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Intro to Stars and Galaxies!!! In the night sky there are thousands of stars Where do stars come from? • The raw material that makes up stars comes from the interstellar medium • Interstellar space is composed of a thin gas laced with microscopic dust particles. • A place where the gas and dust clumps into clouds is called a nebula. The nebulae are 70% hydrogen and 30% helium by mass. The amount of other elements is less than1%. At the end of their life, giant stars die in enormous explosions than send out shockwaves into the interstellar clouds A shockwave from an exploding star rips through the rarefied gas and dust of a nebula. The shockwaves make the clouds collapse. What force do you think makes them collapse? GRAVITY! Spiral galaxies like our own are filled with interstellar dust and gas. The pink glow is caused by hydrogen excited by the radiation from young stars. Colliding galaxies also make interstellar clouds collapse and detonate star formation in their arms Such is the distance between stars, that although each galaxy can contain hundreds of billions of star, a head on collision between galaxies will rarely result in a single collision of stars! So how do the galaxies have an effect on each other? GRAVITY! The Eagle Nebula: a vast region of star formation. So, how big is it? The white circle shows the relative size of our entire solar system The glow is caused by young stars Scientists believe that the pressure and temperature in the centre of the collapsing cloud forms a protostar. How do they know this? Let’s look at the evidence The Orion nebula is a vast cloud of gas and dust 1500 light years from Earth. This image was taken through a small amateur telescope using a digital camera. The following images were taken with the Hubble Space Telescope using filters that only let through wavelengths given out by the glowing atoms of particular elements. Imaged in light given out by excited Hydrogen atoms Imaged in light given out by excited Oxygen atoms Imaged in light given out by excited Nitrogen atoms Click to combine images Combined Can you see where stars are forming? These globules of dense nebula are called proplyds. Could they be hiding protostars inside them? Hubble Space Telescope can sense a wavelength that is given out by hot things and can penetrate the gas and dust – Infrared Infrared This reveals many more stars and hotspots hidden inside the dust and gas. Are any of them inside proplyds? Click to combine images Click to combine images H+O+N+IR What does each proplyd have at its center? H+O+N+IR Each proplyd has a star or hot protostar at its centre. From evidence like this, we know that forces build inside the protostar until they are great enough to fuse hydrogen atoms together into helium. In this conversion a tiny amount of mass is turned into a large amount of energy. For a star of the Sun’s size, 5 million tonnes start to be converted into energy every second. The protostar detonates into a star. Luckily the Sun has a mass of 2 billion billion billion tonnes. So there is enough to last for a very long time. The Sun has been shining for about 5 billion years and in all that time it has only lost 1% of its mass. Understanding how stars evolve requires both observation and ideas from physics. • Our Sun serves as the primary evidence that stars are not permanent. – Nuclear Fuel in the Sun cannot last forever. – Our Sun is similar to the stars we see in the night sky. The new star erodes the surrounding proplyd with its radiation. Eventually the area clears, revealing the star and a surrounding disk that can form a planetary system. Inside, a structure forms and the star settles down to fuse its hydrogen. The force of gravity crushing the star is balanced by the outward force created by fusion in its core. The force of gravity crushing the star is balanced by the outward force of the radiant pressure created by fusion in its core. Draw a copy of this diagram and label the forces Gravity Draw a copy of this diagram and label the forces Pressure of fusion Pressure of fusion Pressure of fusion Pressure of fusion Gravity Gravity Gravity How long do stars live for? Bigger stars have more fuel, so they live longer? What do you think? Discuss this and take a vote What is the pattern between mass and lifetime? What explanation can you find for this in the table? Solar Masses Surface Temp (0C) 25 15 3 1.5 1 0.75 0.5 35000 30000 11000 7000 6000 5000 4000 Lifetime (million yrs) 3 15 500 3000 10000 15000 200000 The bigger the star, the shorter its lifetime. Reason - big stars are hotter, so burn their fuel quicker. More mass, means more gravity, needing more radiant energy to balance the star.. Solar Masses Surface Temp (0C) 25 15 3 1.5 1 0.75 0.5 35000 30000 11000 7000 6000 5000 4000 Lifetime (million yrs) 3 15 500 3000 10000 15000 200000 For Stars up to 1.5 times the size of our Sun: When the hydrogen is used up, the star’s core shrinks and starts to fuse helium. The intense radiation released puffs up the outer layers, which cool and glow red. The star is now a Red Giant The Sun is half way through its hydrogen burning life. How long until it runs out of hydrogen? 