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Galaxy NGC4414 from HST Astronomy 162:
Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30

Lecture 17: The Evolution of High-Mass Stars

Readings: Chapter 22, section 22-5

Key Ideas

High-Mass Stars = O & B Stars (M > 4 Msun)

Stars with 4 < M < 8 Msun
Burn Hydrogen up through Carbon
Blow off their envelope
Core becomes an O-Ne-Mg White Dwarf

Stars with M > 8 Msun
Burn Hydrogen up through Carbon, Neon, Oxygen & Silicon
Iron Core Formation & burning shells
Catastrophic collapse of Iron Core.

High-Mass Stars

O & B Stars (M > 4 Msun):

Main Sequence Phase:


Red Supergiant Phase

After H core exhaustion:

Red Supergiant Star Interior

Moves horizontally across the H-R diagram, becoming a Red Supergiant star

Red Supergiant Branch

Takes about 1 Myr to cross the H-R diagram.


Helium Flash

Core Temperature reaches 170 Million K

Ignites Helium burning to C & O:

Star becomes a Blue Supergiant.

Blue Supergiant Phase

He Core Exhaustion

When He runs out in the core:

End of Blue Supergiant Phase to Second Red Supergiant Phase

C-O Core collapses until:


Carbon Burning:

Nuclear reaction network: 12C+12C fuses to:

Build up an inert O-Ne-Mg core

Very inefficient:

Core of the Star at the end of the Carbon Burning Phase:

Interior of a massive star
     at the end of the Carbon-Burning Phase

Intermediate Mass Stars

Stars with 4 < M < 8 Msun

After 1000 years:

Thermal pulses destabilize the envelope:


High Mass Stars: M > 8 Msun

At the onset of Carbon Burning:

Exception: Strong stellar winds can erode the envelope, changing the outward appearance of the star.

[For example, the massive star Eta Carina has immense stellar winds. Hubble reveals a furiously expanding pair of dust and gas clouds billowing off this star. See this page at STScI for more details & pictures.]


Neon Burning

O-Ne-Mg core contracts & heats up until:

Ignite Neon burning:

Lasts for a few years before Ne runs out.


Oxygen Burning

Ne runs out, core contracts & heats up until:

Ignite Oxygen burning:

Lasts for ~ 1 year before O runs out.


Silicon Burning

O runs out, Si core contracts & heats up until:

Ignite Silicon burning:

Lasts for about 1 day...

Core of a massive star at the end of Silicon Burning:

Core of of a massive star at the end of the Silicon Burning Phase

The Nuclear Impasse

Fusion of light elements is the release of nuclear binding energy.

Iron (Fe) is the most tightly bound nucleus:

Once an Fe core forms, there are no new fusion reactions left for the star to tap.


End of the Road

At the end of the Silicon Burning Day:

Finally, the Fe core exceeds 1.2-2 Msun:


Summary:

Stars with 4 < Mass < 8 Msun

Stars with Mass > 8 Msun


Return to [ Unit 2 Index | Astronomy 162 Main Page ]
Updated: 2006 January 21
Copyright Richard W. Pogge, All Rights Reserved.