Supernovas – or things that Supernovas – or things that go “bang” in the night!go “bang” in the night!

The next-to-last chapter in the The next-to-last chapter in the lives of large stars.lives of large stars.

Large stars begin like any other Large stars begin like any other starstar

Stars that eventually go supernova Stars that eventually go supernova are from are from 3 to 100 times3 to 100 times the mass of the mass of the sun.the sun.

A nebula becomes a protostar, A nebula becomes a protostar, becomes a main sequence star becomes a main sequence star (where it lives most of its life), begins (where it lives most of its life), begins to use up its fuel.to use up its fuel.

continued (2)continued (2)

As the Hydrogen fuel in the core is As the Hydrogen fuel in the core is used up, the core shrinks and heats used up, the core shrinks and heats up, while the outer layers swell and up, while the outer layers swell and cool – the star becomes a red giant.cool – the star becomes a red giant.

As the core shrinks & heats it begins As the core shrinks & heats it begins fusing fusing HeHe (the ash from the previous (the ash from the previous reaction) into reaction) into CC and and OO. .

continued (3)continued (3)

The layer right next to the core begins The layer right next to the core begins fusing H into He – so now 2 fusion fusing H into He – so now 2 fusion reactions are occurring.reactions are occurring.

A small star would stop here, the core A small star would stop here, the core not being hot enough to do anything not being hot enough to do anything with the C and O.with the C and O.

continued (4)continued (4)

A large star’s core can shrink and A large star’s core can shrink and heat up further. The core begins to heat up further. The core begins to fuse fuse CC and and OO into into NeonNeon (Ne), (Ne), MagnesiumMagnesium (Mg), and (Mg), and SiliconSilicon (Si). (Si).

The next layer out fuses He into C The next layer out fuses He into C and O. The layer outside that fuses H and O. The layer outside that fuses H into He. The star starts to resemble into He. The star starts to resemble the layers of an onion.the layers of an onion.

continued (5)continued (5)

The production of Ne, Mg, and Si The production of Ne, Mg, and Si continues only for a few hundred years.continues only for a few hundred years.

Eventually, when all the C and O is used Eventually, when all the C and O is used up, the core shrinks once more, heats to up, the core shrinks once more, heats to over 600,000,000 K and starts fusing over 600,000,000 K and starts fusing MgMg + + SiSi into into ironiron (Fe). (Fe).

Iron is the final ash and doesn’t fuse into Iron is the final ash and doesn’t fuse into anything else.anything else.

Onionlayers

continued (6)continued (6)

The production of iron from Mg & Si The production of iron from Mg & Si happens very quickly, less than 1 happens very quickly, less than 1 day.day.

When the iron core gets massive When the iron core gets massive enough it implodes! (The needed enough it implodes! (The needed mass is mass is 1.4 x the mass of the sun1.4 x the mass of the sun – – the the Chandrasekhar limitChandrasekhar limit!)!)

Inverse Beta DecayInverse Beta DecayThe gravity is so great in the core, The gravity is so great in the core,

that protons & electrons get squashed that protons & electrons get squashed together into neutrons:together into neutrons:

pp++ + e + e-- nnoo

(This is called (This is called inverse beta decayinverse beta decay.).)

The core becomes a neutron star.The core becomes a neutron star.

Neutron StarsNeutron Stars

Neutron stars are formed from stars Neutron stars are formed from stars that were originally 3 – 8 times the that were originally 3 – 8 times the mass of the sun.mass of the sun.

They’re held up against gravity simply They’re held up against gravity simply by the neutrons being jammed in by the neutrons being jammed in tightly next to each other. This is tightly next to each other. This is called “neutron degeneracy”.called “neutron degeneracy”.

Black HolesBlack Holes

What happens if the star was What happens if the star was originally more than 8 solar masses?originally more than 8 solar masses?

