astronomy 1020-h stellar astronomy spring_2015 day-35

Astronomy 1020-HSpring_2015

Day-35Stellar Astronomy

To continue to burn hydrogen with all that helium in the way, the core of the star gets a little hotter, the surface

gets a little bigger and the star gets a little brighter .

Course Announcements• Dark night Alternative exercise is posted.• Reports are due Wed. Apr. 22• Solar Rotation Project due Mon. Apr. 27

• Final Exam (and Exam-4) is (are) scheduled for Thursday, May 7, 1:30-3:30pm.

• IF there is consensus, this could be moved to a different (more convenient) time.

H can collect on the hot outer portions of the white dwarf.

Nuclear reactions can start, and the star gets much brighter temporarily nova.

For a few hours, it can be a half-million times more luminous than the Sun.

The maximum mass for a white dwarf is 1.4 M, called the Chandrasekhar limit.

If material dumped on the white dwarf pushes it over this limit, it will collapse and explode.

•Electron energy levels crowded together

almost continuous

•All low energy levels are full according to

the Pauli Exclusion Principle

•Only place for additional electrons to go is

in high energy levels which meansthey must move very fast…close tothe speed of light

•Adding more mass decreases the volume

•Temperature is same everywhere

This is called a Type Ia supernova.

The explosion is briefly as luminous as 10 billion Suns.

Nothing of that star is left behind; the other star evolves on its own.

This process requires a binary system.

Concept Quiz—Stages

Which of the following is the correct order for the stages of evolution of the Sun?

A. main sequence, white dwarf, planetary nebula, red giant

B. main sequence, red giant, white dwarf, planetary nebula

C. main sequence, red giant, planetary nebula, white dwarf

D. main sequence, planetary nebula, red giant, white dwarf

Massive stars have more hydrogen to start with but they burn it at a

prodigious rate

The overall reaction is still

224 eHeH

There are 3 gamma ray photons instead of two as in the proton-proton cycle and it consumes

hydrogen much faster

Because energy flow in the core of the star is by radiation, the helium ash isn’t being stirred out.

The central helium core is not fusing. It’s just being squeezed by gravity and added to by the hydrogen fusing above it

A star’s escape velocity decreases once it spreads out:

Sun:

Sun as red giant:

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What happens to planets when stars evolve? Planet migration may allow planets to

survive by moving outward, but some may move inward instead.

Planets have been found orbiting red giants and AGB stars.

As the Sun evolves, Earth’s present location will no longer be in the habitable zone.

Earth may move outward, inward, or stay where it is.

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Type Ia Supernovae over time have become very useful.

This could only happen after more scientists with greater technology analyzed their properties and realized connections.

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High-mass stars live different, faster lives. On the main sequence, energy is generated

from the carbon-nitrogen-oxygen (CNO) cycle, with carbon as a catalyst:12C + 4 1H + 2 e = 12C + 4He + gamma rays + neutrinos.

High-mass stars have convection to mix H in the core.

Increases the mass available for fusion.

Once H is exhausted from the core, the star leaves the main sequence and expands and cools.

Move right on the H-R diagram: supergiants.

Ignite He in a nondegenerate core, unlike low-mass stars.

With rising central temperatures, heavier elements (C, Ne, etc.) fuse, generating energy.

The more massive the star, the heavier the elements that can fuse.

As temperature rises and core fuel is used up, heavier and heavier elements will fuse, up until iron.

The fusion shells build up like the layers of an onion.

As high-mass stars expand and cool, they can pass through the instability strip on the HR diagram.

Here, the combination of temperature and luminosity results in the stars’ pulsation.

These pulsating variable stars are extremely important for determining distances.

Specifically, they have a period-luminosity relationship.

Cepheid variables:• High-mass stars

becoming supergiants.• Periods from one to 100

days.• More luminous stars

have longer periods.

RR Lyrae variables:• Low-mass stars on the

horizontal branch.• Less luminous than

Cepheid variables.

Intermediate-mass stars have masses between 3 and 8 M.

Start off evolving as high-mass stars, but finish as low-mass stars do, as white dwarfs.

Very massive stars may shed some mass due to instabilities and go through a luminous blue variable (LBV) phase.

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Concept Quiz—Cepheid Variables

Why are Cepheid variable stars so important?

A. We can know their luminosities.

B. They produce pulsars.

C. They are about to explode as supernovae.

D. They generate most heavy elements.

Fusion of iron or more massive elements requires energy—the star cannot use them for fuel.

Once the star has an iron core, it cannot generate more energy.

Fusion stops, and the core collapses.

Things get so crowded the electrons are squeezed into the nucleus where they combine with protons to make neutrons in a process called Reverse Beta Decay

Each stage of burning is progressively shorter.

Example: Si burning only lasts for a few days.

Why? Huge production of neutrinos, which carry away energy neutrino cooling.

The star cannot access the huge amount of energy produced in neutrinos.

The net energy released by a nuclear reaction is the difference between the binding energy of the products and the binding energy of the reactants.

For the triple-alpha process:

For the fusion of iron, the binding energy of the products is less than that of the reactants, so the net energy is negative.

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