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Sirius, the brightest star in the sky, has a trigonometric parallax of p = arc seconds. What is its distance from Earth? pc light years pc light years light years

Distances of Stars Trigonometric Parallax: Star appears slightly shifted from different positions of the Earth on its orbit The further away the star is (larger d), the smaller the parallax angle p. d = __ p 1 d in parsec (pc) p in arc seconds 1 pc = 3.26 LY

Star A has an apparent magnitude of m A = 5.6 and an absolute magnitude of M A = 2.3. Star B has an apparent magnitude of m B = 0.6 and an absolute magnitude of M B = 2.3. Which of the following statements is true? 1.The flux received from both stars is the same, but star B is 5 times more luminous than star A, so star B must be further away. 2.The flux received from both stars is the same, but star B is 100 times more luminous than star A, so star B must be further away. 3.Both stars are equally luminous, but the flux received from star A is 5 times less than from star B, so star A must be further away. 4.Both stars are equally luminous, but the flux received from star A is 100 times less than from star B, so star A must be further away. 5.Both stars are equally luminous, but the flux received from star A is 5 times more than from star B, so star B must be further away.

Absolute Magnitude The absolute magnitude measures a star’s intrinsic brightness (= luminosity). Absolute Magnitude = Magnitude that a star would have if it were at a distance of 10 pc. If we know a star’s absolute magnitude, we can infer its distance by comparing absolute and apparent magnitudes.

Which of these spectral types describes a Red Giant? 1.O3V 2.F9V 3.B2Ia 4.K5III 5.G2V

Spectral Classification of Stars Temperature

Spectral Classification of Stars Mnemonics to remember the spectral sequence: OhOhOhOhOnly BeBeBoy,Bad AAnAnAstronomers FineFForget Girl/GuyGradeGenerally KissKillsKnown MeMeMeMeMnemonics

Luminosity Classes Ia Bright Supergiants Ib Supergiants II Bright Giants III Giants IV Subgiants V Main-Sequence Stars Ia Ib II III IV V

Masses of Stars in the Hertzsprung- Russell Diagram Masses in units of solar masses Low masses High masses Mass The higher a star’s mass, the more luminous it is. High-mass stars have much shorter lives than low-mass stars Sun: ~ 10 billion yr. 10 M sun : ~ 30 million yr. 0.1 M sun : ~ 3 trillion yr. < 100 solar masses > 0.08 solar masses

In a binary star system … 1.The less massive stars orbits around the more massive one. 2.The more massive star orbits around the less massive one. 3.Both stars orbit on identical orbits around the mid-point between them. 4.Both stars orbit around their center of mass, which is closer to the less massive star. 5.Both stars orbit around their center of mass, which is closer to the more massive star.

The Center of Mass center of mass = balance point of the system. Both masses equal => center of mass is in the middle, r A = r B. The more unequal the masses are, the more it shifts toward the more massive star.

Which law allows astronomers to calculate the masses of stars in binary systems? 1.Newton’s first law 2.Kepler’s third law 3.Einsteins theory of general relativity 4.Newton’s third law 5.Kepler’s second law

Estimating Stellar Masses Rewrite Kepler’s 3. Law as 1 = a AU 3 / P y 2 Valid for the Solar system: star with 1 solar mass in the center. We find almost the same law for binary stars with masses M A and M B different from 1 solar mass: M A + M B = a AU 3 ____ Py2Py2 (M A and M B in units of solar masses)

Which is the most common type of binary star systems? 1.Spectroscopic binaries 2.Eclipsing binaries 3.X-ray binaries 4.Visual binaries (where both stars and their motion can be resolved) 5.Binary neutron stars

Spectroscopic Binaries The approaching star produces blue shifted lines; the receding star produces red shifted lines in the spectrum. Doppler shift → Measurement of radial velocities → Estimate of separation a → Estimate of masses

Which of these fusion mechanisms does NOT fuse Hydrogen to Helium? 1.Proton-proton chain 2.CNO Cycle 3.Triple-Alpha Process

The CNO Cycle In stars slightly more massive than the sun, a more powerful energy generation mechanism than the PP chain takes over: The CNO Cycle. In the sun, energy production is dominated by direct fusion of H into He (PP chain).

Energy Transport Structure Inner radiative, outer convective zone Inner convective, outer radiative zone CNO cycle dominantPP chain dominant

Mass Sun Radiative Core, convective envelope; Energy generation through PP Cycle Convective Core, radiative envelope; Energy generation through CNO Cycle Summary: Stellar Structure

What are “globules”? 1.Small planetary bodies, still in the process of growing into planets (“globes”) 2.Large, cold, uncompressed molecular clouds that may eventually form thousands of stars. 3.Small, compressed pockets of dense gas that may form stars. 4.The remnants of the explosions of sun-like stars. 5.The remnants of the explosions of high-mass stars.

(Bok) Globules ~ 10 – 1000 solar masses; Contracting to form protostars Compact, dense pockets of gas which may contract to form stars.

Jets of gas ejected from protostellar disks are called … 1.Globules 2.Planetary Nebulae 3.Novae 4.Herbig-Haro Objects 5.Pulsars

Herbig-Haro Objects

What happens in the Triple- Alpha Process? 1.Fusion of Hydrogen to Helium 2.Fusion of Helium to Carbon 3.Fusion of Carbon to Neon 4.Fusion of Silicon to Iron 5.Nuclear fission of Uranium

Red Giant Evolution 4 H → He He He-core gets denser and hotter until the next stage of nuclear burning can begin in the core : He fusion: 3 4 He → 12 C “Triple-Alpha Process” Fusion of Helium into Carbon

What is a “white dwarf”? 1.A failed star that does not become hot enough to ignite nuclear fusion. 2.The burned-out remnant of a very low-mass star that never ignites Helium fusion. 3.The collapsed Carbon/Oxygen core of a sun- like star. 4.The collapsed iron core of a high-mass star. 5.The collapsed iron core of a sun-like star.

White Dwarfs Degenerate stellar remnant (C,O core) Extremely dense: 1 teaspoon of WD material: mass ≈ 16 tons!!! White Dwarfs: Mass ~ M sun Temp. ~ 25,000 K Luminosity ~ 0.01 L sun Chunk of WD material the size of a beach ball would outweigh an ocean liner!

Summary of Post-Main-Sequence Evolution of Stars M > 8 M sun M < 4 M sun Evolution of M sun stars is still uncertain. Fusion stops at formation of C,O core. Fusion proceeds to formation of Fe core. Red dwarfs: He burning never ignites M < 0.4 M sun

Which was the first method that allowed astronomers to measure the distances to other galaxies? 1.Light-travel time measurements 2.Gravitational-lensing measurements 3.Trigonometric parallax 4.Using Cepheid Variables 5.Warp-Drive travel

Cepheid Variables: The Period-Luminosity Relation The variability period of a Cepheid variable is correlated with its luminosity. => Measuring a Cepheid’s period, we can determine its absolute magnitude! The more luminous it is, the more slowly it pulsates.

If you plot all stars of a star cluster on a Hertzsprung-Russell diagram: Which feature will allow you to determine the cluster’s age? 1.The brightness of red giants. 2.The number of white dwarfs. 3.The average surface temperature of neutron stars. 4.The turn-off point from the Main Sequence. 5.The minimum mass of stars at the lower end of the main sequence.

Example: HR diagram of the star cluster M 55 High-mass stars evolved onto the giant branch Low-mass stars still on the main sequence Turn-off point

The lower on the MS the turn-off point, the older the cluster.