1. Life Cycle of Stars
Section 26.3

2. How Stars Form
The space around stars contains gas and dust, some areas its spread thinly or packed densely
A nebula is a large cloud of gas and dust spread out over a large volume of space .

3. How Stars Form (cont.)
Some nebulas are glowing clouds lit from within by bright stars. Or they are cold, dark clouds that block light.
Stars form in the densest region of nebula.

4. How Stars Form(cont.)
Stars are created by gravity, it pulls a nebula’s dust and gas into a denser cloud.
The nebula heats up as it contracts.
If that contracting cloud has enough mass then it forms a protostar. As the protostar contracts, its internal pressure and temperature continues to rise.

5. How Stars Form (cont.)
A star is formed when a contracting cloud of gas and dust becomes so dense and hot that nuclear fusion begins.
The pressure from fusion supports the star against the tremendous inward pull of gravity. The new energy source stabilizes the young star.

6. Adult Stars Stars spend 90% of their lives on the main sequence.
A star’s mass determines the star’s place on the main sequence and how long it will stay there.

7. Adult Stars (cont.)
A stars mass is determined by the gas and dust available when the star forms.
Ex: Larger the star, more energy it has.
The more energy it “burns” the faster it will die away.

8. Adult Stars (cont.)
Stars similar to the sun occupy the middle of the main sequence.
Small nebulas produce small, cool stars that live longer than larger stars.
Did You Know: These smaller stars can last for billion years?

9. The Death of a Star
The decreasing supply of fuel in a star’s core leads to a star’s death as a white dwarf, neutron star, or black hole.

10. Low- and Medium-Mass Stars
Planetary nebula- Glowing cloud of gas that surrounds a dying star.
Red giant → Planetary Nebula → White Dwarf → Black Dwarf

11. High-Mass Stars
Supernova- An explosion caused by gravity overcoming the lower pressure in a star. Brighter than an entire galaxy.
Red Supergiant → Supernova →
Neutron star or Black hole

13. High-Mass Stars (cont.)
As a supernova spews material into space, the core continues to collapse.
A neutron star is the dense remnant of a high-mass star that has exploded from a supernova.
Neutron stars are much smaller and denser than white dwarfs.

14. High-Mass Stars (cont.)
Pulsar- A spinning neutron star that appears to gives off strong pulses of radio waves.
Black Hole An object whose surface gravity is so great that even electromagnetic waves, traveling at the speed of light, can’t escape from it.