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The Interstellar Medium and Star Formation

The Creation of Stars and the Interstellar MediumWithin 3 minutes after the moment of creation, the universe was an expanding mass of hydrogen and helium. About 75% was hydrogen and 25% helium. The laws of physics acted in this mass to produce the universe as we see it now. Within galaxies like our Milky Way Galaxy, the birth and subsequent death of stars continues to enrich the universe with heavy elements and life-sustaining planets.

Hydrogen + HeliumProduced in The Big Bang

The cores of high mass stars undergo supernova

l i

First stars were made of H and He. Jupiter-like planets also formed. Heavy elements were produced in the cores of stars.

explosions.

As a result, the interstellar medium becomes richer in heavy elements such as C, N, and O.

New stars are richer in the heavy elements and some planets are Earth-like and capable of supporting life.

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Spectroscopic Notation for Ionization States• Neutral atom of element A denoted by A I• Singly ionized atom denoted by A II• Example: Fe I is a neutral iron atom.

Fe IV is an iron atom with 3 electrons removed.Fe IV is an iron atom with 3 electrons removed.

H II Regions - Fluorescence

• Hot Star emits UV Radiation.• UV radiation ionizes hydrogen.• Electrons recombine with H II to form H I.• Electrons drop from one energy level to another until• Electrons drop from one energy level to another until

they reach the ground state, emitting a photon each timethey drop.

• Some of the photons will be visible (red, blue, and violet).• Because of the hydrogen Balmer alpha line, the nebula has a pink color.

Trifid Nebula (M20)

Reflection Nebula

Emission NebulaDust

National Optical Astronomy Observatory (NOAO)

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Horsehead Nebula in Orion

NOAO

Extinction and Reddening

Extinction near the Sun is about 1.9 magnitudes per thousand parsecs.Interstellar Dust Cloud

Toward Earth

p

Starlight

Blue photons are more frequently scattered than red photons. The starlight received at Earth is reddened.

The intensity of the starlight that passes through the dust is reduced by scattering and absorption. This reduction in the star’s brightness is called extinction.

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Component Temperature(K)

Density(atoms/cm3) Gas

Four Components of the Interstellar Medium

(K) (atoms/cm )

HI Clouds 50 – 150 1 – 1000HI and ions

of other elements

IntercloudMedium 103 - 104 0.01 partially

ionized

Coronal Gas 105 - 106 10-4 – 10-3 highlyIonized

MolecularClouds 20 – 50 103 - 105 molecules

Evidence for Star Formation

• High mass (and therefore young) stars.• Bok globules• Bok globules.• Herbig-Haro objects.• T-Tauri Stars.• T Associations• O Associations.

Bi l j t• Bipolar jets.• Presence of all of the above in regions of

(relatively) high density gas and dust.

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Bok Globule in Reflection Nebula NGC 1999

Bok globule

V380 Orionis-the star that illuminates the nebula.

about 5 light years

The Eagle Nebula

The Eagle Nebula (M16) is an emission nebula 7000 light years from Earth. Young, hot stars inside the nebula ionize hydrogen. The electrons recombine with the nuclei. As the electrons drop from higher energy levels to lower ones, they emit photons. These visible photons are responsible for the pink color.

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Hubble Space Telescope Picture of the Central Region of the Eagle Nebula

Click on the picture to zoom in on the tip of the left pillar. Click outside the picture to proceed to the next slide.

Animation from the Hubble Space telescope website (http://www.stsci.edu)

EvaporatingGaseousGlobule

AboutOneLightYear

New Star

Hot stars in and near the Eagle nebula cause evaporation of gas and dust, leaving behind denser regions called evaporating gaseous globules (EGGS). Some of these EGGS contain protostars.

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The Orion Nebula is the visible tip of a giant molecular cloud in which star formation is occurring.

Belt stars areabout 8 millionyears old.

Trapezium stars (4 stars at the

Orion Nebula(1600 ly away)

center of the nebula) are less than 2 million years old.

Phenomena Resisting Gravitational Collapse of a Cloud Fragment

• Rotation: In the rotating cloud’s frame of reference, there is a centrifugal force pulling the cloud material outward.

