The Milky Way: Crash Course Astronomy #37

Summary

This script explores the structure of the Milky Way galaxy, describing it as a vast, flat disk of gas, dust, and stars with the Sun located halfway from the center. It discusses the galaxy's spiral arms, central bar of old red stars, and halo of old stars, highlighting the ongoing discovery and mapping of these celestial features.

Takeaways

• 🌌 The Milky Way is a vast, flat, disk-shaped galaxy composed of gas, dust, and stars, spanning 100,000 light years across and several thousand light years thick.
• 🌞 The Sun is one of hundreds of billions of stars in the Milky Way, located approximately halfway from the center, in what is considered the 'suburbs' of the galaxy.
• 🌀 The galaxy features two major and two minor spiral arms, which converge roughly in the middle of the galaxy, with a bar of old red stars at the center.
• 🌌 The Milky Way's name originates from its appearance as a faint, white glow in the night sky, which the ancient Greeks called 'Galaxius', meaning 'milky'.
• 🔭 Galileo's observations with a telescope revealed that the glow of the Milky Way was actually composed of countless individual stars, too faint to be seen by the naked eye.
• 🌌 Astronomers initially thought the galaxy was oval or a flattened disk with the Sun near the center, based on star counting, but this method was flawed due to the influence of interstellar dust.
• 🌐 Globular clusters, which orbit the galaxy outside its main body, provided clues that the Sun is not at the center but rather off-center, about 25,000 light years from the galactic center.
• 🌌 The Milky Way's disk is divided into spiral arms, discovered through radio astronomy, where young, massive, and bright stars are predominantly found.
• 🚗 The spiral arms are not fixed structures but rather 'traffic jams' of stars and nebulae, moving through the galaxy and maintaining the spiral pattern through ongoing star formation.
• 🌌 The central region of the galaxy, known as the bulge, is actually a bar of older, redder stars, contrasting with the active star formation in the spiral arms.
• 🌌 The Milky Way also has a halo, a spherical cloud of old stars surrounding the galaxy, with most of the globular clusters orbiting within it.

Q & A

What is the Milky Way galaxy?

The Milky Way is a vast, flat, disk-shaped collection of gas, dust, and stars, spanning approximately 100,000 light years across and several thousand light years thick, with the Sun being one of hundreds of billions of stars within it.

Where is the Sun located within the Milky Way galaxy?

The Sun is situated about halfway out from the center of the Milky Way, in what can be considered the galactic 'suburbs'.

What are the two major components of the Milky Way's disk?

The disk of the Milky Way has two major spiral arms and two minor ones, which converge more or less in the middle of the galaxy.

What is the bar at the center of the Milky Way made of?

The bar at the center of the Milky Way is an elongated, roughly cylindrical collection of old red stars.

How did ancient Greeks describe the glow of the Milky Way?

The ancient Greeks called the glow 'Galaxius' or 'milky' due to its dull white appearance, which is the origin of the name Milky Way.

What was Galileo's discovery when he observed the Milky Way through his telescope?

Galileo discovered that the glow of the Milky Way was actually composed of thousands of stars that were too faint and closely packed to be seen individually with the naked eye.

Why did the initial star counting method used by astronomers to determine the shape of the galaxy have limitations?

The initial star counting method was flawed because it did not account for interstellar dust, which absorbs visible light from stars, hiding them from view and distorting the perception of the galaxy's shape.

What evidence suggested that the Sun is not at the center of the Milky Way?

The uneven distribution of globular clusters and the brighter glow of the Milky Way in the direction of Sagittarius suggested that the Sun is not at the center but is located about 25,000 light years from the galactic center.

How do spiral arms in the Milky Way form and why do they not wind up over time?

Spiral arms form due to disturbances in the disk, such as an overdense region created by the collision of giant molecular clouds. They do not wind up over time because they are not fixed structures but rather moving waves of stars and nebulae that enter and exit the arms.

What is the central region of the Milky Way called, and what is its structure like?

The central region is called the 'bulge,' which is actually a bar-shaped structure of older red stars that is pointed nearly directly at us, making it appear circular.

What is the role of the supermassive black hole at the center of the Milky Way?

The supermassive black hole at the center of the Milky Way, with a mass four million times that of the Sun, is believed to have played a significant role in the galaxy's formation and evolution, although it is a very small part of the galaxy by mass.

What is the halo of the Milky Way, and what types of stars are found there?

The halo of the Milky Way is a vast, spherical cloud of old stars surrounding the galaxy out to great distances, more than 100,000 light years. No star formation occurs in the halo, and it is also where most of the globular clusters orbiting the Milky Way are located.