1 solar mass stars burn hydrogen for 10,000,000,000 years So the Sun has 5,000,000,000 years of hydrogen burning left Orbits of Mars Earth The Sun as a Red Giant in 5,000,000,000 years time Red giant stars in Auriga 300 million km Dormant hydrogen fusing shell Carbonoxygen core Heliumfusing shell The core of the red giant now fuses helium into bigger atoms but it eventually completely runs out of fuel to fuse With no fusion energy to keep the core inflated, it collapses for one last time. The carbon is crushed into a diamond the size of the Earth. The outer layers of the Red Giant are shed into space as giant gas and dust shells. They are illuminated by the remaining tiny White Dwarf star. Astronomers thought these dim clouds looked a bit like planets so they called them planetary nebulae. With modern telescopes like the Hubble Space Telescope, astronomers have shown planetary nebulae to be some of the most beautiful objects in the Universe. What do you thinks these planetary nebulae are called? Astronomers call this the Eskimo Nebula Astronomers call this the Glowing Eye Nebula Astronomers call this the Hourglass Nebula Astronomers call this the Cat’s Eye Nebula Astronomers call this the Ring Nebula Astronomers call this the Spirograph Nebula Astronomers call this the Eight Burst Nebula Astronomers call this the Ant Nebula Astronomers call this the Red Spider Nebula Astronomers call this the Eye Nebula Astronomers have not given this nebula a name yet The life of a star up to 1.5 solar masses Hydrogen runs out and click to start the core shrinks and starts Helium burning. The star swells into a Red Giant. The star burns Hydrogen. Fuel runs out and core shrinks into a White dwarf The Protostar forms in a collapsing layers form a proplyd of a nebula planetary nebula An ancient cluster of over a million stars reveals some remaining red giants and many white dwarfs Stars of more than 1.5 Solar masses have a very different fate They burn fiercely as Blue Giant stars and use their fuel up quickly Betelgeuse Bellatrix Alnilam Mintaka Alnitak Orion Nebula Rigel Saiph The blue giant star Bellatrix, in the constellation Orion, is coming to the end of its hydrogen fuel When the star runs out of hydrogen and starts to burn Helium, it swells into an enormous red star – a Red Super Giant Betelgeuse in the constellation is a red super giant, here imaged by HST Betelgeuse dwarfs all other stars What sort of star do you think Sirius B is? A white dwarf When a Red Super Giant runs out of Helium fuel it has enough heat and pressure in its core to fuse Oxygen and Carbon to make even bigger atoms Eventually the star creates a core of iron 700 million km Iron core Silicon Neon Oxygen Carbon Helium Hydrogen The core of the red giant becomes an onion-like structure with an iron core. As the fuel runs out, the core shrinks and gets hot enough to fuse iron. You don’t want to be in that part of the Universe when a Red Super Giant starts to fuse iron! As the star fused hydrogen, helium, carbon and oxygen it creates enough energy to keep itself supported against the crushing force of gravity. Unlike the fusion of smaller atoms, when iron fuses it sucks in energy. The giant star is millions of years old but it now collapses in seconds. Billions of tons of star material hurtle into the core and then bounce back out in the biggest explosion in the Universe. The energy released outshines the entire galaxy the star is in – up to 300,000,000,000 stars! Here, one star in a galaxy of billions fades after it explodes as a supernova. At the time of the explosion it would have been brighter than the entire galaxy. The Tycho supernova occurred in 1572 but is still a rapidly expanding, seething explosion wiping out a sizeable part of its local universe. The blue shell is the distance travelled by intense x rays at the speed of light, heating interstellar material to 20 million degrees Celsius. The explosion is at least 800 light years across. You would only know it was coming when it hit you. What remains from the explosion depends on the size of the original star • 1.5 – 3 solar masses form one giant, 12 mile diameter atomic nucleus composed only of neutrons (all of the protons and electrons have been squeezed together to form neutrons). • This is a neutron star. The force of its tremendous gravity is balanced by the repulsion of the neutrons. What remains from the explosion depends on the size of the original star • 3+ solar masses and there is no force greater than the star’s gravity. • The star collapse never stops, it disappears leaving only it’s mass to create a gravity field. • Nothing goes fast enough to escape, not even light. This is a black hole. A rotating neutron star (pulsar) is at the heart of the crab nebula supernova corpse The gravity near to a black hole is so strong that even light does not go fast enough to get away Black holes are made up of two features A singularity – all their mass is squeezed into a point An event horizon – the outer edge where the force of gravity is great enough to stop light escaping. The event horizon of the smallest black holes created by supernovae should be about 12km across. But black holes will eat anything, gas, dust, stars, even the centre of galaxies. As material spirals into a black hole, it heats up and blasts radiation out into space before it disappears for ever. A simulated picture of a black hole with a disk of matter spiralling inwards What would happen if you fell into a black hole? The differences in gravity between your feet and your head would stretch you out like spaghetti Do you have a complete diagram showing the life cycles of stars? • It should show what stars form from • What triggers star formation • The different fates of different mass stars • How some star material is recycled This is what your diagram should look like Credits Written by Michael Cripps, Neatherd High School Norfolk UK Website by Michael Cripps and Graham Colman of Taverham High School, Norfolk UK Images by Michael Cripps, Neatherd High School Students, ESA and NASA Project sponsored by the UK Particle Physics and Astronomy Research Council, the European Space Agency and Norfolk Education Business Exchange Technical assistance by the scientists, engineers and educationalists of the European Space Agency and NASA at the Space Telescope Science Institute, Goddard Space Flight Centre and other institutions in the UK. Special thanks to Helen Mason at Cambridge University and Dennis Christopher at NASA, GSFC. This resource and its content may be used freely for non commercial educational purposes.