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Dr. Pamela Gay, host of the popular Astronomy Cast was interviewed for the latest episode of Brains Matter. In this interview Dr. Gay discusses the recent discovery that our Milky Way has only two arms instead of four. I recommend this episode to everyone who enjoyed Pam’s interview in Episode 14 of Books and Ideas and to all the fans of the Astronomy Cast.

Two arms and not four? Two arms are great for a human body but not for a galaxy. Two arms look ugly on a galaxy. The Milky Way (our system of stars) appears to be a mediocre galaxy.

Charles Mudede—who writes about film, books, music, and his life in Rhodesia, Zimbabwe, the USA, and the UK for The Stranger—was born near a steel plant in Kwe Kwe, Zimbabwe. He has no memory...

11 replies on “Only Two Arms”

  3. They’re all beautiful. The Galaxies are not static things, but billions of stars engaged in a beautiful dance among themselves and their neighboring galaxies, all the while being born and dying, and seeding the space around them with heavier elements. In several million years or thereabouts, our nearest galactic neighbor, Andromeda, will intersect our own. The millions-of-years splash from that should be an amazing sight. The Milky Way may end up with some additional arms…or even a tail like the Tadpole Galaxy. I wonder what humanity will look like then…assuming we survive that long…

  4. Correction…I just looked it up again… The collision with Andromeda is thought to be a couple Billion or so years in the future…

    A billion here…a billion there…

  5. I demand an upgrade to a better galaxy! I expect, at a minimum, four plush, upholstered arms on any galaxy where I choose to spend my time.

  7. But isn’t it true that the stars in the outer arms are moving at the same speed as the stars nearer the center? It’s an embrace of it’s center. The black hole. Our own star is also just a mediocre star but it has a planet chock full of life.

  8. True story: I actually named my character in Dragon Warrior (NES) “Dr. Gay”

    “Fortune has smiled on thee, Dr. Gay! Thou hast found a torch!”

  9. But isn’t it true that the stars in the outer arms are moving at the same speed as the stars nearer the center?

    Not quite, but enough close to it. They’re moving so fast that if the visible matter in the galaxies was all there was to them there wouldn’t be enough mass to keep them from flying apart. Which is why the whole “dark matter” thing started. There had to be something else holding the galaxies together besides what they could see.

    I’m told they’re actually starting to map out whole regions of the stuff in intergalactic space now.

  11. @7, 9: Here’s the NASA pictures of the Bullet Cluster. The Bullet Cluster is actually two separate clusters of galaxies that collided “recently” (i.e. only a few million years ago). The picture is actually highly important because it’s direct visual proof of the existence of dark matter.

    Despite all the stars in each individual galaxy, the vast majority of a cluster’s mass (if you look at just the “normal”, “baryonic” matter) comes from the vast, thin clouds of wispy neutral hydrogen gas floating between the galaxies. When these two clusters collided, the galaxies (yellow/white in the picture) passed straight through because a galaxy is much denser than a hydrogen cloud. Meanwhile, the two neutral hydrogen clouds (red in the picture) smacked into each other, heating up enough to ionize and produce X-rays (which the Chandra X-ray telescope can see).

    Funny thing is, though, that General Relativity lets you weigh an object by looking at the things behind the object. The light passing through is distorted because gravity bends the light beams. This “gravitational lensing” causes funny optical effects that you never see in actual galaxies, like rings and curves with sharp edges, so it’s easy to detect with a computer. So they measured the lensing with optical telescopes, then artificially shaded the picture (in blue) to show where the gravitational lensing was strongest — and therefore where the most mass is.

    Since most of the “normal” matter from the cluster was in the gas clouds (red) that smashed together in the collision, we would expect that to be the location where the most gravitational lensing is (blue). Instead, the maximum gravitational lensing is not where the maximum concentration of hydrogen is. The blue and red are separate. The bulk of the mass passed straight through… even though the galaxies themselves don’t weigh enough to account for it.

    The picture is a very elegant proof that dark matter physically exists, and that it doesn’t interact strongly with normal matter. Which is pretty amazing, when you consider that dark matter was originally proposed to explain why the outer edges of galaxies rotated too quickly.

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