# Tag Info

## Hot answers tagged astrophysics

80

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 ...

15

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. ...

13

Every galaxy has to rotate 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. For there to be rotation however, there needs to be an axis, a line about which all matter revolves in the galaxy. Now, the manner in which all the matter revolves ...

11

If it was possible to reflect the energy back at the sun, yes, the location where the energy strikes will become hotter. If fact, if you could insulate the sun from radiating energy, then the sun would get even hotter.

11

LDC3 and Kitchi addressed your main question, but I'd like to comment on your second paragraph. Isn't this something like a machine is running, I am getting work out of it and supplying back to the machine to 'accelerate' it? I am not expert in physics, but intuitively thinks that this may not be possible. Actually, we do this all the time! Electricity ...

9

It is due to the combined effect of rotation and "dissipation". A rotating cloud of gas consists of particles which interact strongly with each other (colliding physically) on relatively short timescales can radiate away some of their energy and momentum by emitting photons. For both of these reasons, a dense cloud of rotating gas will collapse to form a ...

8

There are many problems with this line of reasoning. The most common galaxy types are elliptical galaxies and spiral galaxies, and there might be a parallel with star systems, where the most common types are systems with a single star, and binary systems with two stars in the middle. There is simply no justification for this. The dynamics of stellar ...

7

The route to the answer is somewhat anti-intuitive. By reflecting some of the Sun's energy back towards the sun at a point you are effectively reducing the flux of energy that can emerge from the photosphere and escape. The global effect of this on the Sun must be similar to that of blocking the flux at the photosphere - in other words, similar to the ...

6

First, a star does not become a red giant when helium fusion begins, instead it becomes a red giant earlier when an inert degenerate core of helium forms and a shell of hydrogen begins fusion. When shell hydrogen fusion begins, the star expands to be a red giant. The core is degenerate (sustained from collapse by electron degeneracy pressure) and ...

6

Light always travels at the speed of light when in a vacuum. Space is a pretty good vacuum. So if it's been travelling for 13.7 billion years, then it has travelled 13.7 billion light years. There is no contradiction here. Yes, those galaxies are now 46 billion light years light years away, but this is because the universe has expanded. You can find lots ...

6

It turns out that it is the distribution of birth stellar masses and most importantly, the lifetimes of stars as a function of mass that are responsible for your result. Let's fix the number of stars at 200 billion. Then let's assume they follow the "Salpeter birth mass function" so that $n(M) \propto M^{-2.3}$ (where $M$ is in solar masses) for $M>0.1$ ...

5

Measuring the star formation history of the Universe is a key component in understandng the evolution of galaxies. It is closely related to the other uestion, recently asked by the same person: Are stars getting more metal-rich, less massive and shorter-lived with cosmic time? Although this question pertains to the Universe as a whole, an understanding ...

5

You mentioned elliptical galaxies, which the other answers haven't touched upon. Contrary to your statement about the galaxies being 2D, elliptical galaxies are "3 dimensional" in the sense that the stars are not confined to one plane; You could think of them as being "egg shaped". So why are elliptical galaxies not confined to a plane? Mostly because they ...

4

I think the following image sums up why your model, at least for our galaxy, is wrong rather nicely: These are the orbits for 6 stars in the inner region of the galaxy. The orbital period for S2, for instance, is 15 years for an orbit that is roughly twice the size of Sedna's orbit--which takes it 12 thousand years to complete its orbit. Using Kepler's ...

4

The simplest answer is that in order to maintain helium fusion, a certain pressure and temperature are necessary. Therefore, given the fact that helium fusion is occurring in the core, and the mass pushing down on the core is X from dynamic concerns, you therefore must conclude that the temperature in the core is Y, irrespective of the size of the core.

4

The specific heat capacity of mercury is 140 J/kg/K. Let's suppose you have about 1cc of mercury in your thermometer; let's assume this isn't going to boil and your thermometer isn't going to melt and let's suppose the mercury is in the form of a cube which is 1cm on a side. The density of the mercury is about 7.6 g/cc, so you have 7.6$\times10^{-3}$ kg of ...

