class events: week 11
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DESCRIPTIONClass 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. Stars—basic parameters. Luminosity - PowerPoint PPT Presentation
*Class events: week 11GoalsLearn about starsa primer from basic astronomy Luminosity (and brightness) Distance Temperature Hertzprung-Russell diagram Radii Mass Life expectancies and life histories
*Starsbasic parametersLuminosityThe rate at which electromagnetic energy is emitted - the total amount of power emitted by a star over all wavelengths.
BrightnessHow much energy we receive from a star. It is modified by a stars distance.
DistanceKey in understanding stars. Parallax is the most direct measure, if it can be done. (100,000 stars to 1000 ly)
*Starsbasic parametersTypical stellar temperatures3000 K 50,000 K
SpectraRecall that the peak in a stars continuum spectrum is determined by its temperature (Weins Law).
Spectral linesFurthermore, recall that the presence of absorption lines and emission lines reveal the chemical composition of the luminous gas.
ClassificationBased upon the strength of absorption lines, spectra of stars can be classified: OBAFGKM
*The Hertzprung-Russell diagramThe many parameters of stars are confusing.
Astronomers discovered that order could be revealed by plotting two aspects of stars on a single graph.
1) Temperatureexpressed directly in Kelvins, or by color, or by spectral type.
2) Luminosityexpressed 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.
*The Hertzprung-Russell diagramAn example of the HR diagram in useRecall that we know the temperatures of stars on the HR diagram.
We also know the luminosities of starsthey range from 0.001106 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!L = 4p R2sT 4
*Binary starsRecap: we now know the following about stars:LuminositiesRadiiTemperaturesPopulation distributions
Most stars are not single stars such as our ownmost occur in binary or multiple star systems. Some are just near each other in space, others orbit around each other.
*Binary starsWe 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 stars mass, and we know its luminosity, we can learn how long it will live (since a star is burning itself up, like a campfire).
*Starsa life of gravity vs. pressure supportStar 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, where they 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.
*Starsa life of gravity vs. pressure supportStars more massive than our Sun (M=240 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 implosiona 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.
*Starsa life of gravity vs. pressure supportBinary 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.
*Studying stars from an astrobiological perspectiveKnowing 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 starsTimescalesLife spans of 0.550 million years, too short for the development of life (although possibly enough time for planetary formation with B stars). RadiationOB stars produce enormous amounts of sterilizing, ultraviolet radiation.Habitable zoneLiquid water would be stable over an enormous range of distances.
*Studying stars from an astrobiological perspectiveA-F stars: ~3% of all starsTimescalesLife spans of 12 billion years, enough time for at least primitive life to form. RadiationSignificant 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 zoneVery large compared to our Suns habitable zone.
*Studying stars from an astrobiological perspectiveG stars: ~7% of all starsTimescalesLife spans of 10 billion years, enough time for multicellular life to form? RadiationModerate 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 zoneInner solar system.
*Studying stars from an astrobiological perspectiveK-M stars: ~90% of all starsTimescalesLife span of 20-600 billion years; 2-60 times the Suns lifespan! RadiationSterilizing 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 zoneVery 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).
SummaryK-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.
*Studying stars from an astrobiological perspectiveL-T brown dwarf stars: presumably common?TimescalesLife span not defined by the same standards, but they will stay warm for very long timescales! RadiationVery long wavelength optical and infrared radiation.Habitable zoneNo conventional habitable zone.
SummaryHabitable 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.
*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 spacedyes!If the stars are closely spaced, and the planet has a large orbit around both starsyes! (Such planets, like Kepler 16b and Kepler 47(AB)b, have been discovered.)If the planets orbit is on the same size scale as the stars orbitsno!
Even if they are not, only about 25% of M stars are in multiple star systems.Overall, including M stars, about 30% of the galaxys stars are in multiple systems.