April11

Units to cover: 64, 65, 66, 67

More than 33-A, 30-AB, 27-B, 24-BC, 21-C, 16-D, rest is F

Light Curves

• To characterize the variability of a star, scientists measure the brightness, and plot it as a function of time. – Light Curves

• Different kinds of variability – Irregular Variable • Novae (death) • T Tauri stars (birth)

– Pulsating Variable • Periodic changes in brightness

Yellow Giants and Pulsating Stars

• If you plot the positions of variable stars on the HR diagram, many of them fall in the “instability strip” – Most have surface temperatures of ~5000K, so appear yellow – Most are giants (Yellow Giants) – Instability comes from partial absorption of radiation in the interior of the star • Helium absorbs radiation, and the outer layers of the star get pushed away from core • As the star expands, the density decreases, letting photons escape • Outer layers head back inward toward core • Repeat

– RR Lyrae and Cepheid variables are useful for finding distances to the stars, as the star’s period is proportional to its luminosity.

The Valve Mechanism

Cepheid and RR Lyrae Variables

The Period-Luminosity Relation

Periods of Variable Stars

In terms of nuclear reactions, what is the next stage of a star's life after the end of hydrogen burning in the core?

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a. Hydrogen burning in a thin shell around the core b. Helium burning in the core c. carbon burning d. death

What makes a red giant star so large?

• a. The star has many times more mass than the Sun. • b. The helium-rich core expanded, pushing the outer layers of the star outward. • c. Red giants are rapid rotators, and the centrifugal forces pushes the surface of the star outwards. • d. The hydrogen-burning shell is heating the envelope and making it expand.

A Cepheid variable is

• a. a low mass red giant that varies in size and brightness in an irregular way • b. a big planet • c. a high-mass giant or supergiant star that pulsates regularly in size and brightness • d. a variable emission nebula near a young star

The Fate of Sunlike 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 burning begins, starting the yellow giant phase

– Once helium is consumed, core contracts and outer envelope 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 Life-path of the Sun

Formation of Planetary Nebula

• As a red giant expands, it cools – Outer layers cool enough for carbon flakes to form – Flakes are pushed outward by radiation pressure – Flakes drag stellar gas outward with them

– This drag creates a highspeed stellar wind! – Flakes and gas form a planetary nebula

The Hourglass Nebula

White Dwarf Stars

• At the center of the planetary nebula lies the core of the star, a white dwarf – Degenerate material – Incredibly dense

• Initially the surface temperature is around 25,000 K • Cools slowly, until it fades from sight.

Figure 64.05e

Our Sun will end its life by becoming

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A. a molecular cloud B. a pulsar C. a white dwarf D. a black hole

Mass Transfer and Novae

• A Roche lobe can be seen as a sphere of gravitational influence around a star • Red Giant stars can fill their Roche lobes • In a binary star system, the Roche lobes of the two stars can touch, and mass can pass between them.

• If a white dwarf is in orbit around a red giant companion star, it can pull material off the companion and into an accretion disk around itself • Material in the accretion disk eventually falls to the surface of the white dwarf

Novae

• If enough material accumulates on the white dwarf’s surface, fusion can be triggered, causing a massive explosion • This explosion is called a nova • If this process happens repeatedly, we have a recurrent nova.

A Post-nova expansion

The Chandrasekhar Limit and Supernovae

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If mass is added to a white dwarf, its gravity increases If the white dwarf mass exceeds 1.4 solar masses (the Chandrasekhar Limit), the end of the white dwarf is near. The additional gravity squeezes the degenerate material in the white dwarf, causing it to compress by a small amount

• This compression causes the temperature to soar, and this allows carbon and oxygen to begin to fuse into silicon • The energy released by this fusion blows the star apart in a Type 1a supernova

Type 1a Supernova – Another standard candle!

• The light output from a Type 1a supernova follows a very predictable curve – Initial brightness increase followed by a slowly decaying “tail”

• All Type 1a supernova have similar peak luminosities, and so can be used to measure the distance to the clusters or galaxies that contain them!

Formation of Heavy Elements • Hydrogen and a little helium were formed shortly after the Big Bang • All other elements were formed inside stars! • Low-mass stars create carbon and oxygen in their cores at the end of their lifespan, thanks to the higher temperatures and pressures present in a red giant star • High-mass stars produce heavier elements like silicon, magnesium, etc., by nuclear fusion in their cores – Temperatures are much higher – Pressures are much greater • Highest-mass elements (heavier than iron) must be created in supernovae, the death of high-mass stars

The Lifespan of a Massive Star

Layers of Fusion Reactions

• As a massive star burns its hydrogen, helium is left behind, like ashes in a fireplace • Eventually the temperature climbs enough so that the helium begins to burn, fusing into Carbon. Hydrogen continues to burn in a shell around the helium core • Carbon is left behind until it too starts to fuse into heavier elements. • A nested shell-like structure forms. • Once iron forms in the core, the end is near…

Core Collapse of Massive Stars

• Iron cannot be fused into any heavier element, so it collects at the center of the star • Gravity pulls the core of the star to a size smaller than the Earth’s diameter! • The core compresses so much that protons and electrons merge into neutrons, taking energy away from the core • The core collapses, and the layers above fall rapidly toward the center, where they collide with the core material and “bounce” • The “bounced material collides with the remaining infalling gas, raising temperatures high enough to set off a massive fusion reaction. The star then explodes. • This is a supernova!

Before and After – a Supernova

Light Curve for a Supernova

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The luminosity spikes at the moment of the explosion, and gradually fades, leaving behind a…

The Crab Nebula