A Guide To Your Galaxy

Here is a model of the Galaxy (not too scale!):

The Galaxy is split into three major components. I’ve noted them on the above diagram. Below is a potted guide to galactic structure…

The Bulge
The bulge is the plump, spheroidal region of old stars found at the centre of the Galaxy. (In science fiction, this region is often referred to as ‘the Core’.) The bulge is a dense region, dominated by old and metal-poor* stars, red and orange giants. You can see it quite clearly in our skies, on a dark night. The ‘fat bit’ of the Milky Way, the bit in Sagittarius, is the bulge. Towards the centre, the bulge becomes incredibly dense with stars. If I remember correctly, the central cubic parsec** or so contains something like a million stars. And of course, right in the middle, you have the infamous supermassive black hole.

The bulge is actually elongated, a sort of squashed ellipsoid. I didn’t try to represent it in the above picture, but if you try to imagine a sort of ovular sphere, you’ve got roughly the right idea. This means that our galaxy is what’s called a ‘barred spiral’. (Seen from above, the bulge would look a bit like a sort of central bar.)

The Disc
The galactic disc is where we live. The disc is rather thin, compared to its diametre. If you want to see a reasonably-good representation of it, oddly enough a DVD has about the right proportions. The disc is narrow! (In the jargon, we say it has a lower ‘scale height’ than the other components.) The disc is the region where you find active star formation. It is where you find the hot, young stars like O and B-type giants and dwarfs. It’s also where you find the nebulae and star-forming regions.

Between the disc and the bulge, you have something called the Five-Kiloparsec Ring. The 5 Kpc Ring is a ring of star-forming activity. It seems to be just bigger than the long axis of the Bulge’s bar. One theory is that the rotation of the Bulge is what creates the Ring – it curdles the gas on the Disc’s inner edge, essentially, triggering a circular star-forming burst.

Beyond the Ring, you come to the spiral arms. These appear to be tracing out regions of star formation – the arms are marked out by hot O and B-type stars. Oddly enough, there are almost as many stars inside the ‘voids’ between them. The ‘voids’ only look dark because these stars are older and cooler A to M-type stars. (It used to be thought that there actually were voids between the arms – this isn’t so.) O and B stars have such short life expectancies that they don’t really have time to migrate into the voids before they pop!

We don’t fully understand how the arms form, and there’s even debate over how many of them there are. The problem we have is a woods-versus-trees issue! We live in the disc, so when we try to map its structure we’re looking into the mess of gas, dust, planets and stars. Seeing clearly through all of that is problematic!

However, what we do know is that the structure of the disc is complicated. It supports a wide range of star-forming environments, with a range of differing compositions. However, overall, the disc is definitely the youngest of the three populations.

Disc stars all tend to orbit in the same direction, and do so with low speeds relative to their neighbours. They all orbit more or less in the same plane, too – that is, after all, what the disc is!

In our night sky, the disc is the Milky Way – and indeed most of the other stars.

The Halo
And now we come to the stellar halo. I say ‘stellar’ because the cosmologists have also adopted the word ‘halo’, for their putative structures of dark matter. The stellar halo is quite a different entity – we can see its components, for starters!

The halo contains the oldest, most metal-poor stars. They occupy a vague spheroid surrounding the rest of the Galaxy. Unlike disc stars, halo stars often have out-of-plane orbits. Also, a large number of them are retrogade – they orbit ‘backwards’ relative to the disc! The halo as a whole doesn’t seem to display much (if any) overall rotation, although there may be some components within it that do have a net spin. (There is debate on this, though.)

Age estimates for the halo range as high as 12-13 billion years. It was forming stars when the rest of the galaxy was just a collapsing cloud of gas. However, the halo’s star-forming days are long over. It’s dominated today by old, faint red dwarfs. You don’t expect to see stars hotter than spectral type F in the halo – this is because any such stars will have had more than enough time to evolve into red giants and die.

There are a lot of halo white dwarfs, though, as you would expect! Also, it is beginning to emerge that there does exist a population of halo brown dwarfs as well. One odd aspect of the halo is that some of its stars come from outside – galaxies like ours grow by accreting smaller ones, and one of the places these smaller galaxies deposit their stars into is the halo.

This means the halo is partly the result of galactic cannibalism!

The halo is the trickiest component to see with the naked eye. Halo stars do exist in the solar vicinity – they’re defined by composition and by velocity, not by position! – but they just look like other stars. Nonetheless, they are there. However, the halo star density is the lowest of all three components. Of the stars in the solar neighbourhood, just 0.25% are believed to be halo objects – the rest are all disc stars!

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*‘Metal’ in astronomy is anything heavier than helium. Or, to put it another way, anything that had to come from a star rather than directly from the Big Bang.
**1 pc = 3.26 LY. Or, to put it another way, a sphere 1 PC in radius centred on the Sun, wouldn’t stretch as far as Alpha Centauri.