Light from Distant Stars
Lots of stars out there
The sun is just the closest
Earlier we calculated how many may exist (3 X 10^10 galaxies X 10^11 stars/galaxy = 3 X 10^21 stars)
And this is accepted as a conservative minimum
As we see farther, we will certainly find more galaxies
How can we know anything about them
Dots of light at great distance
Brilliant minds are figuring it out (hope they're as brilliant as the stars)
But can we believe them?
Assumptions based on assumptions based on assumptions based on...
Any small error in the basic assumptions causes the entire "house of cards" to fall
We have only a few class sessions to talk about stars
Obviously, we will skip a great deal!
Said above that they are dots of light at great distance
We will concentrate on the light and the distance
Only recently have we become aware of just how vast interstellar distances are
How can we accurately measure these distances
Why do we have two eyes?
Does the 2nd serve as a spare?
Much more than that
Allow 3-D vision
DO SOME EXAMPLES
Hold pencil close to eyes - close one, then the other - pencil jumps
Move pencil to arm's length and blink again - less "parallax"
Can imagine a distance where the parallax would not be noticeable
Cover one eye, etc.
Set up objects on front desk
Have class record depth order, then uncover both eyes and check
What's happening here?
Each eye views the same object from a slightly different place
Our brain calculates the math
Gives us distances to objects in our field of view
We can do this ourselves with a bit of help from geometry or trig
Assume the following right triangle and formulas (show overhead)
Let's do some practice:
1. A tree across a river. (baseline) a = 100'; C=90 deg.; B=60 deg. (173')
2. A rock across a valley. a = 200'; C=90 deg.; B=70 deg. (550')
3. A good looking guy across a fence. a = 333'; C=90 deg.; B=84 deg. (3168')
4. A good looking guy across a fence. a = 200'; C=70 deg.; B=65 deg.
What do we do here? Best to keep with right triangles
Tomorrow we'll do this for real so be ready
Outside for lab
Estimate distance to remote object. Will need:
Compass to get angles
Tape measure for baseline
Much greater than what we are doing here
What is required for accurate measurements
Accurate angle measurements
No math errors!
We use the earth's orbit! (see fig. 21.4, pg. 368)
Units of distance
Astronomical Unit - average distance earth to sun (1.5 X 10^8 km)
Light year - 299,792.5 km/sec (9.46 X 10^12 km)
Parsec - 206,265 AU (3.1 X 10^13 km)
How do we feel about the accuracy of our distances to stellar objects?
Read pg. 370
What are these other methods?
Based on luminosity of the star (see pg.410)
No way we can study the stars directly
No matter how strong the telescope
Still just points of light
Astronomers have developed methods which give lots of info
Not only location, but composition, velocity, and life cycle
How can they extract so much data from "feeble" points of light?
Again, lots of assumptions based on a little bit of "fact"
Not all stars appear to shine with the same intensity
What can affect how bright they look?
Differences in actual energy output
Like differences in a 60 watt bulb and a 250 watt bulb
Distance from earth
Easy to understand this one
Material "in the way"
Causes refraction and diffusion of the light
Both interstellar and atmospheric
Nothing we can do about the interstellar
Hubble is designed to fix the atmospheric distortion
Astronomers have been comparing relative brightness for thousands of years
Hipparchus (200 BC) compiles a list of 1000 stars
Classified into 6 categories based on brightness (called magnitude)
Brightest were 1st Magnitude stars
Faintest were 6th Magnitude stars
His system is still used today
Values range from -26.5 (sun) to over 30 (Table 22.2, pg. 380)
PROBE CLASS: If the original range went from 1-6, why do we have such a broad range of values now?
Apparent Visual Magnitude
What we see
DIGRESS TO: perception vs. reality
Anyway, apparent magnitude is not reality
Distance and interference affect what we see
A measure of the brightness (magnitude) of stars at a common distance
How bright they really are
Each stars is mathematically moved closer or farther, based upon its assumed distance
We use 10 parsecs (32.6 LY)
Luminosity - a measure of the amount of electromagnetic energy radiated into space by a star
Has the same apparent vs. intrinsic modifiers
Lots of other info is derived from looking at the points of light
Most obtained from "Spectroscopy"
A study of the light itself
Can give info on luminosity, temperature, motion, and composition
Relates back to Newton and his prism - the rainbow effect
Different elements emit energy at different wavelengths
A study of star light can therefore give composition
Not all stars have the same spectra
There are 7 "spectral classes"
See Table 22.3, pg. 385
H-R diagram - describe w/overheads
Main sequence - describe w/overheads
Old age - beyond the main sequence
Red Giant - 1st step in old age
Expansion and cooling as star exhausts fuels
Our sun will expand to the orbit of Mars
White Dwarf - next step
Loss of mass causes the star to "go out"
No more force counteracting the gravitational attraction
Star collapses - very dense (10^6 g/cm^3) but small
Very hot inside
Nova - a last burst of glory
Star/dwarf binary system
Normal star looses mass to the dwarf
This "normal" star is often a red giant
Outer layers enter the dwarf's gravitational field as is expands
Dwarf collects material until there is enough to set off hydrogen explosion
Just like a bomb
Uses up the "new" fuel
Resets and does it again
Very long process - 1000's of years