galaxy

History

Before the 1920s, astronomers debated whether 'nebulae' were separate galaxies. In 1923, Edwin Hubble, observing Cepheids in Andromeda, proved that it was another galaxy. Hubble also created a classification of galaxies by shape.

How it works

Stars in a galaxy orbit around a common center of mass, following the law of universal gravitation. Spiral arms are density waves where gas compresses and new stars are born, like a traffic jam: stars move through the arms, but in the jam zone their density is higher.

History

Before the 1920s, astronomers debated whether 'nebulae' were separate galaxies. In 1923, Edwin Hubble, observing Cepheids in Andromeda, proved that it was another galaxy. Hubble also created a classification of galaxies by shape.

How it works

Stars in a galaxy orbit around a common center of mass, following the law of universal gravitation. Spiral arms are density waves where gas compresses and new stars are born, like a traffic jam: stars move through the arms, but in the jam zone their density is higher.

History

The term 'galaxy' traces back to the ancient Greek 'milky circle'. In the 1750s, Immanuel Kant suggested that faint nebulae were 'island universes'. William Herschel in the late 18th century compiled the first catalog of nebulae. Decisive proof came from Edwin Hubble in 1923–1924 using the 100-inch Mount Wilson telescope, discovering Cepheids in M31 and Andromeda, which allowed distance measurements far exceeding the size of the Milky Way. Hubble classified galaxies in 1926, and later, with his redshift law, proved the expansion of the Universe. Vera Rubin in the 1970s confirmed the existence of dark matter through galactic rotation curves.

How it works

The stellar and gas components of a galaxy move in a common gravitational potential, including the contribution of visible matter and the dark halo. Rotation curves of spiral galaxies show anomalously high velocities at the periphery, indicating the existence of dark matter (the law of universal gravitation would predict a velocity drop if all mass were visible). Spiral arms are explained by the density wave theory of Lin and Shu (1964): they are quasi-stationary structures where stellar orbits intersect and compress gas, triggering star formation. The formation of large-scale galaxy structure follows from primordial density fluctuations observed in the cosmic microwave background, described by the Friedmann equations in cosmology. Hubble's law links the redshift of galaxies to distance, which underlies the Big Bang model.

Open problems & fun facts

Nature of dark matter: WIMPs or axions, alternative theories of gravity.; How do supermassive black holes at galaxy centers reach billions of solar masses in a short cosmological time?; Relation between galaxy morphology and environment: why do clusters have predominantly elliptical galaxies, while the field has spirals?; The role of active galactic nuclei and feedback in regulating star formation.; The cusp problem in dark halo density profiles: simulations predict a steep center, observations show a core.

💡 If you could travel at the speed of light, crossing the Milky Way would take 100,000 years—longer than human history. And the Andromeda Galaxy is approaching us and will collide with the Milky Way in 4 billion years.