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The Suns of Other Worlds

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The Suns of Other Worlds. What stars might be suitable planet hosts? How does the evolution stars affect the origin and evolution of life?. IU Astrobiology Workshop, June 2006 Caty Pilachowski, IU Astronomy. The Sun Today. Image credit: Solar Orbiting Heliospheric Observatory/MDI - PowerPoint PPT Presentation
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The Suns of The Suns of Other Worlds Other Worlds What stars might be suitable planet hosts? How does the evolution stars affect the origin and evolution of life? IU Astrobiology Workshop, June 2006 Caty Pilachowski, IU Astronomy
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Page 1: The Suns of Other Worlds

The Suns of The Suns of Other Other WorldsWorldsWhat stars might be suitable planet

hosts?How does the evolution stars affect

the origin and evolution of life?

IU Astrobiology Workshop, June 2006Caty Pilachowski, IU Astronomy

Page 2: The Suns of Other Worlds

The Sun

Today

Image credit: Solar Orbiting Heliospheric Observatory/MDI www.spaceweather.com

Can our knowledge of the Sun and

stars guide us where to

look for planets?

Page 3: The Suns of Other Worlds

AboutStars

Basic Properties of Stars temperature diameter brightness

The Hertzsprung-Russell Diagram

Page 4: The Suns of Other Worlds

Familiar Stars

Page 5: The Suns of Other Worlds

The Nearest Stars

Page 6: The Suns of Other Worlds

1000 ly

A little farther out

Page 7: The Suns of Other Worlds

Properties of Stars

We can’t see the stars’diameters through a telescope.Stars are so far away that wesee them just as points of light.

If we know a star’s temperature and its luminosity, we can calculate its diameter.

How do we determine a star’stemperature?

Luminosity depends on….

TEMPERATURE -the hotter a star is,the brighter it is.

DIAMETER –the bigger a star is,the brighter it is.

Stars range in size from about the diameter of Jupiter to hundreds of times the Sun’s diameter

Page 8: The Suns of Other Worlds

The Nearest and the Brightest

Exploring the Solar Neighborhood – What types of stars do we see in the sky?

Exploring the HR Diagram:– How do our familiar stars fit into a

Hertzsprung-Russell diagram?– What about the nearest stars?

Page 9: The Suns of Other Worlds

The Brightest

Stars in the Sky

(no need to copy these

down!)

StarDistance

(LY)Temperature

(K)Absolute

Magnitude

Sun  0.000015 5800 4.8

Sirius  9 9600 1.4

Canopus  232 7600 -2.5

Alpha Cen A  4 5800 4.4

Arcturus  37 4700 0.2

Vega  25 9900 0.6

Capella  42 5700 0.4

Rigel  773 11000 -8.1

Procyon  11 6600 2.6

Achernar  144 22000 -1.3

Betelgeuse  427 3300 -7.2

Hadar  335 25000 -4.4

Acrux  321 26000 -4.6

Altair  17 8100 2.3

Aldebaran  65 4100 -0.3

Antares  604 3300 -5.2

Spica  263 2600 -3.2

Pollux  34 4900 0.7

Page 10: The Suns of Other Worlds

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Page 11: The Suns of Other Worlds

The Nearest Stars

StarDistance

(LY) TemperatureAbsolute

Magnitude

Prox Cen 4  2800 15.53

Alp Cen A 4  5800 4.4

Alp Cen B 4  4900 5.72

Barnard’s 6 2800 13.23

Wolf 359 7.5  2700 16.57

Lal 21185  8 3300 10.46

Sirius A  9 9900 1.45

Sirius B  9 12000 11.34

Luyten 726-8A  9 2700 15.42

UV Ceti  9 2600 15.38

Ross 154  10 3000 13.14

Page 12: The Suns of Other Worlds

Plot Absolute

Magnitude vs.

