StarsChapter 13

Stars

Apparent magnitude: is a measure of the brightness of a celestial object as seen from Earth. The lower the number, the brighter the object. Negative numbers indicate extreme brightness. We can see objects up to 6th magnitude without a telescope.

Absolute magnitude is a measure of the actual brightness of a celestial object. The lower the number, the brighter the object. Negative numbers indicate extreme brightness.

Stars

Stars come in different sizes and they produce different amounts of energy.

The energy a star produces is related to its temperature and color.

White and blue stars burn their fuel at the highest temperatures. (most powerful, most luminous)

Mass is everything to a star. More mass means more self-gravity

so the star presses in on itself more strongly.

It gets hotter in the core than a smaller star, so it burns faster and at a much higher temperature.

The Main Sequence

Runs from the upper left to the lower right of the graph.

Hot, blue, bright stars in upper left Cool, red, dim stars in lower right. Yellow sequence are in the middle

(our sun) 90% of all stars

White dwarfs

Hot, not bright Lower left of diagram Small stars About the size of Earth

Giants, Red Giants

Extremely bright, not hot Upper right of the H-R diagram Usually red in color Largest—Supergiants (Antares is

300x the sun)

Hertzsprung Russell Diagram 1910 Ejnar Hertzsprung and Henry

Russell They did this without even knowing

the other was doing it! Both had the same idea.

They put brightness on vertical axis and temperature on horizontal axis

They plotted over a thousand stars before analyzing what they had

They noticed the groupings and began to understand the different stars

H-R Diagram

Some HR diagrams indicate color and spectral class (letter classification)

Instead of brightness---magnitude and luminosity

Magnitude

Absolute magnitude—measure of the amount of light given off by a star

Apparent magnitude---measure of the amount of light received on Earth

The brightness is often deceiving!