Transcript Lecture 1: Welcome to Astronomy 106

Supernova explosions
Supernova explosions
• The lives of stars
• Type I supernovae
– Destruction of white dwarfs
• Type II supernovae
– Core collapse of massive stars
• What’s left behind?
– Pulsars
– Black Holes
Hertzsprung-Russell diagram
Nova Explosions
Hydrogen accreted
through the accretion
disk accumulates on
the surface of the
white dwarf
Nova Cygni 1975
 Very hot, dense layer of
non-fusing hydrogen on the
white dwarf surface
 Explosive onset of H
fusion
 Nova explosion
Artist’s impression of a nova
The Chandrasekhar Limit
The more massive a white dwarf, the smaller it is.
 Pressure becomes larger, until electron degeneracy
pressure can no longer hold off gravity > Type I
supernova
WDs with more than ~ 1.4 solar
masses can not exist!
Evolution of high-mass stars
Time-scales and conditions
No need to memorize these numbers. Main thing is to appreciate the
differences in the numbers for each evolutionary stage.
Core collapse - a star in
gravitational free-fall (Q1)
Artist’s impression of a supernova
Supernova facts
~1/100 yr in our Galaxy
~1/second in Universe
Only 0.01% of energy
released in visible light!
p + e -> n + neutrino
Most of energy is in the
form of neutrinos!
Neutrinos
•Elementary particle postulated in 1930
•First detected in 1942
•Miniscule mass and no electric charge
•Come in three “flavors”
• electron- muon- and tau•Interact only weakly with other matter
Detecting neutrinos
Sudbury neutrino detector
Historical Supernovae (Q 2)
The youngest supernova explosion occurred in ~1868
and astronomers have just discovered the remnant in
the X-ray image shown on the right.
The explosion occurred close to the center of our Galaxy
and the optical light was obscured from view.
www.nasa.gov/mission_pages/chandra/news/08-062.html
The Famous Supernova of 1987:
Supernova 1987A
Before
At maximum
Unusual type II supernova in the Large
Magellanic Cloud in Feb. 1987
How much energy is liberated?
• An application of Einstein’s equation
– Matter and energy are related as follows
E = M c2
• In question 3 we will use this
–
–
–
–
Take mass of original star
Subtract mass of end product
Subtract mass of expanding shell
Apply the equation to what is left over
Extragalactic Supernovae
Why wait? Supernovae are so bright that they can be seen in other Galaxies!
Zwicky started such searches and found a total of 120. Currently 984 known.
One of their uses is to study the expansion of the Universe (see ASTR 106)
Neutron Stars
A supernova The central core will
Pressure becomes so high
collapse into a
explosion of
that electrons and
compact object
an M > 8 Msun
protons combine to form
supported by
star blows
stable neutrons
away its outer neutron degeneracy
throughout the object.
pressure.
layers.
Typical size: R ~ 10 km
Mass: M ~ 1.4 – 3 Msun
Density: r ~ 1014 g/cm3
 Piece of neutron
star matter of the
size of a sugar cube
has a mass of ~ 100
million tons!!!
Black Holes
Just like white dwarfs
(Chandrasekhar limit: 1.4
Msun), there is a mass
limit for neutron stars:
Neutron stars can
not exist with
masses > 3 Msun
We know of no
mechanism to halt the
collapse of a compact
object with > 3 Msun.
It will collapse forming a singularity: A BLACK HOLE!
Summary
• Supernova are violent stellar explosions
– Type I destruction of greedy white dwarfs
– Type II core collapse of massive stars
• Supernovae produce exotic objects
– Neutron stars
– Black holes
• Supernovae probe the size of the Universe
– Type I supernovae act as “standard candles”