The Hertzsprung-Russell Diagram The HR diagram separates The effects of temperature And surface area...

The Hertzsprung-Russell The Hertzsprung-Russell DiagramDiagram

The HR diagram separatesThe effects of temperatureAnd surface area on stars’Luminosity and sorts theStars according to their size


The HR diagram separates

The effects of temperature

And surface area on stars’

Luminosity and sorts the

Stars according to their size


The Main Sequence

- all main sequencestars fuse H into Hein their cores

- this is the definingcharacteristic of a main sequence star.


Red Giants

- Red Giant starsare very large, cooland quite bright.

Ex. Betelgeuse is100,000 times moreluminous than the Sunbut is only 3,500K onthe surface. It’s radiusis 1,000 times that of theSun.


White Dwarfs

- White Dwarfsare hot but sincethey are so small,they are not veryluminous.


Size of Star

Mass ofStar

The HR diagram separates

The effects of temperature

And surface area on stars’

Luminosity and sorts the

Stars according to their size

Mass-Luminosity relationMass-Luminosity relation Most stars appear on the Main Sequence, where

stars appear to obey a Mass-Luminosity relation: L M3.5

For example, if the mass of a star is doubled, its luminosity increases by a factor 23.5 ~ 11.

Thus, stars like Sirius that are about twice as massive as the Sun are about 11 times as luminous.

The more massive a Main Sequence star is, the hotter (bluer), and more luminous.

The Main Sequence is a mass sequence!

Review QuestionsReview Questions

1.1. What is the Hertzsprung-Russell What is the Hertzsprung-Russell Diagram?Diagram?

2.2. Why are most stars seen along the Why are most stars seen along the so-called main sequence?so-called main sequence?

3.3. What makes more massive stars What makes more massive stars hotter and brighter?hotter and brighter?

To calculate a star's radius, you must know its

1) temperature and luminosity.

2) chemical composition and temperature.

3) color and chemical composition.

4) luminosity and surface gravity.

L=4πR2 σT4

3) 4 times larger

4) the same

1) ½ times as large

2) ¼ times as large

If a star is half as hot as our Sun, but has the same luminosity, how large is its radius compared to the Sun?

L=4πR2 σT4

What is burning in stars?What is burning in stars?

Gasoline Gasoline Nuclear fissionNuclear fission Nuclear fusionNuclear fusion Natural gas Natural gas

Stars More in Depth:To fully characterize stars we

need to know their Four Basic

ParametersLuminositySizeMassSurface Temperature

Measurements of Star PropertiesMeasurements of Star Properties

Apparent brightness DistanceLuminosity

TemperatureRadius

Direct measurentParallaxDistance + apparent brightness( L=4D2 f)Spectral type (or color)Luminosity + temperature(L=4R2 T4)

Luminosity and temperature are the two independent intrinsic parameters of stars.

Mass: how do you weigh a Mass: how do you weigh a star?star?

Mass is the single most important property in Mass is the single most important property in how a star’s life and death will proceed.how a star’s life and death will proceed.

We can “weigh” stars that are in binary We can “weigh” stars that are in binary systems (two stars orbiting each other). systems (two stars orbiting each other). Fortunately, most stars fall into this category.Fortunately, most stars fall into this category.

Most stars in binary systems have a mass that Most stars in binary systems have a mass that is very similar to its companion … we’ll see is very similar to its companion … we’ll see why this is soon!why this is soon!

Big p = Small MassesSmall p = Big Masses

ra

Star A

Star B

Binary Stars

-Each star in a binary system moves in its own orbit around the system's center of mass.

-Kepler’s Third Law: the orbital period depends on the relative separation and the masses of the two stars:

p2 = 4π2

G(M1+M2)a3

rb

Ma/Mb = rb/ra

Center of mass(or baricenter)

I. Visual Binaries

Spectroscopic binaries: Doppler Shift tells if it is moving toward or away

1. The total spread (size) of the Doppler shift gives velocities about center of mass (gives orbit sizes, rA+rB )

2. The time to complete one repeating pattern gives period, P

1

2

3

4

5

Eclipsing Binaries:Best binaries to measure mass

Classification of Stars: the H-R Classification of Stars: the H-R diagramdiagram

1) Collect information ona large sample of stars.

2) Measure their luminosities(need the distance!)

