Is the space between stars empty?
No it is just full of particles in lower density
interstellar medium
the matter and radiation that exists in the space between the star systems in a galaxy. made up of Hydrogen, Helium, ~2% “metals”
Nebula
a cloud of dust and gas in space
warm clouds
mostly atoms and ions
Cold clouds
atoms, molecules, dust,
“molecular clouds”
Can IR light shine through nebula?
yes it can shine through dust
How are stars formed?
- Interstellar gas cloud starts to contract (shrink) due to their own gravity
- Getting smaller, denser, and hotter
- Spins faster
- Densest parts of the cloud become opaque and trap heat
- H-fusion begins when core reaches ~5 million degrees
What determines whether the gas cloud will become a star?
Mass (related to gravity + pressure)
What starts the collapse of a gas cloud?
Supernova shockwave, Colliding clouds, Turbulence in galaxy, Spontaneous cooling
Why do stars heat up as they form?
Gravitational Energy -> Kinetic Energy
Kinetic Energy = Thermal Energy
Protostar
a contracting mass of gas which represents an early stage in the formation of a star, before nucleosynthesis has begun.
What determines how small a star can be?
Mass affects temperature, and temperature must be high enough for fusion to occur (Mass > 0.08 Msun required for fusion)
Brown dwarf
no fusion, but still heat from gravitational contraction.
All about the same radius (~0.1 Rsun), regardless of mass.
How big can stars get?
As protostars collapse, core is so hot radiation pressure
blows off their outer layers. Less mass, less pressure
needed to counteract gravity, lower temp. Largest stable(ish) mass is ~120-150 Msun?
Low mass star
< 2 times the Sun
High mass star
> 8 times the Sun
Main Sequence stage
- Where a star begins and is for 90% of its life
- Fusing is occuring
- Stellar thermostat keeps luminosity and temperature stable
Intermediate mass star
2-8 times the Sun
What initially happens when a star runs out of hydrogen
- No more fusion
- Temp not hot enough to fuse helium
- Core will collapse (gravity overcomes pressure)
- Increase in pressure = hotter in shell
- Now hot enough for hydrogen fusion in shell around core.
What answer most accurately describes the end life of a star 8-20 times more massive than our Sun?
Deep in the star’s core, elements fuse into ever heavier elements until no more energy is available to hold itself up against gravity; the star explodes as a Supernovae and the core is crushed into a Neutron Star.
The Sun is a low mass star, what is the most likely sequence of events in its future?
It will use up it’s fuel in another 5 billion years, expanding to become a Red Giant, eventually it’s core will be a White Dwarf.
What are the most common stars in the Universe?
Low mass stars (K, M types) are the most common.
What happens in the core of a star fusing helium into carbon after all the helium is used up in the reaction?
Energy generation through fusion stops because carbon fusion requires higher temperature. The core collapses and helium starts fusing in shell.
The core of a star 1.5 times more massive than our Sun is crushed into a dense object, which of the following most accurately describes this state?
The degeneracy pressure from electrons stops the core’s further collapse, leaving behind a White Dwarf.
The majority of stars in the galaxy formed in which two structures?
Globular cluster and open clusters.
Star formation in giant gas clouds is a result of competition between which forces?
Gravity and gas pressure
There are hundreds of elements in the Universe, which of the following best describes how they were created?
The Big Bang produced mostly Hydrogen and Helium, fusion in stars and during supernova explosions made the rest.
What effect sets the largest size a star can have?
The forming star is shining so strongly that it blows the collapsing gas cloud apart
When astronomers look at globular clusters they don’t see any stars hotter/more luminous than our Sun. What does this mean for the age of the globular cluster?
The globular cluster is older than the typical lifetime of a star like our Sun.
Red giant
- Occurs when star core runs out of hydrogen and hydrogen fusion begins to occur in shell
- Fusion is closer to star surface
- outer layers will expand (increase pressure) and cool
- Radiation can leave more easily
- cooler on average (hence red) but overall larger amount of heat (more luminous)
- Core is continuing to shrink getting denser and hotter eventually will be hot enough to fuse 3 4He -> 1 12C
Helium “flash”
-When core contracts enough to heat to 100 million K,
helium starts to fuse into carbon
-will heat up and expand core (pressure wins)
After the helium flash
- He fused to C in hotter increasing core size
- H fusion in shell decreases
- becomes Horizontal Branch Star
- Core stabilizes, luminosity decreases
- Gravitational equilibrium is restored
Double-Shell Red Giant
- when He in core runs out
- heat not enough for C fusion
- C core will collapse
- Outerlayers expand He fusion in shell (and H)
- Star becomes very luminous
Carbon fusion temp
600 million K, Electron degeneracy pressure becomes a
factor before the core reaches that
temperature
Electron Degeneracy Pressure
-When atoms are subjected to extremely high temperature and pressure, the atoms are stripped of their electrons.
-Restricts how tightly packed particles can
become
Low mass stars planetary nebula stage
-Temperature never gets high enough for carbon fusion
-Shell fusion becomes violent
• (no stellar thermostat)
• Outer layers blown off (due to temp sensitive He): Planetary nebula formed
- white dwarf formed
Planetary nebula
- explosions of dying star shells resulting in shell of gases
- core is a small carbond left over rock, i.e. white dwarf
Where are elements heavier then carbon produced
high mass stars
High mass stars life
- faster fusion in core (high temp and pressure)
- early stages after main sequence similar to low mass stars but faster
- C core will get hot enough to fuse other elements
Onion model of high mass stars
-Concentric shells of increasing temperature and pressure produce heavier and heavier elements.
-Energy created by each new shell is enough to balance
gravity
-Successive shells are hotter, denser, and “burn out” in shorter times, until you reach iron
Supernova
-After core begins to collapse as iron atoms get
compressed into pure neutrons it stops
-casues star to explode
-Energy of the explosion enables the production of elements heavier than iron
-huge energy release from neutrinos
-disperses heavy elements through the galaxy
-Inside may be a neutron star
neutron star
remnant core of pure neutrons
Where did our atoms come from?
Our atoms were once parts of stars that exploded
more than 4.6 billion years ago, whose remains were
swept up into the cloud out of which our Sun (& Solar
System) formed
Why Neutron Stars Spin So Fast
Vast shrinking conserves angular momentum
Collapse to a neutron star
increases?
Both rotation and magnetic field
Neutron star speed over time
Gradually slows down as angular momentum is lost (slower =older)
Pulsar
rotating neutron star