天文学和天体物理学代写 Astronomy and Astrophysics代写

Astronomy and Astrophysics

天文学和天体物理学代写 EXAM DURATION: 2 hours EXAM INSTRUCTIONS: (a) Candidates must attempt ALL questions. (b) Marks for sections and subsections of

EXAM DURATION: 2 hours

EXAM INSTRUCTIONS:

(a) Candidates must attempt ALL questions.

(b) Marks for sections and subsections of questions are shown in square brackets. The total mark for a question is shown at the end of that question. The marks given in this paper are indicative, and subject to change given the different circumstances under which the papers are being sat.

(c) Your answers must be submitted as a pdf file in the manner instructed by the University. One option is to write by hand your answers on paper and use a smartphone with an app such as Microsoft’s “Office Lens” to create a pdf file of your answers, but other means to generating the pdf file are acceptable. Before submitting, please check that your pdf file contains all of your written work.

1. Question total: 25 天文学和天体物理学代写

Assuming that a cometary nucleus is roughly spherical, has an albedo of zero and radiates like a black body, show that the temperature T of the nucleus varies with distance d from the Sun as

where L is the luminosity of the Sun, and σ is the Stefan-Boltzmann constant. [7]

Use the expression above to calculate the temperature of the nucleus of Halley’s comet at perihelion, when the comet is located 0.59 AU from the Sun, and at aphelion, when the comet is 35.3 AU from the Sun. [7]

Describe briefly the composition of the nucleus and the changes that the comet undergoes as it approaches the Sun, including a sketch of the major components. [6]

Discuss briefly the origin of short period comets and their eventual fate.  [5]

2. Question total: 25 天文学和天体物理学代写

Give typical temperatures and note the dominant spectral lines for three spectral types. Explain why stars with different surface temperatures show different lines in their spectra. [9]

A star of spectral type O is found to have a radial velocity of +300 km s⁻¹ . It is observed through an intervening gas cloud which has zero radial velocity and a higher temperature than the star. Describe the appearance of the observed spectrum of the star plus the gas cloud, noting the major spectral features observed. Calculate the wavelength shift of a spectral line in the stellar spectrum with a rest wavelength of 468.6 nm. [8]

Explain why hydrogen lines seen in the spectra of main-sequence stars should be observed to have a significantly different shape to those seen in the spectra of supergiants. [4]

Describe TWO possible ways by which astronomers recognise that apparently single stars in the sky are close binary stars. [4]

3. Question total: 25 天文学和天体物理学代写

(a) Sketch an edge-on view of the Galaxy showing the location of the Sun relative to the bulge and the halo and disk populations. Indicate where open and globular clusters are generally found. Include scales on your sketch. [8]

(b) For each of the halo and disk populations, name THREE types of object that exist there. In addition, comment on the orbits of the different populations about the centre of the Galaxy and the ages and metallicities of the different populations. [12]

(c) Sketch the observed rotational velocity structure of the Galaxy and show how it differs from that of a Keplerian planetary system. Include numerical labels on the axes. [5]

4. Question total: 25 天文学和天体物理学代写

(a) A northern hemisphere survey shows that there are 5000 spiral galaxies brighter than apparent magnitude m = 14. Assuming a typical bright spiral galaxy has an absolute magnitude M = −20, what is the volume density of galaxies (i.e., the number of galaxies per Mpc³ )? [10]

(b) In deriving the volume density of galaxies above, what assumption did you make about how galaxies are distributed throughout the Universe? How does this differ from what we now know about the spatial distribution of galaxies? [5]

(c) Assuming the hot big bang model, sketch the evolution of the scale factor of the Universe, R(t), as a function of time t, for the cases where the energy density of the Universe is

(i) less than,

(ii) equal to, and

(iii) more than the critical density.

[6]

(d) We measure galaxies in Virgo to be receding with a total velocity of 1200 kms⁻¹ . We believe our galaxy is currently falling into Virgo with a velocity of 250 kms⁻¹ . What is the inferred Hubble constant and observed redshift to Virgo if we adopt a direct distance of 20 Mpc? [4]

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