Star Formation

Astronomy 315Professor Lee

CarknerLecture 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

Age -- IR class -- Infall -- Outflow -- Disk -- possible Magnetic activity -- yes

The New Star

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

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 Age -- 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

Age -- IR class -- Infall -- Outflow -- Disk -- thin Magnetic activity -- strong X-ray

activity and starspots

Childhood’s End

Star is now on the zero age main sequence (ZAMS)

Also,

Disk may have formed into planets

Stage 6 -- Zero Age Main Sequence

Age -- 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