Lecture 1 Stellar evolution, spectral features and chemical composition, luminosity, blackbody radiation, color index and Hertzsprung-Russel diagram transitions, protostars.

The Stellar Nursery

Young Stars (Protostars) Once dense parts of a molecular cloud come together to form a ball of gas that is gravitationally independent, it is called a protostar. Gas will continue to flow towards the protostar until it becomes hot enough for the hydrogen in the gas to ionize into hydrogen atoms. These atoms will be pushed together under stronger forces as more and more gas gravitates towards the protostar. Eventually, they will fuse together to create helium atoms. Once fusion begins, the protostar graduates into being a full-fledged star. This young star begins producing stellar wind, clearing all molecules that aren’t hydrogen or helium based from it. This excess matter spirals outward at very long radii from the star—eventually resulting in planetary systems.

Young Stars cont. (Pre-main sequence stars) A pre-main-sequence star (also known as a PMS star and PMS object) is a star in the stage when it has not yet reached the main sequence. Earlier in its life, the object is a protostar that grows by acquiring mass from its surrounding envelope of interstellar dust and gas. After the protostar blows away this envelope, it is optically visible, and appears on the stellar birthline in the Hertzsprung-Russell diagram. (Wiki, pre-main sequence star)

At this point, the star has acquired nearly all of its mass but has not yet started hydrogen burning (i.e. nuclear fusion of hydrogen). The star then contracts, its internal temperature rising until it begins hydrogen burning on the zero age main sequence. This period of contraction is the pre-main sequence stage. An observed PMS object can either be a T Tauri star, if it has fewer than 2 solar masses (M ☉ ), or else a Herbig Ae/Be star, if it has 2 to 8 M ☉. There are also stars which undergo extreme fluctuations in intensity. These are called FU Orionis stars. More on this later. What would the light curve of an FU Orionis star look like compared to the light curve of a normal star? There are also failed stars, called Brown Dwarfs. They grow to be no larger than 80x the mass of Jupiter, but no less than 13x the mass of Jupiter. Low mass brown dwarfs are thought to fuse deuterium, while high mass brown dwarfs (>65 Jupiter masses) may fuse lithium along with deuterium.

Spectral Features and Chemical Composition (

Blackbody Radiation A blackbody is a theoretical body of matter. It is not real. Although some bodies of matter in the universe are very, very close to being a black body. An object that absorbs all incident electromagnetic radiation is a black body. Conversely, a white body is an object that reflects all incident electromagnetic radiation. Since a black body is supposed to absorb anything and everything that is incident upon it, it is sometimes unclear when we see radiation emanating from such a body in the universe. So we call said radiation, simply, blackbody radiation. More on this topic next time.