1. Type 1a Supernovae
Astrophysics Lesson 17

2. 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.

3. 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.

4. 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?

5. Answer (a) Use: 0.15 = - -v ÷ 3 x 108 m/s
v = 4.5 x 107 m/s = km/s
(b) d = v/Ho = ÷ 100 km s-1 Mpc-1 = 450 Mpc (which is quite a long way)

6. 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.

7. 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?

8. Type 1a Supernova
A star and a white dwarf are orbiting each other in a binary system.

9. Type 1a Supernova
The companion to the white dwarf ages, becomes a red giant and starts accreting mass on the white dwarf.

10. 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.

11. 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.

12. Light Curve
Type Ia supernovae follow a characteristic light curve (luminosity vs time).

13. 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.

14. 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:-
...and because they have massive luminosities we can find the distance to very distance galaxies!

15. 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!

16. The Return of λ… Note axes are switched!
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!

17. 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

18. 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?

19. 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.