Galaxies, part 1: Crash Course Astronomy #38

Summary

This script explores the fascinating neighborhood of the Milky Way galaxy, highlighting its vibrant celestial activity. It delves into the historical debate between astronomers on the nature of 'spiral nebulae' and the subsequent revelation of galaxies as 'island universes.' The script also covers the classification of galaxies into elliptical, spiral, peculiar, and irregular shapes, and discusses the cosmic phenomena of galactic collisions and growth through galactic cannibalism. It concludes by reflecting on the humbling yet awe-inspiring scale of the universe.

Takeaways

• 🌌 We live in the Milky Way galaxy, which is part of a vast and active universe filled with various celestial bodies.
• 🔭 In the early 20th century, astronomers were still trying to understand the nature of 'nebulae', which appeared as faint fuzzies in the sky.
• 🌌 The Great Debate of 1920 between Harlow Shapley and Heber Curtis was pivotal in shaping our understanding of the universe's scale, with Curtis arguing for the existence of multiple galaxies.
• 🌌 Edwin Hubble's observations of Cepheid variables in the Andromeda nebula (M31) revealed it to be a separate galaxy outside the Milky Way, significantly expanding our view of the universe.
• 🌌 Galaxies are immense collections of stars, gas, and dust, varying greatly in size and shape, and are classified into four main types: elliptical, spiral, peculiar, and irregular.
• 🌀 Spiral galaxies, like the Milky Way, have rotating disks with central bulges and can be grand design or flocculent, with wide or tightly wound arms.
• 🌍 The structure of spiral galaxies is influenced by the angle from which we observe them, revealing different views from face-on to edge-on perspectives.
• 🌐 Elliptical galaxies range from nearly spherical to elongated shapes and are characterized by older stars and a lack of gas and dust, likely resulting from galactic collisions.
• 💥 Galactic collisions can lead to the formation of peculiar galaxies with distorted shapes or ring galaxies, as well as the growth of larger galaxies by consuming smaller ones.
• 🌌 Irregular galaxies are small and chaotic in shape, possibly due to collisions or simply being too small to form a coherent structure.
• 🌟 The Milky Way has dozens of satellite galaxies, including the Large and Small Magellanic Clouds, which are rich in gas and dust, indicative of ongoing star formation.

Q & A

What is the significance of the Milky Way galaxy in the context of the universe?

The Milky Way galaxy is significant as it is the galaxy in which we reside. It is a bustling neighborhood filled with stars, nebulae, and stellar clusters, and was a key focus in early 20th-century astronomy as scientists began to understand the nature of the universe.

What were the two competing ideas about the Universe in 1920?

In 1920, there were two main theories about the Universe: one suggested that the Milky Way was the entirety of the Universe, while the other proposed that the 'spiral nebulae' observed in the sky were separate galaxies, similar to the Milky Way, known as 'island Universes'.

Who were the astronomers involved in the debate about the nature of the Universe in 1920, and what were their positions?

Harlow Shapley and Heber Curtis were the astronomers involved in the debate. Shapley argued that the Milky Way was all there was, while Curtis believed that there were multiple galaxies, each like an 'island Universe'.

What was the observational evidence that initially supported Shapley's argument about the spiral nebulae?

Shapley noted that one of the spiral nebulae had been seen to rotate, which he believed indicated that it must be small. However, this observation was later proven to be incorrect.

What was the role of Edwin Hubble and Milton Humason in resolving the debate about the nature of the Universe?

Edwin Hubble and Milton Humason observed the great spiral nebula M31 in Andromeda using the largest telescope of their time. They discovered Cepheid variables, pulsating stars, which allowed them to measure the distance to M31, proving it was outside the Milky Way and thus part of a separate galaxy.

What are Cepheid variables and why are they important in astronomy?

Cepheid variables are stars that change their brightness in a regular, periodic manner. They are important because the time it takes for them to pulse is directly related to their luminosity. This relationship allows astronomers to determine the distance to these stars by measuring their apparent brightness.

What are the four major types of galaxies, and how are they classified?

The four major types of galaxies are elliptical, spiral, peculiar, and irregular. They are classified based on their overall shapes and structural characteristics.

What is unique about spiral galaxies, and how do they vary?

Spiral galaxies are characterized by broad, flat rotating disks of stars, gas, and dust, a central bulge of older stars, and sometimes a bar of stars. They vary in the size of their central bulges, the presence of a bar, and the structure of their spiral arms, which can be well-organized or patchy.

How do elliptical galaxies differ from spiral galaxies in terms of structure and composition?

Elliptical galaxies differ from spiral galaxies in that they lack the flat disk structure and have a more uniform, puffy shape. They are typically composed of older stars and lack gas and dust, indicating that star formation occurred eons ago.

What is the current understanding of how elliptical galaxies form?

Current theories suggest that elliptical galaxies form as a result of galactic collisions. The immense gravitational forces during these collisions can lead to the merging of galaxies, resulting in the formation of elliptical galaxies.

What is the role of collisions in the formation and evolution of galaxies?

Galactic collisions play a significant role in the formation and evolution of galaxies. They can lead to the creation of peculiar galaxies with distorted shapes, the merging of galaxies to form ellipticals, and the absorption of smaller galaxies by larger ones, contributing to their growth.

What are irregular galaxies and why are they considered shapeless?

Irregular galaxies are those that lack a recognizable shape, often appearing chaotic. They are typically small and may be the result of collisions that have distorted their structure or are too small to form a coherent shape.

What is the significance of the Large and Small Magellanic Clouds in relation to the Milky Way?

The Large and Small Magellanic Clouds are significant as they are the two largest satellite galaxies of the Milky Way. They are irregular in shape and are rich in gas and dust, with the Large Magellanic Cloud hosting the Tarantula Nebula, a major star-forming region.