Even the pressure of the neutrons is Even the pressure of the neutrons is overcome, and the neutron star overcome, and the neutron star collapses into a black hole.collapses into a black hole.

What about the rest of the star’s What about the rest of the star’s layers?layers?

When the iron core collapses into a When the iron core collapses into a neutron star or black hole (at nearly neutron star or black hole (at nearly the speed of light), the outer layers the speed of light), the outer layers follow it in.follow it in.

The outer layers “bounce” or rebound The outer layers “bounce” or rebound off the immensely hot new neutron off the immensely hot new neutron star and a star and a gigantic explosiongigantic explosion occurs! occurs!

Supernova ExplosionsSupernova Explosions

The most recent The most recent closeclose SN explosion SN explosion happened in 1987 in the Large happened in 1987 in the Large Magellanic Cloud – a satellite galaxy Magellanic Cloud – a satellite galaxy to the Milky Way.to the Milky Way.

For a few days, the SN was brighter For a few days, the SN was brighter than the rest of the satellite galaxy than the rest of the satellite galaxy combined!combined!

How often do SN happen?How often do SN happen?

On average, about every 100 years On average, about every 100 years for any given galaxy.for any given galaxy.

Our own galaxy has had several Our own galaxy has had several during recorded history:during recorded history:

““Recent” SupernovasRecent” Supernovas

July 4July 4thth, 1054 in , 1054 in Taurus, 6500 light Taurus, 6500 light years away. This years away. This resulted in the resulted in the Crab Nebula. It Crab Nebula. It was recorded by was recorded by Anasazi Indians in Anasazi Indians in the American the American southwest.southwest.

The Crab Nebula in Taurus

““Recent” Supernovas (2)Recent” Supernovas (2)

Tycho Brahe saw a supernova in Tycho Brahe saw a supernova in Cassiopeia in 1572 – it was 16,000 Cassiopeia in 1572 – it was 16,000 light years away.light years away.

The Chinese also saw and recorded The Chinese also saw and recorded the appearance of a “guest” star.the appearance of a “guest” star.

TheremnantofCassio-peia A.

This isthebrightestradio-emittingobject inthe sky.

Evidence for Older SN’sEvidence for Older SN’s

When a supernova explodes, it When a supernova explodes, it leaves behind a cloud or supernova leaves behind a cloud or supernova remnant.remnant.

These remnants last for hundreds of These remnants last for hundreds of thousands of years. New (small) thousands of years. New (small) stars can be formed from their gases stars can be formed from their gases and dust.and dust.

This is howthe wholeCygnus LoopSN remnantlooks – remarkablylike the cloudfrom anordinaryexplosion.

Future SupernovasFuture Supernovas

At present, astronomers are waiting At present, astronomers are waiting for at least 2 more stars to go for at least 2 more stars to go supernova: one in Cassiopeia, and supernova: one in Cassiopeia, and another in the southern sky called another in the southern sky called ““Eta CarinaeEta Carinae”.”.

A Very Special SupernovaA Very Special Supernova

The only close supernova that The only close supernova that astronomers have been able to study astronomers have been able to study in detail is in detail is SN 1987ASN 1987A in the Large in the Large Magellanic Cloud, 170,000 light years Magellanic Cloud, 170,000 light years away.away.

The SN happened in a nebula called The SN happened in a nebula called the Tarantula Nebula.the Tarantula Nebula.

The Large Magellanic Cloud

The Tarantula Nebula

SN 1987A (2)SN 1987A (2)This supernova was different than This supernova was different than

many – when it exploded, it was an many – when it exploded, it was an blueblue supergiant. Guess we don’t supergiant. Guess we don’t know know allall the details yet about SN’s. the details yet about SN’s.

The star was originally called The star was originally called Sanduleak -69Sanduleak -69oo 202. The Sanduleak 202. The Sanduleak (pronounced San-doo-lik), was (pronounced San-doo-lik), was named after Nick Sanduleak, who named after Nick Sanduleak, who studied it studied it beforebefore it blew up. it blew up.

