Transcript The Hidden Lives of Galaxies NSTA 2001

The Life Cycles of Stars
Dr. 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
After Helium exhausted, outer layers
of star expelled
Planetary Nebulae
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 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
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
O
Color
Temperature (K)
Violet
> 28,000
B
Blue
10,000 to 28,000
A
Blue
7,500 to 10,000
F
6,000 to 7,500
K
Blue - white
White yellow
Orange - red
M
Red
< 3,500
G
5,000 to 6,000
3,500 to 5000
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
•These lie in the upper right of the HR
diagram, meaning that they are cool but
luminous (bright).
•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.
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 Dwarfs
These 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.