Today, Ansa.it released an article that states:

[...]. In questo suo peregrinare galattico, il Sole ha attraversato anche i due bracci della Via Lattea Perseo e Centauro. "Sono zone di alta densità stellare, in corrispondenza delle quali il Sole e le stelle intorno rallentano e possono anche fermarsi, [...]

(my) translation to English:

In this galactic wandering, the sun traversed the two spiral arms of the Milky Way, Perseus and Centaurus. These are regions of high density of stars, in correspondence of which the Sun and other surrounding stars slow down and can even stop, [...]

The article is complemented with a suggestive video-clip that depicts the Sun orbiting around the galaxy's center and crossing various spiral arms.

Given my lack of an education on the topic, I cannot reconcile the existence of an entity called "spiral arm" with the notion that a star can freely cross several "spiral arms" in its movement around the galaxy's center.

My intuition is that if every star within the Milky Way could freely cross the "spiral arms" several times during their life-span, then there would be no such thing as a "spiral arm" at all (because this is supposed to be a mass grouping wherein all matter moves --please forgive me the abuse of the word-- "together").

  • Where am I wrong?

  • Is the above description of the movement of the Sun accurate?

  • In case of an affirmative answer, is it a very special case or is it a defining characteristic of the entire Orion's arm?

  • In the latter case, can other spiral arms cross each other?


2 Answers 2


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 forward, but for a while, a car behind the truck will be moving slow, until it overtakes and speeds up.

Similarly, stars may overtake, or be overtaken by, the spiral arms. Inside a certain distance from the center of the galaxy called the corotation radius ($R_\mathrm{c}$) stars move faster than the arms, while outside they move slower.

Since the stars and the interstellar gas follow the rotation of the arms for a while once they're inside, the density of the arms is higher than outside, but only by a factor of a few (e.g. Rix & Rieke 1993).

When interstellar gas falls into the potential well it is compressed, triggering star formation. Since the most luminous stars burn their fuel fast, they will mostly have died once they leave an arm. Hence, what we see as spiral arms is not so much the extra stars, but mostly due to the light from the youngest stars which are still inside the arms. Since most luminous also means hottest, their light peaks in the bluish region — hence spiral arms appear blue.

Origin of the spiral structure

At least the most prominent spiral arms (especially grand designs) are thought to be created by these long-lived, quasi-stationary density waves (Lin & Shu 1964). The reason that the density waves exist in the first place is not well-understood, I think, but may have to do with anisotropic gravitational potentials and/or tidal forces from nearby galaxies (e.g. Semczuk et al. 2017).

But in fact even small perturbations may spawn gravitational instabilities that propagate as density waves. In computer simulations of galaxy formation, even numerical instabilities may cause this, so the fact that your simulated galaxy has spiral arms doesn't necessarily mean that you got your physics right.

When the luminous and hence massive stars die, they explode as supernovae. The feedback from this process, as well as that exerted by the radiation pressure before they die, may help maintaining the density waves, at least in flocculent galaxies (Mueller & Arnett 1972). Perhaps this so-called Stochastic Self-Propagating Star Formation may also initiate the density waves (see discussion in Aschwanden et al. 2018).

The rotation speed of the material in the galactic disk is roughly constant with distance from the center (this is mainly due to the dark matter halo hosting a galaxy). Hence, stars close to the center complete a revolution faster than those farther away. In contrast, the spiral pattern rotate more like a rigid disk such that, in an intertial frame, the pattern can be described by a constant angular speed $\Omega_\mathrm{p}$ throughout the disk. However, note that spiral arms are transient phenomena; they appear and disappear with lifetimes of the order of (a few) Myr (e.g. Grand et al. 2012; 2014). Sometimes you also see multiple spiral patterns propagating with different velocities.

The Sun in the Milky Way

Note: This section first contained errors based on dubious values for the angular speed of the spiral arms, as pointed out by @PeterErwin and @eagle275.

In the case of our Sun, we happen to be located very near the corotational radius $R_\mathrm{c}$; we sit at a distance of $R_0 = 8.32\,\mathrm{kpc}$ from the center of the galaxy (Gillessen et al. 2017), while $R_\mathrm{c} = 8.51\,\mathrm{kpc}$ (Dias et al. 2019).

Using Gaia data, Dias et al. (2019) find a pattern angular speed $\Omega_\mathrm{p} = 28.2 \pm 2.1\,\mathrm{km}\,\mathrm{s}^{-1}\,\mathrm{kpc}^{-1}$. At the location of the Sun ($R_0$) this implies a pattern speed of $\simeq235\,\mathrm{km}\,\mathrm{s}^{-1}$. This is only a little bit slower than (and in fact statistically consistent with) the Sun's velocity of $239\pm5\,\mathrm{km}\,\mathrm{s}^{-1}$ (Planck Collaboration et al. 2018). If the spiral arms were "permanent"*, the timescale for the Sun crossing an arm would hence be gigayears.