Astronomer Nick Sanduleak

SN 1987A (3)SN 1987A (3)

The first astronomer to actually The first astronomer to actually observe SN 1987A observe SN 1987A afterafter it exploded it exploded was Ian Shelton, from the University was Ian Shelton, from the University of Toronto, Ontario.of Toronto, Ontario.

At the time, he was observing at Las At the time, he was observing at Las Campanas Observatory in Chile.Campanas Observatory in Chile.

Ian Shelton

Las CampanasObservatory

After the ExplosionAfter the Explosion

The brightness of SN 1987A has The brightness of SN 1987A has been monitored for the past 15 years. been monitored for the past 15 years. It didn’t follow the usual pattern It didn’t follow the usual pattern (suddenly bright, with a quick fall-off).(suddenly bright, with a quick fall-off).

Rather, it got suddenly bright, grew Rather, it got suddenly bright, grew brighter, then faded off gradually.brighter, then faded off gradually.

After the Explosion (2)After the Explosion (2)

A couple of years after the supernova A couple of years after the supernova faded, it suddenly brightened again.faded, it suddenly brightened again.

It wasn’t the supernova itself, but its It wasn’t the supernova itself, but its light reflecting off a cloud of dust light reflecting off a cloud of dust behind the SN. This reflected light is behind the SN. This reflected light is called a “light echo”.called a “light echo”.

After the Explosion (3)After the Explosion (3)

This supernova has been observed This supernova has been observed extensively. Over the years, we’ve extensively. Over the years, we’ve seen shock waves from the explosion seen shock waves from the explosion slam into the clouds of gas that the slam into the clouds of gas that the star gave off just before it exploded.star gave off just before it exploded.

The shock waves heat the gas, The shock waves heat the gas, producing rings.producing rings.

After the Explosion (4)After the Explosion (4)

The shock waves heat the shells of The shock waves heat the shells of gas hot enough to give off X-rays.gas hot enough to give off X-rays.

One last point – supernovas make One last point – supernovas make the chemical elementsthe chemical elements

We’ve already seen how supernovas We’ve already seen how supernovas can make elements up to the mass of can make elements up to the mass of Fe (atomic number 26) before they Fe (atomic number 26) before they explode. However, there are 83 explode. However, there are 83 elements that are heavier than iron.elements that are heavier than iron.

How do supernovas make the heavier How do supernovas make the heavier elements? It’s a process called elements? It’s a process called

nucleosynthesisnucleosynthesis..

Nucleosynthesis (2)Nucleosynthesis (2)

During the explosion, there are a lot During the explosion, there are a lot of very fast, high energy neutrons of very fast, high energy neutrons flying around. Sometimes, one of flying around. Sometimes, one of these neutrons hits an iron nucleus:these neutrons hits an iron nucleus:

56562626Fe + nFe + noo 5757

2626FeFe

Nucleosynthesis (3)Nucleosynthesis (3)

The extra neutron inside the heavy The extra neutron inside the heavy iron nucleus can split into a proton iron nucleus can split into a proton and an electron. This produces the and an electron. This produces the next heavier element:next heavier element:

57572626FeFe 5757

2727CoCo

Nucleosynthesis (4)Nucleosynthesis (4)

This process of adding a neutron, This process of adding a neutron, then the neutron splitting into a proton then the neutron splitting into a proton and electron can happen over and and electron can happen over and over, producing elements heavier over, producing elements heavier than Uranium.than Uranium.

57572727Co + nCo + noo 5858

2828Ni etc.Ni etc.

The EndThe End

The next time you look at your The next time you look at your girlfriend or boyfriend – remember girlfriend or boyfriend – remember that they truly are made of stars.that they truly are made of stars.

So is your lunch today….So is your lunch today….that’sthat’s that that funny taste.funny taste.