# Presentation: 191823

## The Life Cycles of Stars

***Dr. Jim Lochner, NASA/GSFC***

**Notes:**
This version is a combination of the hour long v3 and the shorter “45 min”.  The “45 min” included some updated slides, most significantly an improved HR diagram.

This version was uploaded to the Imagine site on 8/28/03 as life_cycles_v2.ppt
Feb 26, 2004, life_cycles_v2.1.ppt - changed 3rd mnemonic on slide 12.

Imagine Life Cycle Poster Image

## Twinkle, Twinkle, Little Star ...

***Twinkle, Twinkle, Little Star ...***

## How I Wonder What You Are ...	

**Stars have **
**D****i****f****f****e****r****e****n****t**** ****c****o****l****o****r****s**
**Which indicate different temperatures**
**The hotter a star is, the faster it burns its life away.**

**Notes:**
By looking at previous slide, audience should determine that stars have different colors, and deduce that this means different temperatures..
They can deduce that hotter stars burn faster by analogy with a regular fire. 

## Stellar Nursery

**Space is filled with the stuff to make stars.**

## Stars start from clouds

**Clouds provide the gas and dust from which stars form. **

**Rather: Irregular Grains **
**Of Carbon or Silicon**

## Collapse to Protostar

- Stars begin with slow accumulation of gas and dust.
- Gravitational attraction of Clumps attracts more material.
- Contraction causes Temperature and Pressure to slowly increase.

**Notes:**
Protostars grow on the principle that “The rich get richer”.  As the clump grows, the gravitational force it exerts increases, and thus is able to grow more.
The equation gives the gravitational force exerted by mass m1 on mass ms.  As the mass of m1 increases, the force it exerts increases.

## Nuclear Fusion !

- At 15 million degrees Celsius in the center of the star, fusion ignites !
-   4 (**1**H) --> **4**He + 2 e+ + 2 neutrinos + energy
- Where does the energy come from ?
- Mass of four **1**H > Mass of one **4**He

- E = mc2

**Notes:**
Be sure audience members understand what the symbols mean.  Especially that the superscripts for H and He are the atomic weights.

The energy comes from the slight difference in mass between four H atoms and one He atom.  This excess mass gets converted to energy via Einstein’s famous equation.

## How much Energy

- 4 (**1**H) --> **4**He + 2 e+ + 2 neutrinos + energy
- Energy released = 25 MeV
- 		= 4 x 10 -12 Joules
- 		= 1 x 10 -15 Calories
- But the sun does this 1038 times a second !
- Sun has 1056 H atoms to burn !

**Notes:**
1 MeV is the voltage necessary to move 1 million electrons through 1 volt

Re- 1 ev = 1.6 x 10-19 J.  1 cal = 4.184 J
The “Calories” given in the slide are kcal, the same as used on food labels.

We know the rate at which the sun consumes H because we can measure its energy output.  We know how many H atoms it has from knowing the sun’s mass (2 x 1030 kg). 

## A Balancing Act

- Energy released from nuclear fusion counter-acts inward force of gravity.

- Throughout its life, these two forces determine the stages of a star’s life.

**Notes:**
This is an important principle which governs the life stages of a star.

## New Stars are not quiet !

- Expulsion of gas from a young binary star system

## All Types of Stars

***All Types of Stars***

**Recall - **
**  ****Stars have ****D****i****f****f****e****r****e****n****t**** ****c****o****l****o****r****s**
**  ****which indicate different temperatures**

## All Types of Stars

- Out Beyond Andromeda, Fiery Gases Kindle Many Red New Stars

- Oh! Be a Fine Girl - Kiss Me !

- Oh! Be a Fine Girl - Kiss Me Right Now Sweetheart !

## Reprise: the Life Cycle

- Sun-like Stars

- Massive Stars

**Notes:**
Stars are either low mass or high mass.  Their mass determines their fate.
One might note that stars are not quiescent even during the time they steadily fuse Hydrogen.  For example, our own sun is very active.

