2nd Astronomy Exam Fall 2015

Name______

Class Meeting Time: MWF 9:05 to 9:55 or TTh 4:30 to 5:45

Grade Summary

HW#1a ______

HW#1a ______

HW#2 ______

HW#3 ______

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HW#5 ______

Exam #1 ______

Exam #2______

Midterm Grade_____

  1. A star leaves the main sequence when
  2. it stops fusing hydrogen in its core.
  3. neutrinos can no longer escape the star.
  4. its core of iron can no longer support itself and it collapses catastrophically
  5. the t-Tauri winds clear the nebula of gas and dust terminating planet formation.
  1. When a star like the Sun ceases to produce energy in its core and energy production begins in a shell outside the core, its envelope

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  1. Collapses and heats up.
  2. Collapses and cools down.
  3. Expands and heats up.
  4. Expands and cools down.

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The HR Diagram at right is provided to assist with answering the following two questions.

  1. Which ishotter, a main sequence star with an absolute magnitude of M= -5 or a Red Giant.
  2. The main sequence star
  3. The red giant
  4. They have the same temperature.
  5. There is insufficient information to determine this.
  1. Which statement is the most correct about the comparison between a K5 main sequence star and a Red Giant
  2. The K5 star is less luminous, smaller, and will not live as long as the Red Giant star.
  3. The K5 star is less luminous, smaller, and will live longer than the Red Giant star.
  4. The K5 star is less luminous, larger and will not live as long as the Red Giant star.
  5. The K5 star is more luminous, smaller, and will live longer than the Red Giant star.
  1. The element iron in the hemoglobin of your blood was formed
  2. in our Sun.
  3. in the chemistry of Giant Molecular Clouds
  4. at the instant of the Big Bang.
  5. in a distant galaxy in a different part of the early universe.
  6. by the explosive death of a massive star.
  1. Theobject to the right is an example of which of the following
  2. It is the outflow of gas from the envelope of a sun-like star that marks the end of the star’s energy production.
  3. It is the peculiar morphology associated with the collision of two galaxies
  4. It is the giant molecular cloud from which protostars form
  5. It is what is left when a white dwarf star explodes as a supernova.
  1. Which of the following lists, in the correct order, a possible evolutionary path for a sun-like star (i.e. 1 solar mass)?
  2. Main sequence, horizontal branch star, red giant, red supergiant, white dwarf
  3. Main sequence, red giant, horizontal branch star, red supergiant, supernova
  4. Main sequence, proto-star, red giant, white dwarf
  5. Main sequence, red giant, horizontal branch star, red supergiant, white dwarf
  6. Main sequence, red giant, white dwarf, supernova

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Refer to H-R diagram illustrating the evolutionary track of a 1 solar mass star to the right to answer the following questions.

  1. Which of the objects listed below would be observed along the portion of the track marked (e)?
  2. White dwarf
  3. Horizontal Branch Star (a.k.a.Yellow Giant)
  4. Planetary Nebula
  5. Red Giant Star
  6. None of the above
  1. Which of the methods of energy production is at active along the portion of the track marked (d)?

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  1. Core H-burning
  2. Core He-burning
  3. Shell H-burning
  4. Shell He-burning
  5. Proton-proton chain

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  1. In a few sentences, define an HII region and an OB Association. Also describe the relationship between the two of them.

An OB Association is a small group of brand new O & B main sequence stars. An HII region is a cloud of ionized hydrogen gas that glows red as electrons recombine with the hydrogen nucleus. The HII requires a power source to excite and ionize the hydrogen. The power source is the UV photons from the very hot O and B stars. The OB association creates the HII region and maintains it.

  1. The 3-D HR diagram to the right displays the abundance of stellar types in a typical million cubic parsec volume of the galaxy. As this figure indicates, the most common types of stars in a typical location of the galaxy are M main sequence stars (a.k.a. Red Dwarfs).
  1. List two reasons why the most common stars in the galaxy are M main sequence stars.
  1. M stars form more often from a GMC because the Birth Function dictates that there are hundreds of times more smaller (≈ 0.1 solar masses) than there are lager fragments and the smaller fragments from the M main sequence stars.
  2. The M main sequence stars have very long lifetimes…longer than the Universe is old. The lifetime of an M main sequence stars is over 1 trillion years. Thus every M main sequence star that has ever formed in the history of the universe is still an M main sequence star. They never die…we’ll not yet anyway. So their numbers just continue to build up.
  1. State why there are so relatively few Giant and Supergiant stars.

