 billion stars. Counting the stars in the galaxy is inherently difficult because, well, we can't see all of them. We don't really count the stars, though. That would take ages: instead we measure the orbit of the stars we can see. By doing this, we find the angular velocity of the stars and can ...

22

The gravitational potential of the disk of the Milky Way can be approximated as: $$\Phi = -\frac{GM}{\sqrt{r^2 + (a + \sqrt{b^2 + z^2})^2}} \tag{1}$$ where $r$ is the radial distance and $z$ is the height above the disk. I got this equation from this paper, and they give $a$ = 6.5 kpc and $b$ = 0.26 kpc. In the weak field approximation the time dilation ...

19

Short answer The question is a bit ambiguous. If the question is why do star velocity increase with distance close to the galactic centre ? the answer is because their orbit encompass more mass, and this corresponds to a stronger gravity pull. If the question is why does their velocity stays constant and does not decrease at big radii, ...

18

There are several theories on how they form like Density Waves and stochastic self-propagating star formation (SSPSF). But you're interested in how they start. Current debate about the arms have two main points: One holds that the arms come and go over time and a second and widely held theory is that the material that makes up the arms - stars, gas and ...

18

Actually, there are parts of a galaxy that extend beyond the galactic plane: Galactic halo: This is actually the primary part of a galaxy that is not in the main galactic disk. It's made up of multiple sections, and is composed or an array of objects. Dark matter halo: This is a section of the galaxy's dark matter that exists in a semi-spherical shape. ...

16

I was giving a talk about the galactic black hole at the center, Sagittarius A*, back in 1998. At that time, it was already clear to enlightened people that it had to be a black hole. An analysis of a two-temperature plasma helped to bring some new evidence that the object had a real event horizon. The black hole is huge but it is not "galactically" huge. ...

15

To some extent the universe exhibits something called self-organized criticality where a dynamic, non-linear system with many degrees of freedom (the gas after the Big Bang but before the emergence of structure) eventually forms a system with a notable degree of scale invariance (moons orbiting planets, planets orbiting stars, stars orbiting galactic ...

15

Asimov's description is pretty much correct. There aren't many stars out there, so the night sky away from the galactic disk would be fairly dark. Toward the galaxy you you have an edge on view of the galactic disk. As for a sky full of galaxies, you might see a few but probably not. They are intrinsically very faint and moving a few tens of thousands of ...

15

All matter in the galaxy has to rotate (not necessarily in the same direction) so that a centrifugal force acts. Without the centrifugal force, all matter contained in the galaxy will collapse into the center of the galaxy due to gravitation. The rotation happens about an axis, a line about which all matter revolves in the galaxy. Now, the manner in which ...

14

By definition, anything outside of the observable universe is unobservable. This has the annoying effect (eye twitch) of making it so we have practically no idea what the universe is actually like outside of what we can observe. We can assume that it is homogeneous and isotropic and that there are other large galaxies out there, but there is a non-zero ...

14

No one has discovered it. Dark matter is a proposed explanation to some observed phenomena. In particular, Galaxies rotate at a speed that implies they are quite heavy, especially towards the outer edges - but when we look at the mass from stars and interstellar gas, there isn't enough to make them spin the way they do. Gravitational lensing is a ...

14

user6972's answer is great, but I thought I'd add a somewhat more technical footnote. If the mathematics are lost on you, skip to the end where I give a simple physical interpretation. The dispersion relation for a differentially rotating fluid disk (i.e. the rotation frequency changes with radius, as opposed to a uniformly rotating disk) is: ...

14

There certainly are quasars. Obviously, we can't see any as they are today because they aren't nearby. Quasars are a type of Active Galactic Nucleus(AGN), which means we won't find any within our local few million lightyears. However, as the Wikipedia page on Quasars will tell you, any time a supermassive black hole of a galactic nucleus gets a massive ...

13

One interesting fact is that there are some revolving structures in space that aren't mostly flat - they're known as elliptical galaxies. And the difference here is that elliptical galaxies usually don't have much gas or dust in them. Interestingly enough, the orbits of objects in the inner solar system also tend to be coplanar, whereas the orbits of the ...

13

Look at the question a different way: will the Earth get "sucking into" the sun? Answer: no, it's in orbit. Now, black holes are a little different because inside 3/2 of the Schwartchild radius there are no stable orbits, but at very large distances gravity is gravity and orbits are orbits.

13

Most of the big, bright, noticeable galaxies are either spiral or elliptical, yes. However, there are certainly irregular galaxies, and they may very well be the most numerous.1 The thing is, we can only see the very closest "dwarf" galaxies, most of which do not display a nice elliptical or spiral structure. Indeed, the two closest galaxies of note to us ...

13

I feel that exactly the opposite should be the case; that is, dark matter halo should be inside the galaxy rather than outside. Your feeling is entirely correct, and actually agrees with dark matter theories. Your only mistake is in thinking that the dark matter halo of those theories is only surrounding the galaxy; it's also inside the galaxy, and is ...

12

Observing planets in other galaxies is really hard to do because they are so far away and planets are so small. One of our closest neighbors, the Andromeda Galaxy (also called M31), is about $10^{19}$ km away (just under 780 kpc), so finding a planet the size of Jupiter (roughly $10^5$ km diameter) is pretty tough (radius to distance is very small). Even the ...

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