3

You are confused. When the star expands to become a red giant it is burning hydrogen in a shell around an inert He core. How things proceed from there depend on the mass of the star, but generally speaking, the core contracts and heats up to maintain pressure and hydrostatic equilibrium. The shell moves inwards, heats up and the nuclear burning rate ...

3

There won't be a violation of thermodynamics because you are not creating energy from nothing. The total energy of the system is still conserved, it is just fed back into the system. Here's what will probably happen - The concave mirror will not be perfectly reflecting, so will reflect something like $99\%$ of the incident energy. This energy (although very ...

3

it can be said that the γ photon loses energy and also its velocity This isn't a good description of what happens. In a material like plasma that contains free electrons photons are absorbed and their energy converted to kinetic energy of the electrons. The electrons then collide with, or otherwise interact with, other electrons and reradiate photons. ...

3

An ADS search for "star formation" turns up about 142,000 articles with "star formation" in the title or abstract. The first article is a 43 page review paper of Star Formation in Galaxies in the Hubble Sequence, written by Robert Kennicutt, Jr, one of the leaders of the field. He never defines anything else to mean star formation and one of the "key words" ...

2

I think the plot you show is the estimated abundance of the interstellar medium from which the Sun has formed. The chemical abundances of the interstellar medium change with time, so you have to define some point in time at which to estimate them. As the initial chemical abundance in the Universe is basically H, He, with traces of D, Li and Be, then it ...

2

There are several reasons. One is that when a cloud of gas and dust collapse into a star forming region, it becomes unstable to gravitational fragmentation and usually forms filamentary structures. The gas that lies outside of the densest regions is often not dense enough to be itself then gravitationally unstable. This behaviour is clearly shown in modern ...

2

I asked the question because I did not believe in the accepted answer that has been sitting for more than 3 years. I have my own understanding, but since it is not good practice to put it with the question, I am posting it as one possible answer. My problem is that I do not believe the first statement quoted in the question which is contradicted by the ...

2

There is an angular momentum problem with regard to star formation, but you have the sense of the problem completely backwards. The problem is not where the angular momentum arises. The problem is where does it go. Gas clouds a tenth of a parsec across have been routinely been observed to rotate at about one revolution every five or ten million years or so ...

2

You could start from the premise that there was not net angular momentum in the universe at all; but it would still be the case that everything of interest was spinning. On the scales of stars and planets there are (at least) two important mechanisms that result in individual systems having angular momentum. The first is turbulence. If you take a parcel of ...

2

Explanation: Photons, quanta of light always travel at the speed of light: $c$, since you are talking about light, classical kinetic energy equation: $E=\frac{mv^2}{2}$ no longer applies to it, instead you have to use this relativistic energy equation: $$E^2=(m_0c^2)^2+(pc)^2\tag{1}$$ since photons don't have rest mass i.e. their rest mass equals to zero: ...

2

Probably the most direct example is synchrotron radiation. This is the case in which an electron is accelerating by moving in along a curved path (e.g., a helix). As it is accelerated, it emits photons in the radio spectrum: (source) Another big one would be bremsstrahlung in which an electron moving along a path is decelerated near the presence of a ...

2

Nobody knows. That's it, in a nutshell. However, there are some various ideas floating about. Here's a (long) list of some: From this page: Collapse of massive gas clouds Merger of lots of stellar-mass black holes Growth of a stellar-mass black hole to astronomical (pun intended) proportions From Wikipedia: Core collapse of a cluster of stars ...

2

The luminosity of the Galaxy is currently estimated to be around $5\times10^{36}$ W and thus an integrated "mass loss" in the form of radiation of of order $10^{-3} M_{\odot}$/yr. But how much radiation is present in the Galaxy? An order of magnitude estimate could be that the Galaxy (including the dark matter) is of order 100,000 light years in radius and ...

2

Yes, your qualitative argument is correct and the number of stars brighter than the Sun is almost certainly much smaller than 10 billion. The reason is that the luminosities are hugely variable. Due to the mass-luminosity relation, each doubling of the stellar mass corresponds to increasing the luminosity 10 times. So many if not most of the "stars ...

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