* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Star Formation
Observational astronomy wikipedia , lookup
Corona Australis wikipedia , lookup
Cassiopeia (constellation) wikipedia , lookup
Dyson sphere wikipedia , lookup
Spitzer Space Telescope wikipedia , lookup
Formation and evolution of the Solar System wikipedia , lookup
Cygnus (constellation) wikipedia , lookup
Aquarius (constellation) wikipedia , lookup
Perseus (constellation) wikipedia , lookup
History of Solar System formation and evolution hypotheses wikipedia , lookup
Planetary habitability wikipedia , lookup
Open cluster wikipedia , lookup
Directed panspermia wikipedia , lookup
Timeline of astronomy wikipedia , lookup
Corvus (constellation) wikipedia , lookup
Accretion disk wikipedia , lookup
Future of an expanding universe wikipedia , lookup
High-velocity cloud wikipedia , lookup
Astronomical spectroscopy wikipedia , lookup
Nebular hypothesis wikipedia , lookup
Stellar kinematics wikipedia , lookup
10/17/2012 Star Formation Lecture 12 Stellar Birth • Since stars don’t live forever, then they must be “born” somewhere and at some time in the past. • How does this happen? • And when stars are born, so are planets! 1 10/17/2012 Molecular clouds • Stars form in giant clouds of gas and dust called molecular clouds. • The term molecular cloud is used since molecules are present. • The large amount of gas and dust in the cloud shields the molecules from UV radiation from stars in our galaxy. Anatomy of a Stellar Factory • Molecular cloud: • Contains ‒ ‒ ‒ H, He, etc. H2, H20, OH, CO, H2CO, etc. dust of silicates, iron, ices, etc. Collapsing Region 103 to 106 Msun of gas and dust in the cloud. 2 10/17/2012 M8 - Lagoon Neb. Cloud fragmentation • The molecular cloud does not collapse into a single star. • It fragments into many clumps. • These clumps can further collapse to form stars. • 10 - 1000 stars can be formed from the cloud. 3 10/17/2012 Gravitational Collapse • When a fragment of a molecular cloud reaches a critical mass - it collapses to form a star. – Gas and dust pulled together by gravity until a star is formed. • But to get this critical mass is not so easy. Causing collapse: Method 1 • Accretion: – Build up of small clouds of gas and dust into giant ones. • Clouds “stick” together and grow. • Very slow - due to low interstellar densities 4 10/17/2012 Causing collapse: Method 2 • Gravity and Radiation Pressure Pressure of Starlight Problem: But how do the first stars form! High densities & Gravitational Collapse Causing collapse: Method 3 • Compression by supernova blast waves Old Star Nearby Cloud Compressed cloud Exploding Star Shock waves from Supernova 5 10/17/2012 M16 Eagle Nebula NOAO Image Pillars in M16 HST Image 6 10/17/2012 M16: Close-up M16 10 ly 7 10/17/2012 The path to collapse • Gravity makes the cloud collapse. • Two hindrances to collapse 1. Internal heating - Causes pressure build-up 2. Angular momentum - Causes high speeds (like a skater) Internal Heating • Cloud fragments collapse • Potential energy => Kinetic Energy – Gas particles speed up and collide. • The temperature increases. • This causes a pressure build-up which slows (or stops) the collapse. • Energy is radiated away. 8 10/17/2012 Angular Momentum • Angular momentum A = mass vel. of rotation radius = mvr • Conservation of angular momentum. – A = constant for a closed system. • As the cloud fragment shrinks due to gravity, it spins faster. Angular momentum • Collapse occurs preferentially along path of least rotation. • The cloud fragment collapses into a central core surrounded by a disk of material. 9 10/17/2012 Disk Formation Rotating Central Core Infall Material Rotating Disk OMC Proplyds 2 10 10/17/2012 Dust Disks around Stars Planet formation • The disk around the central core will fragment further, producing rings of material. • The particles in these rings can accrete together to form planets! 11 10/17/2012 Protostars • The central core is called a protostar. • It is undergoing continuous gravitational contraction. • Self-compression heats the central core. • Surface ~ 300 K • Energy emitted in the infrared. L = 4 R2 T4 , R is very large. Overview of the build-up • Collapse starts out in free fall controlled by gravity • Central parts collapse more rapidly => central core becomes a protostar. • Core accretes material from the surrounding envelope 12 10/17/2012 A Star is Born • The protostar continues to collapse while the central core heats up to millions of degrees. • Fusion reactions start => A star is born What stops the collapse? • Collapse is halted by the pressure of the heated gas which balances gravity. An equilibrium • Gas and Radiation Pressure balance Gravity HOT • No collapse or expansion. 13 10/17/2012 Entrance into the H-R diagram more massive Luminosity (Lsun) Hayashi Contraction Phase less massive O B A F G K M Temperature Time to form a star • The time to reach the main-sequence varies with stellar mass. Mass (Msun) 15 5 2 1 0.5 Time (106 years) 0.16 0.7 8 30 100 14 10/17/2012 Making the stars visible • After a star is born it heats the gas and dust around it. • Eventually the gas and dust are pushed away. • The star then becomes “visible.” • Prior to this it could be seen only in the radio and the infrared. 30 Doradus (Opt/IR) Massive newborn stars are indicated by the arrows. Note that some (2, 3, & 4) are hidden to visible light. Arrows 1 and 5 indicate a compact cluster of bright young stars. Sources 6 & 7 may be due to outflow jets from the cluster 5. 15 10/17/2012 Dying star showing outflows – may go supernova in a few thousand years Evaporating disks around stars – planet nurseries? Bok Globules – dark clouds that could form stars Young star cluster Newborn stars emerging from their birth clouds NGC 3603 - Star Formation 16 10/17/2012 Stars Die! • The fuel in stars is proportional to the mass, M. • It is found that the luminosity of stars on the main-sequence varies with mass as: L M 3.5 17 10/17/2012 Stellar Lifetimes • Assuming stars “consume” the same fraction of their mass (M), the lifetime, T, is given by: T T T T Amount of Fuel Amount of Fuel Rate of Consumption Rate of Consumption M 1 M3.5 12.5 M M M 3.5 M 2.5 where M is in solar masses and T is in solar lifetimes. The Lifetime of Stars • The mass of a star determines how long it will live. • More massive stars evolve faster. Mass 1 Msun 5 10 Lifetime ~1010 yrs ~108 ~107 18