Astronomy Jargon 101: Galactic Halo

In this series we are exploring the weird and wonderful world of astronomy jargon! You’ll wrap your head around today’s topic: galactic halo!

When you look at a random galaxy, your eye is drawn to the disk. That’s not surprising, considering that the disk contains the vast majority of stars, and thereby gives off the most amount of light.

But there’s a lot more to a typical galaxy than just the main disk of stars.

Beyond the central disk lies something called the galactic halo, which surrounds most galaxies. This galactic halo is roughly spherical and doesn’t have a hard edge. Instead, it just gently fades away into the general intergalactic background. The halo is made of three components:

  1. Stellar Halo – As the name implies, this is just a bunch of stars, although taken together they make up less than 1% of all the stars in a galaxy. The stars in the stellar halo of the Milky Way are generally older and metal-poor compared to the disk stars, though some galaxies appear to have much younger, healthier halos. The stellar halo is also where the globular clusters live, which are clumps of thousands to millions of old, almost-dead stars.
  2. Galactic Corona – This is a sphere of loose gas that lazily orbits the outskirts of every galaxy. It’s the leftover fluff and bits of junk – ejecta from supernova, emissions from central black holes, and so on.
  3. Dark matter Halo – This invisible collection of matter makes up over 80% of the mass of any galaxy, and can extend to many times the size of the main disk. Astronomers can’t see the dark matter halo itself (because it’s invisible) but can infer its existence based on its gravitational influence on everything else.

It’s thought that halos form as leftovers of galactic mergers. When galaxies collide, some material ends up flung far from the main galaxy…and stays that way.