AS101 Week 9 Lecture 1
Spectra: Quick Summary
Continuous (a.k.a. thermal, a.k.a blackbody) spectrum: produced by a hot, dense object.
Hotter objects produce more light overall, and also more short wavelengths compared to
Line spectra (emission or absorption): produced by low-density gases absorbing or
emitting light at specific frequencies only. Line spectra show “chemical fingerprints” for
the particular atoms in the gas.
Ex of continuous spectrum: filament in incandescent light bulb, heating element on
toaster, surface of a star
Three Basic Type of Spectra
The Doppler Effect
When we observe spectral lines from an astronomical source, (for example, all the
spectral lines of hydrogen), we can compare them with a reference spectrum from
hydrogen gas in the lab.
If the source is moving toward us, the spectral lines will be shifted to higher frequencies
(blueshift) compared to the reference spectrum.
If the source is moving away from us, the lines will be shifted to lower frequencies
Note: the doppler effect can only tell us about radial velocities (i.e., motion toward or
away from the observer)
It cannot give us any information about the transverse (sideways motion)
Farther away the galaxy, the quicker its moving away
What are the main objects in the solar system?
The Sun o The sun includes most of the matter in the solar system
o Mostly hydrogen and helium.
o Biggest object, contains most mass
o Terrestrial planets (Mercury, Venus, Earth, Mars) are small, rocky, and closer to
o Jovian planets (Jupiter, Saturn, Uranus, Neptune) are large, gassy (contain a lot
more hydrogen, helium, methane, etc.) and farther from the Sun.
o Jovian planets also have more moons than the terrestrial planets.
o Jovian planets all have rings, but Saturn's are the most noticeable.
o Order of planets by distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter,
Saturn, Uranus, Neptune.
o “My Very Excellent Mother Just Served Us Nachos.”
Meteoroids, Asteroids, Comets and Dwarf Planets
Other Parts of the Solar System: the Asteroid Belt
Asteroid belt is a region at distances approximately 2-4 au from the Sun, between the
orbits of Mars and Jupiter.
Total mass adds up to about 3x10 kg, or 4% of the Moon's mass.
Includes between 700,000-1,700,000 objects with diameters of 1km or more.
Rocky and metallic, like terrestrial planets, but some also contain water ice.
Actually extremely spread out, not crowded together as in a lot of SF movies. (Several
spacecraft have passed through it without hitting anything.) Still, they collide with