Astro 105: The Milky Way

Lecture V: The Light

Modern Astrophysics

"If the doors of perception were cleansed everything would appear to man as it is, infinate. For man has closed himself up until he sees all things through the narrow chinks in his cavern"

William Blake

After Newton, astronomy continued to progress by using the Universal Law of Gravitation as the key piece of physics for understanding the heavens.

Sometime around the mid 1800's a new tools began to be added to astronomer's kit. Each of these changed how the reach and power of astronomy

Astronomy becomes Astrophysics

theory of light = Radiation
theory of interaction of Matter and Light = Black Body radiation
theory of atomic structure (Quantum Mechanics!!!!!)

Afterall, the only message we get from celestial objects comes in the form of light.

Once we learned to "read" those messages we can start telling the stories.

This lecture we will review the properties of light as it relates to Astronomy and astrophysics.

Light -> The Basics

What most people think of as light is the "white light" of sunlight or an incadesent bulb.

This is only optical or visible light. Light of a specfic kind.

Visible light can be broken up into its component rainbow of colors via a prism

Light Through A Prism - While passing through a prism, white light splits into its component colors, spanning red to violet in the visible part of the electromagnetic spectrum. The slit narrows the beam of radiation. The image on the screen is just a series of different-colored images of the slit.

Visible light = Violet, Blue, Green, Yellow, Orange, Red

For years people argues about the wave-like or particle-like nature of light.

Waves
- Sound
- Ocean waves
- Traffic
Particles
- Bullets
- Billard Balls
- Car
By the 1800's it seemed like the wave picture had won.

Waves have two important properties

Wavelength = distance between peaks (Wavelength = m)

frequency = number of peaks passing a point in one second (f = 1/sec)

Typical Wave Motion - A typical wave, showing its direction of motion, wavelength (l), and amplitude (a).

A prism bends light in way that SHORT wavelengths get bent more than LONG ones.

We measure the wavelength of light in Angstroms

1 Angstom = 1x10^-9 m = 10 nanometers

In the middle of the 1800s it was discovered that visible light was an ELECTROMAGNETIC wave.

Electromagneticsm <-> electric charges (electrons), electric fields, magnetic fields, electric currents.

Components Of Electromagnetic Wave - Electric and magnetic fields vibrate perpendicular to each other. Together they form an electromagnetic wave that moves through space at the speed of light.

By shaking an electric charge fast enough you can produce any size (wavelength) electromagnetic wave you want.

Visible Light, Radio Waves, Microwaves, Infrared

It works the other way round too - Particles Collected By TV Antenna - Charged particles in an ordinary household television antenna vibrate in response to electromagnetic radiation broadcast by a distant transmitter. The radiation is produced when electric charges are made to oscillate in the transmitter's emitting antenna. The vibrations in the receiving antenna "echo" the oscillations in the transmitter, allowing the original information to be retrieved.

Thus comes the ELECTROMAGNTIC SPECTRUM - the spectrum of all possible wavelengths of "light".

As we will see later on, shorter wavelengths correspond to higher energies in the light.

Notice that the atmosphere only lets Radio, Optical and some Infrared light in. The rest is screened

For most human history astronomy meant optical astronomy. Only in the last 50 years have new wavelength "windows" been opened.

Radio was first. Then infrared and X-rays etc. Now the sky is being mapped in Gamma-rays.

(Why did it go in this order?)

Objects look very different at different wavelength. Each window has a different "story" to tell.

First of four images of the Sun, this one made using visible light

Second of four images of the Sun, this one made using ultraviolet light.

Third of four images of the Sun, this one made using X rays.

Fourth of four images of the Sun, this one made using radio waves. By studying the similarities and differences among these views of the same object, important clues to its structure and composition can be found.

The Inverse Square Law For Light

Ever notice that things that are farther away look dimmer?

Car lights a long way away appear faint.

This comes about because the "brightness" (B) of a light source decreases with distance (D) the same way gravity does.

