At the end of my previous post on this topic, I left you with a diagram of the solar system’s orientation and approximate trajectory in its orbit around the Milky Way galaxy. Below, we’re looking past the solar system towards the galactic core. The plane of the galaxy runs horizontally across the image, north is at the top, and the solar system is travelling around the galaxy in a clockwise direction when viewed from galactic north. For more on all that, consult my first post via the link above.
This time, I want to give some more detail of the exact direction in which the solar system is moving, to the extent that this is known. And this will bring me to my gripes about Wikipedia‘s discussion of the topic, as I promised last time.
Although my arrow marks the general direction followed by the Sun and other stars in its vicinity, in practice this movement is reminiscent of a swarm of bees—they’re all heading in the same direction, but they’re all moving relative to each other as well. Astronomers find it useful to break this orbital motion down into two components—they separate out the general trend of motion from the smaller random relative movements.
To do this, they invoke something called the Local Standard of Rest, which (somewhat paradoxically) is actually in motion around the galaxy. It applies only to our local solar neighbourhood, and it moves at a speed such that a star at rest in the LSR would follow a circular orbit around the galaxy. There’s some uncertainly about the LSR’s velocity, which depends on getting an accurate measurement of the Sun’s distance from the centre of the galaxy, among other things. But it seems to be somewhere around 220 km/s.
Within the LSR, individual stars have their own random motions, typically of a few kilometres per second—these small deviations from the perfect local circular velocity mean that each star is following its own, slightly elliptical, slightly tilted orbit around the galaxy. The relatively small additional velocity component of each star is called its peculiar motion—“peculiar” because it’s unique to that star, not because it’s odd in some way.
Here’s how it fits together:
While stars have very different peculiar motions within the LSR, they’re actually all heading in pretty much the same direction around the galaxy.
The Sun has its own peculiar motion, too. If we add up all the peculiar motions of stars in our local neighbourhood, they don’t average down to zero—the residual velocity is the result of the Sun’s motion relative to the other stars. In practice, this is tricky to work out—we have to sample only stars that belong to the “thin disc” of the galaxy, which share similar orbits with the Sun, and also allow for some complicated dynamics that I won’t go into here. Because of this trickiness, estimates of the Sun’s peculiar motion vary considerably. But my twenty-year-old copy of the fourth edition of Allen’s Astrophysical Quantities gives some fairly typical values—9 km/s towards the galactic centre, 7 km/s towards galactic north, and moving around the galaxy 12 km/s faster than the LSR—giving it an overall velocity (relative to the LSR) of about 16.5 km/s directed towards a point near the star Mu Herculis.
This point, marking the direction in which the Sun is moving relative to the other stars in its neighbourhood, is called the solar apex. (The astronomer William Herschel called it “the apex of the Sun’s way” in the eighteenth century, and you’ll still see that phrase floating around the internet.) There’s a solar antapex, too, on the other side of the sky in the constellation Columba.
Over the years, the constellation of Hercules has become pretty much peppered with solar apex candidates, as each new study samples different stars and makes different dynamical allowances. But here’s the thing to remember about the solar apex—it’s the point towards which the Sun is moving relative to our local stars. But all those local stars, and the Sun, are also whooshing around the galaxy, with the Local System of Rest, at 220 km/s. So the solar apex is not the Sun’s direction of travel in its orbit around the galaxy. As my diagrams above show, the Sun’s peculiar motion will slightly modify the velocity it inherits from the LSR, but not by much.
Which brings me to my first gripe about the Wikipedia page I mentioned in my first post. (Because Wikipedia changes from time to time, my link goes to a copy of the relevant page capture on 19 October 2023, just so you can see what I’m talking about.) Here’s the vexatious text:
The apex of the Sun’s way, or the solar apex, is the direction that the Sun travels through space in the Milky Way. The general direction of the Sun’s Galactic motion is towards the star Vega near the constellation of Hercules, at an angle of roughly 60 sky degrees to the direction of the Galactic Center.
Well, no, the solar apex is very much not “the direction the Sun travels through space in the Milky Way”, or “the general direction of the Sun’s Galactic motion”. Nor is the solar apex particularly near Vega, though Vega is easier to pick out, for the uninitiated, than is Hercules.
