Astronomy: Chapter 6

Question 1

Give the Basic definition of “parsec” and discuss the relationship between the distance of a star and its parallax shift.

Answer 1

• Parsec (PC): The distance of a hypothetical star whose parallax is 1 second of arch.
• because stars are so distant, their parallaxes are very small angles.
• the farther away an object is, the less it shifts
• A parsec is 206,265 AU, which equals roughly 3.26 LY
  
  • d = 1 parsec = 1 second arc
  • 2 Pc = 1/2 parallax shift
  • 3 Pc = 1/3 d

Question 2

Discuss the apparent and absolute magnitudes of a star. how can knowing both of them give us a star’s distance?

Answer 2

• Absolute Visual Magnitude (MV): Intrinsic brightness of a star. The apparent visual magnitude the star would have if it were 10 Pc away.
• Apparent Magnitude: -4,-3,-2,-1,0,1,2,3,4
  Distance 10pc;
  
  • abs 5; app mg7; distance >10 Pc
  • abs 3, app mg 2, then <10 Pc

Question 3

Why are the hydrogen lines not as strong in type “O” stars and type “K” stars as they are in type “A” stars?

Answer 3

• cool stars have weak Hydrogen Balmer absorption lines in the spectra
• Hot stars have weak hydrogen balmer absorption lines in the spectra - 20,000 K
  Intermediate temperature, 10,000 K: gas absorbs Balmer-wavelength photons well and thus produces strong hydrogen balmer lines.
• Hydrogen lines in “O” and “K” are not as strong because “O” is extremely hot and “K” is very cool. But “A” is in the middle.

Question 4

Discuss the spectral classification of stellar temperatures.

Answer 4

Spectral Classes: seven main types: O,B,A,F,G,K,M. From hot to cool.

• The “O” stars are the hottest, and the temperature continues to decrease down to the “M” stars, the coolest.
• Astronomers divide each spectral class into 10 classes.
  
  • Class “A” consists of A0,A1…A9
  • the finer divisions define a star’s temperature to a precision of about 5%

Question 5

How does the brightness of a star depend on temperature? Give a numerical example.

Answer 5

• the luminosity of a glowing star is determined by its temperature. (Stefan-Boltzmann Law)
• Hotter stars generally are brighter stars, but size of star also takes place.

Question 6

Discuss the H-R diagram, what it plots, and where various kinds of stars are located on the diagram.

Answer 6

• Hertzsprung-Russell (H-R) diagram: A plot of the intrinsic brightness versus the surface temperature of stars. It separates the effects of temperature and surface area on stellar luminosity and is commonly plotted as absolute magnitude versus spectral type but also as luminosity versus surface temperature or color.
• main sequence: the region of the H-R diagram running from upper left to lower right, which includes roughly 90% of all stars generating energy by nuclear fusion.

Question 7

Describe three methods for detecting binary stars.

Answer 7

• Visual binary system: a binary star system in which two stars are separately visible in the telescope.
• spectroscopic binary system: a star system in which stars are too close together to be seen separately. We see a single point of light, and only by taking a SPECTRUM can we determine that there are two stars.
• eclipsing binary system: a binary system in which the stars cross in front of each other as seen from earth. when one star moves in front of the other, it blocks some of the light.

Question 8

What is the chief method of measuring the mass of an object in space?

Answer 8

• Binary systems with large orbits have less mass than those with smaller orbits.
• mass-luminosity relation: the more massive a star is, the more luminous it is.
  
  • Giants, supergiants, and white dwarfs do not follow the mass-luminosity relation.
• objects in space with greater mass orbits other objects in greater speeds.

Question 9

What single, basic property of a star determines its temperature and brightness, and therefore its position on the main sequence of the H-R diagram. Why?

Answer 9

• Based on Main sequence stars, the more massive stars are hot stars.
• the more massive a star, the more luminous it is.