Midterm Exam #2 Tuesday, March 23. Closed book Will cover Lecture 8 (Special Relativity) through Lecture 14 (Star Formation) only If a topic is in the book, but was not covered in class, it will not be on the exam! Some combination of multiple choice, short answer, short calculation
“Heir ist wahrhaftig ein Loch im Himmel”
Image taken in optical / visible light
Milky Way: Radio
Milky Way: InfraredCold clouds are transparent in the infrared and radio
Cold clouds obscure our view at visible wavelengths, but infrared and radio light penetrates the clouds.
Horsehead Nebula (in Orion), optical image
A stable cloud has a balance of two forces:
No net force => No motion
This cold, dark cloud is collapsing and forming cores that will eventually become stars
This is a cloud where gravity has won the tug-of-war!
Most molecular clouds contain MUCH more mass than would make a single star
Most molecular clouds are very LUMPY (not smooth)
Likely scenario is that many lumps (which are more dense than the average) contract to form stars at about the same time
Single star formation is possible but probably very rare (because you need an unusually dense, yet low-mass cloud)
Pleiades Star Cluster
* Cloud collapses
* Protostar forms
* Disk forms
* Planets form
If you compress a gas, it will heat up.
Center of a collapsing cloud becomes denser and hotter.
The energy is gravitational. Half the gravitational energy goes into heating the collapsing clout, the other half escapes as light.
The central object is called a “protostar”, and they are very bright! (Because they have very large radii.)
Protostars are hard to see because they are being formed in very dusty regions of space.
Main difference between a protostar and a genuine star:
Stars generate power by nuclear fusion, protostars generate power by gravitational collapse.
Note: For the most part, stars are stable (neither expanding nor contracting). Protostars are all contracting.
When does the contraction end?
When the core becomes hot enough and dense enough to start nuclear fusion reactions.
The following quantities are conserved:
1. Total Energy
2. Linear Momentum
3. Angular momentum
The total amount of these quantities never changes
Li = mviri
Lf = mvfrf
Li = Lf
vf = vi (ri/rf)
If the radius of the orbit decreases, and angular momentum is conserved, the velocity must increase.
L = mvr
To conserve angular momentum, an object moving perpendicular to the rotation axis must increase speed, and eventually stop because it has a finite energy.
r changes here
HST Image of Beta Pictoris: note star itself has been blocked out
Jets are perpendicular to the disk = rotation plays a role in their formation. Most likely link of rotation and outflowing gas are magnetic field lines. We’ll see this again when we talk about “active galaxies”.
It shouldn’t be that hard….
Portion of Orion Nebula showing “Proplyds” = Protoplanetary Disks