This Set of Slides Continuing Stellar Evolution Star “Death” (general and low-mass stars) Units 62, 63, and 64.

A Reminder of a Star’s Internal Processes The balance of forces in the interior of a star is delicate, though stable for millions or billions of years. –A star acts like it has a thermostat. –If internal temperature decreases, internal pressure decreases, and the star collapses a little, raising the temperature. When hydrogen in the core is exhausted, the thermostat breaks…

Evolution to red giant phase The star collapse causes an increase in internal pressure and temperature. Hydrogen surrounding the core then fuses. Shell fusion increases (once again) the inner pressure. The star expands and cools; luminosity increases even though its surface temperature decreases. Position on the HR diagram shifts up and to the right.

Helium Fusion Normally, the core of a low mass star is not hot enough to fuse helium. –Electrostatic repulsion of the two charged nuclei keeps them apart. The core of a red giant star can reach very high temperatures. –If the temperature is high enough - 100,000,000 K - helium fusion begins.

A (temporary) new lease on life The triple-alpha process (helium fusing at core) provides a new energy source for giant stars. Temperature increases until helium runs out. The stars cool and then expand once again. The end is near!

Variable Stars Variable stars change their brightness over time, dimming and brightening again.

Light Curves To characterize the variability of a star, scientists measure the brightness, and plot it as a function of time. This is a light curve. Different types: –Irregular Variable Nova (death) T Tauri stars (birth) –Pulsating Variable Periodic changes in brightness

A Fluctuating “Balance” Pulsation of a Yellow Giant Star

The Period-Luminosity Relation Cepheid and RR Lyrae Variables Henrietta Swan Leavitt - discoverer

DISTANCE! The period-luminosity relationship tells us the luminosity of a distant variable star if we measure the period. If we know it’s luminosity and apparent brightness, we can calculate the distance to that star. But this works for (variable) stars even in other galaxies – millions of lightyears away! THIS is how we’ve come to know how big this universe really is! (At least one MORE way!) Why do we care about Cepheid Variable stars?

The Sun’s Lifetime: –10 billion years on the main sequence. –Once the hydrogen is consumed, it will enter the red giant phase. –Helium fusing begins, starting the yellow giant phase. –Once helium is consumed, core contracts, reheats, and the outer surface expands, beginning the red supergiant phase. –Core begins to cool and the outer envelope expands again, forming a planetary nebula. –The core remains as a white dwarf. The Fate of Sun-like Stars

The Life-path of the Sun

As a red giant expands, it cools –Outer layers cool enough for carbon flakes to form. –Flakes are pushed outward by the radiation pressure. –Stellar gas is also dragged/forced outwards. –This drag creates a high-speed stellar wind blowing the outer layers outward forming a –Planetary nebula. –A confusing name – it has nothing to do with planets. Formation of Planetary Nebula

Nebula Shape

At the center of the planetary nebula lies the core of the star, a white dwarf. This is the corpse. –Degenerate material. –Incredibly dense. Initially the surface temperature is 25,000 to 100,000 K. (Quite hot!) Cools slowly, until it fades from sight – a black dwarf. White Dwarf Stars