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Swinburne Online Education Exploring Stars and the Milky Way Module : 6: Module Evolution of Stars Activity 2: From Starbirth © Swinburne University of Technology Summary In this Activity, we will learn how stars are created from interstellar gas and dust. In particular, we will discuss: • the best conditions for star formation; • how stellar disks and jets are produced; and • some of the interesting physics that is necessary to describe star formation. Hit the Dust If a nebula contains enough mass, it may begin to collapse because of gravity. Whether it succeeds in collapsing depends on the mass. Pressure within the gas opposes the gravitational collapse. Low-mass cloud High-mass cloud gravity gravity pressure pressure Pressure “wins”, cloud expands Gravity “wins”, cloud contracts Gravity is not enough The collapse of a molecular cloud usually succeeds in creating low- to medium-mass stars (like our Sun), but to create high-mass stars, gravity alone is not enough. There are three factors fighting against it. The particles are in motion (causing heat and pressure) Gravity works to collapse the cloud, but ... magnetism gravity Magnetic forces from moving charges act against collapse A rapidly rotating disk tends to spread out A shocking affair Most molecular clouds cannot contract enough to form high-mass stars under gravity alone. However, shock waves travelling through a nebula can produce overdense regions, sometimes enough to trigger a localised collapse. NGC 604, a huge, star-forming nebula in the M33 Galaxy in the constellation Triangulum Four shocks Astronomers can think of four events that could cause a shock wave to pass through a molecular cloud. An explosion (such as a supernova) can emit a huge, hot, fast-moving blast of gas and dust that will crash into anything nearby as it expands. NGC 6188, a region of molecular dust and hot young blue OB stars More shocks! A shock wave can travel around a galaxy, creating regions of denser molecular cloud A collision with another molecular cloud can create havoc in both clouds The start of fusion in a nearby star can throw off a blast of very hot hydrogen gas (H II) Group of new, big, hot OB stars A domino effect Let’s think through a possible scenario in which star formation can occur: a group of massive, hot stars forming near a molecular cloud. The new stars produce strong stellar winds which eat away and compress the edges of the cloud. Some of the stars even supernova. Parts of the cloud then become so dense that a cluster of new massive stars forms, along with a new stellar wind. The stellar wind causes a compression wave at the edge of the remaining cloud, and the process begins again. Molecular cloud Gas gobblers Once a group of these young stars forms within a molecular cloud, the stellar wind buffets all surrounding objects - it clears a space all around the new star. The molecular cloud that is the Rosette Nebula, 4500 light years from Earth, was the birthplace of the young, hot, blue stars in the centre. Now, they are busy modifying their home. How exhausting The stellar winds consist mostly of hydrogen (H II), with some heavier elements. Although it’s not very dense, this “exhaust” gas moves at hundreds of kilometres per second and sweeps the molecular cloud away. The Some Stellar of winds these (consisting from these aremostly Thisnebula process isstars causing of dust and II) is to actually stars sweeps a lotH clean a into the nebula tocloser expand Earth cavity compressed, and inside so itnebula. inside gets hotter the cold Haren’t I the region the and nebula. emitscompressing light. TheySome just of outside, thethe happen dust concentrated to be in the way. into dark gas is there as well. “lanes”. Star-Forming Regions in our Galaxy Of course, not all stars are young, hot massive stars. Let’s now look at stars more generally. The Hubble Space Telescope is currently filling in vast gaps in our knowledge about the formation of stars by giving us unprecedented images of stars (and probably planets) that are in the process of forming in the Milky Way. One of the richest stellar nurseries in our neighbourhood of the Milky Way is the Orion Nebula. Here is the region of the Orion Nebula around the Trapezium, a group of four, hot, nascent stars. Star formation in Orion The Hubble Space Telescope has been used extensively to study star-forming regions in the constellation Orion, which people in the Southern Hemisphere often call “the Saucepan”. Orion Nebula The next slide is a mosaic of 45 images taken by the HST of 15 separate fields in the centre of the Orion nebula, which is located in the middle of Orion’s sword (the “handle of the saucepan”). At least 153 glowing protoplanetary disks have been detected around young stars being formed in this region. These disks may become planets, hence their name. Protoplanetary disks are made of dust and gas. The ones you see here were first discovered with the HST in 1992. They are also called proplyds, embryonic solar systems that may eventually form planets. Here are HST images of four newly-discovered proplyds around young stars in the Orion Nebula. The red glow in the centre of each disk is a young, newlyformed