Hot answers tagged

84

You shouldn't think of black holes as "sucking things in". Black holes interact with matter through gravity, just the same as any other object. Think of our Solar System. All the planets orbit around the sun because it has a lot of mass. Since the planets have some lateral motion (they're not moving directly towards or away from the sun), they circle around ...


77

Does the Milky Way move through space? Yes it does. I'm very fascinated with space, although I don't have a degree or any formal education, I'm still very in love with everything about it and want to learn constantly. Good man Mike. One thing I ask myself is if our galaxy moves through space? It does. When we look at the Cosmic Microwave ...


58

The object which has less mass and gravitational pull orbits around the nearest object with more mass and gravitational pull. Actually, both the heavier and the lighter object orbit around their common center of mass. It's just that the heavier object doesn't move much (has a tiny orbit), while the lighter object moves a lot (has a wide orbit). E.g. our ...


51

There are two reasons that often — but not always — light from galaxies millions and even billions of lightyears away make it through the Universe and down to us: Particle number and particle size First, the intergalactic medium (IGM) is extremely dilute. The number density of particles out there is of the order $n\sim10^{-7}\,\mathrm{cm}^{-3}$, or roughly ...


38

Galaxies move through space with velocities of the order of a several 100 km per second; small velocities for small groups (~100 km/s; e.g Carlberg et al. 2000) and large velocities for rich clusters (~1000 km/s; e.g Girardi et al. 1993). In addition to this so-called "peculiar velocity", galaxies also also carried away from each other due to the expansion ...


31

There're several pieces of information one needs to understand this. Although stars more massive than 70 solar masses exist, when they become black holes, they usually lose mass in the process. The exact amount of mass lost depends on the metallicity (which is a technical term that describes how much "metals" - the astronomer's definition of metals is ...


31

M33 does not appear to contain a supermassive black hole: in fact there's no evidence that it contains a central black hole at all. The upper limit on the mass of a central black hole based on the dynamics of the core region is a few thousand solar masses. Merritt et al. (2001) "No Supermassive Black Hole in M33?" derive an upper limit of 3000 solar masses ...


26

Presumbably we rotate beacuse of the BH. No. The galaxy is being held in one piece due to its own total gravity. The black hole is only a small fraction of that. Basically, the BH doesn't matter. When the black hole dies in our galaxy The BH will probably be the last thing left of our galaxy at the end. And even then it will take some incredibly long ...


26

The low supernova rate in M31 can be directly attributed to the fact that the galaxy's star formation rate is much lower than the Milky Way's. Andromeda is currently in a relatively quiet phase in terms of star formation, currently experiencing rates of $\sim0.40M_{\odot}\;\mathrm{yr}^{-1}$. The Milky Way, on the other hand, has a star formation rate ...


24

Here's a rough sketch of the Milky Way and the Andromeda galaxy,* showing their approximate sizes and distance from each other to scale:   What the picture (hopefully) illustrates is the incredibly vast gap of empty space — around 2.5 million light years, to be exact — between the galaxies, each of which has a diameter of only(!) around ...


24

As Rob Jeffries says, the universe is mostly empty space. A photon can easily travel thousands of light years without interacting with anything. Most of the interaction would occur when photons entered the earth's atmosphere. The Hubble avoids this. These photos were most likely from combining several viewing sessions giving basically an extended time period ...


23

What will happen to life on earth or human beings on earth? Assuming that human beings, or life, still exists on Earth at that time, they will have survived so much due to the ongoing death of the sun, that the gravitational pertubations due to the galactic collision will be nothing. Keep in mind that in about 1-2 billion years, the sun will be so hot and ...


23

There's a misconception in your question I don't think the other answers have addressed. If light emitted from the galaxy travels in all directions, then how is it that we can still map out the entire galaxy Light is emitted from the galaxy in all directions. Only a tiny, tiny fraction of it is directed to Earth, and of that, an even tinier fraction ...


22

Humphreys & Larsen (1995) suggest, using star count information, a distance of $20.5 \pm 3.5$ pc above the Galactic plane; consistent with, but more precise than the Bahcall paper referred to by Schleis. Joshi (2007) is more guarded, investigating some systematic uncertainties in the estimation techniques and ends up with distances between 13 and 28 pc ...


22

Answer: Not much The Milky Way's central black hole (BH) masses about 5 million suns, while the galaxy masses 100 billion to a trillion suns. Consequently, the central BH is pretty much irrelevant to the dynamics of stellar orbits except very close to the center. But what do you mean by "the black hole dies"? Do you mean evaporates through Hawking ...


