THis is the second part of the Universe formation lecture for the HPU NCS2200 earth science for elementary education majors summer online course

1. Life Cycle of a star

2. 24.3 The birth of a star
• Energy from the Bang was first generation
energy
– Light from that original spreading out
• The light we see from most stars & galaxies is
second generation energy
– Light from concentrated matter

3. 24.3 The birth of a star
• Begins as a nebulae – huge clouds of H & He
• Gravity causes the nebulae to collapse in on
themselves
• As it collapsed it became more dense and began
to spin.
• This spinning motion created gravity which pulled
the gas and dust in tighter creating friction
• This Friction created heat which caused the
Hydrogen atoms to undergo thermo-nuclear
fusion creating Helium and huge amounts of
energy.
• This energy is emitted as light- thus a star is born!!

4. Fig. 24.4, p.612
Balance of forces – gravity and energy from fusion – must exist to maintain the star

5. Fig. 24.5a, p.612
Orion Constellation
One of the most
Recognizable star
formations

6. Fig. 24.5b, p.612
A close-up schematic view
of Orion’s belt region

7. Fig. 24.6, p.613
a close-up view of
stellar nurseries
near Orion’s belt.

8. 24.5 Stars: the main sequence
• Stars initially cataloged just by brightness
– Apparent brightness – luminosity as seen from
Earth
• Brightness as affected by distance
– Absolute brightness – luminosity as if stars were at
a fixed distance
• Brightness at 10 parsec (32.6 light years)
• Scale is set with our Sun as a value of 1.0

9. 24.5 Stars: the main sequence
• Hertz-Sprung Russell diagram
– Main sequence – a sinuous line along which 90%
of stars lie
• Luminosity and temperature are in direct relation
• Larger stars are brighter
– Greater gravity drives fusion reactions faster
– Non-main sequence stars are part of the stars life
cycle….

10. Fig. 24.11, p.618

11. Steps in the death of a
medium mass star
For medium mass stars (size of our Sun)
1. H nuclei fuse to He, He cannot fuse to heavier
elements at these temperatures
2. As fusions slows, temperatures drop
3. Outward pressure drops and star contracts
4. As the core contracts, it heats up
This heat ignites H fusion in outer layers – outer layer
expands and becomes a red giant

12. Fig. 24.4, p.612

13. Steps in the death of a
medium mass star con’t
5. Core of red giant shrinks until He fusion starts
6. Helium fuses to carbon and eventually it runs out
7. Further contraction causes heating again
8. At one solar mass, pressure does not cause any more
fusion

14. Fig. 24.15, p.621
The Ring Nebula is a sphere of
gas and dust expelled as a dying
star exploded.
Death of a Star
5. Blows gas shell off and
creates planetary
nebula
6. Contracts to about
Earth size and glows
from residual heat – a
white dwarf
7. After tens of billions of
years, it will just go
dark

15. Fig. 24.13, p.620

16. The death of a massive star
• Stars with a large mass, >1.44 solar masses
– Star doesn’t go white dwarf
• Carbon fusion initiates and iron forms
– Iron fusion doesn’t release energy, it absorbs energy
• This cools the star very fast, rapidly collapses
– on the order of seconds
• Massive heat build-up (trillions of K) causes the star to go
supernova or explode
• Shockwave from explosion causes fusion reactions that
create the rest of all elements

17. 24.6 The life and death of a star
• First and second generation stars
– All original stars had to form from only H & He
• Some of these old population II stars still exist
– When a star dies (as nebula or supernova) the dusts
and gases form into new stars
• These contain heavier elements
– Thus newer, population I stars, contain small bits of
heavy elements
• Our own solar system was born of this recycled material

18. Fig. 24.16, p.621

19. Fig. 24.17, p.622

20. 24.7 Neutron stars, pulsars, and black
holes
• After a supernova, there is often a large mass left,
larger than a white dwarf
– Subatomic particles squeezed together form a neutron star
– Very dense – 1013 kg.cm3
– Pulsars
• Storms on the spinning neutron star put out regularly-spaced radio
waves
– Black-holes – stars of >5x solar masses
• When these die, the leftover core is so massive that even the
neutrons collapse
• Eventually this compresses to a point-mass, a black hole
– Gravity so strong it bends light around it

21. Fig. 24.30, p.629

22. 24.12 The end of the universe
• The Big Bang theory supports the expansion of
the universe however….
• Gravity pulls on all object everywhere, gradually bringing
things together so…
• How will it end – or will it?
– Closed universe – gravity will eventually win and it
will all collapse on itself one day
– Open universe – it will fly apart forever and
eventually the fusion stops

23. Fig. 24.32a, p.631

24. Fig. 24.32b, p.631