Ask Borodog

[Borodog]

Some of the kinds of things that I might be able to comment on semi-coherently:

- Physics and astrophysics
- Fluid dynamics and CFD
- Accretion disks in binary stars
- Getting a Ph.D.
- Being a college professor
- Lindy Hop/swing dancing and teaching dance
- Austrian economic theory and evolutionary theory (no credentials, but both are major amateur interests)
- Being a pro bono internet a-hole.

[pryor15]

[ QUOTE ]

Accretion disks in binary stars


[/ QUOTE ]

what the hell is it?

[Borodog]

When two stars are gravitationally bound and orbit their common center of mass, we call it a binary star. In certain types of binaries, the stars orbit close enough to each other that material from one star (either a stellar wind blown from the surface, or the actual surface of the star itself if they are close enough) can be gravitationally captured by the other star. Because the system is rotating, the material from the donor star often cannot fall directly onto the mass-gaining star (or the accretor; to gain mass is called "accretion"), although sometimes it can, if the accretor is large enough. But if the accretor is small enough, the transferred mass can lag behind it and be captured in orbit it. This gas shocks and circularizes, and forms a thin disk around the accretor, called an accretion disk.

In this diagram, a massive star, like an OB supergiant, say 10 solar masses, is in close binary orbit with a compact companion like a black hole or a neutron star. The OB is so large that it's surface is in contact with the point between the two stars where the force along the line of centers is zero; this means the gas in the donor's surface is free to expand (due to thermal pressure) towards the compact star. It does so, and "falls" towards the other star, forming what is called a tidal stream. The tidal stream misses the accretor, whips around it in orbit and "bounces" off of the inside of the accretor's Roche lobe (this is technical; the bounce point is really like the height reached by dropped tennis ball after it bounces; it can't escape the system because that would require more energy than it has from being "dropped" from the surface of the donor). After it "bounces", the gas stream collides with itself, creating fluid dynamic shocks that circularize the flow, forming a disk.



This is a snapshot from a 2d simulation of a High Mass X-Ray Binary (HMXB; in this case, LMC-X4) I made:



Here's a movie from a 3d simulation of a disk forming in such a system that I made about 10 years ago:

http://wonka.physics.ncsu.edu/~blondin/Movies/hmxb.mpg

[Coffee]

What is a realistic income for a first-year professor? I realize that it varies based on the discipline, so let's keep it to liberal arts. I'm about to begin teaching high school, but it would be a rather natural transition/goal to earn my Ph.D and begin teaching in college.

[RunDownHouse]

Where do you teach?

EDIT: Directed towards OP, sorry for QR confusion

[Coffee]

I don't, yet...I'm getting my alternative certification, but I'm going to be teaching in Houston, either Spring Branch ISD or Katy ISD.

[Borodog]

[ QUOTE ]
Where do you teach?

EDIT: Directed towards OP, sorry for QR confusion

[/ QUOTE ]

A major state university in North Carolina.

[diebitter]

How long did you take to get your PhD.


I took 42 months, wondered if I burned you or not. [img]/images/graemlins/smile.gif[/img]

[Borodog]

[ QUOTE ]
How long did you take to get your PhD.


I took 42 months, wondered if I burned you or not. [img]/images/graemlins/smile.gif[/img]

[/ QUOTE ]

It took me 13 years from when I started undergrad to when I completed my Ph.D. That included 3 years off in industry, after my department made the (in retrospect) mistake of giving me a Master's Degree after I completed the Ph.D. qualifying examination. I was the first in my department to receive one of these "en route" Master's. The department instituted them to compete with other departments that were offering them (it's a scary thing to contemplate spending 3-5 years on something, busting out and have nothing; the en route Master's was viewed as a "safety net" for potential Ph.D. candidates). I had run into an 18th month wall in my research by biting off too much of a problem to chew for my dissertation topic (I was attempting to simulate radiative warping in accretion disks, without resorting to full radiative transfer, and the reradiation backpressure algorithm I developed, while clever and elegant, was never quite numerically stable enough. Combine that with the collapse of my first marriage, and them just handing me an advanced degree, and I took the money and run, so to speak.

So basically, 4 years undergrad, 5 + 1 years for the Ph.D. with the 3 years in industry thrown in between the 5 and the 1.

[pryor15]

wow. um...wow. i think my head just exploded

i remember now why i never did well in science