Wednesday, November 30, 2011

Sunday last day for blog posts

Jackie here. I don't want there to be pressure on you to write blog posts during exam week, and I need to eventually win the race to grade blog entries faster than you all post them, so I'm not going to grade any blog posts that are posted after Sunday evening 12/4 (ie if they say Monday 12/5, they're still awesome but they're not for points). If you think you really really need to post more blogs next week in order to get the grade you want to get, let me know. But currently you are all pretty much doing fantastic, so don't stress about it too much.

Prof. Johnson on TV

Tomorrow at 6pm and 9pm on the National Geographic Channel. Yes, this will be on the final (just kidding).

Finding the Next Earth

Join astronomers as they enter the final lap in a race to find a planet capable of sustaining life, a world like ours, the next Earth. See the launch of Frances CoRot and Americas Kepler missions, and the smoking hot worlds they discover. See a controversial and tantalizing discovery of a planet where life could exist in a strange twilight zone, that is, if the planet really exists. Astronomers are working to determine what conditions are necessary for life to exist, and they are building the radical James Webb Space Telescope, a spacecraft that can look at the atmosphere around a planet and reveal whether or not life as we know it actually exists. It could be the greatest discovery in human history and it could change how we see ourselves.

Tuesday, November 29, 2011


Please be sure to fill out your TQFR (Teaching Quality something something) surveys at the end of the term. We will listen closely to your feedback, which we'll incorporate into adjustments to the Ay20 course in future terms. Thanks!

Wednesday, November 23, 2011

Essential Topics

Essential topics from the class to study for your exam:

  • What is the relationship between telescope diameter and resolution?
  • Celestial sphere: when will a star at a given RA and dec be visible in the sky?
  • What are the equations of stellar structure, and the story of each one?
  • Scaling relations (what they are and how to derive them): M-R, M-L, M-Teff
  • Use scaling relations to derive the slope of the main sequence in the H-R diagram (log L vs -log Teff)
  • Derive the slope of the white dwarf main sequence
  • Equilibrium temperature of a planet as a function of semimajor axis and stellar properties
  • Blackbodies! Flux, luminosity, Rayleigh-Jeans Law (kT >> h*nu), Wien's Law (kT << h*nu), peak wavelength (relationship between photon energy and temperature)
  • Color and brightness of a star
  • Virial theorem - what does it mean? apply it to:
  • white dwarf (mass-radius)
  • typical temperature in the Sun
  • derivation of Kepler's 3rd law
  • How do we find planets?
  • Relationship between velocity amplitude of a Doppler signal and mass of planet, period of planet, mass of star

Sunday, November 20, 2011

Optional Lab: Exoplanet Transit

The Las Cumbres Observatory Global Telescope Network has a program called Agent Exoplanet where you go through real astronomical images from their telescopes, measure the flux from stars, and then identify stars with transiting planets based on a dip in flux. If you would like to earn extra points, or if you're just curious, please go play with their tools and then blog it up!

Transit Probability

Many of you had some trouble with the worksheet problem about the transit probability of a planet. Consider the sketch below:

The star is the big orange circle in the middle, and the filled blue circles show two extreme planet-orbit inclinations, above and below which the planet does not transit. Note that the orbit planes for the two configurations are parallel to the blue solid lines, not the black lines. The two orbit configurations are separated by and angle of approximately 2 Rstar/(purple trace), obtained using the "skinny angle" property that the sine of a small angle is the small side over the long side.

With those definitions in mind, the transit probability is related to the solid angle traced out by the two extreme transit configurations, which is
as well as the total solid angle at a semimajor axis a, or:

The probability is the ratio of these two solid angles:

For more on all things transit, including eccentric orbits and other properties of the transit geometry, see Prof. Josh Winn's (MIT) excellent book chapter here:

Wednesday, November 16, 2011

RV Plots for Thursday's Worksheet

Here are the radial velocity time series for two exoplanets. The mass of the star is listed under each plot. Problem 1 on the worksheet asks you to measure the masses of the planets in each system (assume e = 0, and i = 90 degrees).

Monday, November 14, 2011

Josh Carter's Exoplanet Talk

Tuesday 3pm in 370 Cahill:

Josh Carter
, Harvard-Smithsonian Center for Astrophysics

Kepler's Multi-Eclipsing Hierarchical Triples: Accurate Masses and Radii, Transiting Circumbinary Planets
The Kepler mission has opened a new era of high-precision time-series photometry. It has allowed for the wholesale detection of planetary systems and the detailed characterization of both stars and planets. The Kepler data quality and restricted mission scope has also led to the unveiling of novel events.  Amongst these are the discovery of hierarchical multi-eclipsing systems including those with transiting circumbinary planets (e.g., KOI-126, Kepler-16). These systems are observationally biased to have small periods and period ratios and, consequently, have short (Kepler mission lifetime) secular variation timescales. This dynamical information is encoded in variable eclipse morphologies.  I describe photometric-dynamical fits to the these light curves.  I present results from these fits; namely, I report accurate absolute bulk parameters (stellar and planetary masses and radii) that are determined free of typical model-dependencies.  I compare these parameters with theoretical expectations and comment on the efficacy of stellar models. I briefly address the search for additional transiting circumbinary planets in the Kepler data and discuss future applications of this work.

Thursday, November 10, 2011

Schedule your final exam!

The final exam will be a half-hour oral exam with Professor Johnson and me (and maybe a guest scientist!), sometime during the exam period from Dec 7-9. Please follow this link to fill out a form indicating your preferred exam times.

Monday, November 7, 2011

LaTeX Math Symbols

It's great seeing so many people use online LaTeX editors! Here's a handy guide to LaTeX math symbols:

Also, FYI, LaTeX is pronounced "Lay-Tek." My fellow grad students and I at Berkeley once spent the better part of a Stellar Structure study session debating this point. Somehow we all did well on the final...

Sunday, November 6, 2011

Prof. is back

Hey everyone, I'm finished with my battle against bronchitis. Short story: I won. Yay! I'm looking forward to being back in the classroom.

Juliette stumbled upon my most recent Astrobites contribution about preparing for grad school. I wrote that post this past summer, and I didn't think to connect it to our course. But it makes perfect sense to do so, so check out Juliette's blog and follow the link from there.

I wrote another Astrobites post last year that went viral, at least throughout the astronomy community. I think all Caltech students should check it out, and talk to me if it strikes a nerve.

Finally, we had our first two blog posts come in as part of the Professional Astronomers series. Write your first post soon, with your initial thoughts and impressions about what the process of going pro is all about. Then get going on your interviews! Talk to me if you need recommendations for interview subjects. But don't wait.