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Goal: To understand how stars form. Objectives: 1) To learn about the properties for the initial gas cloud for 1 star. 2) To understand the collapse and evolution of the gas cloud. 3) To understand how this evolution leads to Stellar clusters. In the beginning • All you have is a large cloud of dust and gas. • This cloud is very large and very cold. • They are called Giant Molecular Clouds. • Somehow (more on this later on in the lecture) the cloud collapses. Then… • A small part of the gas cloud collapses to form the starting solar nebula. • What is this cloud mostly made of (what materials)? The initial cloud • Is made of mostly Hydrogen (~90% by weight). • Most of the rest is Helium (9%) • 1-2% are everything else (in Astronomy we call the everything else “metals” including Oxygen). • The cloud has some spin. What will that do? Spin city • The small amount of spin acts like a merry-go-round. • Much like on a merry-go-round, this spinning motion pushes things outward. • However, nothing stops the collapse in the vertical direction, so the cloud collapses to a disk. • The gas in the disk is literally in orbit around the center of the gas cloud. And then, something special happens • In the core of this bit of cloud, there are a lot of particles falling into the center (everything not lucky enough to start orbiting). • This creates heat from the kinetic energy of the infalling materials. • NOTE: kinetic energy is the energy of motion. If something stops moving, that energy has to go somewhere. In collisions like this it goes into heat! • At some point the central object starts to radiate this newly acquired heat and becomes a protostar. Protostar • A protostar is a newly forming star. • It generates its energy from gravitational collapse and not from nuclear fusion like an adult star. • Eventually, the pressure and density at the core of this protostar increase. This increases the collisions of particles at its core. • This causes the core to heat up quickly. Class 0 Protostar • Protostars go through stages. • The first is stage 0. • Lasts the first 10,000 years of formation • You still have very cold gas in the system. • However Astronomers had long wondered what kept it from collapsing too fast. Magnetic Fields • Are force barriers for charged particles. • They slow down the infalling gas Class I Protostar • At this stage large amounts of material are falling to the central protostar(s). • While they have a disc they still have an envelope • The core star(s) is/are heating up • Some of the inflowing material is thrown out along magnetic field lines that go above and below the disc • 100,000 years T Tauri, Class II Protostar • During this stage the star is variable as the collapsing envelope changes the temperature of the “star” which changes its brightness. • Also, in addition to generating energy through gravitational collapse the upper atmosphere of the protostar will fuse deuterium into Helium T Tauri, Class II Protostar • No more envelope, just a disc • Smaller and therefore dimmer than Class I • 1 million years Class III • Final class before becoming a full star • Core heats up as the collapse finishes • Disc goes away • Want to read more: • Furlan et al, 2008, SPITZER IRS SPECTRA AND ENVELOPE MODELS OF CLASS I PROTOSTARS IN TAURUS, Astrophysical Journel, 176:184Y215 It’s a girl! • Eventually something amazing happens. • The core gets so hot and so dense that fusion begins! • The star is now born! Meanwhile • The birth of the star results in a lot of debris. • This debris will form the planets, asteroids, comets, ect. How long do you think this process took to form our sun (and planets)? • It took about 10 million years. A very short time compared to the 4.5 billion years of age that the earth and sun are currently. • If a star was more massive, would it take a longer time or a shorter time to form? If a star was more massive, would it take a longer time or a shorter time to form? • The seemingly obvious answer would be the more massive star would take longer. • HOWEVER, with more mass means more gravity. • More gravity means the collapse occurs much faster. • As we will see again and again, the bigger the star, the faster it does everything. • So, stars bigger than the sun form FASTER than the sun. • Similarly, stars smaller than the sun take longer to form. Collapse mechanisms • 1) Local supernova – however you need to form a star to do that. • 2) Collision with another cloud of gas – this usually happens when 2 galaxies collide. • 3) Spiral arm – probably the most common. • You get a spiral density wave that shocks the gas cloud. That causes it to collapse – much like sending a seismic wave through an old house make the house collapse. But? • Um, if that cloud collapses, why do we get so many stars. • Due to spin when the cloud collapses, it breaks into pieces. • Imagine a ice skater with arms 100 light years long. • What happens when that skater pulls in their arms? Rotation • The spin becomes so great that the cloud has to break into pieces. • Sort of like spawning a tornado out of a larger rotating storm system. • This is what leads to lots of star systems. Conclusion • A collapsing cloud of dust and gas forms stars • The forming star, or Protostar goes through stages • When fusion starts in the core a star is formed. • This also leads to a star cluster as we will examine next lecture