A star is a hot ball of mostly hydrogen gas held together by gravity. In the core, nuclear fusion reactions generate energy by converting hydrogen to helium. This process is called stellar evolution. As the star's fuel is depleted, its structure changes. Stars evolve through different stages over their lifetimes, from main sequence stars to red giants or supergiants and eventually ending as white dwarfs, neutron stars, or black holes depending on their original mass.

1. What is a star? (Not in Notes Page)
1) A star is a hot ball of mostly hydrogen gas; the Sun is an
example of a typical, ordinary star.
2) Gravity keeps the gas from evaporating into space, and pressure
due to the star's high temperature and density keeps the ball from
shrinking.
3) In the core of the star, the temperature and densities are high
enough to sustain nuclear fusion reactions, and the energy
produced by these reactions works its way to the surface and
radiates into space as heat and light.
•When the fuel for the fusion reactions is depleted, the structure of
the star changes. The process of building up heavier elements from
lighter ones by nuclear reactions, and adjusting the internal
structure to balance gravity and pressure, is called stellar evolution.

3. Why Stars Evolve (Start of Notes)
•The stars' fuel for energy generation is the
stuff they are made of -- hydrogen, helium,
carbon, etc. -- which they burn by
converting these elements into heavier
elements.
•"Burning" in this context does not refer to
the kind of burning we are familiar with,
such as the burning of wood or coal, which
is chemical burning.
• It refers to nuclear burning, in which the
nuclei of atoms fuse into nuclei of heavier
atoms.

4. How Stars Evolve
•When stars start their lives, they consist mostly of
hydrogen, some helium, and small amounts of heavier
elements, such as carbon, nitrogen, and oxygen.
•They generate energy by converting
nuclei of hydrogen into nuclei of helium in their hot,
central cores (Fusion).
•The energy produced by nuclear burning heats its
interior to many millions and, in some instances,
hundreds of millions to billions of degrees Fahrenheit.

5. It all starts with a
nebula
(interstellar
clouds of dust
and gas).
•Some nebulas are
several light years in
diameter and could be
100 times bigger than
our own solar system.
•Emission nebulae are clouds of
high temperature gas and dust.
•Emission nebulae are usually red,
because hydrogen, the most
common gas in the universe, most
commonly emits red light.

6. •Stars begin their lives by burning hydrogen in
their cores. The product of this burning is
helium. A star that burns hydrogen in its core is
called a main sequence star. The Sun is an
example of a main sequence star.
The Sun in white light. Most of the radiation
from the Sun is white light -- namely, light that
is a combination of all of the rainbow colors.
The conspicuous dark splotches are sunspots.
The Sun's brightness is equivalent to four trillion
trillion 100-watt light bulbs! (A trillion trillion is
a one followed by 24 zeros.) All of this energy is
generated in the Sun's core by hydrogen
burning.
Image Credit: National Solar
Observatory/Sacramento Peak, Sunspot, New

7. Life Cycle of a Star
I. Main Sequence Stars (Young Stars)
• Main Sequence Stars are the central band of
stars on the Hertzsprung-Russell Diagram.
• 90% of all stars are Main Sequence stars.
• For these stars, the hotter they are, the
brighter.
• The sun is a typical Main Sequence Star.

8. Life cycle of a star
II. Giants and Supergiant Stars (Old, Large Stars)
Depending of the size of the nebula, as a star approaches
“old” age, it can become a:
1) Red Giant
a) 100 times bigger than it was originally.
b) Is cooler than it was originally.
c) Frequently orange in color.
d) 20 times as massive as the sun and 14,000 brighter.
e) When they die, they become dwarfs.
Example: Betelgeuse

9. Betelgeuse

10. Life cycle of a star
2) Supergiants
a) Largest known type of star. Some are almost
as large as our entire solar system.
b) When they die, they explode as a supernova
and become:
a) black hole
b) neutron star (pulsar)
Example: Rigel

11. The pulsating blue supergiant Rigel has a diameter of about 100
million kilometres, some seventy times that of the Sun. In the far
distance a double blue star is visible - Rigel's much less luminous
companions.

12. Life Cycle of a star
III. Faint, Virtually Dead Stars
1) Dwarf stars (from RED GIANTS)
a) White Dwarf
- occurs after a RED GIANT loses its outer layers (nuclear
cores are depleted)
- small, very dense, made of carbon.
- about the size of Earth, but much heavier.
- They will eventually turn into a cold and dark…
b) Black Dwarf
Example: Sirius

13. Constellation
Sirius are Canicula which is Latin and
means "The Little
Dog"

14. Sirius A is hotter,
bluer, and younger
than our Sun, Sol.
This Hubble image
also shows white
dwarf companion
Sirius B, at lower
left.

15. Life Cycle of a Star
2) Neutron Star
a) Very small, super dense, tightly packed neutrons.
b) Has a thin atmosphere of hydrogen.
c) Has a diameter of 5 – 10 miles.
Pulsar – a rapidly spinning neutron star that emits
energy as pulses.

16. Black Hole Swallowing Neutron Star

17. Life cycle of a star
3) Black Holes
• Occurs after a supernova of a Supergiant.
• A region of space containing a huge amount
of mass compacted into an extremely small
volume.
• A black hole's gravitational influence is so
strong that nothing, not even light, can escape
its grasp.

20. September 5, 2001 (CNN) -- Direct observations are offering the
first authoritative evidence that a black hole resides in the heart
of our galaxy. Black Hole Spotted

21. Burning of Elements Heavier than Helium
1) Stars that start their lives with masses less than about eight solar masses stop
their nuclear burning history with core helium burning. (Red Giants)
2) Stars that start their lives with masses greater than about eight solar masses
continue their nuclear burning history beyond that of core helium burning.
(Super Giants)
a) Eventually, silicon and sulfur ignite in the star's core to form iron, nickel,
and other elements of similar atomic weight.
b) The star's structure now resembles an onion: The central core of the
onion consists of iron. Surrounding it is a shell in which silicon and sulfur burn,
adding more iron to the iron core. In additional shells further out, lighter
elements burn -- oxygen, carbon, helium, and hydrogen.
•
The structure of a highly evolved star
of 20 solar masses.
• The letters H, He, C, O, Ne, Mg, Si, S,
and Fe are the chemical symbols for:
•Hydrogen, Helium, Carbon, Oxygen, Neon,
Magnesium, Silicon, Sulfur, and Iron (Fe
from Ferrum – Latin term for Iron).

22. •The evolution from main sequence to red giant occurs
at different times for different stars.
•Stars that are much heavier and hotter, like O-stars,
become red giants in only 10 million years.
•Cooler, lighter stars like our sun take 10 billion years to
become red giants.