Categories: galaxies

Astronomy Jargon 101: Galactic Halo

This NASA/ESA Hubble Space Telescope image shows a compact and distant globular star cluster that lies in one of the smallest constellations in the night sky, Delphinus (The Dolphin). Due to its modest size, great distance and relatively low brightness, NGC 7006 is often ignored by amateur astronomers. But even remote globular clusters such as this one appear bright and clear when imaged by Hubble’s Advanced Camera for Surveys. NGC 7006 resides in the outskirts of the Milky Way. It is about 135 000 light-years away, five times the distance between the Sun and the centre of the galaxy, and it is part of the galactic halo. This roughly spherical region of the Milky Way is made up of dark matter, gas and sparsely distributed stellar clusters. Like other remote globular clusters, NGC 7006 provides important clues that help astronomers to understand how stars formed and assembled in the halo. The cluster now pictured by Hubble has a very eccentric orbit indicating that it may have formed independently, in a small galaxy outside our own that was then captured by the Milky Way. Although NGC 7006 is very distant for a Milky Way globular cluster, it is much closer than the many faint galaxies that can be seen in the background of this image. Each of these faint smudges is probably accompanied by many globular clusters similar to NGC 7006 that are too faint to be seen even by Hubble. This image was taken using the Wide Field Channel of the Advanced Camera for Surveys, in a combination of visible and near-infrared light. The field of view is a little over 3 by 3 arcminutes.

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.

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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:

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.
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.
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.

Paul M. Sutter

Astrophysicist, Author, Host | pmsutter.com