Hertzsprung-Russell Diagram and the

From a birth to death  All stars form in +/- the same manner the sun did  A nebula collapses (due to its own gravity)  Cloud contracts to form a hot dense center  Friction increases the temp. until it reaches ignition temp. for nuclear reactions (H  He)  Ta da! A new star.

After birth:  If the star is like our sun:  It enters what is called its main sequence.  Main sequence lasts approx. 10 billion years.  During the main sequence the star burns (H  He) and experiences an increase in temperature. (It creates more and more heat as it goes.) It also becomes brighter, or more luminous.  Next it becomes a red giant.

Red Giant

Mira

Red Giants  When a star gets to this phase its outside layers are expanding and cooling as all of the the H has been turned into He.  There is a hard shell of H just outside the He core. This creates a great heat that is spread out across the expanded surface area.  Thus, the luminosity becomes brighter while the temperature becomes lower.  This results in the red color of red giants.  Note: this is a short phase – 100,000,000 m.y.

White Dwarf Stars  The sun sized star then enters the final stage of its life:  Very Dense! About a ton of matter in the size of a table grape.  Energy production ends as He becomes Carbon  The outer layers are shed  Results in a planetary nebula  The core of the PN holds a small, hot, earth sized star – the white dwarf.  It gradually cools, loses its luminosity and becomes:  An undetectable black dwarf – the cinder of a once active star.

Ancient White Dwarf stars – Milky Way

Sirius A and Sirius B

Massive stars  If the star is massive it is higher on the main sequence. (Hotter, brighter, more massive)  Its lifespan is much shorter b/c it uses up all its fuel quickly.  Called a supergiant. (8-10x bigger than Sun)  It makes far more elements: H, He, C, O, Ne, Mg, Si, Fe, Ni  Ends in a supernova explosion.

R136: A Cluster of Massive Stars in Nebula 30 Doradus

Star WR124 detail

Special Note: Black holes  Stars about 20x bigger than the Sun will usually form a black hole.  The collapse forms matter that is so dense nothing, not even light can escape from it.

OVERVIEWOVERVIEW

After it’s born…how do we describe it?  There are many kinds of stars. They are classified by:  Temperature  Luminosity (brightness)  Color (spectrum)  Size (diameter)  Mass (density/cubic area)

It’s all connected…how?  Stars are assigned spectral (visible light arranged according to wavelengths) types  These spectral types describe the temperature and color of the star.  They are: O, B, A, F, G, K, M. Numbers classify them even more specifically. (ex: A5)  The colors are (in order, hottest  coolest):  Dark blue (O5), light blue (B5), yellow (F5), orange (G5), red (M5)

Eta Car STIS Spectrum

Eta Carinae – star on the brink of destruction

The combinations of:  Temperature  Brightness (or luminosity)  Color (or spectra)  Describe something called its magnitude.  All of these things interact to express varying levels of energy output.  Used to be classified/organized on something called the: Hertzsprung-Russell diagram.

Hertzsprung-Russell Diagram