21

The Milky Way Galaxy is a large spiral galaxy with some characteristic features worth mentioning: 1) The bulge - This refers to the collection of tightly packed stars located in the central region of the galaxy. 2) The spiral arms or the disk - This region extends from the inner region of the galaxy (where it meets the bulge) to outskirts of the galaxy, ...


21

Surely if you stared long enough, the light from them would eventually hit your eye? Collecting light over a long span of time is how telescopes can see very dim objects. The human visual system doesn't work that way. For one thing, even when you think you are staring at something, your eyes still dance around a bit. It's a built-in response called ocular ...


20

Dark matter, is just a name for something we know nothing of. It was named to account for an extra gravity source for which there have been indirect observations, but yet we cannot explain. The force of gravity exerted by light is negligibly small yet we have measured the gravitational pull of Dark Matter to be big enough to affect whole galaxies; it is ...


20

Actually, the stars and nebulae that make up the spiral arm are only temporarily part of that spiral arm. Spiral arms are more like sound waves where individual particles move around a more or less stationary position. (Look for instance at the animation of longitudinal waves from Dan Russel, the red dots move a bit to the left and to the right around a ...


20

No. In fact the opposite is the case. (See the last paragraph for an intuitive explanation.) It is a common misbelief that galaxies receding faster than the speed of light are not visible to us. This is not the case; we easily see galaxies moving at superluminal velocities. This does not — as I think most people would think — contradict the theory of ...


19

4 billion years is the same timeframe of the life remaining to our Sun. So if we have not yet invented interstellar voyages, we're screwed, with or without Andromeda. Besides, stars do not interact directly with each other in a galactic collision. What we will notice from the several stars we are on is that star orbits around the galactic centre will be ...


19

In short, you can't make a galaxy-mass ball of iron because there's no way to support the ball against the inward crush of its own gravity. First, it would collapse into a neutron star, but even neutron stars can't hold themselves up beyond about 2-3 solar masses. Your galaxy-sized object would collapse into a black hole. But we do see these! All massive ...


18

A quasar is simply an active galactic nucleus (AGN) that is viewed from a particular angle; see the picture below, in which quasars are labeled "QSO". This is really a remarkable figure because historically all of the names in the figure were thought to correspond to different types of objects, when really they all refer to the same thing! Your question ...


18

First, note that by the time Andromeda is close enough for collisions with wandering stars to become a concern, Earth's average temperature will have changed significantly, and the planet will be unrecognizable. When Sol is 8.5 billion years old, it will still have hydrogen available for fusion, but as it fuses it contracts and expands differentially. The ...


18

The terms refer to the viewing angle, i.e. from which direction do we observe a (disk) galaxy. If we happen to be located roughly in its plane of rotation, we see it from the "edge", whereas if we are more or less above or below its plane, we see its "face". To make a more quantitative statement, the orientation is described by the inclination angle $i$, ...


18

When a galaxy recedes from us, the light we see from it is redshifted. For galaxies at cosmological distances, this redshift is fundamentally different from a Doppler shift; whereas the latter is due to a velocity difference between the emitter and the receiver, a cosmological redshift is due to photons traveling through an expanding space$^\dagger$. Hence, ...


16

What you could think at first, regarding the orientation of any planetary system, is that it should be roughly in the plane of the galaxy, simply by angular momentum conservation. But, when you take a look at observations, you see that protoplanetary disks orientation is not what you would expect, with no preferential orientation (protoplanetary disks are ...


16

The tilt of our solar system (or any star system) is determined by the net angular momentum of the gas cloud from which it formed. This might be a bit of a vague answer, but over time, the formation of stars and their respective planets is thought to look something like this: Other influences (net forces: maybe nearby massive objects, or other components of ...


16

Dark Matter Your understanding of dark matter isn't bad, but here's a few clarifying details. Orbits: The speed of an object's orbit is related to 2 things: the radius of its orbit and the mass inside of it. In the solar system, over 99% of the mass is concentrated at the centre, so radius is the dominant effect on orbital speed. As we look at planets ...


16

I just want to add pictures to augment pela's answer. Just google "edge on galaxy" and "face on galaxy" to find results like these, it is entirely about the viewing angle. Example of a face on galaxy (Messier 74) Example of an edge on galaxy (NGC 891)


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