Monday February 8th 1999
Announcements:

Lecture notes:

Class 13
 

Cassopeia - M shaped constellation formed by 5 bright stars
- Ethiopian queen - wife of Cepheus
- Circumpolar

Pegasus - the winged horse

Andromeda - daughter of Cepheus and Cassiopeia
- Cassiopeia boasted about Andromeda's beauty
- Angered sea Nymphs and they sent a whale to ravage Ethiopia's
coasts
- Perseus freed Andromeda married her and flew off on Pegasus

Contains Andromeda Galaxy - M31 - nearest large galaxy to ours.
 

Today we'll talk about the overall characteristics of the stars.
We'll focus on 2 characteristics in particular
temperature
luminosity - How much light made.

These are two of the most simple and basic characteristics of a star but together they do a good job of characterizing almost
any star in the sky.
We talked about temperature before.  Two ways to get it.

• Color - hotter is bluer cooler is redder.

For example.

• Sun has 1 solar luminosity = 1 L_(.)
• Vega has 50 solar luminosities = 50 L_(.)
• Arcturus has 100 Solar luminosities = 100 L_(.)

Now the first two astronomers who looked at star temperatures and luminosities systematically were named Hertzsprung and Russell. They made a diagram to let them study the relative L T of stars better - the Hertzprung - Russell diagram.

Stars are not randomly scattered throughout this diagram but live along certain tracks.  The goal is to figure out why stars only
live in particular places.

Emphasize main sequence - contains majority of stars and it is where stars spend most of their lives.  Either hot and bright or
cold and dim.

Also stars like Betelgeuse and Arcturus are cool but luminous.  If a star has the same temperature as the sun but is more
luminous then it stands to reason that it must be larger in size so it can have a larger radiating area.

If Arcturus were at the center of the solar system the outer layers would extend almost to Earth.
Red Supergiants are even more extreme.  Betelgeuse would extend almost out to Jupiter.  It is starting to die.

In contrast there are also dwarf stars.  An example is Sirius B.  It is much hotter than the sun but radiates only .3% of the suns
luminosity.  Only the size of a planet.  We will discuss this later.

Let's focus on Main Sequence.
- What controls the location of a star along the main sequence?  Mass
- How much mass a star is born with determines its T L and how long it lives.
Be careful since this is not true for giants and dwarfs - they are oddballs in the process of dying.

How can we learn the masses of stars along the main sequence?
Use binary stars - 2 stars in orbit around each other.  Made close together when forming.  About half of all stars are
binaries.

In some cases we can use Newton's Law of Gravity to get masses.

Mass determines the lifetime of a star.  We might expect more massive stars to live longer since there is more fuel for
fusion.  However this is not the case.  More massive stars burn their fuel faster and live shorter.
- 10 M(.) star lives for 10 million years.
- 1 M(.) star lives for 10 billion years.
This is 1000X longer.

So massive blue stars on the main sequence are generally babies compared to the red stars of the main sequence.

Can find star formation places by finding blue stars.