Galactic orbits

A galaxy is a chaotic place. Sure, they may look silent and motionless, suspended in space, but a galaxy is in constant motion. Hundreds of billions of stars, swirling around at hundreds of metres per second, gravitationally fixed in place by the pull of a supermassive black hole and a huge clump of dark matter surrounding it.

But there’s more than one way to orbit a galaxy. Spiral galaxies like the Milky Way where we live actually have several distinct populations of stars. The youngest stars lie in the galaxy’s disk along with us. These stars undulate and wobble up and down as they orbit the centre of the galaxy, staying loosely in a single plane. A big stellar pancake, if you will.

Then you have the halo stars. These have much larger orbits which carry them up far from the galaxy’s disk. A galactic halo is essentially a diffuse cloud of stars in a loosely spheroid shape around the galaxy. Not many new stars form out in orbits like these. Halo stars are old. Some of them very old indeed. And I bet the view from up there is amazing!

Finally, you have the bulge orbits. The galactic bulge is the bright patch surrounding the core of the galaxy, also roughly spheroidal in shape. This region is packed full of dense interstellar clouds, held fast by the supermassive black hole’s gravity. Along with those clouds are an assortment of stars in fast and chaotic orbits. Some of these stars are young and bright, freshly squeezed from interstellar clouds by galactic tidal forces. Others are as old as the galaxy itself and have spent their entire lives living in the gravitational strangehold of the galactic centre.

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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9 Responses to Galactic orbits

  1. James Harmer says:

    It’s total carnage up there. I live near junction 8 and every time there’s an accident an ambulance goes past up the hill to the motorway. It’s so bad that I don’t even notice them any more, unless three or four go past at once.
    On a more scientific note, last year there was a huge pile up and the motorway traffic was diverted through the local small towns. I was a bit disconcerted when a large truck ended up outside the building I work in. Nothing unusual there, except the truck was laden with large cylinders of Uranium Hexafluoride gas, and lots of radiation symbols, which was a bit disconcerting!

  2. James Harmer says:

    Interesting! Thank you. I’ve heard of the traffic jam analogy and can relate to it particularly well, since I live near the M25. For the uninitiated, this is a huge orbital motorway that goes right round London. It’s known as the biggest car park in Europe…

  3. James Harmer says:

    Thank you for the clarification. I’m a very amateur astronomer, still trying to get my head round the theory that the spiral arms are density waves in the disk. The immensity of the galaxy is fascinating….

    • invaderxan says:

      You’re very welcome! I know full well that many of these things can be a bit puzzling when you first encounter them.

      As density waves go, have you ever seen this video? I found it really helped me to visualise density waves…

      (A few years ago, I remember seeing a video which explained quite well how similar density waves actually cause traffic jams… But I’m afraid I can’t find that one!)

  4. James Harmer says:

    I thought that the Milky Way was a barred spiral?

    • invaderxan says:

      It’s believed to be a barred spiral, yes. Though it still has distinct populations of stars, depending on where in the galaxy you look – the halo, the disk, and the central bulge. It’s just that in a barred spiral, that bulge is stretched out into a bar shape. As far as I’m aware, the overall morphology of spirals is much the same, irrespective of whether or not a bar is present.

      (Though as always, if I’m wrong on that point, I’d appreciate anyone correcting me!)

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