Brown Dwarfs: Crash Course Astronomy #28

Summary

This script explores the intriguing concept of brown dwarfs, celestial objects that lie between planets and stars in terms of mass. Unable to initiate hydrogen fusion, they cool and radiate heat, eventually appearing cold and dark. Discovered relatively recently, brown dwarfs have expanded our understanding of the cosmos, with thousands now identified. They exhibit unique characteristics, such as not increasing in size with added mass, and their atmospheres can even rain molten iron. The script also ponders the possibility of an even closer star system to Earth than Proxima Centauri, challenging our current astronomical knowledge.

Takeaways

• 🌌 Stars are massive celestial bodies that undergo nuclear fusion in their cores, converting hydrogen into helium and emitting energy.
• 🌍 Planets, including gas giants like Jupiter, lack the mass necessary for core fusion and rely on gas pressure to balance their gravity.
• 🔥 Jupiter requires significantly more mass to initiate fusion, being only 1% of the mass needed to become a star.
• 🚀 The concept of a celestial body with a mass between that of a planet and a star, known as a brown dwarf, was introduced in the 1960s.
• 🔮 Brown dwarfs form similarly to stars but do not sustain heat due to the lack of fusion, eventually cooling and appearing black.
• 🏷️ The term 'brown dwarf' was coined by Jill Tarter, despite the fact that these celestial bodies are not actually brown.
• 🌡️ Astronomers classify stars by their temperature, with the coolest being M class stars, followed by L, T, and Y class stars, which include brown dwarfs.
• 🔬 The discovery of lithium in a celestial body's spectrum can indicate the presence of a brown dwarf, as it is not fused in their cores like in regular stars.
• 🌟 The first confirmed brown dwarf, Teide 1, was found in the Pleiades star cluster, with about 50 times the mass of Jupiter.
• 🌌 The Wide-field Infrared Survey Explorer (WISE) mission has discovered hundreds of brown dwarfs, expanding our understanding of these objects.
• 🌈 Brown dwarfs can exhibit unusual colors, including magenta, due to the absorption of specific light wavelengths by molecules in their atmospheres.

Q & A

What is the primary difference between stars and planets in terms of their ability to generate fusion?

Stars are massive enough to fuse hydrogen into helium in their cores, generating energy, whereas planets, even gas giants like Jupiter, are too small to generate fusion.

At what mass does an object become capable of initiating hydrogen fusion?

An object needs to have at least 0.075 times the Sun’s mass, or roughly 75 times the mass of Jupiter, to initiate hydrogen fusion.

What is a brown dwarf, and how does it differ from both stars and planets?

A brown dwarf is an object with more mass than a planet but not enough to initiate hydrogen fusion like a true star. It forms like a star but cools and doesn't sustain significant heat.

Who coined the term 'brown dwarf' and why?

Jill Tarter coined the term 'brown dwarf' to describe these objects that emit light in the infrared and almost nothing in the visible spectrum, even though stars can’t literally be brown.

What significant discovery did astronomers make in the Pleiades cluster in 1995?

Astronomers discovered Teide 1, a sub-stellar object with lithium in its spectrum, confirming it as the first true brown dwarf.

How do astronomers distinguish brown dwarfs from normal stars using lithium?

Brown dwarfs lighter than about 65 times the mass of Jupiter don't fuse lithium, so the presence of lithium in their spectrum helps distinguish them from normal stars.

What was unique about the brown dwarf companion of Gliese 229 discovered in the 1990s?

Gliese 229b had lithium and methane in its atmosphere, making it cooler than Teide 1 and leading to the creation of the T dwarf classification.

How did the Wide-field Infrared Survey Explorer (WISE) contribute to the study of brown dwarfs?

WISE, launched in 2009, scanned the entire sky in infrared light, discovering hundreds of brown dwarfs and identifying a new classification, Y dwarfs.

Why do some brown dwarfs appear magenta instead of red?

Some brown dwarfs have molecules in their atmospheres, like methane and water, which absorb specific colors of light, blocking more red light than blue, making them appear magenta.

What unusual characteristic do brown dwarfs have concerning their size and mass?

As brown dwarfs gain more mass, they get denser instead of larger, so even a brown dwarf twice as massive as Jupiter isn't much bigger in size.