M42 (The Orion Nebula) and M43

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1 3.4b demonstrate an understanding that emission nebulae, absorption nebulae and open clusters are associated with the birth of stars 3.4c demonstrate an understanding that planetary nebulae and supernovae are associated with the death of stars 3.4d describe the nature of neutron stars and black holes Nebulae Nebulae are clouds of gas and dust. Nebulae can show colour, depending on the chemicals that are present in the cloud itself, or because of the stars nearby. The density of material in a nebula is very low. Nebulae are found where stars are being born, or where they have died. There are 3 main types of nebula:- Emission Nebula : A gas cloud that emits/gives off light Picture Credit : NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team M42 (The Orion Nebula) and M43

2 The Orion nebula is where new stars are being formed:- Huge gas clouds which are 30 l.y. across are present Gravity causes the clouds to collapse into a smaller size Fragments of the nebula called PROTOSTARS form Picture credits : Mark McCaughrean (Max-Planck Institute for Astronomy), C. Robert O'Dell (Rice University) and NASA The PROTOSTARS (just visible) are masked by a disc of dust a PROTOPLANETARY DISC. This disc is held in the region of the star as it spins and will probably form planets in a solar system in the time ahead. Spinning PROTOSTARS draw more material on to themselves and eventually the central temperature of PROTOSTARS reaches 15 million K. At this temperature, nuclear fusion of hydrogen atoms to helium begins. Radiation from the centre forces outwards, stopping further gravitational collapse of the PROTOSTAR THE PROTOSTAR RADIATES ENERGY AND IS A NEW MAIN SEQUENCE STAR Remaining gas inside the M42 nebula is excited (energy level raised) by the newly formed stars and the gas emits light by fluorescence RED = HYDROGEN : GREEN = OXYGEN. This is what makes the nebula an example of an EMISSION NEBULA.

3 Absorption (Dark) Nebula : A dark area of the gas cloud Picture credit : T.A.Rector (NOAO/AURA/NSF) and the Hubble Heritage Team (STScI/AURA/NASA) The Horsehead Nebula in Orion Dense gas or areas of dust stop light passing through the nebula these are associated with the birth of stars. This is what makes the nebula an example of an ABSORPTION (DARK) NEBULA.

4 Reflection Nebula : An area of the gas cloud which shines due to reflected starlight Picture credit : Ted Wolfe The Pleiades Star Cluster (M45) showing the blue reflection nebula Newly formed stars are emitting light inside the gas still remaining from the original gas cloud. This light reflects off the nebula making it glow blue. The Pleiades is an example of an open cluster of stars - regions of relatively recent star birth. This is what makes the nebula an example of A REFLECTION NEBULA.

5 The Death of Stars (i) What happens to a star at the end of its life depends on its mass. Up to 11 solar masses, the star will gently fade away, giving off a planetary nebula. Planetary Nebula : A nebula from a star like our sun which has ended its life the gas looking like a planet Picture credit : Hubble Heritage Team AURA/STSC1/NASA/ESA The Helix Nebula in Aquarius

6 Stars like our sun use hydrogen to fuel the nuclear reactions inside it. Eventually the fuel (burning at 4.3 million tonnes/second!) starts to run out Radiation from within slows and less radiation pressure pushes outwards Gravity causes the star to collapse into a smaller size As the particles of gas are now closer together, further reactions in the core take place The outer layers of the star expand and cool A RED GIANT IS FORMED (Our Sun will one day expand and start to swallow up the inner planets) More hydrogen is used up in the core and the star collapses further. The core becomes even hotter (up to 100 million K) The RED GIANT can no longer create enough gravitational pull The outer layers of gas move away in an expanding shell which looks like a planet A PLANETARY NEBULA The remaining star collapses further The star gets hotter and more dense and becomes A WHITE DWARF A star like our Sun will one day expand to beyond the size of the orbit of Mars then it will lose its outer layer of gas in a PLANETARY NEBULA. The Sun will have a mass which is a little less than at present, but the star will be about the size of the Earth.

