Star Formation Astronomy 315 Professor Lee Carkner Lecture 12

Where Do Stars Come From? Eventually will run out of fuel for fusion reactions Born, live their lives and then die How are they formed? Stars must form out of something We find young stars near interstellar clouds

Young Stars What are the general characteristics of young stars? While properties depend on mass and age there are at least three common phenomena: Circumstellar material Outflows Magnetic activity Strong X-ray emission and starspots

The Interstellar Medium Dust makes them mostly opaque Clouds contain the raw material for star formation Gas (hydrogen) and dust (silicates)

Balancing Act There are two types of forces acting on the cloud: Inward Outward Centripetal force from the cloud’s rotation Magnetic pressure from the magnetic field lines running through the cloud

Collapse However, something may overcome those forces and trigger a collapse These force the particles closer together which increases the gravitational force The cloud (or more accurately a piece of a cloud) has overcome the outward forces and begins to contract

Stage 1 -- Cloud Age -- IR class -- Infall -- Outflow -- Disk -- Magnetic activity -- none, but cloud has weak magnetic field

After the Collapse The star consists of a central core of condensing material and a thick envelope of accreting material The star is very cool and only radiates at millimeter wavelengths

Stage 2 -- Protostellar Core IR class -- Infall -- Outflow -- Disk -- possible Magnetic activity -- yes

The New Star How are these effects manifested? The center contracts faster than the edges producing a central core The star rotates faster as it contracts producing a disk How are these effects manifested?

Protostar Properties Envelope blocks out visible light, but the disk and envelope produce infrared radiation Young star and disk are bathed in high energy radiation

Protostellar Jets The disks and magnetic fields focus the material and fire it out the poles Jets regulate the angular momentum of the rapidly spinning young star

Stage 3 -- Protostar Age -- IR class -- Infall -- Outflow -- Disk -- thick disk Magnetic activity -- strong hard X-ray emission

Behind the Veil Eventually the envelope clears off We now can see the star optically Known as a Classical T Tauri star Energy of star comes from gravitational contraction Still rotating rapidly and surrounded by a disk

Circumstellar Disks Disks are common around young stars Produce an infrared excess in the spectra of young stars Disks are common around young stars

Mulitwavelength Star Formation Radio -- Millimeter -- from the cool outer disk Infrared -- Visible -- from the photosphere Ultraviolet -- X-ray -- from magnetic activity

Stage 4 -- Classical T Tauri Star IR class -- Infall -- Outflow -- Disk -- thick disk Magnetic activity -- strong X-ray emission

End of Accretion Disk becomes thin and does not interact with the star (weak interaction) Star is moving out of star forming cloud Hard to find these Weak T Tauri stars except by X-ray emission

Stage 5 -- Weak T Tauri Star IR class -- Infall -- Outflow -- Disk -- thin Magnetic activity -- strong X-ray activity and starspots

Childhood’s End Also, Star is now on the zero age main sequence (ZAMS) Disk may have formed into planets

Stage 6 -- Zero Age Main Sequence IR class -- Infall -- Outflow -- Disk -- possible planetary system Magnetic activity -- weak X-ray emission

As the star moves towards the main sequence: Core gets denser Submillimeter and IR radiation decreases

Next Time Read Chapter 20.1-20.5