Announcements Exam 2 will be returned Monday Dark Sky Observing Night tomorrow night. Meet at the Farm for set-up at 7:30pm. Cancellation notice, if needed, will be posted on by 5:00pm Thursday.

Light is an Electromagnetic Wave (although it can also be a particle called a photon)

A Basic Wave Wavelength = Frequency = fSpeed = c

Sources of Light Any object that has temperature emits a blackbody spectrum. Since everything has a temperature, everything emits light Blackbody Light The spectrum is called a Planck’s Spectrum

Light also comes from electron transitions

The Doppler Effect Watch Doppler Effect Car Horn video

Brightness Versus Distance An Inverse Square Relationship If all stars had the same intrinsic luminosity, the brightest stars would be the closest ones. Unfortunately, stars do not come with the same intrinsic luminosity. Watch YouTube Size Comparison video.

To find distance to an object using the apparent brightness you must also know intrinsic luminosity of the object The intrinsic luminosity of the most distant cosmological objects is one of the most important questions in cosmology

Complicating factor: interstellar reddening Intervening gas and dust will absorb some of the light and scatter the blue light leading to reddening of the color. The more space the light has to pass through, the greater the effect

The life of stars

The Interstellar Medium Reflection Emission HII Region Absorption Nebulae

The Birth of Stars

Stars form in the coldest darkest places in nebulae

Bok Globule Close-up

Something Triggers Collapse and a protostar begins to form

New Star Formation Shock

The collapse from nebula to protostar How long the collapse takes depends on the mass. More massive stars form quickly.

Protostars are shrouded in a dense cocoon Infrared can penetrate into the cloud to reveal the protostars

Protostars continue to collapse until the core temperature reaches 10,000,000°

Not all protostars will reach a core temperature of 10 million K Brown dwarfs aren’t much larger than Jupiter but they can have up to almost 80 times the mass of Jupiter

Stars maintain themselves by hydrostatic equilibrium If energy output from the core is insufficient, the star contracts. As it contracts, it heats up according to the gas law: PV = nRT If energy output from the core is excessive, the star expands. This will be important in the late stages of life.

Once a core temperature of 10 million K is reached, fusion begins The Proton-Proton Cycle The high temperature is needed to overcome the electric repulsion between the protons.

Due to hydrostatic equilibrium, the luminosity depends on the mass The self-gravity depends on the mass. The gas pressure depends on the temperature. The temperature depends on the energy output from the core which eventually comes out as the luminosity of the star

The Hertzsprung-Russell Diagram

Lifetime of Stars

Stellar Populations Population I Stars: like the Sun, relatively young. Lots of “metals” in them. Found in the disk (and some in the nuclear bulge) of the Milky Way Population II Stars: very old, upwards of 10 to 12 billion years old. Most are as old as the galaxy is. Very low “metal” content. Found in the halo and nuclear bulge of the Milky Way. Due to their age, only lower mass (<M sun ) stars can be Pop II stars Population III Stars: the first generation of stars. No “metal” content. Theory predicts there shouldn’t be any left. May have been several thousand solar masses.

H – R Diagram of a relatively young cluster…The Pleiades An even younger cluster would still have protostars at the low end

H – R Diagram of an Old Cluster…Globular Cluster

Stellar Size and the H-R Diagram

On an H-R diagram the mass increases as you move up the Main Sequence