STAR BIRTH

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?

Interstellar Medium

• The matter between the stars

• Made out of Gas & Dust

• Any interstellar cloud of gas & dust is called a Nebula

• Evidence of Nebulae:– Spectral lines. – Reddening of stars.

Types of Nebulae

1.) Emission Nebula: emission line spectrum of a hot, thin

gas.– Found near hot, luminous (Type O & B) stars– emission nebulae have masses ~ 100 M to

10,000 M

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.

2.) 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.

Dark Nebula

Bernard 86 nebula in the constellation of Sagittarius.

3.) 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).

Reflection Nebula

NGC 6726-27-29 in the constellation of Corona Australis.

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

• 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.

Reflection Nebula

Interstellar Reddening

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 105 - 106M.

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: 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

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: – 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

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.