• Thermal Motion: The random motions of atoms and molecules exert a pressure that resists collapsethat resists collapse.

• Magnetic Fields: Ionized atoms falling inward are deflected by the magnetic field which thus opposes gravity. Neutral atoms collide with the ions, so their fall is also obstructed. The magnetic field thus exerts a pressure opposing collapse.

• Turbulence: irregular motions, including whirlpools of gas, oppose collapse.

Phenomena that Produce Shock Waves can Cause Gravitational Collapse of Cloud Fragmentsp g

• Supernova Explosions• Stellar Wind from a Hot, young Star• Radiation Pressure from a Hot, young Star• Cloud Collisions• Spiral Arms of the Galaxy

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Overview of Star Formation

Collapsing cloud fragment

Protostar (converts gravitational energy to heat and light)

Main Sequence Star (generates light and heat by fusion of H to He)

As material surrounding the young star falls toward it, it is channeled along magnetic field lines and heated by the conversion of gravitational

i h d li henergy into heat and light.

After crashing onto the surface near the magnetic poles, much of it is ejected and “focused” by the magnetic field and the pressure of the accretion disk to form a jet of ionized gas emerging from each pole.

450 ly away in Taurus

top of disk

bottom of disk1500 ly away near Orion nebula

Bipolar Jets in Young Stars

1,000 AU

1500 ly away near the Gum nebula

The scale shown in the lower left corner of each image is 1000 AU in each case.

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Properties of an Ideal Gas

The material in a main sequence star is so hot that, even in the core where the density is more than 100 times the density of water, it behaves like an ideal gas. When the atomic nuclei collide (and fusion doesn’t occur) they just bounce without sticking

An ideal gas is a gas of particles that don’t stick to one another when they collide.

A. Ideal Gas Equation of State: The pressure P (the force on a square meter of surface), number of particles per unit volume n, and absolute temperature T are related by the formula PV = NkT. k is a constant.

B. Virial Theorem: When a star contracts, its gravitational potential

atomic nuclei collide (and fusion doesn t occur), they just bounce without sticking. Because of this, you must understand the following properties of an ideal gas in order to understand the behavior of a main sequence star.

energy decreases; half of the lost GPE increases the star’s temperature and half is converted into electromagnetic energy.

C. When an isolated gaseous region expands , its temperature drops.D. When an isolated gaseous region contracts, its temperature rises.E. The average kinetic energy of the particles of a gas is proportional to

its temperature.

Hydrostatic Equilibrium

When all of the forces on an object cancel each other so that the net force is zero, and the object (according to Newton’s first law of motion) is not accelerated, the object is said to be in equilibrium.

When the net force on the parts of a fluid (gas or liquid) is zero the fluid is said to beWhen the net force on the parts of a fluid (gas or liquid) is zero, the fluid is said to be in hydrostatic equilibrium.

A main sequence star is approximately in a state of hydrostatic equilibrium. If we imagine it to be divided into shells that fit together to form a continuous sphere, the net force on each shell is approximately zero.

The white arrows in the figure represent the outward pressure force on the shell. The green arrows represent the force of

i h h ll d ll f h i id igravity on the shell due to all of the mass inside it.

If something causes the pressure to drop, the force of gravity is then greater than the pressure force. The shell accelerates inward.

If something causes the pressure to rise, the pressure force is then greater than the force of gravity. The shell accelerates outward.

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Principles Responsible for the Stability of a Main Sequence Star

A. The rate of energy production by fusion in the star’s core increases with gy p ytemperature.

B. The absolute temperature is proportional to the average kinetic energy of the particles of the gas.

C. When the star expands, its gravitational energy increases. The law of conservation of energy and the virial theorem require that this increase be compensated for by a decrease of some other energy. In this case, the average kinetic energy of the atoms decreases and photons are absorbed. Since temperature is proportional to the average kinetic energy of the p p p g gynuclei, the expansion of the star is accompanied by cooling.

D. When the star contracts, its gravitational energy decreases. In this case, the average kinetic energy of the atoms increases and photons are emitted. Therefore, the star heats up.

