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 Can our knowledge of the Sun and stars guide us where to look for planets?

About Stars  Basic Properties of Stars  temperature  diameter  brightness  The Hertzsprung-Russell Diagram

Familiar Stars

The Nearest Stars

1000 ly A little farther out

Properties of Stars We can’t see the stars’ diameters through a telescope. Stars are so far away that we see 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’s temperature? 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

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?

The Brightest Stars in the Sky (no need to copy these down!) Star Distance (LY) Temperature (K) Absolute Magnitude Sun Sirius Canopus Alpha Cen A Arcturus Vega Capella Rigel Procyon Achernar Betelgeuse Hadar Acrux Altair Aldebaran Antares Spica Pollux

Plot Absolute Magnitude vs. Temperature An HR Diagram for the Solar Neighborhood

The Nearest Stars Star Distance (LY)Temperature Absolute Magnitude Prox Cen Alp Cen A Alp Cen B Barnard’s Wolf Lal Sirius A Sirius B Luyten 726-8A UV Ceti Ross

Plot Absolute Magnitude vs. Temperature

Adding the Nearest Stars to the HR Diagram

The HR Diagram Giants and Supergiants White Dwarf Main Sequence The most common stars in the Solar Neighborhood are dim and cool

Luminosity: L Sun Temperature: 3,000 K - 50,000 K Mass: M Sun Summarizing Stellar Properties

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

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

What are the typical masses of newborn stars? Observations show that star formation makes many more low-mass stars than high-mass stars

Why does the Sun Shine? Nuclear fusion reactions Hydrogen fuses into helium Mass converted to energy Luminosity ~ 10 billion years Nuclear Potential Energy (core)

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

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?

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

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

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

Mass & Lifetime Sun’s life expectancy: 10 billion years Life expectancy of 10 M Sun star: 10 times as much fuel, uses it 10 4 times as fast 10 million years ~ 10 billion years x 10 / 10 4 Life expectancy of 0.1 M Sun 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

Main-Sequence Lifetimes High Mass: High Luminosity Large Radius Blue Short-Lived Low Mass: Low Luminosity Small Radius Red Long-Lived

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.

Star Clusters Stellar Evolution in Action Stars in clusters tell us about stellar evolution Star clusters tell us the ages of stars

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

Constructing a Star Cluster HR Diagram We measure the brightness and temperature of each star in the cluster.

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

The slope of the spectrum is different at different temperatures

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

The HR diagrams of clusters of different ages look very different

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

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

We can determine ages from the “color” of the main sequence “turnoff”

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

How Old Are the Jewels? Create a color-magnitude diagram of the Jewelbox and estimate its age

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 Planetary Nebula 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

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”

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

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

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

Iron builds up in core until degeneracy pressure can no longer resist gravity Core then suddenly collapses, creating supernova explosion

Is life on Earth safe from harm caused by supernovae? Earth is safe at the present time because there are no massive stars within 50 light years of the Sun. But other types of supernovae are possible…

What stars might be good hosts for life?  Low mass stars –long-lived –stable  More massive stars –short lives –often variable