Announcements

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Slide 1

Announcements

Homework 7 due today

Pick up Homework 8

Next test will be next week

Slide 2

Our Solar System and Others

16 October 2006

Slide 3

Today:

Solar system patterns

Age of the solar system (and a crash course in nuclear physics)

Formation of the solar system

Planets around other stars

Slide 4

Solar System Patterns

The solar system is very flat. Why?

Nearly all the planets orbit and spin in the same direction. Why?

Inner planets are small; outer planets are big. Why?

Inner planets are mostly solid; outer planets are mostly gas and liquid. Why?

Inner planets have little hydrogen and helium; outer planets have lots. Why?

Partial answers are not hard to guess…

Detailed answers require an account of how the solar system formed.

Slide 5

How old is the solar system?

Layered rocks imply an age of at least millions of years.

Earth’s hot interior implies an upper limit on its age (as does sun’s energy output).

Age of the earth?

William Thomson, Lord Kelvin

Slide 6

How old is the solar system?

To get an actual number, we need nuclear physics.

Each chemical element has a different number of electrons (and an equal number of protons).

Slide 7

The Periodic Table

Masses and rarities increase (mostly) toward the bottom of the table.

Slide 8

Nuclear Isotopes

Same element, different numbers of neutrons (hence different masses).

Examples: Hydrogen-1 (1p, 0n); Hydrogen-2 (1p, 1n); Uranium-235 (92p, 143n); Uranium-238 (92p, 146n).

Slide 9

Nuclear Isotopes

Of all the thousands of possible isotopes, only a few hundred are stable. These tend to have slightly more neutrons than protons. Others spontaneously decay.

Slide 10

Nuclear Decay (“Radioactivity”)

Unstable nuclei spontaneously disintegrate, usually by emitting a helium-4 nucleus (2p+2n) or an electron (converting a neutron into a proton).

The time when any particular nucleus will decay is random and cannot be predicted.