The birth of the galaxy in 60 seconds - Scott Hershberger

195,816 views ・ 2024-09-19

TED-Ed


Please double-click on the English subtitles below to play the video.

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Within the Milky Way, the Earth resides on the inner edge of a spiral arm.
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But this isn’t our permanent address.
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When most dinosaurs went extinct,
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our solar system was in a different arm than it is today!
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The Milky Way never stops changing;
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even its structure continues to evolve.
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To see why, let's start at our galaxy's very beginning.
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13 billion years ago,
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propelled by supernovae and the general chaos of the early universe,
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the gas and dust particles that eventually became our Milky Way
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were whizzing around in every direction.
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Each particle had a certain angular momentum—
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the magnitude and direction of its rotation
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around the emerging galaxy’s center.
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Just as the new galaxy’s total mass
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was the sum of the individual particles’ masses,
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the galaxy also had an angular momentum
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that was the sum of the particles’ angular momenta.
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So, despite the wild motion of its parts,
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the galaxy as a whole was rotating about an axis.
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Meanwhile, the gas and dust particles frequently collided,
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losing some energy to heat.
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Because they slowed down,
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gravity was able to pull them closer to the galactic center.
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So, why did the Milky Way, like most galaxies, become flat,
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and not spherical like stars and planets?
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The answer lies in its angular momentum.
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The laws of physics dictate that in the absence of external forces,
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the total angular momentum of an object or system has to stay the same over time.
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In a star or planet, the spinning material is so dense
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that the outward pressure partners with gravity
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to create a mostly spherical shape.
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But emerging galaxies tend to have low densities and high angular momenta,
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meaning that the spinning motion is a more significant factor
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than the outward pressure.
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Indeed, if a galaxy has a lower angular momentum,
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an egg shape can form.
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In galaxies like the Milky Way,
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particles, instead of falling directly towards the center,
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tended to fall parallel to the axis of rotation,
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as to keep the larger total angular momentum constant.
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Over billions of years the cloud of particles gradually fell,
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sped up in their orbits, and created a spinning disc.
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Now what about the arms?
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Spiral arms, like the one Earth is in,
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are regions where stars and gas are packed more tightly together.
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But the arms aren't static structures.
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They are caused by zones of compression that travel through the galaxy as waves.
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Just as a whirlpool has multiple peaks and troughs,
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a density wave in a galaxy has multiple regions of high density—
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the bright spirals— separated by regions of lower density.
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And as the wave travels,
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different stars are constantly entering and leaving the spirals.
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Density waves can form in several ways.
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For some galaxies, a nearby companion galaxy stirs the pot.
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Its gravitational pull breaks the disc’s symmetry,
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generating a wave that could last for a billion years.
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For other galaxies,
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the presence of a small clump of tightly packed stars and gas within the galaxy
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can have a ripple effect, spontaneously giving rise to a wave.
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This is thought to be the cause of the Milky Way's spirals.
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In both scenarios,
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the galaxy’s overall rotation bends the dense region into spirals,
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which rotate around the galaxy’s center.
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Our solar system is orbiting the galactic center faster than the spiral arms.
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We’ll be moving deeper into our current arm for millions of years
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before eventually putting it in our rearview mirror.
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And recent observations have added another wrinkle to the picture.
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Rather than just one density wave,
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a typical spiral galaxy likely has two or more waves that overlap with each other
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and travel at different speeds.
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The result would be spiral arms that last for tens or hundreds of millions of years
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before breaking apart and re-forming.
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This may be happening in the Milky Way,
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meaning that when the Earth formed 4.5 billion years ago,
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the spirals themselves may have looked entirely different than they do today.
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In any case, our spirals won't last forever.
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About 5 billion years from now,
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the Milky Way will start to merge with the Andromeda galaxy,
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throwing off the balance of angular momentum and creating an egg shape—
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the birth of a new era in our galaxy’s history.
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