The Milky Way and Other Galaxies

Seeing the Milky Way

On a clear night away from light pollution, the Milky Way is visible as a faint, irregular band of light arching across the sky. This is not a separate object in space: it is our own galaxy, the Milky Way Galaxy, seen edge-on from our position within it. The band is broader and brighter towards the constellation Sagittarius, which lies in the direction of the galactic centre. Even with binoculars, the band resolves into countless individual stars and dark rifts of dust.

Structure of the Milky Way

The Milky Way is a barred spiral galaxy with several distinct components:

• The disc — a flattened region containing most of the galaxy’s stars, gas, and dust, including the spiral arms. New star formation is concentrated in the disc. The disc is about 100,000 light-years across but only ~1,000 light-years thick.
• The central bulge — a roughly spherical concentration of older stars at the galactic centre. At the very centre lies a supermassive black hole known as Sagittarius A*, with a mass of approximately 4 million solar masses.
• The central bar — a bar-shaped concentration of stars passing through the galactic centre, from which the spiral arms emerge. This feature gives the Milky Way its classification as a barred spiral.
• The halo — a roughly spherical volume surrounding the disc, populated by old stars and globular clusters (over 150 are known around the Milky Way). The halo also contains the majority of the galaxy’s dark matter.

The Sun is located about 26,000 light-years from the galactic centre, in the Orion Spur — a minor arm between two major spiral arms. The Sun takes about 225-250 million years to complete one orbit of the galactic centre (the galactic year).

Mapping the Milky Way with the 21 cm Line

Visible light cannot penetrate the dense dust of the galactic disc, so optical telescopes cannot map the full structure of our galaxy. Radio waves, however, travel through dust unimpeded. Neutral hydrogen in the interstellar medium emits a characteristic radio wave at a wavelength of 21 cm. By measuring the Doppler shift of this emission in different directions, astronomers can map the distribution and velocity of hydrogen gas throughout the disc, revealing the galaxy’s spiral arm structure that would otherwise be hidden.

The Local Group

The Milky Way is not alone. It belongs to a gravitationally bound collection of galaxies called the Local Group, which contains roughly 80 galaxies spread across about 10 million light-years. The dominant members are:

• Andromeda Galaxy (M31) — the largest galaxy in the Local Group, about 2.5 million light-years from Earth. It is a spiral galaxy slightly larger than the Milky Way and is approaching us at roughly 110 km/s. In about 4 billion years it will merge with the Milky Way.
• Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) — irregular dwarf galaxies orbiting the Milky Way, visible in the Southern Hemisphere sky as separate misty patches. The LMC is about 160,000 light-years away.
• Triangulum Galaxy (M33) — a smaller spiral galaxy about 2.7 million light-years away, the third-largest in the Local Group.

Hubble’s Galaxy Classification — the Tuning Fork

Edwin Hubble devised a classification system for galaxies, often illustrated as a Hubble tuning fork diagram. Galaxies are grouped by shape:

• Elliptical galaxies (E0-E7) — smooth, featureless ellipses ranging from nearly circular (E0) to strongly flattened (E7). They contain mostly old red stars and little gas or dust, so star formation has effectively ceased.
• Spiral galaxies (Sa-Sc) — disc galaxies with a central bulge and spiral arms containing young blue stars and active star formation. Sa spirals have tightly wound arms and a large bulge; Sc spirals have loosely wound arms and a smaller bulge.
• Barred spiral galaxies (SBa-SBc) — like spirals but with a bar of stars through the centre from which the arms emerge. The Milky Way is classified as SBb.
• Irregular galaxies — no regular shape; often the result of galactic collisions or gravitational disturbances. The Large and Small Magellanic Clouds are examples.