But first, a little background Kirchhoff's Laws
Kirchhoff's laws ,there are three types of spectra: continuum, emission line, and absorption line. High pressure, high temperature gas Low pressure, high temperature gas Cool gas in front of continuous spectra source
Hydrogen Helium Oxygen Neon Iron
Hydrogen Continuum Absorption Lines
similar in light and sound Doppler effect • Waves compressed with source moving toward you Sound pitch is higher, light wavelength is compressed (bluer) • Waves stretched with source moving away from you • Sound pitch is lower, light wavelength is longer (redder)
Inverse square of light If two stars are similar and one star is 3 times as far away, as the other, its intensity will be 1/9.
Spectra of Stars OBAFGKM
A5 K7 Sun(G2) • Annie Cannon classified stars according to the strength of the hydrogen absorption lines in the sequence A, B, C….P Spectral Classification • These spectral classes were changed to a temperature-ordered sequence and some were discarded, finally leaving : Subclasses O B A F G K M 35,000 K 3,000 K • Oh, Be AFine Girl (Guy) Kiss Me
OBAFGKM L Bluest Reddest Hottest Coolest 50,000K 1300K The Spectral Sequence Spectral Sequence is a Temperature Sequence
O Stars Hottest Stars: T>30,000 K; Strong He+ lines; no H lines T = 11,000 - 30,000 K; Strong He lines; very weak H lines B Stars T = 7500 - 11,000 K; Strongest H lines, Weak Ca+ lines. A Stars T = 5900 - 7500 K; H grows weaker Ca+ grows stronger, weak metals begin to emerge. F Stars
T = 5200 - 5900 K; Strong Ca+, Fe+ and other metals dominate, G Stars T = 3900 - 5200 K; Strong metal lines, molecular bands begin to appear K Stars T = 2500 - 3900 K; strong molecular absorption bands particularly of TiO M Stars 4000 A7000 A Solar Spectrum
Spectra Part II Quantum Mechanics Electrons can only orbit the nucleus in certain orbits. • n =1First orbital: Ground State) • Lowest energy orbit .
Hydrogen Spectrum Down emission Up absorption • Hydrogen (1H) consists of: • A single proton in the nucleus. • A single electron orbiting the nucleus.
Emission Lines: Balmer Lines When an electron jumps from a higher to a lower energy orbital, a single photon is emitted with exactly the energy difference between orbitals. No more, no less.
Absorption Lines: Balmer Lines An electron absorbs a photon with exactly the energy needed to jump from a lower to a higher orbital. No more, no less.
Hydrogen lines absent in the hotteststarsbecause,photons ionize electrons. They are also absent in the coolest stars because, photons don’t have enough energy to move the electrons from n=2 to higher energy levels. No electrons, no lines.
HR Diagram In 1905, Danish astronomer Hertzsprung, and American astronomer Russell,noticed that the luminosity of stars decreased from spectral type O to M. • To bring some order to the study of stars, they organize them in the HR diagram.
106 104 102 Luminosity (Lsun) 1 10-2 10-4 40,000 20,000 10,000 5,000 2,500 Temperature (K) H–R Diagram Supergiants Giants Main Sequence White Dwarfs
As you move up the H-R diagram on the Main Sequence from M to O, the stars get hotter and larger
Back to this is your life Star Formation “All we are is dust in the wind” - Kansas
Protostars form in cold, Giant Molecular Clouds (GMC) in Orion • About 1000 GMCs are known in our galaxy • These clouds lie in the spiral arms of the galaxy
The Cone Nebula Examining a Star Forming Region
Giant Molecular Clouds (GMC) are mostly composed of molecular hydrogen. • Properties: • Radius ~50 pc (~160 ly) • Mass ~105 Msun • Temperature: 10-30 K • Also, small amounts of He,and others
Size of cloud – large, Compression area - small GMC’s resist forming stars because of internal pressure (kinetic energy) so, a cooler gas is needed. • A shockwave is needed to trigger formation, and to compress the material .
Sources of Shockwaves: 1.Supernova explosions: Massive stars die young . 2. Previous star formation can trigger more formations 3.Spiral arms in galaxies like our Milky Way: Spirals arms are probably rotating shock waves.
View all images An expanding supernova explosion , occurring about 15,000 years ago.
As the cloud is compressed, cool blobs contract into individual stars. Gravity Contraction The blobs glow faintly in radio or microwave light. As they heat up, blobs glow in the infrared, but they remain hidden .
As protostar compresses: Density increases Temperature rises. Photospheres (~3000K) Rotation increases as it shrinks in size. What types of stars form ? OB - Few AFG - More KM - Many, Many
Many of the cooler stars, spectral classes G,K,M, become heavy gas-ejecting stars called T-Tauri stars. Stars blows away their cocoon Leave behind a T Tauri star with an accretion disk and a jet of hot gas.
A T-Tauri star can lose up to 50% of its mass before settling down as a main sequence star. False Color: Green = scattered starlight and red = emission from hot gas.
Motion of Herbig-Haro 34 in Orion • You can actually see the knots, called Herbig-Haro objects, in the jet move with time • They can have wind velocities of 200-300 km/s. This phase lasts about 10 million years.
Low-Mass Protostars • Collapse is slower for lower masses: • 1 Msun (solar Mass) ~30 Myr • 0.2 Msun ~1 Billion years • When core temperature ~ 10 Million K: • Core ignites, P-P chain fusion begins • Settles slowly onto the Main Sequence • Has a rotating disk, from which planets • might form .
The disks are 99% gas and 1% dust. • The dust shows as a dark silhouette against the glowing gas of the nebula. Actual Protoplanetary Disks
High-Mass Protostars Collapse is very rapid: 30 solar mass protostar collapses in ~30,000 years When core Temperature >10 Million K:Ignite first P-P Chain then CNO fusion in the core.
Clouds are blown away from the new stars near the stars
The Cocoons of proto-stars are exposed when the surrounding gas is blown away by winds and radiation from nearby massive stars. Protostars!
The Main Sequence • Core temperature & pressure rise • Collapse begins to slow down • Finally: • Pressure=Gravity & collapse stops. • Becomes a Zero-Age Main Sequence • Star, (ZAMS).
Pre-main sequence evolutionary tracks Most everything about a star's life depends on its MASS.
Stars Form in Clusters Our own Sun is part of an open cluster that includes Alpha Centauri and Barnard's star. Gravitational interactions will cause some stars to eventually leave over time