3) Measure their surface temperatures(need their spectra)

The Interstellar Medium

Assigned ReadingAssigned Reading

Chapter 10Chapter 10

A World of Dust and GasA World of Dust and Gas

We are interested in the interstellar medium because

a) dense interstellar clouds are the birth place of stars

b) Dark clouds alter and absorb the light from stars behind

them

The space between the stars is not completely empty, but filled with

very dilute gas and dust, producing some of the most beautiful objects

in the sky.

Bare-Eye Nebula: OrionBare-Eye Nebula: OrionOne example of an

interstellar gas cloud (nebula) is visible to the

bare eye: the Orion nebula

The ISMThe ISM

Space between stars not emptySpace between stars not empty Gas, dust; Gas is mostly Hydrogen (80%) and Helium Gas, dust; Gas is mostly Hydrogen (80%) and Helium

(20%)(20%) Physical status of the gas characterized by: Physical status of the gas characterized by:

TemperatureTemperature DensityDensity Chemical compositionChemical composition

ISM and stars are the components of the ISM and stars are the components of the ““machinemachine”” that makes the universe evolve: the cycle of star that makes the universe evolve: the cycle of star formation and death, and the chemical enrichment formation and death, and the chemical enrichment of the cosmos.of the cosmos.

ISM also ISM also ““disturbsdisturbs”” observations, since it absorbs observations, since it absorbs light and modifies light and modifies (reddens) (reddens) colors colors

Red light can more easily penetrate the cloud, but is still absorbed to some extent.

Interstellar ReddeningInterstellar ReddeningBlue light is strongly scattered and

absorbed by interstellar clouds.

Infrared radiation is hardly absorbed at all.

Interstellar clouds make background stars appear redder.

Visible

Barnard 68

Infrared

Interstellar Reddening (2)Interstellar Reddening (2)

The Interstellar Medium absorbs light more strongly at shorter wavelengths.

Interstellar Reddening (3)Interstellar Reddening (3)

Nebulae that appear as dark nebulae in the optical, can shine brightly in the infrared due to

blackbody radiation from the warm dust.

The ISM Main Components The ISM Main Components (Phases)(Phases)

PhasePhase DustDust

Present in all phasesPresent in all phases ““MetalsMetals””

Everything that is not Everything that is not hydrogen or Helium is a hydrogen or Helium is a metalmetal

HI CloudsHI Clouds Inter-cloud MediumInter-cloud Medium Coronal GasCoronal Gas Molecular clouds Molecular clouds

This what forms starsThis what forms stars

T (K) Density T (K) Density a/cma/cm33

20-100 size: a few 20-100 size: a few mm

50-500 1-100050-500 1-1000 101033-10-1044 0.01 0.01 101055-10-1066 10 10-4-4-10-10-3-3

20-50 1020-50 1033-10-1055

How did a star form?How did a star form? A cloud of hydrogen gas began to gravitationally A cloud of hydrogen gas began to gravitationally

collapse.collapse. To start the collapse, the gas needs to loose To start the collapse, the gas needs to loose

pressure, i.e. needs to become coldpressure, i.e. needs to become cold If the gas does not cool, it cannot collapseIf the gas does not cool, it cannot collapse

It also needs to become dense, i.e. to have more It also needs to become dense, i.e. to have more gravitygravity

As more gas fell in, itAs more gas fell in, it’’s potential energy was s potential energy was converted into thermal energy.converted into thermal energy. As it collapses, the gas gets hotter and hotterAs it collapses, the gas gets hotter and hotter

Eventually the in-falling gas was hot enough to Eventually the in-falling gas was hot enough to ignite nuclear fusion in the core.ignite nuclear fusion in the core.

Gas that continued to fall in helped to establish Gas that continued to fall in helped to establish gravitational equilibrium with the pressure gravitational equilibrium with the pressure generated in the core.generated in the core.

Conservation of Angular Momentum (L)L is conserved during the collapse: if R decreases, T has to decrease, too,

to keep L constantOnly, the rate of change of T is faster to keep pace with the rate of

change of R2 (because of the second power)The collapsing object spins up rapidly during the collapse

Molecular cloud

Molecular cloud

Cool molecular cloudsgravitationally collapseto form clusters of stars

Stars generatehelium, carbonand iron throughstellar nucleosynthesis

The hottest, mostmassive stars in thecluster supernova –heavier elements areformed in the explosion.

New (dirty) molecularclouds are leftbehind by thesupernova debris.

