THE HR DIAGRAM

1. THE HR DIAGRAM The Life Cycles of Stars

2. HR-Diagram- What is it? • Stands for the Hertzsprung-Russell Diagram • Graphs surface temperature (color) vs. Absolute Brightness • Horizontal Axis- Surface Temp/Color • Vertical Axis- Absolute Brightness • Mass can also be estimated when Stars are placed on the HR Diagram. Typically the higher the star is on the diagram the larger the mass with a couple of exceptions.

3. Main Sequence stars • This is the diagonal band of stars that start with the bright hot stars in the upper left and the cool, dim stars in the lower right corner • 90% of the stars in the universe lie on the main sequence • The sun is located in the middle

4. Giants VS. Dwarfs • The HR diagram can estimate size and mass of the star • If you compare two stars of the same temperature the brighter star must be larger • If you compare two stars of the mass/size the hotter stars are brighter than the cooler stars. • http://aspire.cosmic-ray.org/labs/star_life/hr_interactive.html

5. Super Giants And White Dwarfs • GIANTS- large bright stars a bit smaller and fainter than Super giants • Super giants • In the Red temp range tend to be in their last stages of life. They run out of hydrogen and are now fusing Helium into Carbon. • In the Blue temp range they are hot bright main sequence stars. • White Dwarfs- are the small, dense remains of low or medium mass stars. • They are very hot, but dimmer then the main sequence stars of the same temperature. • They are no longer producing energy or light, they are lit because of the remaining energy from the collapse of the original star. They will eventually cool to a black dwarf. • Most stars fall on the main sequence • Close to 9% are White Dwarfs • Less than 1% are Giants or Super giants

6. Lifecycle of a star-BIRTH and Formation • FORMATION- Space contains gas and dust and stars are formed in nurseries called Nebulas or a contracting cloud of dust and gas • Some Nebulas glow while others are dark • Stars are created from Gravity pulling the nebula together and making a dense ball of gas • PROTOSTAR- enough gas and dust to form a star is available, but it is not an actual star until it gets hot enough to start nuclear (hydrogen) fusion

7. Adult star • When a star is on the main sequence it is considered an adult star. It will spend approx. 90% of its life span on as an adult star on the main sequence. • This begins the moment Hydrogen fusion begins • The more massive the star, the more nuclear fusion takes place to produce energy • The mass of the star determines the place it lies on the main sequence. • Larger stars die quicker than smaller stars because it uses its fuel quicker • Large- live a few million years • Medium- 10 billion (like our Sun) • Small- 100 billion or more

8. DEATH • At some point the star runs out of fuel. Now remember the star is stable because of its balance between inward and outward forces. • The outward force- Thermal Pressure • The inward force- Gravity • The star now begins to lose thermal pressure and gravity squeezes the core which starts hydrogen fusion in the shell of the star. This causes the outer layers of the star to expand. • The star cools down and turns red • The core continues to shrink causing helium fusion. For a moment ( few million years) the star stabilizes in the red giant region of the HR diagram.

9. Death of a LOW or MEDIUM mass star • As the Helium supplies dwindle, the outward pressure is not enough to balance the inward pressure of gravity • The star collapses • The hot core remains with all of the gas around it. We call this a Planetary Nebula • Eventually the gas surrounding the core blows off and we are left with a hot core called a White Dwarf • This will cool off after 20 billion years leaving a cold core called a Black dwarf

10. High Mass Star- more than 8x the Sun • The star runs out of energy and turns into a Red Super Giant the same way a low mass star does. • The Star Collapses • This time there is so much energy because of the large mass of the outer star that the collapse causes a Super Nova Explosion. NOW…if the CORE of the star has a mass of 3x or less the size of the sun ( but has a much smaller diameter than the sun) it creates a Neutron Star which spins and emits a steady beam of radiation and light out of its poles. *Neutron stars are so Dense that a teaspoon of a neutron star on EARTH would weigh a billion tons. * If the neutron star is spinning it will appear to pulse giving it the name PULSAR FINALLY…if the CORE of the star is more than 3x the mass of the Sun ( and still a much smaller diameter than the Sun) the star collapses with so much energy and as the star get smaller, the gravity gets much larger. In Fact it is too large even for light to escape the gravity. This is a BLACK HOLE!