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Galaxy NGC4414 from HST Astronomy 162:
Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30

Lecture 3: Matter & Light

Readings: Chapter 5, sections 5-3 & 5-4

Key Ideas

Temperature (Kelvin Scale) Kirchoff's Laws of Spectroscopy

Continuous (Blackbody) Spectrum

Emission-and Absorption-Line Spectra Each atom has a unique spectral signature

The Interaction of Light & Matter

Light & Matter can interact in a number of different ways: The last two (absorption and emission) bear on the internal energy of the matter.


Temperature is a measurement of the internal energy content of an object.
Higher temperature means higher average vibrational energy per atom or molecule.
Higher temperature means more average kinetic energy (faster speeds) per atom or molecule.

Kelvin Temperature Scale

An absolute temperature system:

Absolute Kelvin Scale (K):

The principal advantage of the Kelvin scale compared to the more familiar Celsius and Fahrenheit scales is that the temperature measured in Kelvins is directly proportional to the amount of internal energy in an object. If you double the internal energy, you double the temperature in Kelvins. This is why the Kelvin scale is said to measure absolute temperature. Both the Celsius and Fahrenheit systems are difficult to use for relating the absolute energy content of objects because they are tied arbitrarily to the freezing and boiling points of water on the surface of the Earth.

We will primarily use the Kelvin scale in this course.

Kirchoff's Laws of Spectroscopy

  1. A hot solid or hot, dense gas produces a continuous spectrum.
  2. A hot, low-density gas produces an emission-line spectrum.
  3. A continuous spectrum source viewed through a cool, low-density gas produces an absorption-line spectrum.

Graphic of Kirchoffs Laws

Gustav Kirchoff formulated these laws empirically in the mid-19th century. While they adequately describe the different kinds of spectra that are observed, they do not explain why these spectra appear in these circumstances. It was not until the early 20th century, with the development of quantum mechanics to explain the nature of the atom, when we fully understood the origins of spectra.

Stefan-Boltzmann Law

Energy emitted per second per area by a blackbody with Temperature (T):
Stefan-Boltzmann Law

s is Boltzmann's constant (a number).

In Words:

"Hotter objects are Brighter at All Wavelengths"

Wien's Law

Relates the wavelength of maximum emission by a blackbody to its Temperature:
Wien's Law
In Words:


Heating a bar of Iron Use a torch to heat an iron bar from 300K (room temperature) up to 600K. The net result of heating any blackbody is that

Person: Body Temperature = 310 K

Sun: Surface Temperature = 5770 K

In Astronomy 162, we will use the properties of blackbodies, via the Stefan-Boltzman and Wien Laws, to help us understand some of the observed properties of stars.

Hydrogen: The Simplest Atom

An atom of Hydrogen (1H) consists of:

First orbital: Ground State (n=1)

Higher orbitals: Excited States (n=2,3,...)

Schematic Energy Level diagram for Hydrogen
(Click on the image to view at full scale [Size: 8Kb])

Emission & Absorption Lines

Emission Lines:

When an electron jumps from a higher to a lower energy orbital, a single photon is emitted with exactly the energy difference between orbitals. No more, no less.
Formation of Hydrogen Emission Lines (Balmer Series)
(Click on the image to view at full scale [Size: 19Kb])

Electrons can get into the excited states by either

Absorption Lines:

When an electron absorbs a photon with exactly the energy needed to jump from a lower to a higher orbital.
Formation of Hydrogen Absorption Lines (Balmer Series)
(Click on the image to view at full scale [Size: 51Kb])

Absorption is very specific:

The excited states decay by emitting photons in random directions.

Fingerprinting Matter

Other atoms have more electrons, and hence more complex electron orbital structures.

Every element has its own, distinctive spectral signature.

The Importance of Spectroscopy

From the emission or absorption lines in an object's spectrum, we can learn many things of importance, especially: These data can give us a nearly complete picture of the physical conditions in the object, even though that object is separated from us by vast, unbridgable cosmic distances.

Spectroscopy is what transformed Astronomy into Astrophysics.

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Updated: 2006 January 3
Copyright Richard W. Pogge, All Rights Reserved.