If I_o is the intrinsic amount of light an object is pumping out and D is the distance to the object the its apparent brightness is

Inverse-Square Law For Radiation - The amount of detectable radiation (apparent brightness) varies inversely as the square of the distance from an emitting object. As radiation moves away from a source, it is steadily diluted as it spreads over progressively larger surface areas (depicted here as sections of spherical shells).

This relation is very important for astronomers because sometimes we actually know the intrinsic amount of light celestial objects emit. (Lets call it the intrinsic brightness).

So if we know I_o from physics and we observe B at the Earth then we can D the distance to the object.

Remember that determination of astronomical distances is always one of the most difficult problems.

Any object which we know I_o is called a Standard Candle.

Standard Candles:
- Some kinds of Pulsating Stars (Cepheid Variables)
- Some kinds of Supernova
- Some kinds of Galaxies
Standard Candles are worth their weight in gold!

Blackbody Radiation

There are different ways for matter to emit light.

That means different physical processes.

Each process will produce a different spectral signature

Spectral Energy Distribution (SED)

Energy vs Wavelength

Observing an objects SED provides astronomers with a way of diagnosing what's going on physically in the object.

Here is one important kind of SED: the Blackbody Curve

Black Body Curve - The black-body, or Planck, curve represents the distribution of the intensity of radiation emitted by any heated object.

"Blackbody radition" is thermal radition.

It is the light emitted simply because of the random jiggly motions of atoms and molecules due to heat.

The hotter an object is the more jiggly motions the atoms experience.

Technical Point - The term blackbody refers to the way something dark, like a black piece of construction paper, will absorb and emit light. A blackbody is an object that is in thermal balance with its radiation.

Why are blackbodies important? Because big collections of stuff like stars, planets and people all have a temperature and so they all emits radiation like blackbody.

Here is the kicker. The "peak" wavelength of a blackbody (the wavelength at which most of its light is emitted) depends on its temperature.

Wien's Law

The meaning of Wien's Law

Black Body Curves For Different Temperatures - As an object is heated, the radiation it emits is still described by the black-body curve, but the curve shifts to peak at higher and higher frequency as the temperature rises. Shown here are the curves corresponding to temperatures of 600 K, 6000 K, and 60,000 K, which peak, respectively, in the infrared, the visible, and the ultraviolet regions of the electromagnetic spectrum.

Now you know what red hot means.

What is great about the blackbody radiation is just by looking at somethings color you can tell how hot it is.

Blue stars are hotter than Red stars

Comparison Of Black Body Curves For Four Cosmic Objects - Comparison of black-body curves for four cosmic objects having different temperatures. (a) A cool, invisible galactic gas cloud called Rho Ophiuchi. At a temperature of 60 K, it emits mostly low-frequency radio radiation. (b) A dim, young star (shown here in red) near the center of the Orion Nebula. The star's atmosphere, at 600 K, radiates primarily in the infrared. (c) The Sun's surface, at 6000 K, is brightest in the visible region of the electromagnetic spectrum. (d) A cluster of very bright stars, called Omega Centauri, as observed by a telescope aboard the space shuttle. At a temperature of 60,000 K, these stars radiate strongly in the ultraviolet.

The Whole Universe is a Blackbody!

The Big Bang left a wisper of radiation left over from the first epoch of time.

The Universe was hot. Radiation and matter where strongly coupled. As the Universe cooled the radition and matter stopped interacting. This blackbody radiation became a fossil existing everywhere and everywhere cooling as the Universe expanded. Now it corresponds to a temperature of just about 3 degrees.

COBE Satellite Map Of Microwave Background - A COBE map of the microwave sky reveals that the microwave background appears a little hotter in the direction of the constellation Leo and a little cooler in the opposite direction. The maximum temperature deviation from the average is about 0.0034 K, corresponding to a velocity of 400 km/s.

COBE Satellite Measurement Of Background Radiation - The intensity of the cosmic background radiation, as measured by the COBE satellite, agrees very well with that expected from theory. The curve is the best fit to the data, corresponding to a temperature of 2.735K. The experimental errors in this remarkably accurate observation are smaller than the dots representing the data points.

Spectral Lines

The blackbody curve provides a continious distribution of light.