Temperature

Hertzsprung Russell Diagram - Brightest Stars

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Page 13: The Suns of Other Worlds

Adding the

Nearest Stars to the HR Diagra

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Hertzsprung Russell Diagram

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Page 14: The Suns of Other Worlds

The HRDiagram

Hertzsprung Russell Diagram

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Giants andSupergian

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White Dwarf

MainSequence

The most common

stars in the Solar

Neighborhood are dim and cool

Page 15: The Suns of Other Worlds

Luminosity:10-4 - 106 LSun

Temperature: 3,000 K - 50,000 K

Mass:0.08 - 100 MSun

Summarizing Stellar Properties

Page 16: The Suns of Other Worlds

The Sun is an ordinary main sequence star

Only certain sizes and colors are allowed• Most stars fall on the “main sequence”• Main sequence stars are fusing hydrogen into

helium in their cores

Page 17: The Suns of Other Worlds

Main-sequence stars like the Sun are fusing hydrogen into helium in their cores

•Massive main-sequence stars are hot (blue) and luminous•Less massive stars are cooler (yellow or red) and fainter

The mass of a main sequence star determines its luminosity and temperature

Page 18: The Suns of Other Worlds

What are the typical masses of newborn stars?

Observations show that star formation makes many more low-mass stars than high-mass stars

Page 19: The Suns of Other Worlds

Why does the Sun Shine?

• Nuclear fusion reactions • Hydrogen fuses into helium• Mass converted to energy

Luminosity~ 10 billion years

Nuclear Potential Energy (core)

Page 20: The Suns of Other Worlds

How does

nuclear fusion

occur in the Sun?

• The core’s extreme temperature and density are just right for nuclear fusion of hydrogen to helium through the proton-proton chain

• Gravitational equilibrium acts as a thermostat to regulate the core temperature because fusion rate is very sensitive to temperature

Page 21: The Suns of Other Worlds

•Neutrinos created during fusion fly directly out of the Sun•These neutrinos can be detected on Earth

How do we know nuclear reactions are going on in the Sun?

Page 22: The Suns of Other Worlds

Gravitational contraction:Provided energy that heated core as Sun was forming

Contraction stopped when fusion began

Gravitational equilibrium:Energy provided by fusion maintains the pressure

Balancing Gravity

Page 23: The Suns of Other Worlds

Solar Thermostat – STABILITY!

Decline in core temperature causes fusion rate to drop, so core contracts and heats up

Rise in core temperature causes fusion rate to rise, so core expands and cools down

Page 24: The Suns of Other Worlds

Stellar Mass and Fusion• The mass of a main sequence star

determines its core pressure and temperature

• Stars of higher mass have higher core temperature and more rapid fusion, making those stars both more luminous and shorter-lived

• Stars of lower mass have cooler cores and slower fusion rates, giving them smaller luminosities and longer lifetimes

Page 25: The Suns of Other Worlds

Mass & LifetimeSun’s life expectancy: 10 billion years

Life expectancy of 10 MSun star:

10 times as much fuel, uses it 104 times as fast

10 million years ~ 10 billion years x 10 / 104

Life expectancy of 0.1 MSun star:

0.1 times as much fuel, uses it 0.01 times as fast

100 billion years ~ 10 billion years x 0.1 / 0.01

Page 26: The Suns of Other Worlds

Main-Sequence LifetimesHigh Mass:

High Luminosity Large Radius Blue Short-Lived

Low Mass:

Low Luminosity Small Radius Red Long-Lived

Page 27: The Suns of Other Worlds

Explaining the HR Diagram

Energy

Gravity

Energy Transport

During hydrogen burning, basic physics forces a star to lie on the main sequence. A star’s position on the MS depends on its mass.

Page 28: The Suns of Other Worlds

Star Clusters

Stellar Evolution in ActionStars in clusters tell us about stellar evolutionStar clusters tell us the ages of stars

Page 29: The Suns of Other Worlds

Star Clusters and Stellar Lives

• Our knowledge of the life stories of stars comes from comparing mathematical models of stars with observations

• Star clusters are particularly useful because they contain stars of different mass that were born about the same time

Page 30: The Suns of Other Worlds

Constructing a Star Cluster HR Diagram

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Ap

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We measure the brightness and temperature of each star in the cluster.

