Meet the Milky Way, our big, weird and kind of scary home
Science | Gregory Beatty | Oct. 27, 2022
“The sun and you and me and all the stars that we can see /Are moving at a million miles a day /In an outer spiral arm, at 40 thousand miles an hour /Of the galaxy we call the Milky Way” —Eric Idle, “Galaxy Song”
With all the great space science going on these days, from probes studying all manner of planets, asteroids, comets and the Sun to space telescopes providing breathtaking views of the cosmos, it’s easy to forget about Voyager 1 and 2. Both probes launched in 1977, and were finished their primary missions by 1989 — a Jupiter/Saturn flyby for Voyager 1, and Voyager 2’s “Grand Tour” of Jupiter, Saturn, Uranus and Neptune.
That wasn’t it for the probes, though. They kept going… and going… and going. Now, 30 years on, both have passed through the heliopause, which is the boundary where the Sun’s solar radiation (the heliosphere) pushes out against incoming cosmic radiation. Scientists consider that point (which the probes hit at 121/122 AU from the Sun) the dividing line between the solar system and Milky Way Galaxy.
It was a landmark moment, and a spooky one too. Because as weird and wonderful as our solar system is, the Milky Way is a whole other level of crazy.
Now, distances are so vast that it’s not likely the Voyagers will encounter anything unusual anytime soon (unless they have a random encounter with an Oort Cloud object).
But the Milky Way has long captivated humanity, and while we have learned a lot about our galaxy in recent decades, mysteries remain.
The Milky Way’s name dates to ancient times, says University of Calgary astronomer Jeroen Stil. “The term Milky Way comes from the apparent white glow that suggested a roadway made of milk,” he says. “Via Lactea is the Roman term.” That, in turn, is derived from the Greek galaktikos kýklos which means “milky circle”.
Pretty poetic, right? But the scientific explanation is cool too.
“The Milky Way Galaxy is mostly a flat pancake-like structure and we are inside it,” says Stil. “Because we are, we see more stars if we look along the pancake then if we look out of the pancake. We are not near the centre, and that’s why when we look toward the centre we see more stars. And it’s the combined glow of all those stars that creates the Milky Way.”
That comes with a caveat, as parts of the inner Milky Way are obscured by giant gas clouds, so their light is blocked. In the northern hemisphere, the Milky Way is visible from February to October. But the southern hemisphere has the superior view. That’s because the solar system is tilted at a 60-degree angle to the galactic plane. The solar system’s underside faces inward, so on Earth the southern hemisphere has the best view.
As late as the 1920s, we thought the Milky Way was the whole enchilada. But then Edwin Hubble realized that these dim glowing balls that had previously been mistaken for nebulae were actually “island universes” akin to the Milky Way. Current estimates put the number of galaxies in the universe at two trillion.
Galaxies come in three types: elliptical, spiral and irregular. The Milky Way is a spiral (picture a pinwheel with four main arms and a few minor ones). The Sun is in a minor arm called Orion-Cygnus, which puts us 26,000 light years from the galactic centre.
Like planets around a star, the Milky Way is rotating. Because the Sun is far out from the centre, it takes about 225 million years to complete one orbit. Earth is 4.5 billion years old, so crunch the numbers, and that makes Earth about 20 in “galactic years”.
Independent of the galaxy, the Sun is moving towards the constellation Hercules at 19 km/s. That motion, scientists once thought, would give the Sun’s heliosphere a tear-drop shape. But new research suggests it might instead be croissant-shaped, with a blunt nose and trailing edges that twist toward each other. Also, instead of being pancake flat (with a central bulge), a new 3-D map from Gaia space observatory suggests the Milky Way is warped at the edges.
Voyagers 1 and 2 are on different courses — the former moving in the same direction as the Sun, the latter peeling downward on a southern heading. And while both have left the solar system, they’re still very much in the Sun’s gravitational grip.
They’re joined there by the Oort Cloud, which ranges between 30,000 and 100,000 AU from the Sun. It’s like the asteroid and Kuiper belts, but since it’s so far out the objects have been disrupted by galactic gravity and formed a cloud. The Oort Cloud is the suspected source of long-period comets, and the fact they come into the Sun from all angles confirms the cloud shape.
From there, Proxima Centauri is the closest star. It’s 4.2 light years distant, but neither Voyager is headed its way. Both are one day destined to have close encounters of the stellar kind, though: Voyager 1 with an obscure star in Ursa Minor in 40272 CE (so 38,000 years from now), and Voyager 2 with Ross 248 in Andromeda around the same time.
The Milky Way has an estimated 100 billion stars. Some resemble our Sun, being class G with a projected lifespan of 11 billion years. Others are class O and B giants that cycle through life in 10 to 100 million years before exploding in fiery supernovae. On the opposite end of the spectrum, there are red dwarfs with projected lifespans of trillions of years.
