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The Hertzsprung-Russell Diagram The HR diagram separates The effects of temperature And surface area on stars’ Luminosity and sorts the Stars according to their size The Hertzsprung-Russell Diagram The HR diagram separates The effects of temperature And surface area on stars’ Luminosity and sorts the Stars according to their size The Hertzsprung-Russell Diagram The Main Sequence - all main sequence stars fuse H into He in their cores - this is the defining characteristic of a main sequence star. The Hertzsprung-Russell Diagram Red Giants - Red Giant stars are very large, cool and quite bright. Ex. Betelgeuse is 100,000 times more luminous than the Sun but is only 3,500K on the surface. It’s radius is 1,000 times that of the Sun. The Hertzsprung-Russell Diagram The Hertzsprung-Russell Diagram White Dwarfs - White Dwarfs are hot but since they are so small, they are not very luminous. The Hertzsprung-Russell Diagram The HR diagram separates The effects of temperature Mass of Star And surface area on stars’ Luminosity and sorts the Stars according to their size Size of Star Mass-Luminosity relation Most stars appear on the Main Sequence, where stars appear to obey a Mass-Luminosity relation: L M3.5 For example, if the mass of a star is doubled, its luminosity increases by a factor 23.5 ~ 11. Thus, stars like Sirius that are about twice as massive as the Sun are about 11 times as luminous. The more massive a Main Sequence star is, the hotter (bluer), and more luminous. The Main Sequence is a mass sequence! Review Questions 1. 2. 3. What is the Hertzsprung-Russell Diagram? Why are most stars seen along the so-called main sequence? What makes more massive stars hotter and brighter? L=4πR2 σT4 To calculate a star's radius, you must know its 1) temperature and luminosity. 2) chemical composition and temperature. 3) color and chemical composition. 4) luminosity and surface gravity. L=4πR2 σT4 If a star is half as hot as our Sun, but has the same luminosity, how large is its radius compared to the Sun? 1) ½ times as large 2) ¼ times as large 3) 4 times larger 4) the same What is burning in stars? Gasoline Nuclear fission Nuclear fusion Natural gas Stars More in Depth: To fully characterize stars we need to know their Four Basic Parameters Luminosity Size Mass Surface Temperature Measurements of Star Properties Apparent brightness Direct measurent Parallax Distance Distance + apparent brightness Luminosity ( L=4D2 f) Spectral type (or color) Temperature Luminosity + temperature Radius (L=4R2 T4) Luminosity and temperature are the two independent intrinsic parameters of stars. Mass: how do you weigh a star? Mass is the single most important property in how a star’s life and death will proceed. We can “weigh” stars that are in binary systems (two stars orbiting each other). Fortunately, most stars fall into this category. Most stars in binary systems have a mass that is very similar to its companion … we’ll see why this is soon! Binary Stars Center of mass (or baricenter) Star A ra rb Star B Ma/Mb = rb/ra -Each star in a binary system moves in its own orbit around the system's center of mass. -Kepler’s Third Law: the orbital period depends on the relative separation and the masses of the two stars: 2 4π 3 2 a p = G(M +M ) 1 2 Big p = Small Masses Small p = Big Masses I. Visual Binaries 1 4 2 5 1. 3 2. The total spread (size) of the Doppler shift gives velocities about center of mass (gives orbit sizes, rA+rB ) The time to complete one repeating pattern gives period, P Spectroscopic binaries: Doppler Shift tells if it is moving toward or away Eclipsing Binaries: Best binaries to measure mass Classification of Stars: the H-R diagram 1) Collect information on a large sample of stars. 2) Measure their luminosities (need the distance!) 3) Measure their surface temperatures (need their spectra) The Interstellar Medium Assigned Reading Chapter 10 A World of Dust and Gas The space between the stars is not completely empty, but filled with very dilute gas and dust, producing some of the most beautiful objects in the sky. We are interested in the interstellar medium because a) dense interstellar clouds are the birth place of stars b) Dark clouds alter and absorb the light from stars behind them Bare-Eye Nebula: Orion One example of an interstellar gas cloud (nebula) is visible to the bare eye: the Orion nebula The ISM Space between stars not empty Physical status of the gas characterized by: Gas, dust; Gas is mostly Hydrogen (80%) and Helium (20%) Temperature Density Chemical composition ISM and stars are the components of the “machine” that makes the universe evolve: the cycle of star formation and death, and the chemical enrichment of the cosmos. ISM also “disturbs” observations, since it absorbs light and modifies (reddens) colors Interstellar Reddening Blue light is strongly scattered and absorbed by interstellar clouds. Red light can more easily penetrate the cloud, but is still absorbed to some extent. Barnard 68 Visible Infrared radiation is hardly absorbed at all. Interstellar clouds make background stars appear Infrared redder. Interstellar Reddening (2) The Interstellar Medium absorbs light more strongly at shorter wavelengths. Interstellar Reddening (3) Nebulae that appear as dark nebulae in the optical, can shine brightly in the infrared due to blackbody radiation from the warm dust. The ISM Main Components (Phases) Phase