White Dwarfs & Planetary Nebulae: Crash Course Astronomy #30

Summary

This script from Crash Course Astronomy explores the fascinating final stages of low mass stars like our Sun. After helium fusion ceases, these stars become white dwarfs—hot, dense remnants about Earth-sized. They may also form planetary nebulae, intricate glowing structures from the gas expelled during the star's death throes. The nebulae's shape and color reveal the star's life story, offering insights into stellar evolution. Despite their beauty, these nebulae are fleeting, lasting only a few thousand years, a brief moment in the cosmos.

Takeaways

• 🌞 The Sun, like other low mass stars, will eventually become a white dwarf after a series of expansions and contractions, and then fade away over billions of years.
• 🤔 White dwarfs are dense and small, with a size comparable to Earth but with a mass close to the Sun's current mass, making them incredibly dense.
• 🔥 Newborn white dwarfs are extremely hot, with temperatures exceeding 100,000 degrees Celsius, causing them to glow white and emit radiation in the ultraviolet and X-ray spectrums.
• 🌌 Planetary nebulae are formed when the outer layers of a dying star are expelled and then illuminated by the intense radiation from the central white dwarf.
• 🌌 The name 'planetary nebula' is a misnomer, originating from their appearance through early telescopes, which resembled small green disks.
• 🌌 Planetary nebulae display a variety of shapes, from round to elongated, spiral, and with jets or tendrils, indicating complex processes during their formation.
• 🌀 The shapes of planetary nebulae can be influenced by binary star systems or the presence of orbiting planets that may have been swallowed by the expanding star.
• 💫 The glow of a planetary nebula is primarily due to hydrogen and oxygen in the expelled gas, with oxygen emitting a characteristic green color.
• 📚 The study of planetary nebulae provides insights into the life and death of stars, contributing to our understanding of stellar evolution.
• 🌠 The visibility of a planetary nebula is fleeting, lasting only a few thousand years before the gas disperses and the glow fades, making them rare sights in the cosmos.
• ☀️ Unlike more massive stars, the Sun is unlikely to form a visible planetary nebula when it dies, as it will not be energetic enough to excite the surrounding gas.

Q & A

What is the ultimate fate of low mass stars like our Sun?

Low mass stars like the Sun will eventually expand and contract, blow off their outer layers, become white dwarfs, and then fade away over billions of years.

What is a white dwarf and why is it considered an awesome object?

A white dwarf is the dense, hot remnant of a star that has exhausted its nuclear fuel. It's considered awesome due to its incredible density and the fact that it's supported by electron degeneracy pressure, a quantum mechanical phenomenon.

How does the Sun's core change as it ages?

As the Sun ages, it fuses hydrogen into helium and eventually helium into carbon, with traces of oxygen and neon. When it runs out of helium, fusion stops, and the core becomes nearly pure carbon, leading to the cessation of fusion.

What is electron degeneracy pressure and why is it significant in white dwarfs?

Electron degeneracy pressure is a quantum mechanical effect where electrons resist being squeezed together more strongly than simple electric repulsion. It becomes the dominant force supporting the core of a star after helium fusion stops, leading to the formation of a white dwarf.

What is the size of a white dwarf in comparison to the original star?

A white dwarf is about the size of Earth, which is only 1% of the original width of a star like the Sun.

How dense is a white dwarf and what does a cubic centimeter of its material weigh?

A white dwarf is incredibly dense, with a single cubic centimeter of its material weighing about a million grams, or one metric ton.

What is the gravity on the surface of a white dwarf compared to Earth's gravity?

The gravity at the surface of a white dwarf is extremely high, easily topping 100,000 times the Earth's gravity.

What causes the gas around a newly formed white dwarf to glow?

The intense radiation emitted by the hot white dwarf causes the nearby gas, which was expelled during the star's final stages, to glow in response, forming what is known as a planetary nebula.

What is a planetary nebula and why is it called so?

A planetary nebula is a glowing cloud of ionized gas expelled from a star during its final stages, illuminated by the ultraviolet radiation from the central white dwarf. It was named so because early astronomers, observing them through telescopes, mistook them for small green disks similar to planets.

Why do planetary nebulae come in various shapes and not just spherical?

Planetary nebulae can have various shapes due to factors such as the star's rotation, the presence of a binary companion, and possibly interactions with orbiting planets, which can cause the expelled gas to form complex structures.

What element is responsible for the green glow in planetary nebulae and why was it initially a mystery?

Oxygen is responsible for the green glow in planetary nebulae. Initially, astronomers couldn't identify the element and called it nebulium, but later it was discovered to be extremely diffuse oxygen.

Why is the phase of a planetary nebula brief and rare to observe?

The phase of a planetary nebula is brief because the gas expands and thins out over a few thousand years, eventually stopping to glow. Although billions of stars die this way in the galaxy, this phase is short, making it rare to observe.

Will our Sun eventually form a planetary nebula when it dies?

It is unlikely that our Sun will form a planetary nebula when it dies. Most planetary nebulae start off as stars more massive and hotter than the Sun, and when our Sun dies, it will do so quietly without much visible fanfare.