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Dark matter alters autumn

Unseen forces are at play in the November sky.
berman
For countless millennia, the sky has provoked questions — and inspired the hard work that yields answers. What are those moving planets? Why do the stars shine? Resolving such ancient riddles has let today’s backyard observers gaze into familiar constellations where much is known.

But scientific comfort has always been ephemeral. The conception of dark matter in the 1930s and dark energy in the 1990s meant the universe is dominated by powerful-but-elusive entities. These raise a basic and valid question: Do these major, mysterious headline-makers show themselves in the visible heavens? 

Dark energy does not. The universe may be blowing itself apart at an accelerating tempo, but this enigmatic anti-gravity force simply doesn’t manifest itself locally within any particular cluster of galaxies, including ours. So the multimillion light-year range of the naked eye, even under ideal unpolluted rural skies, yields no trace of dark energy, whatever it might ultimately prove to be.

Dark matter is a different story. First recognized by Fritz Zwicky 75 years ago, it is an invisible substance that exerts enough gravitational force to glue together our Local Group of galaxies despite each member’s high speed. Without its pull, Andromeda wouldn’t continue to zoom in our direction on a collision course at 70 miles (113 kilometers) per second. Seen as a naked-eye smudge, its brightest companion galaxies easily show up through backyard telescopes. Our own galaxy is likewise escorted like an aircraft carrier, by small galaxies trapped within our gravitational epoxy.

Arcturus star
Observe Arcturus while you can! Thanks to dark matter, the solar system’s motion through the galaxy means this bright star will be visible only for another 50,000 years.
Mikulski Archive for Space Telescopes (MAST), STScI, and NASA
Three years ago, a research team published a new analysis of the Milky Way’s inner rotation. That’s the section that fills your southern sky this month. The study, published in the journal Nature Physics, concluded that the Sun and Earth couldn’t be zooming along at 143 miles (230 km) a second, keeping pace with all the galactic contents around us, if lots of dark matter didn’t pervade our galaxy. If the galaxy’s spin were dictated solely by the baryonic matter that comprises stars, planets, and black holes, it would rotate much differently from the way it does. The unseen matter isn’t concentrated, but diffusely scattered as an immense halo. So the familiar spiral we imagine ourselves to be in is like a ship in a bottle.

The Milky Way is best seen at nightfall, especially during November’s more moonless first half. Without dark matter critically influencing the way we spin, stars closer to the center — in the direction of Sagittarius — would orbit faster than we do. Conversely, stars farther away, like those in Orion now rising by 10 P.M., would get left behind. Instead, the Milky Way rotates as if it were a solid structure, a vinyl record, so that the same stars tend to keep us company.

This makes the constellations far more durable. And on a larger but still visible scale, there’s our Local Group of about four dozen galaxies, blithely defying the universe’s expansion. While the universe blows itself apart, we and neighbors like M33 are so glued together that we’ll hang out together for the entire duration. Dark matter’s proof sprawls around us on November nights.

But one aspect of dark matter will change our view of the autumn sky. While Andromeda is aligned a mere 13° from the plane of tilt of our galaxy, it lies at a galactic longitude offset 90° from our core. This is changing. As the Milky Way spins, our brisk rotation at 143 miles per second (230 km/s) will whirl us around to the other side of our galaxy in about 100 million years. Then the thick central bulge will block Andromeda behind opaque clouds of carbon dust for about 15 million years. Our sister galaxy will be gone.

Another change: While the strange uniformity to our galaxy’s rotation means that most familiar stars like Vega and the Dipper gang will keep hanging out near us like adjacent horses on a carousel, Arcturus will be gone. It circles our galaxy’s center not by orbiting along the dusty crowded disk, but instead traveling perpendicular to it. For the past 500 centuries, it’s been diving down toward us from the north, from the empty “halo” region. These days it’s about as close to us as possible, a mere 36 light-years away. It will keep going until it reaches the limits of naked-eye visibility in a mere 50,000 years. When it returns to its present location, we will be somewhere else. We will never meet again.

So before we get too carried away with either dark matter’s promise of permanence or the suggestion of celestial evanescence, we should keep things in perspective. This strange entity is indeed making many celestial favorites stick around. It’s preserving the constellations. But it can’t promise us the stars.

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