Galaxies, part 2: Crash Course Astronomy #39

Summary

Phil Plait discusses galaxies, focusing on their shapes, behavior, and environments. He explains the discovery of quasars, extremely luminous galaxies powered by supermassive black holes at their centers. Plait describes how these black holes generate immense energy through accretion disks and jets. He also explores galaxy clusters, superclusters, and the vast cosmic structures, emphasizing the scale and interconnectedness of the universe. The Hubble Deep Field reveals the staggering number of galaxies, underscoring the universe's immensity and the significance of human curiosity in understanding it. The Milky Way's future collision with the Andromeda galaxy is also highlighted.

Takeaways

• 🌌 Galaxies are vast collections of gas, dust, and hundreds of billions of stars, with the Milky Way being a gigantic disk galaxy with spiral arms.
• 🔭 In the 1960s, the discovery of 3C273 revealed a new type of luminous object in the universe, which was later named 'quasar', indicating an active galaxy.
• 🌌 Active galaxies emit enormous amounts of energy due to supermassive black holes at their centers, which can outshine the light of billions of stars.
• 🌀 The Hubble Space Telescope confirmed that every large galaxy has a supermassive black hole in its core, with these black holes likely forming at the same time as the galaxies themselves.
• 🔥 The intense gravitational pull of supermassive black holes can create an accretion disk, where friction and other forces heat the infalling matter to millions of degrees, emitting light across the electromagnetic spectrum.
• 💥 Active galaxies can also have jets of matter and energy launched from their poles, which can travel for hundreds of thousands of light years.
• 🌌 The Milky Way contains a supermassive black hole, but it is quiescent and not actively feeding, which is why we are not considered an active galaxy.
• 💥 Galactic collisions can trigger active galactic behavior, as gas and dust are funneled towards the central black holes, potentially activating them.
• 🌌 The Andromeda galaxy is on a collision course with the Milky Way, which will result in a merger to form a larger galaxy called 'Milkomeda' in a few billion years.
• 🌌 Galaxies are not isolated; they exist in groups like the Local Group and in larger clusters, which in turn can form superclusters, the largest known structures in the universe.
• 🌌 The universe is estimated to contain hundreds of billions of galaxies, each with billions of stars, highlighting the immense scale and complexity of the cosmos.

Q & A

What are galaxies and what are some of their classifications based on shape?

Galaxies are vast collections of gas, dust, and hundreds of billions of stars. They can be classified based on their shape into disk galaxies like the Milky Way with sprawling spiral arms, elliptical galaxies, irregular galaxies, and peculiar galaxies.

What is a quasar and how was it discovered?

A quasar is an extremely luminous active galactic nucleus, which was initially discovered as 3C273. It appeared as a blue star through optical telescopes but was found to be a powerful source of light in the radio spectrum, leading to its discovery as a very distant and energetic galaxy.

How do astronomers classify active galaxies?

Astronomers classify active galaxies based on their behavior, emission of light, and spectra. They are known to emit high-energy light such as X-rays and gamma rays, indicating their highly energetic nature.

What powers the immense energy output of active galaxies?

The immense energy output of active galaxies is powered by supermassive black holes at their cores. These black holes accrete matter, which forms an accretion disk and heats up due to friction, emitting light across the electromagnetic spectrum.

How do supermassive black holes form and grow?

Supermassive black holes are believed to form at the same time as galaxies. As material coalesces to create a galaxy, some of it falls to the center and feeds the black hole, causing it to grow as its host galaxy does.

What is the role of an accretion disk in powering active galaxies?

An accretion disk is a flat disk of matter that forms around a black hole as it accretes material. The matter in the disk heats up due to friction and other forces, emitting light across the electromagnetic spectrum, which powers the active galaxy.

What are jets in the context of active galaxies and how are they formed?

Jets in active galaxies are beams of matter and energy that are launched away from the black hole, along the poles of the accretion disk. They are formed by magnetic fields coupled with the rotation of the accretion disks, and can extend for hundreds of thousands of light years.

How does the viewing angle of an accretion disk affect the appearance of active galaxies?

The viewing angle of an accretion disk affects the type of light observed from the active galaxy. If seen edge-on, the dust in the disk blocks high-energy light, but infrared is visible. If the disk is tipped, more optical and high-energy light is seen. If the poles are aimed at the observer, X- and gamma rays are visible.

What is the Local Group and how does it relate to the Milky Way?

The Local Group is a small collection of galaxies that includes the Milky Way and the Andromeda Galaxy, among others. It is an elongated group with the Milky Way and Andromeda on opposite ends, and is part of the larger cosmic structure of the Universe.

What is the fate of the Milky Way and Andromeda galaxies?

The Milky Way and Andromeda galaxies are on a collision course. They will eventually merge into a single, larger galaxy, possibly an elliptical one, referred to as 'Milkomeda', in several billion years.

What is the significance of the Hubble Deep Field and how does it contribute to our understanding of the Universe?

The Hubble Deep Field is an image taken by the Hubble Space Telescope that shows thousands of galaxies in a tiny section of the sky. It demonstrates the vast number of galaxies in the Universe and helps astronomers estimate the total number of galaxies, which is around a hundred billion.