Once we factor in the relatively high velocity of the Local Standard of Rest, the Sun is heading pretty much at right angles to the direction of the galactic centre. Here’s a map of the relevant bit of sky, with significant locations superimposed on a base map from In-The-Sky.org:
Around the big label “SOLAR APEX” in Hercules, I’ve dotted a few estimates of the solar apex from different sources—my old Allen’s Astrophysical Quantities (2000), Schönrich et al. (2010) and Ding et al. (2019). The grey horizontal line marks the plane of the galaxy, and the black cross is at 90 degrees galactic longitude—the direction in which the Sun would be headed if it were at rest in the LSR, in a perfectly circular orbit around the galaxy. The point marked “True direction of motion” is what happens when we added 220 km/s of tangential velocity to Ding’s solar apex—the large orbital velocity dominates over the relatively small northward and inward components of the Sun’s peculiar motion. So forget Vega—we’re headed towards Deneb, in the constellation Cygnus.
But, you’ll see, we’re heading slightly inwards and rising northwards out of the galactic plane as we go. Back in 1986, the astronomer Frank Bash wrote a chapter in the textbook The Galaxy And The Solar System, entitled “The Present, Past and Future Velocity of Nearby Stars: The Path of the Sun in 108 Years”, in which he used the current orbital motion of the Sun to predict its future movement around the galaxy. In astronomical terms, this is a positively prehistoric reference, but Bash’s calculations are still commonly quoted, despite the fact our understanding of the galaxy has moved on considerably since he wrote. And I haven’t been able to find a more recent reference. What follows is therefore probably accurate in its broad picture, but wrong in the details.
So (Bash writes), the Sun is moving gently inwards, but will reach its closest approach to the galactic centre (perigalacticon) in 15 million years’ time, at which point it will be at about 99.5% of its current distance. It will then move outwards, reaching a maximum distance (apogalacticon) of about 114.5% after completing half an orbit of the galaxy—so about 135 million years from now. And it’ll continue to move between those extremes, tracing out a rosette pattern around the galactic centre, unless it has some sort of catastrophically close encounter with another star.
Meanwhile, its velocity will loft it northwards at a steadily slowing rate until (14.6 million years from now) it reaches a peak at about 77 parsecs (250 light years) above the galactic plane, after which it will fall back to, and through, the galactic plane, making an excursion to a similar distance on the southern side. And so on. Bash calculated that a full cycle would take about 66 million years. The exact number of north-south oscillations per galactic orbit depends on the vertical mass distribution within the plane of the galaxy—Bash’s figures suggest about 3½ cycles per orbit, but you’ll see other figures quoted.
And that concludes my post about the movement of the solar system within the galaxy. But I now need to briefly return to that annoying Wikipedia page, to highlight another bit of misinformation. Having just assured us we’re all heading for Vega, the page goes on to contradict itself:
The Solar System is headed in the direction of the zodiacal constellation Scorpius, which follows the ecliptic.
Scorpius is a long way from Vega. It’s aligned with the centre of the galaxy—in fact, you can see it in the background of my diagram of the solar system’s orbital motion, above—pretty much at 90 degrees to the Sun’s real direction of orbital travel. Where on Earth does that piece of misinformation come from?
It comes from a 2011 article in National Geographic, headed:
Solar System’s “Nose” Found; Aimed at Constellation Scorpius
A NASA craft has uncovered the solar system’s “nose,” which points in the direction our sun is moving through the Milky Way, a new study says.
The study cited actually says no such thing—the whole bit about the “direction our sun is moving through the Milky Way” seems to be a bit of improvisation by the journalist who wrote the National Geographical article.
This is all about the shape of the heliosphere, which is defined by the location of the heliopause—the surface at which the solar wind of particles radiating outwards from the Sun runs into the gas and dust of the interstellar medium. As the Sun ploughs through the interstellar medium, the heliosphere experiences an opposing drag, and develops a “nose” and a “tail”, like this:
The “nose” of the heliosphere therefore points in the direction from which the interstellar medium is flowing past the Sun—and that “nose” is aimed at the constellation Scorpius. But if the solar system were actually “moving through the Milky Way” in that direction, we’d all be falling towards the centre of the galaxy, rather than orbiting around it. So this is just another case of relative velocities—the interstellar medium is also in orbit around the galaxy, and what we’re seeing is the result of the “peculiar motion” of the interstellar medium relative to the LSR.