Stellar Evolutionup to the Main Sequence

Stellar EvolutionRecall that at the start we made a

point that all we can "see" of the stars is:

• Brightness• Color (Spectra)• Position• Distance (if we are lucky or clever)

Let's see if there are any correlations

Stellar EvolutionUsing distance (when we know it) we can

convert the Brightness (apparent magnitude) into the absolute magnitude, or even the Luminosity

To make things easy we can write the luminosity relative to that of the Sun, L/L

Stellar EvolutionThe Color, or spectra, we can convert to

– A Spectral Class – A Temperature– A B-V value

• V is the visible magnitude• B is the magnitude as seen on photographic

plates– Photographic plates are more sensitive to blue light –

blue stars will appear brighter• B-V gives a numerical "Color" index• For comparison

– the yellowish Sun (G2) has a B-V index of 0.656 and a surface temperature of about 6000K

– the bluish Rigel (B8) has B-V index of -0.03 and a surface temperature of about 11000K

Stellar EvolutionWe can plot the Luminosity ratio versus the color:

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The Sun would go here

Stellar EvolutionThis plot was independently

discovered by Hertzsprung and Russell

It is now called the Hertzsprung-Russell, or H-R, Diagram

Ejnar Hertzsprung (1873-1967) Henry Norris Russell (1877-1957)

The HR Diagram

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The Sun

The 50 Nearest Stars

The HR Diagram

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The 50 Brightest Stars

The HR Diagram

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The HR Diagram

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There appears to be three main areas where the stars are grouped

The HR Diagram

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This curve is where 90% of the stars appear

The HR Diagram

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These are pretty dim, but also very hot…white hot

This implies that they are very small

The HR Diagram

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These are cool, but very bright - the size must be huge

HR Diagram

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White Dwarfs

Blue Giants

Main Sequence

Red Dwarfs

Starbirth

Protostars form in cold, dark nebulae

The ‘central Star’ in Orion’s sword is a stellar nursery

Evidence of Star Formation• forming stars are usually embedded in clouds

Distant dark nebulae are hard to observe, because they do not emit visible light

However, dark nebulae can be detected using microwave observation, because the molecules in nebulae emit at millimeter wavelengths

Giant molecular clouds are immense nebular so cold that their constituent atoms can form molecules.

Giant molecular clouds are found in the spiral arms of our Galaxy.

Giant Molecular Clouds

Star-forming regions appear when a giant molecular cloud is compressed

This can be caused by the cloud’s passage through one of the spiral arms of our Galaxy, by a supernova explosion, or by other mechanisms

Giant Molecular Clouds and Star-forming Regions

Molecular Clouds

DisorderlyComplex

Giant Molecular Cloud in OrionInfrared view

From IRAS satellite

Molecular or Dark Clouds

"Cores" and Outflows

Stages of Star Formation

Jets and Disks

Extrasolar System

1 pc

Protostar:PROTOSTAR

SHRINKS AND HEATS UP

PROTOSTAR BEING ASSEMBLED

NUCLEAR FUSION STARTS

“pre-star” without nuclear fusion

Stages of Starbirth# t to Next

Stage (yr)

Core Temp.

(K)

Surface Temp

(K)

Diameter(Km)

Description

1 2,000,000 10 10 100,000,000,000,000

Interstellar Gas Cloud

2 30,000 100 10 1,000,000,000,000 Cloud Fragment

3 100,000 10,000 100 10,000,000,000 Cloud Fragment

4 1,000,000 1,000,000 3,000 100,000,000 ProtoStar

5 10,000,000 5,000,000 4,000 10,000,000 ProtoStar

6 30,000,000 10,000,000 4,500 2,000,000 Star

7 10,000,000,000 15,000,000 6,000 1,500,000 Main Sequence

Lifetime on the Main Sequence

• Luminosity basically describes how fast the star is ‘burning’ its fuel.

• This is clearly related to how much fuel there is because the greater the mass the higher the pressures and temperatures:

L M3

• Lifetime is “how much fuel / how fast it’s used”

T = M/L 1/M2

Lifetime on the Main Sequence

Here are some comparison values:Mass(Msun)

Lifetime(Tsun)

Lifetime(years)

100 0.0001 1 million

10 0.01 100 million

1 1 10 billion

0.1 100 1 trillion

Main Sequence Stars

TemperatureLu

min

osity highest mass

lowest mass

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SUN

SPICA:

107 yr lifetime

PROXIMA CENTAURI

lifetime greater than age of universe

1010 yr lifetime

The Path to the Main Sequence

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The T Tauri phase

The T Tauri phase• Gravity causes the gas/dust cloud to condense. • The situation then usually becomes quite complex

• Some of the infalling gas is heated so much by collisions that it is immediately expelled as an outgoing wind.

• Jets and disks form as the infalling and outflowing gas collide and interact with changing magnetic fields.

• Temperatures and masses are similar to the Sun, but they are brighter

• They have fast rotation rates (few days)• Variable X-ray and radio emission• Not yet a 'star', but will be in a few million years

During the birth process, stars both gain

and lose massIn the final stages of pre–main-sequence contraction, when

thermonuclear reactions are about to begin in its core, a protostar may eject large amounts of gas into space

Low-mass stars that vigorously eject gas are called T Tauri stars (age ~ 1 million year)

A Magnetic Model for Jets (Bipolar Outflow)

Jets: A circumstellar accretion disk provides material that a young star ejects as jets

Jets: Clumps of glowing gas are sometimes found along these jets and at their ends

Known as Herbig-Haro Objects

Starbirth in NCG 281

M16 in Infrared

Bok Globules

Bok GlobulesNote how spherical some have become

The Life of a StarGAS CLOUD

PHASES: WHAT CHANGES THINGS:

PROTOSTAR

gravity pulls part of cloud together

MAIN SEQUENCE STAR

nuclear reactions begin in star’s core