Galaxies: Island Universes

## Galaxies: Island Universes

A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The observable universe contains an estimated 2 trillion galaxies, ranging from tiny dwarf galaxies with a few million stars to giants with several trillion. Our own Milky Way is a large barred spiral galaxy, and yet it is merely an average-sized member of the cosmic community.

### The Great Debate

Before 1924, astronomers debated whether the 'spiral nebulae' seen through telescopes were nearby gas clouds within the Milky Way or distant 'island universes' comparable to our own galaxy. The dispute was settled by Edwin Hubble, who identified Cepheid variable stars in the Andromeda 'Nebula' (M31) and used their pulsation periods to calculate a distance far beyond the edge of the Milky Way. The universe of galaxies was confirmed.

### The Hubble Classification Sequence

Hubble classified galaxies along a 'tuning fork' diagram based on visual morphology:

**Elliptical galaxies (E0-E7)** are smooth, featureless, ellipsoidal systems containing predominantly old red and yellow stars with little interstellar gas or dust. They range from nearly circular (E0) to highly elongated (E7). Giant ellipticals like M87 in Virgo contain trillions of stars and are the largest galaxies in the universe. Ellipticals are thought to form primarily through galaxy mergers.

**Lenticular galaxies (S0)** are intermediate — they have a disk and central bulge like spirals but lack prominent spiral arms and contain little cold gas.

**Spiral galaxies (Sa-Sd, SBa-SBd)** have a central bulge surrounded by a flat disk with prominent spiral arms. The 'S' series has a round central bulge; the 'SB' series has an elongated bar through the center. As the letter progresses from a to d, the bulge shrinks, the arms become more open and knotty, and star formation activity increases. Our Milky Way is classified as SBbc — a barred spiral with moderately open arms.

**Irregular galaxies (Irr)** don't fit the tuning fork — they have chaotic morphologies, often the result of gravitational interactions. The Large and Small Magellanic Clouds, satellites of the Milky Way visible to the naked eye from the southern hemisphere, are classic irregular galaxies.

### The Milky Way

Our galaxy is difficult to study because we are embedded within it — observing the Milky Way is like trying to map a forest from inside a single tree. Nevertheless, decades of radio astronomy, infrared surveys, and the Gaia mission have built up a detailed picture.

The Milky Way contains 100-400 billion stars and is roughly 100,000 light-years in diameter. It has a central barred structure (the Galactic Bar), four main spiral arms (Perseus, Norma-Outer, Scutum-Centaurus, and Sagittarius), and a minor arm (the Orion Spur) in which our solar system resides, about 26,000 light-years from the center.

At the very center of the Milky Way is Sagittarius A* — a supermassive black hole of 4.1 million solar masses. Its existence was confirmed over decades by tracking the orbits of stars (the S-star cluster) that swirl around the invisible central mass. The Event Horizon Telescope captured the first direct image of Sgr A* in 2022.

### The Local Group

The Milky Way is one of about 50 galaxies in the Local Group, a gravitationally bound association spanning about 10 million light-years. The three largest members are:

- **Milky Way**: ~300 billion stars, SBbc spiral
- **Andromeda (M31)**: ~1 trillion stars, the largest spiral in the Local Group at 220,000 light-years across; approaching the Milky Way at 110 km/s and will collide in about 4.5 billion years
- **Triangulum Galaxy (M33)**: ~40 billion stars, a loosely wound spiral and the third largest member

Each major galaxy is attended by satellite galaxies. The Milky Way has about 60 known satellites, including the Magellanic Clouds and the Sagittarius Dwarf Galaxy (currently being disrupted and absorbed into the Milky Way).

### Active Galactic Nuclei

Some galaxies have unusually bright, luminous central regions — active galactic nuclei (AGN) powered by supermassive black holes actively accreting material. AGN come in multiple flavors depending on viewing angle and accretion rate: Seyfert galaxies, quasars, blazars, and radio galaxies. The most luminous quasars outshine their entire host galaxies by factors of hundreds, and the most powerful are detectable across the entire observable universe.

### Galaxy Types and Star Formation

The division between elliptical and spiral galaxies maps roughly onto a division in star formation activity. Spirals are typically star-forming — their spiral arms are traced by young blue stars, HII regions, and dark dust lanes. Ellipticals are 'red and dead' — their star formation largely ceased after a period of intense activity billions of years ago.

This 'quenching' of star formation in elliptical galaxies remains an active area of research. AGN feedback — the energy released by a supermassive black hole's activity driving gas out of the galaxy — is a leading explanation, as it heats and expels the gas reservoir needed for new star formation.