Nebula

Fuzzy Patch Any source of light in the night sky that was not a point was called a nebula. comets gas clouds dust clouds galaxies star clusters In order to help comet hunters, Charles Messier began to catalog these objects in the 1700’s.

Interstellar Medium Interstellar space is 99% gas. Hydrogen 74% Helium 25% All other elements 1% Dust (1%) includes chemical compounds. methane, ammonia, water, monoxides Atoms are widely spaced, about 1 atom per cm3, a nearly perfect vacuum. 1019 atoms/cm3 in air The temperature is cold, less than 100 K. Voyager I reached the ISM in 2012.

Molecular Clouds The small mass of atoms creates very weak gravity. Pull atoms and molecules together Concentrations equal to 1 million solar masses can form giant molecular clouds over 100 ly across.

Diffuse Nebulae Clouds of gas do not naturally emit light. Light blocked beyond The Coal Sack is a dark nebula. Some nebulae reflect the light of nearby stars. H I region The Pleiades are in a reflection nebula.

Emission Nebulae Gas clouds can be heated by nearby stars. Emits own light H II region Like a star the spectrum can be analyzed. chemical composition Temperature and density velocity (by Doppler shift) The Orion Nebula is an emission nebula.

Star Formation A cool (10 K) nebula can be compressed by shock waves. Shock waves from new stars and exploding stars. exploding star shock waves nebula with areas of higher density

Protostars Local concentrations in a nebula can be compressed by gravity. Keeps together at low temperature Contracts to form centers and radiates energy These are protostars, called T Tauri stars (G, K, M).

Birth of the Sun Gravity continues to pull the gas together. Hydrogen fusion begins at 5 million K. At this point the star has reached the main sequence. 1 M

Birth of Other Stars Large masses become brighter, hotter stars. Fusion starts sooner, about 100,000 years Small masses become dimmer, cooler stars. Fusion starts later, up to 100 million years 10 M 3 M 0.5 M 0.02 M