Giant and supergiant stars are so rare because their lifetimes are so short. Giant stars as a class only exist for a few percent of their main sequence lifetimes. Their numbers never have an opportunity to grow due to their high “death rate”.

  1. The HR Diagram to the right is of an open cluster. Note the horizontal axis is labelled as B-V which is another way in which temperature is expressed. Suffice it to say that hot is to the left and cool is to the right as in our usual practice of drawing HR Diagrams. The main sequence is clearly outlined.

  1. Carefully sketch in the ZAMS line and define what the acronym ZAMS means.

The ZAMS line lies to the lower left of the main sequence and represents the point on an HR diagram where conditions within the core of the star are sufficient to initiate fusion.

ZAMS is an acronym for Zero Age Main Sequence.

  1. The five stars that appear in nearly a straight line in the lower left of the HR diagram are what type of objects?

These five objects are white dwarf stellar remnants the “tombstones” of low mass stars (M < 9 solar masses).

  1. Our Sun is halfway through its main sequence lifetime. During the first half of its main sequence lifetime, the Sun’s luminosity was expected to be
  2. Greater than it is now
  3. The Same as it is now
  4. Less than it is now
  5. No predictions of past conditions are possible.

  1. The evolutionary track of a protostar evolving toward the main sequence is shown on the HR diagram to the right.
  2. Describe the changes in the proto-stars luminosity, temperature and radius as it evolves through the two points marked by the circles labeled A and B.

From point A to point B the

  • Luminosity of the proto-star is nearly constant,
  • Temperature is increasing, and
  • Radius must be decreasing following the Stefan-Boltzmann Law. (If L is constant then since an increasing T would drive up the luminosity, the radius must be decreasing to counteract the effect of temperature.
  1. If nuclear fusion has not yet started in this proto-star, then what is the source of energy that is causing the temperature to change? Answer in a sentence.

The proto-star is generating energy by converting gravitational potential energy into thermal energy.In other words, it is creating heat as it collapses and compresses itself.

  1. The following four sketches represent the four types of binary stars. Note that the spectral type and distances are provided for each star in the sketch. Write the type of binary star the picture represents under the appropriate sketch.

Optical Double Star True Binary Star Spectroscopic Binary Star Eclipsing Binary Star

  1. One of the sketches of binary stars from the previous problem represents an eclipsing binary star. What information, besides the combined mass of the system, can be extracted from an eclipsing binary star system? Answer in a sentence.

The diameters of both the primary and secondary star in an eclipsing binary star system can be extracted from the light curve of the two stars.

  1. The image to the right shows several Bok Globules.
  1. What is the mass limit for the smallest globule that will form a star and why can no smaller fragments form stars? Answer in a sentence.

The smallest mass fragment that can go to form a star is about 0.1 solar masses. If a cloud fragment is less massive than 0.1 solar masses, then it will not have sufficient temperature sand density to initiate fusion in its core when it has collapsed fully.

  1. Why is the largest fragment that will form a star limited to 100 solar masses? Answer in a sentence.

The largest fragment that will form a star limited to 100 solar masses because any larger mass fragment will generate so much energy as it collapses that it literally blows itself apart into smaller fragments.

  1. In the August 2015 edition of Physics Today magazine, a new book is advertised as being available. Its cover is shown on the right. This book will no doubt challenge what empirical law that predicts how a Giant Molecular Cloud fragments? Just name the empirical law that predicts how a giant molecular cloud should break up.

The Birth Function or The Initial Mass Function
The following question requiresa detailed quantitative answer.

  1. Describe the nature of the bright stars in the night sky, contrasting their properties with the Sun. Be quantitative in your description. An HR diagram of the brightest stars in the sky appears to the right to assist your memory.


  1. The spectrum of a star appears below.
  1. Estimate the wavelength at which the star emits its maximum intensity.

The wavelength at which the star emits its maximum intensity is about 600 nm.

  1. Using your estimated wavelength of maximum emission, calculate the temperature of this star. (Note: If you did not estimate the wavelength of maximum emission in part A, then use λMax = 950 nm for this part B. It is not the right answer to part A, but will allow you to do this part B.)