However, because as described above they're quite transient, we might once in a while overtake a spiral arm. I haven't been able to find firm evidence for whether we have or haven't crossed any arms; Gies & Helsel (2005) argue that we have crossed an arm four times within the last 500 Myr, but base this on matching glaciation epochs with passages through spiral arms (and admit that this requires a lower but still acceptable pattern speed).

The article you link to

I now wrote to Jesse Christiansen (who the linked article quotes) and asked her if she knows whether or not we are moving in and out of spiral arms; she replied within roughly 8 seconds, tagging Karen Masters who chimed in even faster — they both agree that this is an ongoing debate with no conclusive evidence.

Anyway, the article seems to have misunderstood the tweet from Jesse Christiansen. In her animation she shows the journey of the Sun, but shows the galaxy itself as being static, which she did on purpose to keep it simple. Hence, you see the Sun traversing the arms unnaturally fast.

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    $\begingroup$ +1 This simulation shows stars entering and leaving the denisty waves (sprial arms) youtube.com/watch?v=9B9i4vjj5D4 $\endgroup$ Commented Nov 12, 2019 at 21:04
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    $\begingroup$ @DaveGremlin Very nice illustration, but after watching it my room spins in the opposite direction. $\endgroup$
    – pela
    Commented Nov 12, 2019 at 22:41
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    $\begingroup$ youtube.com/watch?v=C8BHLgfFx7I is good too $\endgroup$ Commented Nov 14, 2019 at 16:09
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    $\begingroup$ @LightnessRaceswithMonica Interesting, yours has the spirals themselves rotating around the centre, while the earlier link has the spirals remain in a fixed position. I wonder which is correct $\endgroup$
    – JBentley
    Commented Nov 14, 2019 at 16:50
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    $\begingroup$ @JBentley -- The "simulation" video linked to by DaveGremlin isn't a real spiral galaxy simulation; it's a kludge using a Solar System simulator where the elliptical orbits are carefully aligned to produce a stationary spiral pattern. The second video (based on an actual N-body simulation) is much more correct. $\endgroup$ Commented Nov 14, 2019 at 17:59

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 super nova - And as the other answer tells - likewise the sun after forming never again was close to a region where new stars form in gas clouds.

After what I learned the sun co-rotates at the same speed as the spiral arms around the center bulge of the milky way. So overtaking in the grand scheme of things seems unlikely - although most stars have a distinct movement direction that make them slowly change their position even though we call them fix stars - So over long periods of time our view of the closer stars changes

As you ask for sources and references .. I base my lines on several presentations by german theoretical astrophysicist Prof. Dr. Harald Lesch from Munich university

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    $\begingroup$ Yes, our Sun happens to lie quite close to the corotation radius (there might be some entropic argument of whether or not this is a coincidence). I don't know about whether or not we crossed any arms (but as I understand from the article, they might have misunderstood the animation they show, where the arms are stationary); do you have any references for the evidence? $\endgroup$
    – pela
    Commented Nov 13, 2019 at 14:52
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    $\begingroup$ I don't think this is true, see e.g. Gies & Helsen 2005. $\endgroup$
    – pela
    Commented Nov 13, 2019 at 16:45
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    $\begingroup$ "On the other hand the found materials (minerals / isotopes ) show clear evidence that the sun did NOT cross those spiral arms." According to what/who? And according to what/who is it that crossing a spiral arm is inconsistent with never having been closer than 30ly to a supernova? Basically what Pela asked for - references. $\endgroup$
    – Beanluc
    Commented Nov 14, 2019 at 2:22
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    $\begingroup$ May I ask which presentations? I went through Harald Lesch's publication record (it's not too long) and I didn't find anything related to this (I'm not trying to debunk your answer, I'm just interested). $\endgroup$
    – pela
    Commented Nov 14, 2019 at 11:09
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    $\begingroup$ Hi @eagle275, I downvoted your answer because I doubted your statement of "clear evidence", which you didn't provide. I still do, but I understand now that there is no evidence for the opposite either. I'd like to remove my downvote, but can't unless you make an edit, so if you do that, I'll remove :) Also, thanks for questioning my calculation — it was indeed wrong or at least based on wrong/dubious data. $\endgroup$
    – pela
    Commented Nov 15, 2019 at 20:57

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