Binary and Multiple Stars: Crash Course Astronomy #34

Summary

This script explores the fascinating world of binary and multiple star systems, debunking the myth that stars like our Sun are solitary. It explains how many stars are gravitationally bound in pairs or groups, with some even merging into contact binaries. The episode delves into visual, spectroscopic, and eclipsing binaries, revealing how these systems contribute to our understanding of stellar masses, sizes, and life cycles. It also touches on the dramatic phenomena of stellar novae and supernovae, highlighting the importance of binary stars in astrophysics.

Takeaways

• 🌞 The Sun is the only star in our solar system, with planets, moons, asteroids, and comets orbiting it, but no other star is part of our cosmic family.
• 🔭 Telescopes reveal that many stars in the sky are not single, but travel with companions, contrary to what the naked eye might suggest.
• 👥 Some stars appear close together by chance and are called 'optical double stars', while others are physically orbiting each other and are known as 'binary stars'.
• 🪐 It's estimated that about a third to half of all stars are part of binary or multiple star systems, indicating that stars often form in groups.
• 🌌 The naked eye can see a binary star system in the Big Dipper's handle, where Mizar and Alcor are two stars that have been historically used as an eye test.
• 🌟 Binary stars are thought to form together from dense lumps in a gas cloud, collecting material until they become true stars.
• 🔬 Visual binaries can be observed with telescopes and, over time, their orbital motion can provide crucial information about their masses and other properties.
• 🌈 Spectroscopic binaries are close enough that they cannot be separated with telescopes, but their motion is revealed through the Doppler shift in their light spectra.
• 💫 Mizar and Alcor, part of the Big Dipper, are part of a sextuple star system, with Mizar being a binary of spectroscopic binaries and Alcor being a spectroscopic binary itself.
• 🌠 Stars can form in larger groups beyond binaries, such as triple, quadruple, and even sextuple systems, with Polaris being an example of a pentuple system.
• 💥 Some binary stars, like 4U 1820-30, have such strong gravity that they orbit each other in minutes, while others can have orbits that last centuries.
• 🌑 Eclipsing binaries are systems where one star passes in front of the other, causing a dip in the total light observed, providing insights into their sizes and distances.
• 🤝 Contact binaries are stars that have become so close they physically touch, sometimes leading to mass transfer and dramatic changes in their evolution.
• 💥 Mass transfer in close binaries can lead to periodic explosions called 'stellar novae', and if a white dwarf reaches a critical mass, it can result in a supernova explosion.

Q & A

What is the Sun's status in the solar system regarding other stars?

The Sun is the only star in our solar system, with planets, moons, asteroids, and comets orbiting it, but no other stars are part of our solar system family.

What is the common misconception about stars based on visual observation?

The common misconception is that stars, like the Sun, are single entities, but in reality, many stars have companions and travel the universe together.

What are 'optical double stars'?

Optical double stars are stars that appear close together in the sky by coincidence, even though they are actually very far apart in space.

What distinguishes binary stars from optical double stars?

Binary stars are stars that are physically orbiting each other, as opposed to optical double stars, which only appear close together due to line-of-sight coincidence.

What fraction of stars are estimated to be part of a binary or multiple star system?

Approximately a third to a half of all stars in the sky are part of a binary or multiple star system.

Which well-known binary star system is visible to the naked eye and has historical significance?

The binary star system consisting of Mizar and Alcor, visible in the handle of the Big Dipper, is known for being used as an eye test in ancient times.

How do binary stars typically form?

Binary stars almost certainly form together from two dense lumps in the same gas cloud, each collecting material until they become true stars.

What is the significance of visual binaries in the field of astrophysics?

Visual binaries are important because observing their orbital motion allows for the determination of the actual size and shape of their orbits, and subsequently, the masses of the stars, which is crucial for understanding stellar properties.

How do spectroscopic binaries differ from visual binaries?

Spectroscopic binaries are stars that orbit so closely together they cannot be split even with the largest telescopes; their binary nature is revealed through the Doppler shift observed in their spectra as they move towards and away from us.

What is unique about the binary star system Mizar and Alcor in terms of their classification?

Mizar and Alcor form a sextuple star system, with Mizar being a visual binary of spectroscopic binaries and Alcor being a spectroscopic binary.

What is the phenomenon where a binary star system's orbit appears edge-on from Earth, causing periodic dimming?

This phenomenon is known as an eclipsing binary, where one star passes in front of the other, blocking its light and causing a dip in the total light observed from the system.

What are contact binaries and how do they affect the stars involved?

Contact binaries are binary stars that are so close together they physically touch, potentially stretching into teardrop shapes due to tidal effects and leading to mass transfer between the stars, which can alter their evolution.

What is the Algol Paradox and how does it occur?

The Algol Paradox refers to a contact binary system where the more evolved star is the white dwarf, which is the opposite of what is typically expected. This occurs due to mass transfer from the initially more massive star to the initially less massive star, reversing their roles over time.

What are recurrent novae and how do they form?

Recurrent novae are a type of binary star system where a white dwarf accretes matter from a companion star, and when enough hydrogen accumulates on the white dwarf's surface, it undergoes a thermonuclear explosion, creating a bright flare. This process can repeat periodically.

What catastrophic event can occur if a white dwarf in a binary system accumulates enough mass?

If a white dwarf in a binary system accumulates a mass around 1.4 times that of the Sun, it can undergo a runaway thermonuclear event, leading to a supernova explosion, which completely destroys the star.