1. The Life Cycle of Stars
Starry, Starry Night

2. A star is formed in a nebula, an interstellar cloud of dust and gas.
In these “stellar nurseries”, dense parts of these clouds undergo something called gravitational collapse where everything compresses to form a rotating gas globule.
The globule is cooled by giving off radio waves and radiation. It is compressed mainly by gravitational forces within the globule. These forces cause the globule to collapse and rotate.
As the collapse proceeds, the temperature and pressure within the globule increases. Also, the globule rotates faster and faster. This spinning action causes an increase in centrifugal forces (a force on spinning objects) that causes the globule to have a central core and a surrounding flattened disk of dust (called a protoplanetary disk). The central core becomes the star.

3. The contracting cloud heats up due to friction and forms a protostar; this stage lasts for roughly 50 million years.
If there is enough material in the protostar, the star’s collapse and heating continue. If there is not enough material in the protostar, it might become is a brown dwarf (a large, not-very-bright sky object).
When a temperature of about 27,000,000°F is reached, nuclear fusion begins. This is a reaction in which hydrogen atoms are converted to helium atoms plus energy. This energy production stops the star from compressing any more.
The protostar is now a stable main sequence star which will remain in this state for about 10 billion years.

4. The steps so far… after life, comes…….
So a star is born in a (ask for answer): nebula
There, the gases collapse, and spread into a disk shape. The core’s temperature heats up and a _______ is formed (ask for answer) protostar
Then, pressure and temperature continue to increase and what happens? (ask for answer): nuclear fusion
From the nuclear fusion process, there is now a stable (ask for answer): main sequence star
Stars spend about 90% of their life as a main sequence star!
after life, comes…….

5. Stars die in different ways depending on their size.
A Sun-like star, that is around the size of our Sun or smaller, converts hydrogen to helium in its core during its main sequence phase to continue burning brightly.
However, eventually it runs out of fuel. When that happens, the core contracts and the outer layers expand creating a Red Giant.
The outer layers start to drift off into space, cooling and becoming less bright as they go. This is called a Planetary Nebula.
The star cools and shrinks until it becomes a White dwarf which only radiates heat into space.
Eventually the star will have lost all of its heat and is just a cold and dark lump. This is called a Black Dwarf.

6. Huge stars are those that are 1.5 to 3 times as massive as the sun!
A huge star has a main sequence phase. It is shorter than that of a smaller star however because the forces that want to pull the gases into the core (gravity) and the forces that want to push them away are much greater and cause the star to lose its stability much sooner.
When a huge star runs out of fuel it becomes a Red Supergiant.
Then, because the forces are so great, the core just collapses in a single instant. This looks like a short-lived explosion and it blows the outer layers away. This is called a Supernova.
A Neutron star is the final result. The core contracts so much that it makes a tiny (in star sizes!), very dense star that has a very strong magnetic field and a rapid spin.

7. Giant stars are more that 3 times the mass of the sun!
They have almost the same death cycle as a huge star: the main sequence is shorter, it turns into a Red Supergiant, then experiences a Supernova.
However, the last step is different. Because the star was so massive, it contracts even more after a Supernova, creating a Black Hole. This Black Hole is so dense and has such a strong gravitational field that even light cannot escape its pull!

8. Supernova of a Huge or Giant Star

9. The final steps… The Death of a star changes with regard to size.
A star up to the size of our sun turns into a ________, then the outer layers drift off into space, creating a ___________ (ask for answers): Red Giant, planetary nebula
The star only emits heat as a _________ and eventually becomes a cold dark lump called a __________ (ask for answers): white dwarf, black dwarf.
Huge and Giant stars first turn into a ____________ (ask for answers): red supergiant
Then, they experience an explosion of forces called a ____________ (ask for answers): supernova
A huge star becomes a dense star called a ___________, and a giant star becomes so dense and magnetic that it becomes a _____________ (ask for answers): neutron star, black hole.

10. References