star, roughly one million years old. The largest disk in this photo is nearly edge-on, with a diameter approximately the same as Pluto’s orbit. Surrounding the disk is diffuse hot gas which has been evaporated from the disk surface by radiation from nearby hot young stars. Click here to see a Hubble animation showing where these proplyds are located in the Orion Nebula. Here we see the centre of the Trapezium cluster, with four, massive, energetic stars evaporating a number of nearby proplyds (the small white blobs). The star within the proplyd shelters the dust behind it, producing the apparent tail effect. Casting a shadow It’s almost as if the proplyds have shadows, made of dust. They may have been sitting there quietly minding their own business when a new young star popped up nearby ... … and blasted them with stellar wind! YOW! The Eagle Nebula You can see pillars of dust being formed this way in many nebulae, such as the Eagle Nebula. There are also many proplyds there. Inside the top of each pillar, there should be a young star or protostar that has so far sheltered the dust below it from the stellar wind coming from above. In time, the dust may be exposed completely.* * Note that the directions in this description only apply to the photograph - not the physical object! Here is a Hubble close-up of M16, the Eagle Nebula, which contains these magnificent pillars. The pillars are columns of gas and dust, protected from the strong winds of young hot stars (out of the picture, top right) by young protostars cocooned in gaseous envelopes. However, Hubble observations were done at visible wavelengths, so we couldn’t actually “see” inside the cloud. This region has recently been observed with the VLT* at near-infrared wavelengths, which can penetrate the dense dust and reveal the newly born stars. Very young, relatively massive stars are found at the tips of two of the main three pillars, and about a dozen lower mass young stars are observed within the “evaporating gaseous globules” (the denser knots seen as small bumps) in one of the pillars. This observation confirms that the pillars are indeed regions of star formation. *The European Southern Observatory Very Large Telescope (VLT) at the Paranal Observatory (Atacama, Chile) . Gravity causes gravidity Let’s look at how molecular clouds become gravid - that is, how they become the site for the formation of new stars. A cold molecular cloud, only a few degrees warmer than the near-zero of space, begins to collapse under its own gravitational field. Funny, that .. When WE’re pregnant, we get LARGER! Potential, kinetic, angular Momentum and energy have to be conserved during all of this, so various terrific changes take place as the molecular cloud gets smaller. • To conserve energy, the smaller the cloud gets the hotter it gets (heat is related to the kinetic energy of a gas). • To conserve angular momentum, the cloud spins faster as it Tell me collapses. about momentum and energy As a big cloud: • high potential energy • low kinetic energy • low spin speed As a smaller cloud: • lower potential energy • higher kinetic energy • higher spin speed Inside a cocoon of dust Once the nebula is sufficiently contracted, a rotating disk with a protostar in the centre remains. Young protostar is forming in centre Start with a vast, rotating, contracting cloud of gas, dust & molecules Why a disk? Nebula contracts to form a disk Disk gets cooler as you go further out Our Solar System This is why our own Sun and the Solar System planets are all spinning in the same direction. It’s the direction that the original cloud of gas and dust was spinning. The hotter the gas in the core of the protostar, the more it will try to expand against the force of gravity. So there is a constant balance being sought between the pull of gravity inwards and the pressure of hot gas outwards. But if we are lucky, there will be enough mass around to make sure that gravity wins ... … that we escape! … and sometimes we get so fast and furious We move faster, too, so we hit each other more Gravity Hot hot hot As our cloud gets smaller we get packed closer Hot enough! If gravity is strong enough, then the core gets hot enough and the gas is under enough pressure ... …for fusion reactions to start and you have a baby star. The baby star, or Young Stellar Object (YSO), consists of a hot, tight core and a dusty cloud. 1 + 4 ++ 6 H He + 2e+ + 2 + 2 + 21H+ 6 hydrogen atoms fuse to become one helium nucleus, two positrons, two neutrinos, two gamma rays and two spare hydrogen atoms to keep the fusion going Headstrong youth Hot gas contains lots of charged particles (ions) The temperature of the gas will not be high enough yet for it to be visible to human eyes, but infra-red telescopes will see it clearly, and can detect its hot centre. The smaller the cloud becomes, the more concentrated the magnetic effects will be. Tell me about magnetism Infra-red radiation Moving charge creates a magnetic field Easiest escape route from the core is along the rotational axis Stellar winds and jets Gas and dust will be radiated from the YSO but the disk of dust disk channels most of this outflow along the axis in two jets. core There will also be two cones of material escaping