1. Stars
Life Cycles

2. What is a star?
A star is a luminous ball of plasma (ionized gas) mostly hydrogen and helium held together by its own gravity. 
Stars shine by burning hydrogen into helium in their cores (reaction similar to a nuclear reactor) and later in their lives create heavier elements

3. How do stars form?
All stars form from a cloud of dust and gases called a nebula.
Gravity can pull some of the gas and dust in a nebula together.

4. LIFE CYCLE Stars do not remain the same forever.
An average star, like our sun, goes through four stages during its life:

5. Protostar
From a nebula, gravity begins to pull the dust and gases together into a ball and its center becomes denser and hotter.
Not a true star

8. Main-Sequence Star
Eventually, the gas becomes so hot that it begins to react.
A star is born when the core temperature gets hot enough (10 M K) that hydrogen reacts to form helium, producing huge amounts of energy (nuclear fusion)
Longest stage of stars life
 protostar becomes a main sequence star when its core temperature exceeds 10 million K. This is the temperature needed for hydrogen fusion to operate efficiently. The more massive the star, the faster everything happens.

9. Main Sequence Star
As long as a main-sequence star has enough hydrogen to react, its size will not change very much.
Stable, lasts about 5 billion years
90 % of stars in the universe
Duration here depends on star’s mass
Depending on mass, can be a low to medium mass star or a high mass star
Next stage depends on the star’s mass.

11. Red Giant or Super Red Giant
When a main sequence star uses up all its hydrogen, the core contracts and gets even hotter, and begins burning helium, causing outer layers to expand.
The expanded layers have a cooler surface temperature of around 5000K, making the star appear red.

12. Our Sun will become a Red Giant
In about 5 billion years our Sun will become a Red Giant, and will swell up and engulf the orbits of the first 3 planets.
Red giants don't last long, only a few million years.

13. Red Giant or Super Red Giant
Have high magnitudes due to their large size. 
The star then begins to radiate more light, appearing brighter.
When the core gets hot enough, helium nuclei fuse in a reaction that forms still heavier nuclei such as oxygen and carbon.

14. Super Red Giant p386
If temperatures rise continues after the helium nuclei are used up, oxygen, carbon and their nuclei fuse to form elements as heavy as iron.
Stars that are 10 times bigger than the sun (or larger) will turn into supergiants when they run out of fuel.
Massive stars are hotter, give off much more light, and use up their hydrogen more quickly than smaller stars. Therefore do not live as long as other stars.

17. Planetary Nebula
After a low or medium mass star has become a red giant the outer parts grow bigger and drift into space, forming an expanding cloud of hot gas (plasma) with a blue-white hot core (called a planetary nebula).
nothing to do with planets, but were so named because early astronomers thought they looked a bit like planets through a small telescope
Despite the name, they have nothing to do with planets, and were so named because early astronomers thought they looked a bit like planets through a small telescope

19. White Dwarf p386
Very small, hot, leftover depleted core of a red giant made mostly of carbon.
about the size of the Earth (but tremendously heavier)
can shine for billions of years.
The white dwarf eventually runs out of fuel and dies (cools, no longer gives off light and becomes as a black dwarf
Our sun will someday turn into a white dwarf and then a black dwarf

20. Massive Stars  Supernova
When fusion of a Super Red Giant stops, iron core remains, star cools and collapses, causing pressures and temperatures within the core to rise, iron nuclei become fused into heavier elements.
In a rush toward further collapse the star explodes so violently that half its mass is blown away as a great cloud.
The star flares up into an intensely bright object called a supernova.

21. Supernova
Extremely dense: A single teaspoon of matter from a neutron star would weigh 100 million metric tons!
For a brief time, a supernova can shine as brightly as an entire galaxy but will fade again over a matter of days.
After such an explosion, a massive star may become a neutron star, a pulsar, or a black hole

23. Modelling a Supernova
Demo Probe 9

25. Neutron Star p387
Remaining half of supernova’s exploding mass becomes a neutron star
Electrons are crushed into the nucleus and combine with protons to form neutrons to form a single dense mass of neutrons
So dense can become a black hole.

