1. Stars

2. 1. Where Are We??
We live in the Milky Way Galaxy, which is one of at least billion (maybe up to !) galaxies
in the universe.

3. 1. Where Are We??
The Milky Way is a barred spiral galaxy, which is much more complex than the earliest galaxies. There are at least billion stars and nebulae in the Milky Way (maybe as
many as 500 billion).
Our solar system is
on the Orion Arm
of the Milky Way.

4. Nebulae – The Birthplace of Stars
Nebulae are large clouds of gas and dust 4

5. Stellar nurseries
Stars are born in gaseous bright nebulae…

6. Stellar nurseries
and in dusty dark nebulae.

7. 2b. The Birth of a Proto-Star
Within a nebula, a ball of hydrogen gas collapses under the force of gravity and begins to heat up – a proto-star is born! 7

8. Basic Star Formation….
A star is formed when a contracting cloud of gas and dust becomes so dense and hot that nuclear fusion begins. 8

9. Our Sun is a small to medium -size yellow star

10. Our Sun
1) A star is a huge nuclear furnace, turning million tons of hydrogen into helium every second!!

11. 1. Fusion – A Star’s Power Source
Inside the sun, Hydrogen nuclei fuse to form 1 Helium nucleus, releasing 2 positrons along with starlight, heat and radiation.

12. 1. Fusion – A Star’s Power Source
Note: Positrons (anti-electrons) and electrons annihilate each other to create gamma rays & photons of light energy (starlight)

13. Proton-proton chain reaction
Step 1: 2 Hydrogen nuclei fuse to form Deuterium:
2 1H1  1H2 + n (neutrino)
+ positron (e+)

14. Proton-proton chain reaction
Step 2: Deuterium and Hydrogen nuclei fuse to form light Helium He-3
1H2 + 1H1  2He3
+ g (gamma ray)

15. Proton-proton chain reaction
Step 3: 2 light Helium He-3 nuclei fuse to form He-4, releasing 2 protons.
2He3 + 2He3  2He4
+ 2 1H1

16. Proton-proton chain reaction
Net Reaction:
4 1H1  2He4
(Side Reactions create light , heat and radiation)

17. 4. Stellar Tug-of-War
1) The life of a star is marked by 2 opposing forces (see next page):
a) The pressure created by thermonuclear
reactions in the
core of a star
pushing outwards
( ) and...

18. 4. Stellar Tug-of-War
2) The crushing force of gravity pulling the star’s
surface inwards.
( )

19. 4. Stellar Tug-of-War Pressure from Nuclear Reactions pushing outward
Gravity pulling inward

20. At first, gravity dominates and causes the core of the star to
4. Stellar Tug-of-War
At first, gravity dominates and causes
the core of the
star to
collapse
inward.

21. As the core starts to shrink, the core temperature rises to 40
4. Stellar Tug-of-War
As the core starts to shrink, the core
temperature
rises to 40
million C
and Hydrogen
fuses to make Helium.

22. The outward flow of energy increases, causing the star to
4. Stellar Tug-of-War
The outward flow of energy
increases,
causing the
star to
expand outward.

23. When a star runs out of hydrogen, its nuclear “fuel”, the force of
4. Stellar Tug-of-War
When a star runs out of hydrogen,
its nuclear
“fuel”, the
force of
gravity takes over and the core of the star collapses .

24. 4. Stellar Tug-of-War
7) This time, the compression caused by the gravitational collapse heats the star’s core even more & starts a whole new chain of thermonuclear reactions with Helium replacing Hydrogen as the nuclear fuel.

25. 4. Stellar Tug-of-War
8) When stars finish “burning up” both their hydrogen and helium nuclear fuel, they begin to die out again and, once again, they collapse inward.
What happens next depends on the size of the star…
(to be continued...)

26. Supernovas Supernovas
White
Dwarfs
Supernovas
Supernovas
Black
Dwarfs
Red
Giants
Black
Holes
Black
Holes

27. 5. How Far Away are the Stars?
It depends on which star you are talking about!!
The distance to the stars in measured in light-years (ly) – the distance a ray of light travels in 1 year
At 186,000 miles per second, a light-year is about 6 trillion miles!!
The Sun is our star. It is about 8 1/3 light-minutes (500 light-seconds) from Earth.

28. 5. How Far Away are the Stars?
It depends on which star you are talking about!!
Our nearest neighbor stars are:
Proxima Centauri – 4.24 ly away and
Alpha Centauri – 4.37 ly away

29. 5. How Far Away are the Stars?
It depends on which star you are talking about!!
Sirius, one of the brightest stars in the sky is 8.7 ly away

30. 5b. Parallax Method for Distance
The closer a star is, the more it “shifts” position as it is observed from opposite side of Earth’s orbit around the Sun.
The distance of the star can be measured by observing the number of degrees of shift. (parallax angle)

31. 5b. Parallax Method for Distance

32. 5b. Parallax Method for Distance
An increase in the angle of one arc second (1/60th of a 1/60th of a degree) equates to an increased distance of one parsec (parallax arc second)

33. 5b. Parallax Method for Distance
This only works for stars closer than 100 parsecs.
(1000 pcs from the
Hippacarus Satellite)

34. 6. How Big are Stars?
They come in all sizes, but they are all pretty big. The sun is an average size star, but 1 million Earths would fit inside it!!

35. 6. How Big are Stars? One measure of a star’s size is its radius:
The easiest way to measure star radii is to compare the radius of the star to the radius of the Sun.
This is called a Solar Radius, whose symbol is R☉.

36. Yellow Dwarf (Main Sequence) Yellow/Orange Sub-giant
6. How Big are Stars?
One measure of a star’s size is its radius:
Red/Orange Giant
Yellow Dwarf (Main Sequence)
Yellow/Orange Sub-giant

37. Yellow Dwarf (Main Sequence)
6. How Big are Stars?
One measure of a star’s size is its radius:
Red Supergiants
Yellow Dwarf (Main Sequence)
Red Giant

38. 6. How Big are Stars?
The most important property of a star is its mass, which determines its temperature, its size, its brightness or luminosity, and how long it will eventually live.
The mass of a star is measured in Solar Masses (M☉).

39. 6. How Big are Stars?
The mass of a star is measured in Solar Masses (M☉).
By definition, our sun has a mass of 1 M☉ (2x1030 kg - nearly a million times the mass of the Earth), and a radius of 1 R☉ (7x105 km - about 100 times greater than the radius of Earth).
Even so, the sun is just a typical, average- sized star.

40. 6. How Big are Stars?
The Sun belongs to a category of dwarf stars, which includes all stars with masses less than 5 M☉ (that is, less than five times the mass of the sun).
They are better known as Main Sequence Stars, because they are all still in the main-sequence, “hydrogen-burning” phase (where they spend 99% of their life).

41. 6. How Big are Stars?
Stars can form with masses ranging from 0.08 M☉ (dwarfs) to around 100 M☉ (which would place a star in the giant or even supergiant category).
A huge 265 M☉ supergiant star was discovered recently.
However, the process of star formation seems to create more small stars than large stars.

42. 6. How Big are Stars?
The most common type of star turns out to be one with about M☉.
Even though dwarf stars may not be as spectacular as giant stars, there are far more of them in the universe.

43. The mass of a star determines its fate
Brown Dwarf
Tiny Stars
Red Giant
Sun-Sized Stars
White Dwarf
3 – 10 M Stars
Supergiant
20 – 30 M Stars
Neutron Star
Supernova
Supergiant
Black Hole