Dust T (K) 20-100 50-500 103-104 105-106 20-50 Density a/cm3 size: a few mm Present in all phases “Metals” Everything that is not hydrogen or Helium is a metal HI Clouds Inter-cloud Medium Coronal Gas Molecular clouds This what forms stars 1-1000 0.01 10-4-10-3 103-105 How did a star form? A cloud of hydrogen gas began to gravitationally collapse. To start the collapse, the gas needs to loose pressure, i.e. needs to become cold It also needs to become dense, i.e. to have more gravity As more gas fell in, it’s potential energy was converted into thermal energy. If the gas does not cool, it cannot collapse As it collapses, the gas gets hotter and hotter Eventually the in-falling gas was hot enough to ignite nuclear fusion in the core. Gas that continued to fall in helped to establish gravitational equilibrium with the pressure generated in the core. Conservation of Angular Momentum (L) L is conserved during the collapse: if R decreases, T has to decrease, too, to keep L constant Only, the rate of change of T is faster to keep pace with the rate of change of R 2 (because of the second power) The collapsing object spins up rapidly during the collapse O The Stellar Cycle Cool molecular clouds gravitationally collapse to form clusters of stars New (dirty) molecular clouds are left behind by the supernova debris. Molecular cloud Stars generate helium, carbon and iron through stellar nucleosynthesis The hottest, most massive stars in the cluster supernova – heavier elements are formed in the explosion. The Four Components of the Interstellar Medium Component Temperature [K] Density [atoms/cm3] Main Constituents HI Clouds 50 – 150 1 – 1000 Neutral hydrogen; other atoms ionized Intercloud Medium (HII) 103 - 104 0.01 Partially ionized H; other atoms fully ionized Coronal Gas 105 - 106 10-4 – 10-3 All atoms highly ionized H Molecular Clouds 20 - 50 103 - 105 Neutral gas; dust and molecules Interstellar Absorption Lines These can be distinguished from stellar absorption lines through: The interstellar medium produces absorption lines in the spectra of stars. a) Absorption from wrong ionization states Narrow absorption lines from Ca II: Too low b) Small line width ionization state and too narrow for the O (too low star in the background; multiple components temperature; too low density) c) Multiple components (several clouds of ISM with different radial velocities) Observing Neutral Hydrogen: The 21-cm (radio) line Electrons in the ground state of neutral hydrogen have slightly different energies, depending on their spin orientation. Opposite magnetic fields attract => Lower energy Magnetic field due to proton spin 21 cm line Magnetic field due to electron spin Equal magnetic fields repel => Higher energy The 21-cm Line of Neutral Hydrogen Transitions from the higher-energy to the lowerenergy spin state produce a characteristic 21-cm radio emission line. => Neutral hydrogen (HI) can be traced by observing this radio emission. Observations of the 21-cm Line G a l a c t i c p l a n e All-sky map of emission in the 21-cm line Observations of the 21-cm Line HI clouds moving towards Earth HI clouds moving away from Earth Individual HI clouds with different radial velocities resolved (from redshift/blueshift of line) Interstellar Dust Formed in the atmospheres of cool stars Mostly observable through infrared emission Spitzer Space Telescope (infrared) image of interstellar dust near the center of our Milky Way (Right:) Infrared Emission from interstellar dust and gas molecules in the “Whirlpool Galaxy” M51. Molecules in Space In addition to atoms and ions, the interstellar medium also contains molecules. Molecules also store specific energies in their a) rotation b) vibration Transitions between different rotational / vibrational energy levels lead to emission – typically at radio wavelengths. The Most Easily Observed Molecules in Space • CO = Carbon Monoxide Radio emission • OH = Hydroxyl Radio emission The Most Common Molecule in Space: • H2 = Molecular Hydrogen Ultraviolet absorption and emission: Difficult to observe! But: Where there’s H2, there’s also CO. Use CO as a tracer for H2 in the ISM! Molecular Clouds • Molecules are easily destroyed (“dissociated”) by ultraviolet photons from hot stars. They can only survive within dense, dusty clouds, where UV radiation is completely absorbed. “Molecular Clouds”: UV emission from Molecules nearby stars destroys survive molecules in the outer parts of the cloud; is Cold, dense molecular absorbed there. cloud core Diameter ≈ 15 – 60 pc HI Cloud Temperature ≈ 10 K Largest molecular clouds are called “Giant Molecular Clouds”: Total mass ≈ 100 – 1 million solar masses Molecular Clouds (2) The dense cores of Giant Molecular Clouds are the birth places of stars. The Coronal Gas Additional component of very hot, low-density gas in the ISM: T ~ 1 million K n ~ 0.001 particles/cm3 Observable in X-rays Called “Coronal gas” because of its properties similar to the solar corona (but completely different origin!) Our sun is located within Probably originates in supernova explosions (near the edge of) a coronal gas bubble. and winds from hot stars The Gas-Star-Gas Cycle All stars are constantly blowing gas out into space (recall: Solar wind!) The more luminous the star is, the stronger is its stellar wind. These winds are particularly strong in aging red giant stars. The Gas-Star-Gas Cycle Stars, gas, and dust are in constant interaction with each other.