We saw how the Solar System was formed – Solar System lecture
-the birth of the Sun
Recap: Formed from a gas and dust cloud.
Stars go through different phases in their life time.
1. Protostar – Stage before fusion starts.
2. Fusion: Hydrogen into Helium - Most of a star’s life
- Known as the Main Sequence
3. Red Giant – Dying star
4. Planetary Nebula – Fusion ends 5. White Dwarf – remains of the star.
6. Neutron Star 7. Black Hole
Discovering the Universe Ninth Edition Discovering the Universe Ninth Edition Neil F. Comins William J. Kaufmann III
Protostar: before fusion has started
• Collapsing cloud – dense core - growing
• Temperature starts rising due to compression
• Emit radiation in infrared
– not hot enough to be seen in visible light.
http://hubblesite.org/news_release/news/1997-13
← Infrared image
The protostar keeps on contracting
When the temperature at the core reaches 107K
→ fusion starts (not a protostar any longer)
→ large amounts of energy released Hydrostatic Equilibrium is achieved
It is called a Main Sequence Star
Most of the star’s life is spent in this stage
Main Sequence
• Stage in which stars convert Hydrogen in Helium through fusion.
• Note: Fusion takes place in the core, where the temperatures are high enough (107K or more).
• After a long time, the hydrogen runs out.
• The time this takes depends on
• the amount of fuel available: mass
• the luminosity (rate at which fuel is burnt)
• More massive stars burn energy at a higher rate.
• As a result, instead of having longer lifetimes, they have shorter lifetimes.
• The lifetime of the Sun = 1010 years
Red Giant Phase: Running out of Hydrogen (in the core)
After spending most of its lifetime burning Hydrogen into
Helium, the star eventually runs out of Hydrogen in the core.
→ Enters the Red Giant Phase
Running out of Hydrogen (in the core)
-As the hydrogen in the core runs out, the energy released from fusion decreases and the gravity causes the core of the star to collapse.
-The external shell, where there is still Hydrogen contracts too.
-This causes it to heat up.
→Fusion stars in the Hydrogen shell.
-More energy is released
→The gases on the outside expand
→The star becomes giant
-The outer most layers are far from the fusing shell therefore cooler
→ star turns red
Red Giant Phase
Red Giant Phase
Eventually, a star like the Sun will expand up to 1 AU.
→ Up to the Earth’s orbit.
NB: this is a simplified presentation
→ In reality, this several steps are involved The Helium in the core fuses to
Carbon -Oxygen too at some point.
https://maas.museum/app/uploads/sites/6/2013/07/Present-Sun-Earth-orbit-and-the-future-red-giant_Nick-Lomb.jpg
Red Giant to Planetary Nebula
In the Red Giant phase, the outer most layers of the star lose mass constantly.
At some point, the mass loss is so great that fusion in the shell stops.
Ejected material → expands → cools
→ electrons and ions recombine
→ dust and gas condense
→ eventually, the interior of the star is revealed
→ This is known as a Planetary Nebula
Planetary Nebula
-The hot interior of the star is revealed.
The name Planetary Nebula does not have anything to do with planets →historical
Planetary Nebula
-Some can be spectacular
-The central star can be seen at the centre of the nebula -They are very common in our Galaxy
Planetary Nebula → White dwarf
-After about 50 000 years, the gases of the nebula spread so far from the cooling central star that they are not visible any longer.
-All that remains is the slowly cooling core of the original star.
-This is known as a White Dwarf.
It is white because once the outer gases spread away, the cooling core that is revealed is at higher temperature.
White dwarf
-Typically, the size of the Earth but much denser.
-Some H and He may remain (but mostly converted to Carbon-Oxygen after fusion.
-Its luminosity decreases further as it cools down.
-Calculations predict that it takes billions of years for a white dwarf to cool down.
-Currently, the oldest white dwarfs still radiate at a few thousand Kelvin.
-Because of their low luminosity, they are hard to observe.
For more massive stars
If the star is massive enough, then the temperature increases enough in the core to allow carbon fusion.
8 solar masses and above
The cycle can repeat itself, fusing heavier elements each time provided the temperature required is reached
Heavier elements fusion
-Carbon, Neon, Oxygen, Silicon
The cycle repeats, fusing heavier elements each time, until the core temperature cannot rise any higher.
A Red Supergiant is formed – size of Jupiter’s orbit Iron core
-At some point, the core cannot sustain the pressure from above and collapses violently.
→ Massive explosion occurs as a result.
This is known as a Supernova explosion
• Star can brighten to 1010 Lsol within minutes
• Can outshine an entire galaxy
• Among the most violent phenomena in the universe
• Neutron stars, pulsars, black holes can be formed as a result of supernova explosions
• Elements heavier than iron are forged in supernova explosions
The gas and dust will eventually be recycled into new star and solar systems
→We come back full circle to the birth of stars
Life cycle of stars
Black hole
• Generally, stars having mass > 3 times than the solar mass is regarded as massive stars.
Question: Why massive stars burns quickly?
• This can be defined by Eddington limit as Balance between gravity and radiative pressure.
• Massive stars have more gravitational potential energy, so they can collapse faster.
• Massive stars, even when they start nuclear fusion, have relatively higher pressures in their centers (because the larger mass is exerting a relatively higher pressure), thus higher central temperatures.
HR Diagram