# Tag Info

25

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 ...

20

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 ...

19

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 ...

17

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 ...

15

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, ...

14

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 ...

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

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 ...

14

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 ...

13

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 ...

11

There are two elements to why the universe appears to be so orderly: the physical laws of that govern the universe are the same everywhere, and astronomical objects are very, very, very far from each other. Consider two objects, one much larger than the other, and both very far from anything else. Because of gravity (which works the same everywhere), the ...

11

If you have a properly aligned telescope with good setting circles, you can easily use the RA and Dec of the galaxy to locate it (or any other deep space object you have the coordinates for). However, many times you don't have those properly dialed in or you're using an alt-az telescope mount (like a Dobsonian telescope) and need another way to find your ...

11

The idea of the existence of galaxies is certainly not new, and quite a bit older than the field of modern astrophysics. In 1750, Thomas Wright, an English astronomer correctly speculated that the Milky Way was a flattened disk of stars and that some of the nebulae astronomers viewed in their telescopes were separate "Milky Ways". In 1755 Immanuel Kant ...

10

Yes. Since the gravitational force is long-range, one star traveling through a star field tends to leave behind a slightly denser "wake" of stars that moved slightly towards its previous locations. This increased density acts to pull backwards on the fast-moving star, creating a sort of fluid friction, or viscosity. The Virial Theorem is based on a ...

10

The simple answer is that the average galaxy spacing is around a few megaparsecs, while the biggest galaxies are around 0.1 megaparsecs in size. So the average spacing is somewhere in the range of 10 - 100 times the size of the biggest galaxies. The peas I had for lunch today were (at a guess - I didn't measure them!) 5mm in diameter so the interpea spacing ...

10

The spiral arms don't mean that the mass is getting sucked to the center. They're just wave-like density patterns. The bodies in orbit around the center of the galaxy are in stable orbit; just like the Earth around the Sun and the Moon around the Earth. What happens is that gravity accounts for the centripetal force (in the orbiting frame, gravity is ...

9

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. ...

9

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.

8

"The Milky Way Galaxy" is indeed the correct name for our Galaxy in western culture, the name "Milky Way" comes from the Latin Via Lactea (translated from Greek). The name varies from culture to culture, for example the Chinese use "Silver River" to represent our Galaxy. Wikipedia has a good list of name for the Milky Way.

8

It takes millions of years for galaxies to collide so even though they're mainly gas and dust they have a weak force of attraction. Also as you mentioned in your question they have a super massive black hole in the middle that pulls them together and drags the rest of the galaxy with them. It's also incredibly unlikely that even one of the stars will ...

8

According to this link (now dead): In the solar neighborhood, the stellar density is about one star per cubic parsec (one parsec is 3.26 light-years). At the Galactic core, around 100 parsecs from the Galactic center, the stellar density has risen to 100 per cubic parsec, crowded together because of gravity. So we'd see about 100 times as many ...

8

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 ...

8

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 ...

8

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: ...

8

A black hole is "just" a massive object. Interesting things happen when close to the black hole, because the high gravity makes all the Einsteinian effects more apparent, including the "horizon" and the trapping of light, and so on. But from afar, this is "just" a massive object, which other objects handle like any other, i.e. by orbiting it. If you replace ...

8

The velocity calculation was distance*(change in angle). However, this does not take into account the changing time-delay of light: we see it sped-up because the time delay is decreasing, like a TV recording where you are fast-forwarding as you gradually catch up with real time. Fortunately, all we need to do to calculate the real speed is to account for the ...

Only top voted, non community-wiki answers of a minimum length are eligible