from the disk itself. Hot gas and radiation is released by the disk Twin exhausts However because a lot of the stellar wind is hot and ionised (H II), it is strongly funnelled by magnetic effects along the axis. This explanation of why there are two jets is often called the twin exhaust model. Charged particles end up spiralling along the axis Jets and a disk This photo of HH30, a young star, taken by the Hubble Space Telescope in 1995 clearly shows the dark line of the disk where it is most cool and dense (we are looking at the outer edge). disk axis The red lines are the thin, fast jets of hot material, and the paler areas show the glowing upper and lower parts of the disk, and some of its escaping material. What shape did you say? There is some conjecture that the disk might actually be a slightly different shape from a simple disk. Surfaces Surfacesof ofthe thedisk? disk? It is thought that dust and gas further out from the centre might take longer to drift towards the disk, and so the surface should be concave (“innie”) rather than convex (“outie”). A pulsed jet HH30 and other young stars appear to be emitting their exhaust in pulses. This may be because the star goes through a regular series of phases: chunks of matter fall from the disk towards the star, and the material is expelled along the jet; then there is a pause while the disk prepares the next batch. HH30 In this HST image, we see the motion of distinct clumps of matter in the HH30 jet over a period of just 11 months. Two views of HH47. Top: Multi-colour. Right: As seen in the S II emission line. The protostar is at top right. The jet, moving at 125 km s-1 from top right to bottom left, is sporadic and ends in a bow shock region. Summary In this Activity, we had a closer look at how star formation follows the collapse of a cold, molecular cloud. This process is a struggle between the competing forces of gravity and pressure, with magnetic and rotational effects playing a secondary role. Shockwaves are often involved, and are necessary if high mass star formation is to occur. We then looked at some observations of starbirth regions in our Galaxy, identifying characteristic features such as proplyds and jets. In the next Activity, we shall follow a complete life-cycle of a star. Image Credits MSSSO © M. Bessell (used with permission) Lagoon Nebula NGC 6188 Rosette Nebula NGC 2004 Orion nebula Eagle Nebula NGC 604, courtesy of Hui Yang at the University of Illinois, and NASA http://oposite.stsci.edu/pubinfo/gif/NGC604.gif Orion and the Aurora Australis, taken from the Space Shuttle http://antwrp.gsfc.nasa.gov/apod/image/orion_aurora_sts59.gif AAO © David Malin (used with permission) http://www.aao.gov.au/local/www/dfm/image/aat094.jpg Image Credits HST: Proplyds in Orion http://antwrp.gsfc.nasa.gov/apod/image/proplyds_hst.gif Four protoplanetaries in Orion http://oposite.stsci.edu/pubinfo/gif/OriProp4.gif Pillars of creation in a star-forming region, star-birth clouds http://www.stsci.edu/pubinfo/pr/95/44.html Jets and a disk http://oposite.stsci.edu/pubinfo/gif/JetDisk3.gif Double jets of HH30 evolve with time http://oposite.stsci.edu/pubinfo/gif/HH30.gif HH47 in S II http://huey.jpl.nasa.gov/~bode/mprl/SII.model.gif Hit the Esc key (escape) to return to the Module 6 Home Page Momentum and energy Over the centuries, physicists and astronomers have found that there are a few laws that they can rely on. One of these is to do with the conservation of momentum, and another is to do with the conservation of energy. Let’s start with the more familiar one first... Sir Isaac Newton Energy In physics, energy has pretty much the same meaning that it has in daily life. The traditional definition is: energy is the capacity for doing work. An object has energy if it is moving, or can cause something else to move, or would rather like to move (given half a chance). ? There’s a lot more heat and light in Summer, right? Yep And heat and light are forms of energy, right? You’re not wrong And energy is the capacity for doing work, isn’t it? Sure is So, how come everyone gets holidays in Summer? Potential Energy (PE) Apple at rest: no kinetic energy lots of potential energy This last situation is of particular interest to astronomers (and other physicists), as most astronomy is involved with the question of how and why things are moving the way they are, and Close to Earth’s surface, changing the way they do. PE = mgh If gravity (or another force) “wants” an object to where m = mass, g = 9.8 ms-2 move, then we say that the object has potential and h = height above ground energy. A slight change of circumstances (such pushing this apple from behind) could see the object suddenly lose its potential energy and have real kinetic energy instead. Kinetic Energy (KE) The term kinetic comes from the Greek kinetos, and means motion. Any object with mass, in motion, has kinetic energy. The more mass the object has, the higher its kinetic energy. The faster it is going, the higher its kinetic energy. In fact, the kinetic energy is related to the square of the speed. If mass = m and speed = v, then kinetic energy = 1/2 m v2 Conservation of Energy If potential energy is defined in this way, it is found that energy is also conserved