## A Red Giant You Know

## The Beginning of the End: Red Giants

- After Hydrogen is exhausted in core ...
- Energy released from nuclear fusion
-     counter-acts inward force of gravity.
- Core collapses,
- Kinetic energy of collapse converted into heat.
- This heat expands the outer layers.
- Meanwhile, as core collapses,
- Increasing Temperature and Pressure ...

**Notes:**
Stars on main sequence have finite lifetime.  Why ? -limited amount of fusion fuel.  
What happens when fuel exhausted ?  Fusion ceases, and the “Balancing Act” comes into play.

As core collapses, the kinetic energy of the collapse is converted into heat.  This heat expands the outerlayers

## More Fusion !

- At 100 million degrees Celsius, Helium fuses:
- 3 (**4**He) --> **12**C + energy
- (Be produced at an intermediate step)
- (Only 7.3 MeV produced)
- Energy sustains the expanded outer layers
- of the Red Giant

**Notes:**
Note that fusion of He requires a much hotter temperature than fusion of H.

## The end for solar type stars

***The end for solar type stars***

**Planetary Nebulae**

- After Helium exhausted, outer layers of star expelled

## White dwarfs

- At center of Planetary Nebula lies a
- 	White Dwarf.
- Size of the Earth with Mass of the Sun 	“A ton per teaspoon”
- Inward force of gravity balanced by repulsive force of electrons.

**Notes:**
Basic characteristics of white dwarfs: about the size of the earth, with a mass of about the sun. 1 million g/cm3 = “1 ton/teaspoon”
White Dwarfs are stable because inward force of gravity is balanced by the repulsive force of the electrons.

## Fate of high mass stars

- After Helium exhausted, core collapses again until it becomes hot enough to fuse Carbon into Magnesium or Oxygen.
- 	 12C + 12C --> 24Mg
- 		OR 12C + 4H --> 16O
- Through a combination of processes, successively heavier elements are formed and burned.

**Notes:**
After the red giant stage, there is a series of collapses and further nuclear burning.  Fusion creates heavy elements from light elements.

## Periodic Table

## The End of the Line for Massive Stars

- Massive stars burn a succession of elements.
- Iron is the most stable element and cannot be fused further.
- Instead of releasing energy, it uses energy.

**Notes:**
Fusion stops at Iron, and star collapses under its own weight.
The star contains products of the fusion processes.

## Supernova !

## Supernova Remnants: SN1987A

**a**

**b**

**c**

**d**

**a) Optical - Feb 2000**
**Illuminating material ejected from the star thousands of years before the SN**
**b) Radio - Sep 1999**
**c) X-ray - Oct 1999**
**d) X-ray - Jan 2000**
**The shock wave from the SN heating the gas**

## Supernova Remnants: Cas A

**Optical**

**X-ray**

## Elements from Supernovae

**All X-ray Energies**

**Silicon**

**Calcium**

**Iron**

## What’s Left After the Supernova

- Neutron Star (If mass of core < 5 x Solar)
- Under collapse, protons and electrons combine to form neutrons.
- 10 Km across
- Black Hole (If mass of core > 5 x Solar)
- Not even compacted neutrons can support weight of very massive stars.

**Notes:**
Neutron Stars and black holes
Neutron Stars  form as protons and electrons in the “superdense” core combine to form neutrons.  Re- the core is collapsing under it’s own weight.
If there’s too much mass, the formation of neutrons cannot stop the collapse.  The neutrons themselves combine and “disappear” under the collapse.

## A whole new life: X-ray binaries

- In close binary systems, material flows from normal star to
- Neutron Star or Black Hole.  X-rays emitted from disk of gas around Neutron Star/Black Hole.

## Black Holes - Up Close and Personal

## SN interaction with ISM

- Supernovae compress gas and dust which lie between the stars.  This gas is also enriched by the expelled material.
- This compression starts the collapse of gas and dust to form new stars.

## Which Brings us Back to ...

**Notes:**
This brings us back to our life cycle !

## Materials for Life Cycles of Stars

- This presentation, and other materials on the Life Cycles of Stars, are available on the Imagine the Universe! web site at:
- http://imagine.gsfc.nasa.gov/docs/teachers/lifecycles/stars.html