Supernova!

•The fate of stars with mass greater than 9 solar masses.

•Principally O and B stars.

The context

• Stars like the Sun (M<9 Msolar) recycle about 50% of their mass back into the ISM through Planetary Nebula leaving behind a White Dwarf as a stellar remnant.

• Stars more massive (O and B main sequence stars) recycle 95% of their mass back into the ISM through an event called a super nova (“super star”).

• Sun-like stars (M< 9 Msolar) stop producing energy with Shell Helium Burning and leave behind a carbon core (White Dwarf).

• Stars more massive continue to fuse heavier elements in their cores as they evolve.

Carbon burning at 600 Million KNeon burning at 1.2 Billion K

Oxygen Burning at 1.5 Billion KSilicon Burning at 3 Billion K

…

The Event

• Finally an Iron core with a mass of about 2 solar masses and a radius of 500 kilometers develops.

• At this stage, the star’s envelope has swelled to 5 AU (Supergiant).

• The iron core is so dense that its own gravity causes it to collapse on itself.

The Event

• Iron atoms are reduced to individual protons, neutrons and electrons in a fraction of a second.

• Collapse continues and individual protons and electrons are squeezed together to form neutrons and neutrinos.

• In immense flood of neutrinos attempts to leave the core but cannot escape the incredible dense matter in the core and they exert an outward pressure on the star.

Collapse of the Iron Core

Core Rebound• The collapsing core of neutrons reaches

nuclear density and stiffens.• The sudden onset of stiffening causes the

collapsing core to rebound and bounce out to meet the infalling envelope.

• The combined effect of the rebounding core and the pressure from neutrinos propels the inner layers of the star outward at near light speed velocities.

A Supernova is formed!

The Implications• The remains of the star are NOT recycled

back into the ISM but remain as a neutron star or a black hole.

• These stellar remnants do not emit radiation and are essentially the end of the line for these high mass stars.

• Within the exploding envelope of the star fusion occurs creating new heavy elements.

We are Children of the Stars

• The new heavy elements are dispersed into the ISM and will later be part of a new star forming system.

• All elements in the ISM heavier than hydrogen are created by these supernova. – Oxygen in the water of our bodies– Carbon in the proteins of our cellular chemistry– Calcium in our bones and teeth– Iron in our hemoglobin– Silicon in the very rocks we walk on

Recall the Aristotelian View of the Universe.

• The Heavens were a spiritual place that represented the ultimate source and destination of mankind.

• In the modern scientific view, the stars are the physical source of the material that we are made of, and as the Sun evolves, our ashes will be sent back into the ISM.

It is understandable that some people have replaced a faith-based religious view with a scientifically-based worldview.

The parallels are clear.

• Any astronomical object with a known luminosity is considered a “standard candle”.

• When a standard candle is observed it’s distance can be determined from the difference between it’s apparent magnitude and its known absolute magnitude: (m-M).

The Standard Candle Concept

The Standard Candle Concept

• Supernova’s are good standard candles because– They have a uniform peak absolute magnitude,

and– They are VERY luminous.

• The absolute magnitude of a supernova is

M=-17

The Standard Candle Concept

How luminous is a Supernova?

• Note that the full Moon has an apparent magnitude of about –12 and that it can cast shadows.

• A supernova at a distance of 10 parsecs (32 light years) would appear to be 100 times brighter than the full Moon!

• It would cast shadows on the Earth from this distance!

How bright would a supernova be at various distances?

• At 10 parsecs, m= -17

• At 100 parsecs, m= -12

• At 1000 parsecs, m= -7

• At 10,000 parsecs, m= -2

• At 100,000 parsecs, m= 3

• At 1,000,000 parsecs, m= 8

With the Hubble Space Telescope that can “see” to m=28, a supernova can be seen to a distance of 10 billion parsecs!

Supernova are so luminous that they are useful standard candles for

exploring the distant (and early) universe.

What you need to know about Supernovas for the exam

• What types of main sequence stars will eventually supernova? What types do not?

• What is the interior of the star like just before the Supernova event?

• How are supernovas important for the chemical evolution of the Universe?

• What do astronomers use supernova for?