26. Black Holes p388
After a large mass star explodes, a large amount of mass may remain.
The gravity of the mass is so strong that gas is pulled inward, pulling more gas into a smaller and smaller space,
Eventually, the gravity becomes so strong that nothing can escape, not even light.

27. BLACK HOLES
Because black holes do not give off light, it can be hard for scientists to locate them. Gas and dust from a nearby star may fall into the black hole and give off X rays. When scientists find these X rays, they can infer that a black hole is close by.
Albert Einstein predicted existence of black holes. That gravity does warp space and time.

28. Event Horizon
April 2019 Event Horizon Telescope took the first image of a black hole.
The 'event horizon' is the edge around a black hole from which nothing (not even light) can escape. In other words, the escape velocity for an object within the event horizon exceeds the speed of light.
The visible disc is super charged/heated gas around rim that makes it visible. Light is actually, the torrent of ultraviolet radiation is being emitted by gas that is heated as it is pulled into the black hole's powerful gravity.

29. Hertzsprung-Russell Diagram
The H-R Diagram classifies stars according to their luminosity, color, temperature and evolutionary stage.
As stars progress through their life cycle, their position changes on the graph.

31. Practice
Life Cycle of a Star Package

32. Summary
Main sequence stars - band running through middle of diagram, including our Sun Red Giants - cool, but very large and bright stars (upper right of diag.) Supergiants - extremely large, bright stars (top of diagram) White Dwarfs - hot, but very small and dim stars (lower left of diag.)

33. Extras

34. TYPES OF STARS p382

35. PULSARS A rapidly spinning neutron star
send out beams of radiation that sweep through space.
A radio telescope, an instrument that can pick up radiation with long wavelengths, can detect pulsars.
Every time a pulsar’s beam sweeps by Earth, scientists hear rapid clicks, or pulses, in the radio telescope.

36. Red Giant
A red giant is a relatively old star whose diameter is about 100 times bigger than it was originally, and had become cooler (the surface temperature is under 6,500 K).
They are frequently orange in color. Betelgeuse is a red giant.
It is about 20 times as massive as the Sun about 14,000 times brighter than the Sun, and about 600 light-years from Earth. 

37. Binary Star System
A binary star is a system of two stars that rotate around a common center of mass (the barycenter).
About half of all stars are in a group of at least two stars.  Polaris (the pole star of the Northern Hemisphere of Earth) is part of a binary star system

38. Variable Stars - Stars that Vary in Luminosity:
Cepheid variables are stars that regularly pulsate in size and change in brightness. As the star increases in size, its brightness decreases; then, the reverse occurs. Cepheid Variables may not be permanently variable; the fluctuations may just be an unstable phase the star is going through. Polaris and Delta Cephei are examples of Cepheids. 

39. RED DWARF
A red dwarf is a small, cool, very faint, main sequence star whose surface temperature is under about 4,000 K.
Red dwarfs are the most common type of star.
Proxima Centauri is a red dwarf. 

40. Other Possibilities
Stars times more massive than our Sun can end as neutron stars after going supernova. These superheated, super massive dead stars can take trillions of years to cool.
Stars 25 times as massive as our Sun can become black holes instead of neutron stars. The same process that produces a neutron star produces an area so massive and yet so small that the gravity it produces traps everything - even light!
Stars can vary greatly in size. Although our Sun is an average size, many of the stars we see in the night sky are up to 3000 times as large as the Sun.

41. Blue Giant A blue giant is a huge, very hot, blue star.
It is a post-main sequence star that burns helium. 

42. Supergiant
A supergiant is the largest known type of star; some are almost as large as our entire solar system. Betelgeuse and Rigel are supergiants. These stars are rare. When supergiants die they supernova and become black holes. 

46. Life Cycle of a Star

47. Which star is
hotter—Antares or Polaris?
9. Read a Graph Is
Betelgeuse on the main
sequence?

48. Where in the
H-R diagram are the brightest
stars located?
diagram are the hottest stars
located?

49. The Hertzsprung-Russell Diagram
By studying stars, astronomers have created an evolutionary ‘lifespan’ that stars progress through.
The Hertzsprung-Russell diagram was developed to show the different stages of a star’s life.
90% of stars are in the main sequence, where energy is produced combining hydrogen atoms into helium.