# Tag Info

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What is a spiral arm? The reason that the Sun, in principle (but see below), may cross spiral arms is that galactic spiral arms are not rigid entities consisting of some particular stars; rather they are "waves" with a temporary increase in density. An often-used analogy is the pile-up of cars behind a slow-moving truck: At all times, all cars are moving ...

10

The answer to the question depends on precisely where you are in the Milky Way. According to density wave theory, which appears to describe the formation and evolution of spiral arms quite well, the arms move relative to some static inertial frame at a global pattern speed $\Omega_{gp}$. At some defining radius $R_c$, known as the corotation radius, stars ...

6

The Milky Way has arms that form due to density waves. Like the majority of spiral galaxies, the arms are trailing. Individual stars orbit in circles (roughly), neither towards or away from the centre. If you consider a common map of the Milky way (imagined from a point North of the Earth, Celestial North is not the same as Ecliptic North, which are both ...

5

If stars were actually about to escape, they would do so on a so-called dynamical timescale: $$t_\mathrm{dyn} \sim \sqrt{\frac{R^3}{8GM}},$$ where $G$ is the gravitational constant, and $R$ and $M$ are the radius and the mass of the galaxy, respectively. For a Milky Way-sized galaxy, this is equates to roughly 30 million years$^\dagger$. But disk galaxies ...

4

"Galactic bars develop when stellar orbits in a spiral galaxy become unstable and deviate from a circular path. The tiny elongations in the stars’ orbits grow and get locked into place, forming a bar. The bar becomes even more pronounced as it collects more and more stars in elliptical orbits. Eventually, a high fraction of the stars in the galaxy’s inner ...

4

The short answer is: yes, magnetic fields in spiral galaxies are generally spiral in orientation. (But it probably isn't directly correlated with your solar model.) For example, this is a map of radio polarization (a reflection of magnetic field direction) in the galaxy IC 342 by Beck et al. (2015): (The short yellow lines are indicators of polarization, ...

3

Take a look yourself! Below is an image detailing the Milky Way's magnetic field structure as measured from the Planck satellite (and thus necessarily from within the galaxy). Although of course this has some projection effects going on and is integrated along the line of sight, so it's not the same as a view of another galaxy or as a 3D model, but it gives ...

3

We know how long stars take to orbit in a spiral galaxy, because we know how big galaxies are (we can measure the size by knowing the distance using various methods such as Cepheid variables, and type Ia supernovae) and we know how fast the stars are moving. (We can measure the velocity of stars using the doppler method) We know how long stars last for ...

2

On the other hand the found materials (minerals / isotopes ) show clear evidence that the sun did NOT cross those spiral arms. Spiral arms as areas with higher star density pose the threat of super novae in close proximity. All we have found on earth leads to the conclusion that the earth - and with it the sun - never were closer than 30 light years to a ...

2

Yes. Antoja et al. (2011) created two models: The Tight-Winding Approximation model (TWA) and the PERLAS model. They decided to use just the $m=2$ term for their TWA, based on data from the Spitzer Space Telescope. The amplitude $A(R)$ was $$A(R)=A_{sp}Re^{-R/R_{\Sigma}}$$ Their $\phi_2(R)$ (denoted $g(R)$ in the paper) was of the form g(R)=\left(\frac{2}{...

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