12.5 Key Terms

Barred spirals: spiral galaxies that also have bar-shaped concentrations of stars in their central regions. 12.1

Central bulge (nuclear bulge): the central (round) part of the Milky Way or a similar galaxy. 12.1

Dark matter halo: the mass in the Milky Way that extends well beyond the boundary of the luminous stars to a distance of at least 200,000 light-years from the centre of the Galaxy; although we deduce its existence from its gravity, the composition of this matter remains a mystery. 12.1

Differential galactic rotation: the idea that different parts of the Galaxy turn at different rates, since the parts of the Galaxy follow Kepler’s third law: more distant objects take longer to complete one full orbit around the centre of the Galaxy. 12.4

Elliptical galaxies: galaxies whose shape is an ellipse and that contain no conspicuous interstellar material. 12.3

Halo: the outermost extent of our Galaxy (or another galaxy), containing a sparse distribution of stars and globular clusters in a more or less spherical distribution. 12.1

Irregular galaxies: galaxies that do not have the regular shapes associated with either spiral or elliptical galaxies. 12.3

Local Group: a small cluster of galaxies to which the Milky Way Galaxy belongs. 12.2

Milky Way Galaxy: the band of light encircling the sky, which is due to the many stars and diffuse nebulae lying near the plane of the Milky Way Galaxy. 12.1

Monolithic Collapse Model: according to this model, the Milky Way Galaxy initially formed from a rotating cloud of gas that collapsed due to gravity. 12.4

Protogalactic cloud: a rotating cloud of gas like the one that gave birth to the Milky Way Galaxy. 12.4

Spiral arm: a spiral-shaped region, characterized by relatively dense interstellar material and young stars, that is observed in the disks of spiral galaxies. 12.4