The Stellar CycleThe Stellar Cycle

The Four Components of the The Four Components of the Interstellar MediumInterstellar Medium

Component Temperature [K]

Density [atoms/cm3]

Main Constituents

HI Clouds 50 – 150 1 – 1000 Neutral hydrogen; other atoms ionized

Intercloud Medium (HII)

103 - 104 0.01 Partially ionized H; other atoms

fully ionized

Coronal Gas 105 - 106 10-4 – 10-3 All atoms highly ionized H

Molecular Clouds 20 - 50 103 - 105 Neutral gas; dust and

molecules

Interstellar Absorption LinesInterstellar Absorption LinesThe interstellar medium

produces absorption lines in the spectra of stars.

These can be distinguished from stellar absorption

lines through:

a) Absorption from wrong ionization

statesNarrow absorption lines from Ca II: Too low

ionization state and too narrow for the O star in the background; multiple components

b) Small line width (too low

temperature; too low density)

c) Multiple components

(several clouds of ISM with different radial velocities)

Observing Neutral Hydrogen:Observing Neutral Hydrogen:The 21-cm (radio) lineThe 21-cm (radio) lineElectrons in the ground state of neutral hydrogen have

slightly different energies, depending on their spin orientation.

Magnetic field due to proton spin

Magnetic field due to electron

spin

Opposite magnetic fields attract => Lower energy

Equal magnetic fields repel => Higher energy

21 cm line

The 21-cm Line of Neutral HydrogenThe 21-cm Line of Neutral Hydrogen

Transitions from the higher-energy to the lower-energy spin state produce a characteristic 21-cm

radio emission line.

=> Neutral hydrogen

(HI) can be traced by observing this radio emission.

Observations of the 21-cm LineObservations of the 21-cm Line

All-sky map of emission in the 21-cm line

G a l a c t i c p l a n e

Observations of the 21-cm LineObservations of the 21-cm Line

HI clouds moving towards Earth

(from redshift/blueshift of line)

HI clouds moving away from Earth

Individual HI clouds with different radial velocities resolved

Interstellar DustInterstellar Dust

Formed in the atmospheres of cool stars

Mostly observable through infrared emission

Spitzer Space Telescope (infrared) image of interstellar dust near the center of our Milky Way

(Right:) Infrared Emission from interstellar dust and gas molecules in the “Whirlpool Galaxy” M51.

Molecules in SpaceMolecules in SpaceIn addition to atoms and ions, the interstellar

medium also contains molecules.

Molecules also store specific energies in their

a) rotation

b) vibration

Transitions between different rotational / vibrational energy levels lead to emission

– typically at radio wavelengths.

The Most Easily Observed The Most Easily Observed Molecules in SpaceMolecules in Space

• CO = Carbon Monoxide Radio emission

• OH = Hydroxyl Radio emissionThe Most Common Molecule in Space:

Difficult to observe!

Use CO as a tracer for H2 in the ISM!

• H2 = Molecular Hydrogen Ultraviolet absorption and emission:

But: Where there’s H2, there’s also CO.

Molecular CloudsMolecular Clouds

• Molecules are easily destroyed (“dissociated”) by ultraviolet photons

from hot stars.

They can only survive within dense, dusty clouds, where UV radiation is completely absorbed.

“Molecular Clouds”:

Largest molecular clouds are called “Giant Molecular Clouds”:

Diameter ≈ 15 – 60 pcTemperature ≈ 10 K

Total mass ≈ 100 – 1 million solar masses

Cold, dense molecular cloud core

HI Cloud

UV emission from nearby stars destroys molecules in the outer parts of the cloud; is

absorbed there.

Molecules survive

Molecular Clouds (2)Molecular Clouds (2)

The dense cores of

Giant Molecular

Clouds are the birth

places of stars.

The Coronal GasThe Coronal Gas

Additional component of very hot, low-density

gas in the ISM:

n ~ 0.001 particles/cm3

Observable in X-rays

Called “Coronal gas” because of its

properties similar to the solar corona (but completely different

origin!)

Probably originates in supernova explosions and winds from hot stars

Our sun is located within (near the edge of) a coronal gas bubble.

T ~ 1 million K

The Gas-Star-Gas CycleThe Gas-Star-Gas CycleAll stars are constantly blowing gas out

into space (recall: Solar wind!)

The more luminous the star is, the stronger is its stellar wind.

These winds are particularly strong in aging red giant stars.

The Gas-Star-Gas CycleThe Gas-Star-Gas CycleStars, gas, and dust are in constant interaction with each other.

The Hertzsprung-Russell Diagram The HR diagram separates The effects of temperature And surface area...

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