Astronomy 294
Life in the Universe
Prof. Scott Gaudi

Lecture 3: Basic Physical Concepts

Key Ideas


: Scientific Notation, Units and prefixes; The Universe is governed by four forces; Matter is made up of atoms and electrons; Light is electromagnetic radiation.; Luminosity is intrinsic, Brightness is apparent; Interaction of light and matter; Temperature is a measure of internal energy; Spectra are diagnostic indicators of composition

Scientific Notation

Very large and very small numbers are unwieldy.

We use scientific notation as a compact way of expressing large and small numbers.

Example:

Scientists Use the Metric System (and so will we!)

Length: meters

Mass: kilograms

Time: seconds

Standard Prefixes

Astronomical Units of Length

Astronomical Unit (AU): Mean distance from the Earth to the Sun

Light Year (ly): Distance light travels in one year.

Parsec (pc): Distance at which a star's parallax would be one arcsecond

The Four Forces

Strong Force: binds nuclei

Weak Force: radioactive decay

Electromagnetic: binds atoms, light, magnetism

Gravity: binds the solar system, galaxies, universe

Atomic Structure

Nucleus: Protons(+) and Neutrons

Electrons(-): Orbit the nucleus

Elements

Each element has a different number of protons in the nucleus.

One proton: Hydrogen

Two protons: Helium

Three protons: Lithium

Ten Most Abundant Elements

1. Hydrogen

2. Helium

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3. Oxygen

4. Carbon

5. Neon

6. Nitrogen

7. Silicon

8. Iron

9. Magnesium

10. Sulfur

Only Hydrogen and Helium were produced in appreciable amounts in the big bang.

The rest of the elements were produced by stars.

Carbon, Hydrogen, Oxygen, and Nitrogen play important roles in the chemistry of life.

Isotopes

Same number of protons, but different number of neutrons.

Chemistry is the same, but radioactivity different (governed by weak force).

Important for age dating.

Radioactivity

Longevity of a species is quantified by its Half-Life

The amount of time when half of the atoms decay.

Electromagnetic radiation

Light is electromagnetic radiation

Waves or particles.

Speed of light is constant

speed = frequency times wavelength

Low energy = low frequency = long wavelength

High energy = high frequency = short wavelength

Luminosity versus Brightness

Luminosity: total energy (photon) output of a source.

Brightness: apparent brightness from a distance.

Luminosity is independent of distance.

Brightness follows an inverse square law:

A light source that is 10 times further away will be 100 times fainter

A light source that is 2 times closer will be 4 times brighter.

Interaction of Light and Matter

Matter can transmit light.

Matter can reflect light.

Matter can absorb light.

Matter can emit light.

Temperature

Temperature is a measurement of the internal energy content of an object.

Gases - higher temperature means average speed per atom or molecule.

Spectrum

Distribution of photons energies (wavelengths or frequencies) emitted by a light source.

When light passes through matter, the matter can imprint its chemical composition on the spectrum.

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.

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