1. Star Formation
Astronomy 315
Professor Lee Carkner
Lecture 12

2. 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

3. 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

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

5. 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

6. 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

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

8. 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

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

10. 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?

11. 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

12. 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

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

14. 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

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

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

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

18. 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

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

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

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

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

23. Next Time
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