Then you have stuff like multiple star systems, globular clusters, neutron stars, pulsars, nebulae, black holes and more. Depending on where you are in the Milky Way, conditions can vary greatly.
In fact, we owe our very existence to the Sun’s location in the Orion-Cygnus Arm, as that puts us in the Galactic Habitable Zone (GHZ).
“That’s a different concept than a habitable zone of a solar system,” says Stil. “There, it’s mainly about temperature, so that if a planet has an atmosphere, there could be liquid water on the surface.”
The GHZ has more to do with distance from the galaxy’s centre. If you go too far out, heavier elements that are needed to make terrestrial planets get scarce. But the closer you get to the centre the more crowded things get.
On Earth, it took three billion years for complex life to start to evolve. That’s a luxury of time that isn’t necessarily available in the more crowded inner galaxy, where conditions are more volatile.
Not that Earth is entirely home free. Asteroid impacts, like the one that caused the dinosaurs’ extinction, remain a threat. And scientists studying ocean sediments have detected a special isotope of iron (60FE) that is only produced in a supernova, says Stil.
“The sediments are about two million years old. Even early hominids were around then, and they were not impacted. [This means] we can be close enough to a supernova to be sprinkled by what it produced, and maybe not even know about it.”
That comes with a qualifier, though. “We have come to understand that supernova explosions can be asymmetric, so they can blast out something a bit more dangerous in a particular direction,” says Stil. “We know there are gamma ray bursts associated with supernovae too. They are highly directional, and if you happen to be in the line of fire, that can be far more serious.”
Heart of Darkness
What lies at the centre of the galaxy was another mystery that once bedevilled astronomers. In the 1980s, they finally determined it was a giant black hole with a mass of 4.1 million Suns. Last April, they even released a photo of Sagittarius A* (our second galactic black hole image after M87* in Andromeda in 2019).
But there are still plenty of mysteries surrounding Sgr A*. Fermi bubbles being one.
“When they were discovered [in 2010], it was quite a surprise,” says Stil. “They’re named after the Fermi Observatory that has a gamma ray telescope in space. It detected this flow on opposite sides of the Milky Way’s centre that stretches out 25,000 light years from the galactic poles.
“Judging by their expansion rate, the total energy behind them must be like 10,000 supernovae,” says Stil. “Something substantial must have fell into Sgr A*. Not everything disappeared. Some of it got ejected, and that typically happens in opposite directions with a jet of material.”
A Fermi bubble was spotted in the M87* photo too, so they could be common among the supermassive black holes that lurk at the heart of most galaxies, devouring everything they can get their gravitational claws into.
Good thing we’re in the GHZ, right? Although that does raise an intriguing question — one linked to yet another galactic mystery that was finally solved in 1992 with the discovery of the first exoplanet orbiting another star. Logic said they surely existed. But since that clashed with creationist beliefs about Earth being a special gift from God, a smidgen of doubt remained.
To date, we’ve detected over 5,200 exoplanets. Most are gas giants, but that’s to be expected, as they’re easiest to see. But we have found plenty of terrestrial exoplanets, including some described as Earth-like.
Whether life exists elsewhere in the Milky Way is not a question the Voyagers can answer. But the recently launched J.S. Webb telescope might have a shot, says Stil.
“Webb’s main benefit is to look at very distant objects that exist far back in time that could allow us to witness the formation of galaxies similar to the Milky Way,” he says. “Another area it’s designed for is to gather information on different solar systems. Already, it’s produced results studying the chemical composition of atmospheres on exoplanets.”
When exoplanets in our line of sight pass in front of their star, the light is filtered through their atmosphere. Webb can read the spectrum and look for elements that could indicate life.
Oxygen is a key one. It’s highly reactive, so if it exists on a planet, it gets quickly locked up, as happened on Mars, where it bonded with iron to give Mars its rusty red colour.
“A substantial presence of oxygen in an atmosphere would not stay there long unless it was being maintained,” says Stil. “If you see oxygen in the atmosphere, especially at higher percentages, it indicates it’s being produced on an ongoing basis, like plants do with photosynthesis.
“The case would get stronger if we saw other things that we associate with terrestrial life,” Stil adds. “Methane would be one, although you have to be careful, because a gas giant like Jupiter has lots of methane too. [Deciding] which elements to rely on is a point of active debate.”
Let’s end on a dark note. Dark matter, to be specific. Scientists aren’t exactly sure what it is, but it’s theorized to exist. That’s because they’ve calculated the visible mass of the galaxy, and there doesn’t seem to be enough matter to hold everything together. Something else must be out there.
“Dark matter is something we can indirectly detect because it affects the motion of stars. But nobody has found dark matter and people have done their best. Research is ongoing,” says Stil.
As it is with Voyager 1 and 2. Happy Halloween trails to them. ■