First, let’s review some tools we have learned about:

How do we know what we do about the universe?

- or, how does science operate? (“if you can’t measure it, it’s not science!”)

Parallax – what is it, what can it tell us?

Temperature and colors of warm objects

Finding distances: the first step, Stellar parallax

:

This image shows both blue and yellow stars. It was taken with a camera that mimics the human eye. Which of the following is true about the stars with arrows pointing to them?

A. The blue star is hotter than the yellow star B. The blue star is cooler than the yellow star C. The blue and yellow star are the same temperature D It is impossible to tell from this information which star is hotter

How do astronomers measure brightness?

Brightness isn’t a quantity like length, or weight, or speed…

We can compare two sources to say which is brighter

We can measure the effect of radiation on phototgraphic film, or a modern CCD (charged couple device) like your digital camera

Question: do you think the fainter stars are just farther away? Do you think some are actually brighter than others?

If we can calculate a star’s distance from parallax, and measure how bright the stars is,

- then we can calculate how much brighter, or fainter, it would be at any distance.*

- and if we do this for every stars whose distance we know, then we can answer our question!

(I have a demo of this I would like to try later, if you are interested…)

The common scale in astronomy is called the “magnitude scale”

- each number differs from the next by about 2.5 times

(Astronomy jargon)

Take a look at your handout: Dist; how far in light years

Ap mag : how bright it seems in the sky

Abs mag: how bright it would be at a distance of ~30 light years

Sp ty: later in the course!

Temp: how hot it is

Luminosity/sun: how bright it is compared to the sun

Does this answer our question? Does it tell us anything else?

How can we understand big lists of data?

Scientists very often make a plot or graph

Class exercise: plot, on graph scale I provided, the absolute magnitude against the temperature of the 20 nearest stars

You may work as a team, or alone

Discuss!

We will see that most (about 90%) of the stars lie along the red line: we call this the main sequence

What questions could we ask about this plot?

The H-R diagram (named after 2 astronomers,

Hertzsprung and Russell)

Lecture tutorial, p. 117, H-R Diagram: reading graphs

Summary of important ideas from this section:

1.  We can measure properties of stars: temperature (from color) and actual brightness (from knowledge of distance of nearby stars)

2.  With this information, astronomers plot temperature vrs actual brightness (called the H-R diagram) and find an interesting relationship.

Time for a 10 minute break:

Anyone wanting to see my demo that “farther means fainter, scientifically”, come on outside… assuming the sun is out!

If you are taking algebra, this may help you.