The Stability of a Main Sequence Star

As we’ve seen on the surface of the Sun, a star is in continual turmoil. Pressure, temperature, and volume of a star are continually fluctuating. Why doesn’t the star collapse or explode?

Consider a shell of material in the core. Suppose that the shell is initially in hydrostatic equilibrium and there is a random increase in the rate of energy production in the shell.

The increased energy production rate increases the temperature of the shell. Since the pressure is proportional to the temperature, this results in a pressure increase.

Because of the increased pressure, the shell is no longer in hydrostatic equilibrium. The outward pressure force is greater than the inward force of gravity, so the shell expands.

The expansion of the shell causes its temperature to drop.

The lower temperature slows the rate of energy production. As a result of the lower pressure, the shell contracts.

The contraction causes the temperature to increase, which causes the rate of energy production to increase.

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Why is there a mass-luminosity relation?

A protostar contracts until it is in hydrostatic equilibrium. The pressure force on any shell is then balanced by the force of gravity on that shell. The higher the mass of the star, the stronger the force of gravity and the higher the pressure required for hydrostatic equilibrium.

Since pressure is proportional to temperature, the higher pressure needed by high mass stars implies that they also have higher temperatures and therefore a higher rate of energy production (luminosity).

The observed mass luminosity relation is approximately given by L ∝ M3.5. The amount of mass converted into energy is proportional to M. The lifetime of a star is therefore related to mass by

.1MLEt 5253 =∝∝

MML 5.25.3

If we use solar mass units and accept the value 1010 years as the main sequence lifetime of the Sun, then

.yearsM10t 5.2

10= Example: the lifetime of a 16 solar mass

star is10 10

72.5 5

10 10t 10 years16 4

= = ≈

Why is the main sequence a band rather than a line?

For an ideal gas, PV = NkT. Solving this for P, we get .V

NkTP =

Every time 4 protons fuse to form 1 helium nucleus, the number N of nuclei in the core decreases without an immediate change in V and with little immediate change in T. Therefore the pressure decreases and the outward pressure force no longer balances the inward force of gravity.

Since the gravitational force is now greater than the pressure force, the core of the star contracts.

This contraction heats up the core and increases the rate of energy production and therefore the luminosity of the star. The star moves upward away from the zero age main ity

→

Schematic HR Diagram

The star moves upward away from the zero age main sequence.

The increased luminosity of the core increases the radiation pressure on the outer layers of the star. This causes them to expand and (if M ≤ 1 solar mass) get cooler. Thus, the star moves to the right away from the zero age main sequence. Temperature →

Lum

inos

i

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Hydrogen Fusion in Main Sequence Stars

A star spends about 90% of its life as a main sequence star. This phase of its life is characterized by the production of energy by the fusion of 1H1 into 2He4. Stars with masses between 0.08 and 1.1 times the mass of the Sun do this by means of the proton – proton chain, which was described in the lecture on the Sun. Although stars more massive than this also fuse hydrogen by this means, most of their energy is due to a more complicated y g y , gy pprocess called the CNO cycle. The net effect of one round of the CNO cycle is to covert 4 protons into one helium nucleus, but there are a number of intermediate steps that involve the use of 6C12 nuclei as a catalyst.

12 1 136 1 7C H N+ → + γ

13 137 6 eN C e+→ + + ν

1 1 21 1 1 eH H H e++ → + + ν

2 1 31 1 2H H He+ → + γ

13 1 146 1 7C H N+ → + γ

14 1 157 1 8N H O+ → + γ

15 158 7 eO N e+→ + + ν

15 1 4 127 1 2 6N H He C+ → +

1 1 2H H He+ → + γ

3 3 4 1 12 2 2 1 1He He He H H+ → + +

4rate cT=

20rate cT=

Energy Transport and the Internal Structure of a Star

• Radiation - photons transfer energy• Convection - hot gas has a low density so it rises;

cool gas has a high density, so it sinks• Conduction - particles of a hot material collide with

particles of a cooler material, transferring kinetic energy to them.

• High mass stars have convective cores, surrounded by a radiative zone.

• Medium mass stars have radiative cores, surrounded by a convective zone.

• Low mass stars are completely convective.