7 The Death of Stars (ii) What happens to a star at the end of its life depends on its mass. Above 11 solar masses, the star explodes in the most dramatic of ways a supernova. Picture credit : NASA/ESA and Allison Loll/Jeff Hester (Arizona State University). Acknowledgement: Davide De Martin (ESA/Hubble) The Crab Nebula (M1) in Taurus

8 Stars much bigger than our Sun use hydrogen to fuel the nuclear reactions inside them, in a similar way to our Sun. However, as the star begins to collapse towards the end of its life, the extra matter near the centre of the star causes the core to become much hotter than in a smaller star like our Sun Radiation from within eventually slows down and less radiation pressure pushes outwards Gravity causes the star to collapse into a smaller size Further heating of the core takes place and nuclear fusion of hydrogen atoms to helium continues The outer layers of the star expand and cool The helium produced when the hydrogen atoms fuse is heated so much that instead of making new elements up to carbon, the process continues further and IRON is produced Iron cannot be compressed to continue nuclear fusion reactions to further elements in the Periodic Table. The iron core collapses in a fraction of a second to form a NEUTRON STAR. The outer layers of the star are no longer supported and collapse, bounce off the incredibly dense NEUTRON STAR and suddenly the star explodes with as much energy being released in 10 seconds as our Sun would release in 10 billion years! A SUPERNOVA The material in the outer core shoots out at an incredible speed THE CRAB NEBULA IS THE REMAINS OF A STAR WHICH BLEW UP IN THE GAS MOVES A DISTANCE OF 80 MILLION km A DAY. The sudden rise in brightness (which may allow us to see the star in daylight hours) is followed by a steady drop in brightness over several months WHAT IS LEFT OF THE STAR?

9 As the outer core explodes away, by pushing on to the inner core, the material remaining forms into a very dense NEUTRON STAR NEUTRON STARS, as their name suggests, are made of neutrons from the nucleus of atoms. The intense heat of the supernova causes protons and electrons to combine to form even more neutrons. The NEUTRON STARS have a mass just greater than our Sun, but are squashed into a diameter of about 20 km Strong gravitational fields cause the NEUTRON STAR to spin, as well as giving off radio waves This causes the remains of the star to release pulses of radio waves PULSARS The CRAB NEBULA contains a PULSAR which spins at 30 rotations a second If what remains of the NEUTRON STAR is more than 3 solar masses, the gravitational field of the spinning NEUTRON STAR is so large A BLACK HOLE IS FORMED (pulling in matter and light that gets too close) The diameter of the BLACK HOLE would only be about 1.5 km! (To have the material of more than three stars like our Sun in this tiny volume means that the density of the BLACK HOLE is almost beyond comprehension) The evolution of stars has taken place since the first stars were formed about 0.3 billion years after the Big Bang. This means that some stars have been in the universe since 13.4 billion years ago. Stars are continually forming from material in nebulae. Young stars are often seen in Open Clusters. The very oldest of stars are found in Globular Clusters. When stars are first formed (first generation stars), they are made of hydrogen and helium. As the stars age, new elements form within the star. When eventually the star dies out, the elements drift into space and may become contained in second, third. generation stars. Newer stars therefore have a wider range of elements contained within their structure.

10 Star Clusters Many stars occur in groups called Star Clusters. Astronomers divide Star Clusters into two categories:- Open Clusters Open Clusters contain a few (between 20 to a few thousand) young stars, with individual stars easily identified. The Jewel Box is found near the Southern Cross and contains about 50 stars. The new, bright stars are just unable to be seen with the naked eye. Open Clusters are found within the plane of the Milky Way. Picture credit : The Jewel Box in Crux (NGC 4755)

11 Globular Clusters Globular Clusters are much older clusters, formed in the early stage of the universe, containing many more stars (typically 100,000 to 1,000,000) in which the central stars are so close that we cannot resolve individual stars using telescopes. These clusters are found outside the Milky Way. They appear as a glob of light - An American phrase! Picture Credit : NASA, The Hubble Heritage Team, STScI, AURA The Globular Cluster (M80)