Guiding Questions Why do astronomers think that stars evolve? What kind of matter exists in the spaces between the stars? Where do new stars form? What steps are involved in forming a star like the Sun? When a star forms, why does it end up with only a fraction of the available matter? What do star clusters tell us about the formation of stars? Where in the Galaxy does star formation take place? How can the death of one star trigger the birth of many other stars?

Low vs. high mass stars Mass is important! –Evolution of a star depends on its original mass. Sun-like stars - mass close to Sun’s Massive stars - mass much larger than Sun’s (M > 5 M  ) –Mass determines the location of a star on main sequence.

Review: Basic concepts Hydrostatic equilibrium - a balance of gravity pulling in and pressure from heat pushing out. Luminosity - total energy radiated by a star (total brightness) –depends on size and temperature. Nuclear fusion - joining of two nuclei together to form different one. –Requires high temp. and density to overcome electrical repulsion of protons.

Birthplace of Stars The matter between the stars are collectively termed as interstellar medium. It is made out of two components: –Gas & Dust Any interstellar cloud of gas & dust is called a Nebula (plural Nebulae) Evidence of Nebulae: –Spectral lines. –Reddening of stars.

Birthplace of Stars Emission Nebula: A nebula with the characteristic emission line spectrum of a hot, thin gas. – Found near hot, luminous (Type O & B) stars –emission nebulae have masses ~ 100 M  to10,000 M  An example of such Nebula is the Orion Nebula: –The middle star in Orion’s sword. Star-Forming Regions(Nebulae)

Birthplace of Stars About 450 pc from Earth. about 300 M .

Birthplace of Stars

The vast amounts of UV radiation emitted by the close by Hot, type O or type B stars are absorbed by the Hydrogen atoms in the Nebulae – these high energy photons strips the H atoms of its electron leaving H ions - H II. –Emission nebulae are referred to as H II regions H II regions emit visible light (red) when some of the free electrons recombine with protons, and the re-captured electrons cascade down to lower orbits. Star-Forming Regions(Nebulae)

Most important among these transitions is the n=3 to n=2 transition. – emits 656nm - red photons (H  photons). –This gives the distinctive red color to the H II regions. Dark Nebula: A nebula so opaque that it blocks visible light that are emitted from stars behind the nebula. –Higher concentration of Dust grains –These look like dark patches. –Example: Horsehead Nebula. Star-Forming Regions(Nebulae)

Emission, Dark and Reflection Nebulae near the Star Alnitak in the Orion constellation.

Dark Nebula Bernard 86 nebula in the constellation of Sagittarius.

Reflection Nebula: Do not produce its own light like emission nebulae, but scatters star light. – The scattering is due to the dust grains. –Lower concentration of dust grains than dark nebulae. –Scattering gives rise to Blue color (similar to the blue sky on Earth). Star-Forming Regions (Nebulae)

Reflection Nebula NGC in the constellation of Corona Australis.

Interstellar Extinction: the intensity of star light is reduced as light passes through the interstellar medium. Interstellar Reddening : When light from a star pass through interstellar medium, dust particles absorb or scatter blue light allowing red light to pass through (like a Sun set on Earth) – the star appears red. Star-Forming Regions(Nebulae)

Reflection Nebula Interstellar Reddening

NGC 3603, 7200 pc NGC 3576, 2400 pc

Spiral Galaxy: M83 The reddish regions in the spiral arms are H II regions.

Spiral Galaxy: NGC891 The dark band is caused by dust.

Giant Molecular Clouds: In certain cold regions of interstellar space atoms combine to form molecules. –Molecular H is hard to detect, since they do not emit light (radiation) –However, carbon monoxide present in these clouds emit millimeter wavelength light, and thus can be detected by radio telescopes. – these giant clouds have masses ranging from M . Star-Forming Regions

Giant Molecular cloud in Orion Constellation About 1000 giant molecular clouds are known in our galaxy. These clouds lie in the spiral arms of the galaxy.

