Interstellar Distance

Light from Distant Stars

Stellar Evolution



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


Interstellar 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


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

Graph paper


Stellar distances

Much greater than what we are doing here

What is required for accurate measurements

Accurate angle measurements

No math errors!

Longer baseline

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)


Light from Distant Stars

Stellar Magnitudes

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

Absolute Magnitude

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

Spectral Sequence

Not all stars have the same spectra

There are 7 "spectral classes"

See Table 22.3, pg. 385


Stellar evolution

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