Star Formation. Stellar Birth

Transcription

1 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

2 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. H 2, H 2 0, OH, CO, H 2 CO, etc. dust of silicates, iron, ices, etc to 10 6 M sun of gas and dust in the cloud. Collapsing Region 2

3 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 stars can be formed from the cloud. 3

4 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

5 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

6 M16 - Eagle Nebula NOAO Image Pillars in M16 HST Image 6

7 M16: Close-up M16 10 ly 7

8 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

9 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 Disk Formation Rotating Central Core Infall Material Rotating Disk OMC Proplyds 2 10

11 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

12 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 R 2 T 4, 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

13 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 No collapse or expansion. HOT 13

14 Luminosity (L sun ) 10/17/2012 Entrance into the H-R diagram more massive 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 (M sun ) Time (10 6 years)

15 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

16 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 Star Formation 16

17 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

18 Stellar Lifetimes Assuming stars consume the same fraction of their mass (M), the lifetime, T, is given by: T T Amount of Fuel Amount of Fuel Rate of Consumption Rate of Consumption M M M2.5 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 Lifetime 1 M sun ~10 10 yrs 5 ~ ~