Lecture 2 January 14 th, 2016 Planet Formation (exoplanets), T Tauri variables, FU Orionis variables, Herbig Ae/Be stars, protoplanetary disks, debris disks, H I/II regions, molecular clouds, and identifying exoplanets.

Adding a Detail to Star Formation As we know, stars are born inside of molecular clouds called nebulae. We know that they are born when particles become gravitationally attracted to one another, and form dense gaseous pockets. But let’s add a very important detail: every nebula begins with a certain amount of angular momentum. This means that the formation of dense gaseous pockets is not an inward migration of gas particles, but a circular development. When protostars form in these swirling disks, and eventually grow dense enough to become stars, they now exist in what is called a protoplanetary disk (sometimes circumstellar disk, or accretion disk.)

Planet Formation After the process of fusion begins, the star (either T Tauri or Herbig Ae/Be) still exists within this accretion disk. Inside of protoplanetary disks, the same process of star formation happens, but on a much smaller scale: There are gravitational perturbations (just like everywhere in the universe), and smaller swirls begin to gravitationally come about on the disk. This is the first of four steps of planet formation.

The Four Steps of Planet Formation 1.Gravitational collapse of a star (star birth) 2.The Condensation of the gas cloud and the formation of chondrules Chondrules are small pebbles that form in dust clouds due to dust particle collision. They eventually accrete into larger asteroids called chondrites. 3.The accretion of chondrites to form small bodies between 1-10km in diameter 4.More violent and rapid impact accretion leading to planet-sized objects.

Image of Chondrules and a Chondrite

Identifying Beta Pictoris (For test purposes, Beta Pictoris will be any image of a star with an accretion disk)

The Fomalhaut Debris Disk Fomalhaut is the brightest star in the constellation Piscis and one of the brightest stars in the sky.

Intrinsic Variable Stars Intrinsic variable stars are stars that change in luminosity due to physical changes in the star; there are three types of Intrinsic Variable Stars: 1. Pulsating Variable Stars These are stars that swell and shrink. 2. Eruptive Variable Stars Mostly pre-main-sequence stars because these are stars that change in magnitude while condensing (during gas-ball-formation inside of a molecular cloud). 3. Explosive Variable Stars This only occurs in extremely massive and old stars, or in white dwarfs that gain enough mass to supernova.

Extrinsic Variable Stars These are stars that change in luminosity due to external changes such as the rotation of the star or the eclipsing of the star. 1.Rotation: this change in luminosity is due to that a large sunspot exists on the surface of the star, and as it rotates, it darkens and brightens periodically. 2.Eclipsing: this change in luminosity is much more common, and is due to a secondary object passing in front of and behind the main star in the system. a.Visual binaries appear to the unaided eye to be one star, but can be seen as two through a telescope. An example is Polaris, which is made up of Polaris A ( which is two more stars in itself) and Polaris B. b.Eclipsing binaries appear to be single stars through a telescope; however, by measuring the brightness of an eclipsing binary, one can determine that the brightness changes over time. This change of brightness is because the plane of these stars' orbit lies along our line of sight. c.Spectral binaries are stars that are so close that they cannot be distinguished by either a telescope or by measuring brightness. They can only be separated with spectral analyses, which is where they get their names.

All that’s left are Kepler’s Laws!