Stage 1: An interstellar cloud –Star formation begins when part of the interstellar cloud contracts under its own mutual gravitational attraction - denser regions in the clouds are favorable for star formation –The gravitational collapse overwhelms the pressure - colder regions are more favorable since they are low pressure regions. – These cold dense regions of clouds collapse under its own weight to form clumps known as - protostars. The Formation of Stars

Stage 1: Contracting Cloud - star formation is triggered when a sufficiently massive pocket of gas is squeezed by some external event. –Material flowing out of protostars cause shock waves that trigger regions nearby to collapse. –A supernova explosion of a dying star can compress the surrounding gas triggering a collapse. The Formation of Stars

Stage 2: Fragmentation - Contracting interstellar cloud fragments into smaller pieces due to gravitational instabilities. The pieces continue to collapse and fragment, eventually to form many tens of hundreds of separate stars.

The Formation of Stars Stage 3: Several tens of thousands of years after its first began contracting, a typical stage 2 fragment has shrunk by the start of stage 3 to roughly the size of our solar system (still 10, 000 times the size of our Sun). The dense, opaque region at the center is called a protostar – an embryonic object at the dawn of star birth.

Stage 4: Protostellar Evolution- –As a protostar evolves, it shrinks, its density increases and it temperature rises.. –Some 100,000 years after start of the cloud collapse, its center gets heated to 1 million K - purely due to compression of the gas. –This hot protostar produce substantial luminosity and can be plotted on a H-R diagram. –As the protostar evolves it collapses, and thus its luminosity and temp. changes. The Formation of Stars

Pre-main sequence evolutionary tracks in the H-R diagram:

protostar evolution of a Sun like star:. –Outer layer is cooler and opaque - temperature does not increase much. –However, due to the collapsing of the protostar the radius decreases and thus the Luminosity decreases. –The star moves to down(less luminous) and slightly to the left (hotter) in the H-R diagram. –Caution: this does NOT represent an actual movement of the star in space. The Formation of Stars

protostar evolution of a Sun like star: –After about 10 million yrs. the internal temperature gets high enough for Hydrogen burning to take place and the pressure due to this process will stop the collapse of the protostar - the evolutionary track now reaches the main sequence. The Formation of Stars

protostar evolution of a massive star: – a more massive star collapses faster and it heats more rapidly and therefore, the H- burning takes place sooner. - evolutionary tracks are horizontal. protostar evolution of a low mass star: –Does not contain the required mass to develop the necessary pressure and temperature to start H-burning - end up as Brown Dwarfs - Low luminosity & low temp. They occupy the bottom right corner of the main sequence. The Formation of Stars

The Evolution of a Star

Stage 5: A Newborn Star: – About 10 million years after its first appearance, a protostar (comparable in mass to our Sun) will become a true star. –It would have shrunk to about the size of our Sun (starting from about 10,000 times the size of our Sun) the contraction would raise the temperature to 10 million Kelvin - enough to start thermonuclear reactions. When thermonuclear reactions start at the center of a protostar, we say a new star is born. The Formation of Stars

Evidence of these processes We have observed –bipolar flow from young stars. –star forming regions (e.g. Orion Nebula). –Young Star clusters.

When the star forms by collapsing due to its gravity, the protostar also emits much of the cold dark matter into space. –This is seen in T Tauri stars. Bipolar outflow - many young stars loose mass by ejecting gas along two narrow jets that are oppositely directed. –These objects are referred to as Herbig- Haro objects. Mass Loss from a Young Stars

Bipolar outflow Hubble telescope image of a bipolar out flow. These are clouds of glowing ionized gas created when fast moving gas jets ejected from the protostar slams into the surrounding interstellar medium.

Star clusters –A cluster of stars forms when a large gas cloud collapses into many stars of many different masses. Each cluster is a snapshot of stellar evolution. Stars in a cluster start forming almost simultaneously. However, they do not become main- sequence stars at the same time. –That depends on the mass of the star. Star Clusters

M16 Star Clusters in Eagle Nebula

Young Star Clusters A young star cluster in a H II region and its H-R diagram. Stars are still forming - all are not main sequence stars yet.

The Pleiades cluster: An older (50 mill. Yrs.) cluster. All the cool, low mass stars have become main sequence stars