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Type 1a Supernovae
Astrophysics Lesson 17
Learning Objectives
To know: What causes a Type 1a Supernovae.
 Why they are ‘standard candles’ and why this is
useful.
 The implications of some recent measurements.
Homework
 Collecting – last week’s questions on Doppler
Shift.
 Complete past paper questions and questions
from book on Hubble’s Law and Type 1a
Supernovae.
Question
• A distant galaxy has a red-shift of 15 %.
(a) What is its speed of recession?
(b) If Ho has a value of 100 km s-1 Mpc-1, what
is its distance?
Answer
• (a) Use: 
v

•

c
• 0.15 = - -v ÷ 3 x 108 m/s
• v = 4.5 x 107 m/s = 45 000 km/s
• (b) d = v/Ho = 45 000 ÷ 100 km s-1 Mpc-1 = 450
Mpc (which is quite a long way)
Supernovae
 Supernovae are classified into different types: Type 1a  we will discuss today
 Type 1b & 1c  don’t need to know about
 Type II  What we have discussed before.
Type 1a Supernova
 Recall that a carbon-oxygen core with a mass
less than the Chandrasekhar limit (about
1.4 solar masses) is a white dwarf.
 But what happens if by some mechanism mass
is added to the white dwarf and it starts
approaching the Chandrasekhar limit?
Type 1a Supernova
• A star and a white dwarf are orbiting each other in a
binary system.
Type 1a Supernova
• The companion to the white dwarf ages, becomes a red
giant and starts accreting mass on the white dwarf.
Type 1a Supernova
 The white dwarf reaches a larger mass, approaching
the Chandrasekhar limit
 But just before it would collapse into a neutron star
(within 1% of the limit), the temperature and density
inside the core increase enough to allow the fusion of
carbon to take place.
Type 1a Supernova
• Within a few seconds, a substantial fraction of the
matter in the white dwarf undergoes nuclear fusion,
releasing enough energy (1–2 × 1044 J) to produce a
supernova explosion.
Light Curve
• Type Ia supernovae follow a characteristic light curve
(luminosity vs time).
Light Curve
• The peak value of absolute magnitude is -19.3,
and occurs about 20 days from the start of the
increase in brightness.
• Notice the convention to define t=0 as when
the peak occurs.
Why do we care?
• It’s a standard candle! That means it is a known
absolute magnitude & the apparent magnitude can be
measured.
• And so we can use:-
d 
m  M  5 lg  
 10 
• ...and because they have massive luminosities we can
find the distance to very distance galaxies!
Quick point
• When we observe distance galaxies it takes the
light a substantial amount of time to reach us.
• The light we observe from the nearest star
shows us what was happening 4 years ago.
• We are looking back in time…billions of years
ago!
The Return of λ…
 The Type 1a supernovae
don’t seem to obey
Hubble’s Law (gravity?).
 The further galaxies
have redshifts that are
too small  the
expansion was slower in
the past i.e. expansion is
accelerating!
 Note axes are switched!
An Accelerating Universe
• What?!!!
• Gravity is an attractive force so the rate of
expansion should be slowing.
• But it appears that expansion is accelerating…
• How can this be?  Dark energy
Dark Energy
• Type 1a Supernova from distant galaxies are
dimmer than expected  larger distance.
• To try and explain the accelerating expansion
some scientists have introduce the idea of dark
energy.
• No one knows what this is! Negative vacuum
pressure? Quantum field effect?
The Return of λ?
Remember Einstein’s greatest blunder?
Maybe λwasn’t such a stupid idea after all?
In fact, it could be that it dominates over gravity.
The point is that there is evidence for dark energy but
no one knows what it is so it is considered
controversial.