Galaxies

Galaxies are massive systems of stars, gas, dust, and dark matter bound together by gravity. They’re the fundamental building blocks of the universe on the largest scales. Our own Milky Way contains somewhere between 200 to 400 billion stars, and there are roughly 2 trillion galaxies in the observable universe.

Galaxies come in three main types based on their structure. Spiral galaxies like the Milky Way and Andromeda have flat rotating disks with spiral arms and a central bulge. Elliptical galaxies are smooth, featureless spheroids ranging from nearly spherical to highly elongated. Irregular galaxies lack any organized structure - the Large and Small Magellanic Clouds visible from the southern hemisphere are examples.

The spiral arms in spiral galaxies aren’t fixed structures. They’re density waves - regions where stars, gas, and dust bunch up temporarily as they orbit the galactic center. Stars move through the spiral arms like cars passing through a traffic jam. The arms appear bright because massive, short-lived stars form in the compressed gas within them. These stars are so luminous and die so quickly that they never leave the spiral arms before exploding as supernovae.

Elliptical galaxies are very different. They contain mostly old, red stars with little gas or dust left for new star formation. They’re thought to form through galaxy mergers - when two spiral galaxies collide and merge, the collision disrupts their ordered rotation and scrambles their structure into an elliptical shape. The central regions of galaxy clusters are dominated by giant elliptical galaxies that have cannibalized their neighbors over billions of years.

At the center of nearly every large galaxy sits a supermassive black hole. These monsters range from millions to billions of times the mass of the sun. The black hole at the center of the Milky Way, Sagittarius A*, has a mass of about 4 million solar masses. The black hole in the giant elliptical galaxy M87 weighs in at 6.5 billion solar masses - this is the black hole imaged by the Event Horizon Telescope in 2019.

BTW galaxies aren’t evenly distributed through space. They cluster together in groups and larger structures called galaxy clusters. The Local Group contains about 80 galaxies including the Milky Way, Andromeda, and Triangulum. The Virgo Cluster, about 50 million light-years away, contains over 1,500 galaxies. These clusters are themselves organized into superclusters forming vast filaments and walls separated by enormous voids.

Galaxies are moving. The universe is expanding, carrying distant galaxies away from us. The farther away a galaxy is, the faster it recedes - this is Hubble’s Law. Andromeda is an exception. It’s moving toward the Milky Way at about 110 kilometers per second. In roughly 4.5 billion years, the two galaxies will collide and merge into a giant elliptical galaxy.

Photographing galaxies requires serious equipment and technique. Most galaxies are faint, extended objects requiring long exposure times. The Andromeda Galaxy spans about 3 degrees of sky - six times the width of the full moon - but most of it is too faint to see without long exposures. Deep-sky imaging often involves stacking dozens or hundreds of exposures to reveal faint details in galactic structure.

The distances to galaxies are staggering. The Andromeda Galaxy is 2.5 million light-years away - the light we see left Andromeda when early humans were evolving in Africa. The most distant galaxies observed by telescopes like James Webb formed less than 400 million years after the Big Bang, over 13 billion years ago. We’re seeing them as they were in the early universe.

Galaxies evolve over cosmic time. Early galaxies were smaller and more irregular. They grew through mergers and by accreting gas from their surroundings. Star formation rates were much higher in the early universe. Most galaxies today are in a quieter phase, forming stars at much lower rates than their ancestors billions of years ago.