Type 1a Supernovae

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 = 45 000 km/s • (b) d = v/Ho = 45 000 ÷ 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… • 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.