The Life Cycles of StarsDr. Jim Lochner, NASA/GSFC

Stellar Nursery

Space is filled with the stuff to make stars.

Nebulas--Stars start from clouds

Clouds provide the gas and dust from which stars form.

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.

Nuclear Fusion ! A Main Sequence Star is born.

At 15 million degrees Celsius in the center of the star, fusion ignites !

4 (1H) --> 4He + 2 e+ + 2 neutrinos + energy

Where does the energy come from?

Mass of four 1H > Mass of one 4He

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.

The Beginning of the End: Red Giants

After Hydrogen is exhausted in core ...

Gravity is no longer counteracted by the energy of nuclear fusion, so…

1. Core collapses, Kinetic energy of collapse converted into

heat. This heat expands the outer layers.

2. Meanwhile, as core collapses,

Increasing Temperature and Pressure cause…

More Fusion !

At 100 million degrees Celsius, Helium fuses:

3 (4He) --> 12C + energy

New energy output sustains the expanded outer layers of the Red Giant

The end for Medium-Sized 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.

A Supergiant You Know

Fate of high mass stars--Supergiants

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.

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.

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 1999c) X-ray - Oct 1999d) X-ray - Jan 2000• The shock wave from

the SN heating the gas

Supernova Remnants: Cas A

Optical X-ray

What’s Left After the Supernova

Neutron Star (If mass of core < 5 x Sun)

• Under collapse, protons and electrons combine to form neutrons.

• 10 Km across

Black Hole (If mass of core > 5 x Sun)

• Not even compacted neutrons can support weight of very massive stars.

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.

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

The Hertsprung-Russell Diagram

The Sun seen in X-rays

The Hertzsprung-Russell Diagram

Usually abbreviated to HR Diagram.

This is a plot of luminosity (brightness) against color for a selection of stars.

Observable properties

Brightness:

Measure by absolute magnitude, or by the total power output of the star.

Color:

Measure by spectral class (OBAFGKM), by temperature, or by color index.

Spectral class

Color Temperature (K)

O Violet > 28,000

B Blue 10,000 to 28,000

A Blue 7,500 to 10,000

F Blue - white 6,000 to 7,500

G White - yellow

5,000 to 6,000

K Orange - red 3,500 to 5000

M Red < 3,500

Types of star

Stars are not scattered randomly throughout the HR diagram, but fall into classes. They are

The main sequence

Giants and supergiants

White dwarfs

The Main Sequence

Most stars reside in a broad band stretching from the top left (hot and luminous) to the bottom right (cold and faint).

The Sun lies pretty close to the centre of this band.

The Main Sequence

The main sequence consists of stars whose principal source of energy is the nuclear fusion of hydrogen to form helium in the star’s core.

Giants and Supergiants

•Their luminosity is high because they are very large, and so have a big surface area to radiate from. Typically they may have a radius one hundred times that of the Sun.

•The most luminous are known as supergiants.

•These lie in the upper right of the HR diagram, meaning that they are cool but luminous (bright).

Giants and Supergiants

The giants and supergiants are stars which have exhausted their supply of hydrogen fuel in their cores, and which produce energy by burning heavier nuclei such as helium.

White DwarfsThese lie in the lower left of the HR diagram, meaning that they are hot but faint.

There are probably very large numbers of these, but they are not easy to detect.

White Dwarfs

White dwarfs are remnants of stars which have completely exhausted their core nuclear fuel and which have too little gravity to contract further. They have no new source of energy and are cooling into obscurity.