(3,053 K)

The temperature of the star is about 4,833 K. (3,053K)

  1. The luminosity of the star is known to be 100 solar luminosities. What is the radius of the star compared to the Sun’s radius? (Note: If you did not calculate the temperature in part B, then use T = 2,500 K for this part C. It is not the right answer to part B, but will allow you to do this part C.)

Use the Stefan-Boltzmann Law to solve for the radius of this star relative to the Sun

Stefan Boltzmann Law:

Now, that we have a simple expression with ratios, we’ll fill in the known ratios and find the unknown ratio

The star has a radius14.4 times that of the Sun.

  1. The orbit of the binary star Kruger 60 is shown to the right. The radius of the companion star’s orbit is 8.9 AU and the period of the companion star’s orbit is 44. 6 years.
  1. What is the combined mass of the two stars in the Kruger 60 binary system?

The combined mass of these two stars is about 0.35 solar masses.

  1. Assuming that both stars in the system are main sequence stars, what would be a reasonable guess as to their spectral types given the value of the combined mass?

Since the combined mass is only 0.35 solar masses both stars must be very low mass stars that would be consistent with M main sequence stars. I would guess that bot these stars are some sub-class of M main sequence star.

Use the HR diagram to the right to answer the following questions. Note: The only calculations needed are to convert absolute magnitude into solar luminosities. Most answers can be read off the HR Diagram.

  1. The bright star Deneb has a luminosity of 54,000 solar luminosities (M = -7.0) and a temperature of 8,525 K (spectral type A2). What is its approximate radius?

The radius of this star is about 60 solar radii.

  1. The star nearest the Sun is an M5.5 V star. What are its luminosity and its radius, approximately?

The absolute magnitude of this star is about 12 which corresponds to a luminosity of

The radius of this star is about 0.2 solar radii.

  1. The companion star to the brightest star in the sky is designated Sirius b. It has a spectral type of A2 and has a radius of 0.01 that of the Sun. What is its approximate luminosity?

The absolute magnitude of this star is about 12 which corresponds to a luminosity of

  1. What would be the spectral type of a main sequence star with 10 times the mass of the Sun?

The spectral type of a main sequence star with 10 times the mass of the Sun would be about a B3 main sequence star.

  1. The core of a high mass star at the end of its energy production lifetime is composed of which of the elements listed below?

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  1. Hydrogen
  2. Helium
  3. Carbon
  4. Oxygen
  5. Silicon
  6. Iron

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  1. In a sentence or two, explain what a “standard candle” is and why are supernova’s considered very good “standard candles.”

A standard candle is any astronomical object of a priori know luminosity (i.e. the luminosity is known as soon as the object is identified).

Type II supernova are goods standard candles because they all seem to have the same peak luminosity (about M = -17) and that peak luminosity is VERY luminous allowing supernovas to be observed at very great distances.

  1. Why do high mass main sequence stars like O and B stars go on to fuse heavier elements in their cores, while stars of lower mass, like the Sun, stop with the fusion of helium? Answer in a couple sentences.

High mass main sequence stars like O and B stars go on to fuse heavier elements in their cores, while stars of lower mass, like the Sun, stop with the fusion of helium because the high mass stars have higher central temperatures and densities in their core that can overcome the electrostatic barriers to fusion that heavier elements like carbon, oxygen, neon etc. present. The larger central temperatures and pressure allow more types of fusion to occur in these higher mass stars.

  1. The image to the right illustrates a typical open cluster of stars. In a few sentences, describe what an open cluster of stars is giving typical dimensions and stellar content. Answer in a few sentences.

An open cluster is a brand new group of stars fully emerged from the Giant Molecular cloud it formed from. An open cluster may contain 100’s to 1,000’s of visible stars. The visible stars are mostly B and A main sequence stars but there are far more invisible K and M main sequence stars whose luminosities are too low to allow us to see them. The typical Open Cluster is about 15 light years across.

  1. On Tuesday 06 Oct 2015, The Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics for 2015 to Takaaki Kajita of University of Tokyo, Kashiwa, Japan and Arthur B. McDonald of Queen’s University, Kingston, Canada “for the discovery of neutrino oscillations, which shows that neutrinos have mass.” For a little extra credit, in what area of study did we encounter the issue of neutrino oscillations?

We encountered neutrino oscillations in our study of the Sun’s source of power.

Astronomy Formula and Constants Sheet for Exams

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