Page 31: The Suns of Other Worlds

What’s this B-V color?

• Astronomers measure the brightness of stars in different colors– Brightness measured in blue light is called “B”

(for “Blue”)– Brightness measured in yellow light is called “V”

(for “Visual)

• Astronomers quantify the “color” of a star by using the difference in brightness between the brightness in the B and V spectral regions

• The B-V color is related to the slope of the spectrum

Page 32: The Suns of Other Worlds

The slope of the spectrum is different at different temperatures

Page 33: The Suns of Other Worlds

Cluster HR Diagrams

Hotter stars are brighter in blue light than in yellow light, and have low values of B-V color, and are found on the left side of the diagram.

Cooler stars are brighter in yellow light than in blue light, have larger values of B-V color, and are found on the right side of the diagram.hotter cooler

Page 34: The Suns of Other Worlds

The HR diagrams of clusters of

different ages look very different

Jewelbox

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Page 35: The Suns of Other Worlds

Ages of Star Clusters

The “bluest” stars left on the main sequence of the cluster tell us the cluster’s age.

As the cluster ages, the bluest stars run out of hydrogen for fusion and lose their “shine”

hotter cooler

Page 36: The Suns of Other Worlds

Main Sequence Turnoffs of Star Clusters

Burbidge and Sandage 1958, Astrophysical Journal

Here we see a series of HR diagrams for sequentially older star clusters that have been superimposed

Page 37: The Suns of Other Worlds

We can determine ages fromthe “color” of the main

sequence “turnoff”

Page 38: The Suns of Other Worlds

The Jewels of

the Night

NGC 4755 is an open star cluster in the southern constellation Crux

It is popularly known as the Jewel Box because an early catalog described it as a "superb piece of jewelry“

Distance ~7500 light years

Image from the Cerro Tololo Inter-

American Observatory's 0.9-

meter telescope

Page 39: The Suns of Other Worlds

How Old Are the Jewels?• Create a color-magnitude

diagram of the Jewelbox and estimate its age

Page 40: The Suns of Other Worlds

The End of Solar-type Stars

When the carbon core reaches a density that is high enough, the star blows the rest of its hydrogen into space.

Main Sequence Red

Giant

PlanetaryNebula

White Dwarf

The hot, dense, bare core is exposed!

Surface temperatures as hot as 100,000 degrees

The hot core heats the expelled gas and makes it glow

Page 41: The Suns of Other Worlds

Planetary Nebulae

• Fusion ends with a pulse that ejects the H and He into space as a planetary nebula

• The core left behind becomes a “white dwarf”

Page 42: The Suns of Other Worlds

Earth’s Fate

• Sun’s radius will grow to near current radius of Earth’s orbit

Page 43: The Suns of Other Worlds

Earth’s Fate

• Sun’s luminosity will rise to 1,000 times its current level—too hot for life on Earth

Page 44: The Suns of Other Worlds

What about Massive Stars?

• Massive stars continue to generate energy by nuclear reactions until they have converted all the hydrogen and helium in their cores into iron.

• Once the core is iron, no more energy can be generated

• The core collapses and the star explodes

Page 45: The Suns of Other Worlds

Iron builds up in core until degeneracy pressure can no longer resist gravity

Core then suddenly collapses, creating supernova explosion

Page 46: The Suns of Other Worlds

Is life on Earth safe from harm caused by supernovae?

Earth is safe at the present timebecause there are no massive stars

within 50 light years of the Sun.

But other types of supernovae are possible…

Page 47: The Suns of Other Worlds

What stars

might be good

hosts for life?

Low mass stars– long-lived– stable

More massive stars– short lives– often variable


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