Outline

1. The H-R Diagram (T, L, R, M of stars)
2. Review of H-R Diagram
3. A Beautiful Balance: Gravity vs. Pressure

Terms to Know

1. HR diagram

Temperatures and luminosities of stars are observed to be highly correlated (like weight and height for humans)

The Hertzsprung-Russell diagram (1911-1913) is a plot of temperature (or color -- basically the same thing as color, for black bodies) vs. luminosity. Every star can be plotted somewhere on the H-R diagram. It is one of the most important graphs in astronomy, and shows the complete life cycles of stars of all different masses, ages, temperatures, colors, and luminosities.

90% of all stars have luminosities (or absolute magnitudes) and temperatures (or colors) that place them in a narrow diagonal band in the HR diagram, called the Main Sequence. This makes sense, since stars are almost perfect black bodies, and we know that cold black bodies are dim and red, while hot black bodies are luminous and blue.

But there are some stars that are dim and blue, and others that are brilliant and red! What's going on?

There must be a third characteristic coming into play: the radius of the star! Bigger stars have more surface area, so they appear brighter, while smaller stars have less surface area and appear dimmer, even if they are very hot.

Which is more important: T or r?

Recall: The area of a sphere is given by A = 4 r².

Now remember the Stefan-Boltzmann Law ( Lecture 9 )? To be more accurate, we should actually write it:

L R2T4

So they're both important, but the temperature is more important.

Here are all the parts of the H-R diagram:

The Main Sequence

temperature and luminosity are correlated

Giants

more luminous (and larger size) than Main Sequence stars of same temperature

Super Giants

even more extreme than giants

White Dwarfs

very small, hot, and faint

How are they all related?

An evolutionary sequence? Sequences?

Note 1:

Don't confuse white dwarfs with luminosity class V ``dwarfs.''

They are NOT the same!

Red dwarfs are the cool end of the Main Sequence, i.e. luminosity class V, and are NOT the same as white dwarfs.

Note 2:

Don't confuse "motion" in the HR diagram with true motion in space! A star that stays in one place all its life but changes temperature and luminosity will move around in the HR diagram. Meanwhile, a star that is zooming around in the Milky Way Galaxy could stay perfectly still in the HR diagram, if its luminosity and temperature don't change.

2. Review of H-R Diagram (T, L, R, M of stars)

• The Hertzsprung-Russel Diagram is a plot of temperature T (or color) vs. luminosity L (or absolute visual magnitude M_V -- don't confuse with mass, also denoted M!) of stars.
• But it also contains information on radius R and mass M, as well as the lifetimes and relative numbers of stars! All stars, including the Sun, can be plotted somewhere on the H-R Diagram.
• The masses of stars are measured from binary systems (more than half the stars you can see with your naked eye are double stars!) -- it's like weighing our Sun using the Earth's orbit.
• Most stars, including the Sun, appear on the Main Sequence in the H-R Diagram. On the Main Sequence, stars appear to obey a Mass-Luminosity relation:

  L M3.5

  The more massive a Main Sequence star is, the hotter, bluer, and more luminous!
• The spectral types of stars are related to their temperature (or color).

3. A Beautiful Balance: Gravity vs. Pressure

The force of gravity at the center of a star is immense, billions of times greater than at the bottom of the deepest ocean trench on Earth. How can a star possibly support that weight?

Gravity compresses gas gas heats up star ignites nuclear fusion in core releases energy provides pressure support (and makes star shine!) stops gravitational collapse.

Lectures

Table of Contents

Astro 100