ASTR 503 - Class 22 - Video 8 - Horizontal Branch and AGB stars

Summary

The video discusses the evolution of stars, particularly focusing on the horizontal branch and asymptotic giant branch (AGB) phases. Initially, helium burns steadily in the core, pushing the hydrogen burning shell outward. As the star transitions, it settles on the horizontal branch, showing constant brightness. Once helium is exhausted, a core of carbon ash is formed, and the star climbs the red giant branch again to become an AGB star. This phase is marked by thermal pulses and unstable behavior, ultimately leading to the ejection of a planetary nebula and the formation of a white dwarf.

Highlights

• Helium core burning is crucial for stars on the horizontal branch. 🔥
• Horizontal branch stars shine about 50 times brighter than the Sun. ☀️
• Stars move back to the red giant branch, evolving into AGB stars. 🌟
• Thermal pulses cause stars to become highly unstable as AGB stars. 🔄
• A planetary nebula is formed from the star's outer layers ejecting, leaving a white dwarf. 🌌

Key Takeaways

• Stars undergo fascinating transformations, moving from the red giant branch to the horizontal branch! 🌟
• The horizontal branch is where stars burn helium steadily in their cores. 🔥
• Massive stars can shine 50 times brighter than the Sun on this branch! ☀️
• Undergoing thermal pulses, stars transition into the asymptotic giant branch. 🔄
• The pulsating phase culminates in the formation of a beautiful planetary nebula. 🌌

Overview

Stars are always on the move through different stages of their life, and in this video, we explore one of the most crucial phases - the transition from giant stars. First off, helium burns steadily in the core during the horizontal branch phase, a pattern that makes our star shine bright like a diamond – or rather, about 50 times brighter than our Sun!

Once the helium is exhausted, things start to heat up with the star moving back up the red giant branch, eventually turning into asymptotic giant branch stars. It’s like a stellar rollercoaster! This phase is marked by thermal pulses that shake things up, leading to periods of dormancy followed by bursts of activity.

Finally, when the star can handle no more, it releases its outer layers in a stunning farewell, creating a planetary nebula. This grand finale leaves behind a dense and hot white dwarf, fittingly marking the end of a star's cycle. These stages shed light on the future of stars, giving us a glimpse into the universe’s life cycle.

ASTR 503 - Class 22 - Video 8 - Horizontal Branch and AGB stars Transcription

• 00:00 - 00:30 helium burns steadily in a core of ideal gas the hydrogen burning shell is pushed outwards by the new luminosity of the core which cools down and reduces the nuclear burning rate and does the luminosity the temperature gradient drops accordingly becoming sub-adiabatic and the deep convection zone that made the star a red giant seizes the star contracts and settles in the horizontal branch so the star would go here this is the
• 00:30 - 01:00 tip of the red giant branch it's going to then settle in this region which is a horizontal branch so the star contracts and saturn the horizontal branches the analog or the mean sequence for helium burning the horizontal branch is called horizontal branch because they appear at roughly constant absolute magnitude now this is because even though the spreading luminosities in this branch is roughly the same as in the main sequence stars of same
• 01:00 - 01:30 mass the average of luminosity is much much higher so the horizontal branch stars are about like 50 solar luminosities and because of magnitude our logarithmic scale or luminosity the same degree of linear dispersion at a larger decade will appear more constant if you plotted this in linear so in luminosity you'd see pretty much the same scatter in the horizontal branch that you'd see in the main sequence so this is what happened to the star the
• 01:30 - 02:00 star had the helium flash and then jumped to an ideal gas again where you have stable helium core burning and this is the structure of a horizontal brain star you have helium burning in the core that is depositing inert carbon and oxygen in the center that builds up you have a shell around here which is inert helium uh and then you have a hydrogen burning shell and the inert envelope that is not burning the hydrogen envelope in the hr diagram the
• 02:00 - 02:30 starcettos and the horizontal branch which is essentially the helium sequence now once a helium is exhausted an inert core carbon ash is left surrounding it you have a helium burning shell and a hydrogen burning shell during shell burning the core has zero luminosity and the situation is very similar to the hydrogen burning phase and as you exhausted the hydrogen and you have a leftover healing wash
• 02:30 - 03:00 and then this helium ash is contracting and the star climbs the red giant branch as a result the same thing is going to happen here the star is going to climb the red giant branch again and it's going to become a star of the asymptotic giant branch the carbon oxygen core contracts and heats up you have a phase where helium is being burned in a shell around the core so this is the helium burning shell and then after that you have the hydrogen burning shell and the hydrogen envelope from the
• 03:00 - 03:30 outside if you plot in uh hr diagram what you're gonna see is that the stars climbing again the red giant branch is going from the horizontal branch and it's climbing again here the chime branch now it's called the asymptotic giant branch in this final stage the star is very unstable having thermal pulses as the hydrogen burning layer ignites
• 03:30 - 04:00 and then dumps healing ash on the helium layer underneath the mass of helium increases and making it slightly degenerate what you're looking here is time and the luminosity this is the same plot as just continuing here have 10 here and have 12 which is time in a hundred thousands of years so 10 is about one million year and this is the time between the pulses again this pulses happen because you have hydrogen that is burning and
• 04:00 - 04:30 then it is depositing helium ash in the layer below making it slightly degenerate as you increase the density and then when the temperature at the base of the helium layer increases sufficiently a healing flash occurs in the shell the helium flash drives the hydrogen shell outwards which cools the star and turns off the hydrogen burning for a while as the helium is consumed in the triple alpha the star contracts heats up again and
• 04:30 - 05:00 the hydrogen shell then ignite again and drops helium ash on the helium layer and the process repeats in this thermal pulses this is a series of helium flashes occurring at the helium shells on the helium burning shells so these thermal pulses are essentially a series of helium flashes eventually as a star hits up the last helium flash produces enough energy to blow up the star ejecting the planetary nebula
• 05:00 - 05:30 so the planetary nebula are the gracious death of low mass stars that's when the last helium flash has enough energy to eject the outer layers of the atmosphere this is another planetary enabler and another planetary ambler what is left in the center is what is called a white dwarf