Friday February 5th 1999
Announcements:

Lecture notes:

Class 12

Now we will move beyond the sun and start talking about other stars
We shall see that the same basic principles we learned about for the sun apply to others stars too.
For example gravity holds them together fusion powers them.

However there are important differences too.

We'll start with 2 relatively nearby stars in constellation Centarus
Hadar and Menkent

Hadar is 4.5 times brighter than Menkent.  Now we learned that not all stars in a given constellation are at the same distances.

You might think Hadar is closer.  This is a reasonable thing to think since closer objects will tend to be brighter.

Inverse square law for apparent brightness - the apparent brightness of an object depends on the inverse square of its distance

• twice as far away 4 times fainter
• 3 times as far away 9 times fainter
• half as far away 4 times brighter

Like Newton's Law of Universal Gravitation.

However it actually turns out that Hadar is further away.  We can measure distances the parallax and find that Hadar is 6.5 times further away than Menkent.

The problem is that we assumed that Hadar and Menkent have the same luminosities - that is they emit the same amount of light.
Actually Hadar emits about 10000 times as much light as Menkent.

So we can make an analogy here to clarify the situation.
Hadar is like a big lighthouse light bulb.
Menkent is like a 25 W light bulb.
Even if the lighthouse is further away than the 25 W bulb the lighthouse could will still appear brighter.

Voyager 2 shows that Hadar is blue and Menkent is red/orange.
Why might the stars have different colors?
Different temperatures.

As we learned before

• hotter objects = shorter wavelength higher frequency bluer color
• cooler objects = longer wavelength lower frequency redder color

So the color of a star can tell us about its temperature.
Now to measure colors of a star we don't just look at them subjectively.  We have built machines called spectroscopes that let us precisely measure the amount of light emitted at a given
wavelength.  We then classify the star to accurately figure out its temperature and the amount of light it emits.

Hadar emits about 1000 solar luminosites
Menkent emits about .1 solar luminosites.
So Hadar is about 10 000 times more luminous than Menkent.

Now we also have other ways to get star temperatures that do not depend just on color.
Stars also have absorption lines in their spectra and these can be used to figure out the temperature of the star.

The precise pattern of absorption lines tells the temperature since the amount of excitation of different types of atoms depends on temperature.

OBAFGKM - historical accident mnemonic
For example A stars have the most prominent absorption lines since temperature is just right.

Now will introduce very important diagrams.
Hertzsprung Russel diagram.  Relates how luminosity is related to temperature and spectral class