The bulge

The bulge is a spheroidal stellar system with radius of $\sim 1{\hbox{\rm kpc}}$, located at the centre of the MW. In fact, its properties are quite similar to that of an elliptical galaxy: it is as if there is a small elliptical galaxy at the centre of each spiral. The surface brightness profile (in $L_\odot$ per unit area) of this component goes like

$$I(r)=I_e \exp[-7.6(r/r_e)^{1/4}-1]\,.$$ (3.2)

This is a famous fit to the intensity profile of elliptical galaxies, introduced by de Vaucouleurs in 1948, and called the `de Vaucouleurs' or `$r^{1/4}$'(r-to-the-one-quarter) profile. ( $r=(R^2+z^2)^{1/2}$ is the 3D distance to the centre.)

$r_e$ is called the effective radius. Note that $I(r=r_e)=I_e$. The factor 7.6 is chosen such that half of the light comes from stars within $r_e$. For the MW, $r_e\approx 0.7{\hbox{\rm kpc}}$.

Although the bulge is bright, we can't see much of it, due to the large amount of obscuring dust (in particular, you can't see it with the naked eye!). The total amount of extinction toward the centre is about 28 magnitudes in the visual! In other spirals, we can see the bulge very well (and recall that the classification of spirals is based on the bulge/disk ratio), but for the MW we need to go the the IR to get a nice picture of it. The MW's bulge is rather elongated with axis ratio 5:3, with strong evidence for a bar.

The stellar content of the bulge is a bit of a mixed bag, with both very old, low metallicity stars, but also young, metal rich stars. The mean metallicity is about [Fe/H]$\sim 0.3$, i.e. twice the solar value. The mass of the bulge is $\sim 10^{10}\hbox{$M_\odot$}$, with a B-band luminosity of $\sim 3\times 10^9$$L_\odot$.