# Tag Info

83

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 ...

36

In a Newtonian/Galilean world, where $c$ is infinite, you could not escape Olbers' paradox with an infinite universe. Any line of sight would eventually intersect the surface of a star, and so the whole sky would be as bright as the Sun. This is true whenever two hypotheses are satisfied: The universe is spatially infinite (or rather, the distribution of ...

16

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. ...

15

In an "ordinary" gas of protons and electrons, nothing would happen - we call that ionized hydrogen! However, when you squeeze, lots of interesting things happen. The first is that the electrons become "degenerate". The Pauli exclusion principle forbids more than two electrons (one spin up the other spin down) from occupying the same momentum eigenstate ...

14

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

In vacuum $$\nabla \times \vec{B} = \frac{1}{c^2} \frac{\partial \vec{E}}{\partial t} = 0$$ so a changing E-field does not beget a changing B-field. Larmors formula for radiation from accelerating charges also has $c$ in the denominator. Therefore no (star)light at all ? [Or at least no electromagnetic waves].

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 ...

6

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 ...

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 $M>0.1$ ...

6

The stellar mass distribution is the distribution of numbers of stars within a range of masses in a galaxy (or cluster or what have you), not the mass of the stars. So if you looked at the $\sim10^{11}$ stars in the galaxy, you would observe that about $4\times10^{10}$ of them will have a mass less than 0.25 $M_\odot$, and so on with the rest of the masses. ...

5

Terrific question. You had it right in your first sentence: “the same amount of energy must have been released during the Earth's history,” but then it gets a little mixed up when you look at various energies, some of which aren’t related to the question at hand (for example, the current internal energy contributes positive mass-energy to the Earth, rather ...

5

Changing c to infinite changes some important things. The actual effect depends on how you want to propose magnetic forces work (they're normally fictitious forces induced by relativity). If we assume the coupling constant (this constant doesn't appear in the equation as it's value is normally 1) goes to infinity as c goes to infinity so that magnetostatics ...

5

The CMB (cosmic microwave background) is a snapshot of the oldest light in our Universe, imprinted on the sky when the Universe was just 380,000 years old. It shows tiny temperature fluctuations that correspond to regions of slightly different densities, representing the seeds of all future structure: the stars and galaxies of today. The anisotropies ...

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 ...

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 Einstein equation says: $${\bf G} = {\bf T}$$ where $\bf G$ is the Einstein tensor that describes the curvature, i.e. the gravity, while $\bf T$ is the stress-energy tensor. So the origin of gravity is the stress-energy tensor. This is typically dominated by mass, but includes less obvious contributions like pressure and momentum. Actually solving ...

3

For an object close to you, the speed of light is effectively infinite - i.e. the time taken for the light bulb 10m away from you to get to you is so close to zero that it can be considered immediate, and thus the speed of light is assumed to be infinite. With this in mind, this would mean that the sky would be brighter. In reality, the speed of light is a ...

3

Yes the energy $u$ stored in a field $B$ in a region with permiability $\mu$ is given by: $$u = \frac{1}{2}\frac{B^2}{\mu}$$ So if you double $B$ then $u$ gets quadrupled and if you increase $B$ by a factor of $10^{10}$ then $u$ increases by $10^{2\times 10} = 10^{20}$. I'm not quite sure about the assumptions that go into the above formula however (I'll ...

3

According to this source, 100% is the number of stars, not the total mass. Same from another source. The reason is that they usually calculate these pies straight from the H-R diagram. The H-R diagram plots individual stars and shows how stellar mass varies along the main sequence. Actually the mass distribution tends to reverse. Even if larger stars are ...

3

The answer is that 41% of the stars have masses below 0.25$M_{\odot}$. To check this I integrated the Kroupa initial mass function. This is that $N(m)$ the number of stars per unit mass is proportional to $m^{-1.3}$ for $0.08<m/M_{\odot}<0.5$ and proportional to $m^{-2.3}$ for higher masses. If I integrate this I find that the ratio of stars with ...

3

White dwarfs with strong magnetic fields ($>$1MG) make up only about 10 per cent of the population of white dwarfs. A further few per cent have fields in the 10-1000 kG range (e.g.Liebert et al. 2003). So it is not clear that the Sun will end up as a "magnetic white dwarf" at all. The production of magnetic white dwarfs is thought to arise via at least ...

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" ...

3

There are several commonly used analytic approximations for the initial (birth) mass function (IMF) that cover both stars and brown dwarfs. It is not yet absolutely certain which of these is more correct at the low-mass end, whether there is a lower mass cut-off as one approaches planetary masses, or whether the fraction of brown dwarfs (BDs) to stars varies ...

3

Ironically, it's actually harder to measure the mass of the Milky Way than that of other galaxies. You'd think that with it being RIGHT THERE it would be easy, but alas. Most of the difficulty comes from (1) the galaxy spans a huge part of the sky, so it takes an extremely long time to observe any particular feature in detail across the whole thing (say ...

3

This is difficult to answer in an unarguable way because the old bimodal classification of population I and II is more nuanced these days - e.g. thin disk, thick disk, bulge population etc. However, if you define population II as meaning those stars that were born in the first billion years of our Galaxy's evolution, then the following rough calculation ...

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 ...

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

In another closely related question (According to the initial mass function, should there be more brown dwarfs than red dwarfs? ), I showed that the number of brown dwarfs (with $M<0.075M_{\odot}$) is a factor of five smaller than the number of red dwarf stars (stars with $0.075<M/M_{odot}<0.5$), using the widely adopted Chabrier (2005) lognormal ...

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