Class events: week 11

1. Class events: week 11 Goals Learn about stars—a primer from basic astronomy Luminosity (and brightness) Distance Temperature Hertzprung-Russell diagram Radii Mass Life expectancies and life histories

2. Stars—basic parameters Luminosity The rate at which electromagnetic energy is emitted - the total amount of power emitted by a star over all wavelengths. Brightness How much energy we receive from a star. It is modified by a star’s distance. Distance Key in understanding stars. Parallax is the most direct measure, if it can be done. (100,000 stars to 1000 ly)

3. Stars—basic parameters Typical stellar temperatures 3000 K — 50,000 K Spectra Recall that the peak in a star’s continuum spectrum is determined by its temperature (Wein’s Law). Spectral lines Furthermore, recall that the presence of absorption lines and emission lines reveal the chemical composition of the luminous gas. Classification Based upon the strength of absorption lines, spectra of stars can be classified: OBAFGKM

4. The Hertzprung-Russell diagram The many parameters of stars are confusing. Astronomers discovered that order could be revealed by plotting two aspects of stars on a single graph. 1) Temperature—expressed directly in Kelvins, or by color, or by spectral type. 2) Luminosity—expressed directly in solar units (L), or in a system called absolute magnitude. The Hertzprung-Russell diagram is one of the most powerful tools stellar astronomers have in understanding stars.

5. The Hertzprung-Russell diagram L =4pR2sT4 An example of the HR diagram in use Recall that we know the temperatures of stars on the HR diagram. We also know the luminosities of stars—they range from 0.001—106 L. Note the Stefan-Boltzmann law: We can therefore determine the radii (sizes) of stars, and plot that on the HR diagram. Some stars are huge!

6. Binary stars Recap: we now know the following about stars: Luminosities Radii Temperatures Population distributions Most stars are not single stars such as our own—most occur in binary or multiple star systems. Some are just near each other in space, others orbit around each other.

7. Binary stars We can observe binary stars different ways. For examples… …Astrometry charts the orbits of stars directly, over time. …Spectroscopy is used to observe how the motions of the stars affect their spectra. The power of binary stars is that we can learn about the masses of stars. Once we know a star’s mass, and we know its luminosity, we can learn how long it will live (since a star is burning itself up, like a campfire). t=1010(M/L) yrs

8. Stars—a life of gravity vs. pressure support Star formation begins from interstellar material, which collapsed into dark nebulae. The lowest mass proto-stars never quite initiate nuclear fusion. These objects are called brown dwarfs. More massive objects settle onto the main sequence, wherethey burned hydrogen into helium. After burning helium into carbon, stars run out of fuel and collapse into white dwarf stars, producing beautiful planetary nebulae in the process.

9. Stars—a life of gravity vs. pressure support Stars more massive than our Sun (M=2—40 M) have cores that are so hot they can burn further elements, extending their lives a few percent. The most massive stars even attempt to burn the element iron. This results in a catastrophic core implosion—a supernova. If the star does not completely blow itself apart, it may remain as an extremely dense, compact object. This object may be a neutron star (such as a pulsar) or a black hole.

10. Stars—a life of gravity vs. pressure support Binary stars can have even more complicated lives. When the giant interstellar clouds fragment into stars, they tend to form many low-mass stars, a medium number of moderate-mass stars, and extremely few high-mass stars.

11. Studying stars from an astrobiological perspective Knowing what we do of stars, we can predict their rapid formation, long main sequence lifetimes, and speedy death processes. What are the astrobiological ramifications? O-B stars: ~0.1% of all stars Timescales Life spans of 0.5—50 million years, too short for the development of life (although possibly enough time for planetary formation with B stars). Radiation OB stars produce enormous amounts of sterilizing, ultraviolet radiation. Habitable zone Liquid water would be stable over an enormous range of distances.

12. Studying stars from an astrobiological perspective A-F stars: ~3% of all stars Timescales Life spans of 1—2 billion years, enough time for at least primitive life to form. Radiation Significant amounts of sterilizing ultraviolet radiation. Life would need to seek shelter under ice, rocks, or perhaps under a thick ozone layer that might form in response to the heavy ultraviolet irradiation. Habitable zone Very large compared to our Sun’s habitable zone.

13. Studying stars from an astrobiological perspective G stars: ~7% of all stars Timescales Life spans of 10 billion years, enough time for multicellular life to form? Radiation Moderate amounts of sterilizing ultraviolet radiation. Life must seek shelter under ice, rocks, or under a moderate ozone layer that might form in response to the moderate ultraviolet irradiation. Habitable zone Inner solar system.

14. Studying stars from an astrobiological perspective K-M stars: ~90% of all stars Timescales Life span of 20-600 billion years; 2-60 times the Sun’s lifespan! Radiation Sterilizing ultraviolet radiation produced in dangerous flares that may be blocked by a resultant ozone. Most of the radiation is produced at low energies (red, infrared) that does not readily power biological activity, at least as it occurs on Earth. Habitable zone Very small zone near the star solar system. Planets within this zone would be tidally locked with the star; a thick circulating atmosphere might be required to avoid the freeze-out of the atmosphere on the night side. This might be somewhat challenging to develop with a rotation period of 70 days (a=0.5a.u., M*=1/5 M). Summary K-M star planets, if habitable, might represent a huge reservoir of life that has had 10+ billions of years to develop, compared to 5 billion years for our Sun.

15. Studying stars from an astrobiological perspective L-T brown dwarf stars: presumably common? Timescales Life span not defined by the same standards, but they will stay warm for very long timescales! Radiation Very long wavelength optical and infrared radiation. Habitable zone No conventional habitable zone. Summary Habitable only in Europa-type conditions (i.e., sub-surface tidal heating).2M1207b is an example of a 12 Jupiter-mass planet orbiting brown dwarf 2M1207.

16. Are multiple star systems habitable? 60% of O-K star systems are multiple. Are orbits in such systems stable? If the planet has a small orbit around one star, where the stars are widely spaced—yes! If the stars are closely spaced, and the planet has a large orbit around both stars—yes! (Such planets, like Kepler 16b and Kepler 47(AB)b, have been discovered.) If the planet’s orbit is on the same size scale as the star’s orbits—no! Even if they are not, only about 25% of M stars are in multiple star systems. Overall, including M stars, about 30% of the galaxy’s stars are in multiple systems.