evolution of stars

Astronomy Notes — Stars and Stellar Evolution

6

Formation of Stars

Stars form inside nebulae:

• Giant clouds of gas and dust

• Mostly hydrogen and helium

Example:

• Orion Nebula

Formation process:

1. Gas cloud collapses due to gravity

2. Collapse may be triggered by a supernova shockwave

3. Dense hot core forms

4. Protostar is created

Protostar

Early stage of star formation.

• Temperature rises rapidly

• Lasts about 10410^4104 to 10710^7107 years

• Not yet undergoing hydrogen fusion

Protoplanetary Disk

Heavy particles around protostar form a disk.

This disk may create:

• Planets

• Asteroids

• Other objects

Beginning of a True Star

When core temperature reaches about:

• 10 million K

Hydrogen fusion begins:

• Hydrogen → Helium

This marks:

• End of protostar stage

• Beginning of main sequence star

Main Sequence Stars

Stars spend about 90% of their life here.

Main process:

• Hydrogen fusion into helium

Key idea:

• Bigger stars burn fuel faster

• Bigger stars have shorter lifespans

Star colors:

• Blue = hottest

• Red = coolest

Solar Mass

Used to compare star masses.

1 solar mass=1.9891×1030 kg1\text{ solar mass}=1.9891\times10^{30}\text{ kg}1 solar mass=1.9891×1030 kg

Brown Dwarfs

“Failed stars”

• Too little mass for hydrogen fusion

• Less than needed to sustain fusion

IAU minimum:

• At least 13 Jupiter masses

Low-Mass Stars

Range:

• About 0.01–0.5 solar masses

Life cycle:

1. Hydrogen fusion stops

2. Helium core remains

3. Outer hydrogen shell expelled

4. Forms helium white dwarf

Medium-Mass Stars

Range:

• About 0.3–8 solar masses

After hydrogen runs out:

1. Core contracts

2. Outer layers expand

3. Becomes red giant

If core reaches:

• 10810^8108 K

Then:

• Helium fusion begins

• Helium → Carbon

Later:

• Outer layers expelled

• Forms planetary nebula

• Core becomes white dwarf

Asymptotic Giant Branch (AGB)

Late giant stage with:

• Hydrogen fusion shell

• Helium fusion shell

Two fusion shells surround the core.

White Dwarfs

Hot dense stellar remnants.

Usually:

• Carbon and oxygen core

Over time:

• Cool down

• Eventually become black dwarfs

White dwarf mass limit:

• About 1.39 solar masses

Massive Stars

Range:

• Above 8 solar masses

Become:

• Red supergiants

• Blue supergiants

Can fuse heavier elements:

• Carbon

• Neon

• Oxygen

• Silicon

This process is called:

• Nucleosynthesis

Produces heavy elements.

Supernova

Violently exploding star.

Occurs when:

• Massive core collapses

Results:

• Shockwave ejects material

• Heavy elements spread into space

Neutron Stars

Formed after supernova collapse.

• Extremely dense

• About 20 km diameter

Neutron star limit:

• About 1.5–3 solar masses

Black Holes

If core exceeds neutron star limit:

• Gravity collapses core completely

• Black hole forms

Hertzsprung–Russell (HR) Diagram

Graph showing relationship between:

• Temperature

• Luminosity or absolute magnitude

Axes

X-axis:

• Temperature decreases left → right

• Hot stars on left

• Cool stars on right

Y-axis:

• Luminosity increases upward

• Brighter stars at top

Regions of HR Diagram

Main regions:

• Main sequence

• Giants

• Supergiants

• White dwarfs

Sun on HR Diagram

Current stage:

• G-type main sequence star

Future:

1. Red giant

2. White dwarf

3. Black dwarf

Variable Stars

Stars whose brightness changes.

Two main categories:

Intrinsic Variables

Brightness changes because of internal changes.

Pulsating Variables

Star expands/contracts.

Examples:

• Cepheid variables

• RR Lyrae variables

• Mira variables

Eruptive Variables

Brightness changes from flares or mass ejections.

Examples:

• Protostars

• Luminous blue variables

Cataclysmic Variables

Explosive changes.

Examples:

• Novae

• Supernovae

Extrinsic Variables

Brightness changes because of outside effects.

Eclipsing Binaries

Two stars orbit and block each other’s light.

Rotating Variables

Brightness changes as star rotates.

Example:

• Pulsars

Important Relationships

• Larger mass → shorter lifespan

• Blue stars → hotter

• Red stars → cooler

• Massive stars → explosive deaths

• Small stars → white dwarfs

• Very massive cores → black holes