1. You can often predict how a baby will look as an adult by looking at other family members.
Astronomers observe stars of different ages to infer how stars evolve.

2. How Stars Form
A star is formed when a contracting cloud of gas and dust becomes so dense and hot that nuclear fusion begins.
A nebula is a large cloud of gas and dust spread out over a large volume of space.
Some nebulas are glowing clouds lit from within by bright stars.
Other nebulas are cold, dark clouds that block the light from more-distant stars beyond the nebulas.
Stars form in the densest regions of nebulae.
Gravity pulls a nebula’s dust and gas into a denser cloud.
As the nebula contracts, it heats up.

3. How Stars Form
A contracting cloud of gas and dust with enough mass to form a star is called a protostar.
As a protostar contracts, its internal pressure and temperature continue to rise.
Pressure from fusion supports the star against the tremendous inward pull of gravity.
A group of bright young stars can be seen in the hollowed-out center of the Rosette Nebula.

4. Adult Stars
A star’s mass determines the star’s place on the main sequence and how long it will stay there.
Stars spend about 90 percent of their lives on the main sequence.
In all main-sequence stars, nuclear fusion converts hydrogen into helium at a stable rate.
There is an equilibrium between the outward thermal pressure from fusion and gravity’s inward pull.
The amount of gas and dust available when a star forms determines the mass of each young star.

5. Middle of the main sequence:
Adult Stars
The most massive stars have large cores and therefore produce the most energy:
High-mass stars become the bluest and brightest main-sequence stars.
These blue stars are about 300,000 times brighter than the sun.
Because blue stars burn so brightly, they use up their fuel relatively quickly and last only a few million years.
Middle of the main sequence:
A yellow star like the sun has a surface temperature of about 6000 K and could remain stable on the main sequence for about 10 billion years.

6. Small nebulas produce small, cool stars that are long-lived:
Adult Stars
Small nebulas produce small, cool stars that are long-lived:
A star can have a mass as low as a tenth of the sun’s mass.
The gravitational force in such low-mass stars is just strong enough to create a small core where nuclear fusion takes place.
This lower energy production results in red stars, the coolest of all visible stars.
A red star, with a surface temperature of about 3500 K, may stay on the main sequence for more than 100 billion years.

7. The Death of a Star
The dwindling supply of fuel in a star’s core ultimately leads to the star’s death.
When a star’s core begins to run out of hydrogen, gravity gains the upper hand over pressure, and the core starts to shrink.
The core temperature rises enough to cause the hydrogen in a shell outside the core to begin fusion.
The energy flowing outward increases, causing the outer regions of the star to expand.
The expanding atmosphere moves farther from the hot core and cools to red.
The star becomes a red giant.

8. The Death of a Star
The collapsing core grows hot enough for helium fusion to occur, producing carbon, oxygen, and heavier elements.
In helium fusion, the star stabilizes and its outer layers shrink and warm up.
The final stages of a star’s life depend on its mass.

9. Low- and Medium-Mass Stars
The Death of a Star
Low- and Medium-Mass Stars
Low-mass and medium-mass stars, which can be as much as eight times as massive as the sun, eventually turn into white dwarfs.
Stars remain in the giant stage until their hydrogen and helium supplies dwindle and there are no other elements to fuse.
The energy coming from the star’s interior decreases.
With less outward pressure, the star collapses.

10. The Death of a Star
The dying star is surrounded by a glowing cloud of gas, called a planetary nebula.
As the dying star blows off much of its mass, only its hot core remains.
This dense core is a white dwarf.
A white dwarf is about the same size as Earth but has about the same mass as the sun.
White dwarfs don’t undergo fusion, but glow faintly from leftover thermal energy.

11. The Death of a Star
Planetary nebulas, such as the Hourglass Nebula, are clouds of gas that surround a collapsing red giant.

12. The Death of a Star
The Crab Nebula is the remnant of a supernova explosion that was observed on Earth in A.D The supernova was so bright that people could see it in the daytime.

13. The Death of a Star High-Mass Stars
The life cycle of high-mass stars is very different from the life cycle of lower-mass stars.
As high-mass stars evolve from hydrogen fusion to the fusion of other elements, they grow into brilliant supergiants, which create new elements, the heaviest being iron.
A high-mass star dies quickly because it consumes fuel very rapidly.

14. The Death of a Star
As fusion slows in a high-mass star, pressure decreases.
Gravity overcomes pressure, leading to a dramatic collapse of the star’s outer layers.
This collapse produces a supernova: an explosion so violent that the dying star becomes more brilliant than an entire galaxy
Supernovas produce enough energy to create elements heavier than iron
These elements, and lighter ones such as carbon and oxygen, are ejected into space by the explosion.
As this occurs, its core continues to collapse.

15. The Death of a Star
If the remaining core of a supernova has less than about three times the sun’s mass, it will become a neutron star: dense remnant of a high-mass star that has exploded as a supernova.
Electrons and protons are crushed together to form neutrons
Much smaller & denser than white dwarfs
Spins more rapidly as it contracts
Some neutron stars spin hundreds of turns per second!
Emit steady beams of radiation in narrow cones
A spinning neutron star that appears to give off strong pulses of radio waves is called a pulsar.

16. The Death of a Star
Pulsars emit steady beams of radiation that appear to pulse when the spinning beam sweeps across Earth.

17. The Death of a Star
If a star’s core after a supernova explosion is more than about three times the sun’s mass, it collapses beyond the neutron-star stage to become a black hole.
A black hole is an object whose surface gravity is so great that even electromagnetic waves, traveling at the speed of light, cannot escape from it.

18. Assessment Questions
As a protostar contracts, what happens to its pressure and temperature?
They stay constant.
The temperature increases while the pressure stays constant.
They both decrease.
They both increase.
ANS: D

19. Assessment Questions
Which type of main sequence star would be likely to remain in the main sequence for about 100 billion years?
red
yellow
white
blue
ANS: A

20. Assessment Questions
Based on its position on the H-R diagram, what will the sun become when it finally runs out of fuel?
nebula
supernova
neutron star
black dwarf
ANS: D