during collisions and interactions. Over the centuries, different types of energy have been identified, and it is the work of scientists, mathematicians and engineers to study, predict and even use the various possibilities. For instance: humans convert the chemical energy stored in coal, gas and oil to electrical energy, and then to the energies of heat, light and motion. Lots of PE No KE Some PE Some KE No PE Lots of KE Oomph! Oomph! Momentum Momentum is the amount of oomph something has because • it has mass, and Oomph! • it is moving. The more mass an object has, the more momentum it has. The more speed an object has, the more momentum it has. If mass = m and speed = v, then momentum = mv Conservation of momentum If there is a collision or interaction between two (or more) objects then the total momentum before the event is the same as the total momentum afterwards. You have to take direction into account! Initially, this ball has a little bit of momentum to the right Finally, the small ball has a bit of momentum to the left Initially, the two together have a total momentum to the left Finally, the two together have a total momentum to the left Initially, this ball has a lot of momentum to the left Finally, the big ball has a bit of momentum to the left Angular momentum If something is spinning, it also has angular momentum and that is conserved as well. The angular momentum of an object depends on how the mass is arranged about the axis it is spinning around, and the speed at which it spins. radius r m Speed v For each part of an object, angular momentum = mvr where m = mass, v = speed, and r = distance from centre The classic ice skater concept If an iceskater is spinning with his arms and a leg out, he will spin slowly. But if he pulls these limbs closer to his body, he will spin faster. Angular momentum is conserved Distance large, speed small axis Distance small, speed large This is because the total sum, for each part of his body, of the angular momentum mass x speed x distance from axis must stay the same. If the distances of his hands, arms and legs from the axis get smaller, the speeds must get bigger to compensate! Angular momentum in space Distance large, speed small Distance small, speed large If a molecular cloud, for instance, contracts under gravity, it will spin faster. Return to Activity Magnetic fields in astronomy Most of the bodies that we observe in space are rotating, some of them very fast indeed. If the inside of such a body is fluid (as in a molecular cloud, protostar, star or planet such as the Earth) it can slosh around in complex and turbulent motion. Rotation causes eddies inside the body Magnetic fields In warm or hot objects there will be ions (charged particles), and as these move they create magnetic fields: that is, the object develops a North and South pole and you can draw magnetic field lines to show where a magnet would move if you were to let it go near the object. N S S N The direction of the field depends on how the insides are sloshing, and can change with time. While our Sun’s magnetic field reverses every 22 years, that of the Earth flips only once every million years or so. Return to Activity Why a disk? Rotation axis Why does the dust contract into a disk shape, and not a sphere? The answer is that the nebula is spinning. This doesn’t greatly affect the movement of gas and dust along the general direction of the rotation axis. If it is attracted towards the centre by gravity, it can move there. However, if a particle moves inwards from the side (in our picture) there is a problem. As the distance from the axis decreases, conservation of angular momentum means that the particle will spin faster. Distance large, speed small Distance small, speed large It takes a greater force to keep a fast spinning object moving in a circle than it does for a slowly moving object ... … which is why if you whirl a weight on a string around your head fast, you’d better make sure that the string is strong! Slow spinning gas Fast spinning gas Relatively small force needed to keep it spinning in circle Relatively large force needed to keep it spinning in a circle So as gas and dust moves in towards the rotation axis, it spins faster and faster, and needs a greater and greater force to keep it from “spinning out”. This force is provided by gravity, but it has its limits. The attractive force of gravity that the cloud can provide depends on its mass and size ... … but a point will be reached where, to move any closer to the axis, the gas and dust would need a greater attractive force than gravity in this situation can provide. So the collapse of the cloud is halted relatively soon in the direction perpendicular to the rotation axis, but can continue along the axis, turning a spherical cloud gradually into a disk-shaped one. For the maths enthusiasts: For each part of a rotating object, angular momentum = mvr where m = mass of that part, v = its orbital speed, and r = distance from the rotation axis and An object (e.g a gas molecule or dust particle), mass m, moving in a circle under the attractive force of gravity at orbital speed v, and distance r from the rotation axis, obeys: mv2/r = GMm/r2 (the force due to gravity) Return to Activity