1. This Question Is About Hubble S Law

ROUND 2 - QUESTIONS

1. This question is about Hubble’s law.

The light received from many distant galaxies is red-shifted.

(a) State the cause of this red-shift (1)

expanding universe

(b) State Hubble’s law(1)

recessional speed of a galaxy is proportional to its distance from the Earth

/ galaxies move away from each other with a speed proportional to their separation

/v =Hd with symbol’s explained

State any assumptions you have made(1)

galaxies have moved distance d from the Big Bang in time T

if the speed of recession is constant

2. One of the most intense sources known to radio astronomers is the Galaxy NGC5128.

Long exposure photographs show it to be a giant elliptical galaxy crossed by a band of dark dust.

It lies about 1.5×107 . light years away from Earth.

(a) Describe any differences between this galaxy and the Milky Way. (2)

Milky Way is a spiral galaxy with “concentration” of stars in the centre;

NGC5128 is an elliptical galaxy - form is different;

Hubble’s law predicts that NGC5128 is moving away from Earth.

(b) (i) State Hubble’s law. (1)

recession velocity is proportional to the distance away

(ii) State and explain what experimental measurements need to be taken in order to determine

the Hubble constant.(3)

a measurement to get recession velocity;

e.g. red shift measurement

a measurement to get distance away;

e.g. Cepheids

repeat procedure for many galaxies to get relationship from graph

(i) the recession speed of NGC5128. (2)

(ii) the age of the universe. (2)

T = 5.1×1017 s 5×1017 s

3. This question is about galaxies.

(a) Draw a sketch showing the basic structure of the Milky Way galaxy. Mark the approximate position of the solar system. (3)

(b) A particular spectral line of light from a laboratory source has a wavelength of 658 nm. The wavelength of this line of light from a distant galaxy is 670 nm. Calculate the recessional speed of the galaxy (2)

(c) (i) Draw a sketch graph to show how the recessional speed of galaxies varies with distance from Earth. (This is a sketch graph; you do not need to put any values on the axes.) (2)

(ii) Deduce how may be used to estimate the age of the universe. (2)

two points separated by d have speed

T = age of Universe

(iii) Outline how Hubble’s constant H0, is determined from your graph.

The slope of the graph is Hubble’s constant.

4. This question is about extragalactic astrophysics.

(a) In an observation of a distant galaxy, spectral lines are recorded. Spectral lines at these wavelengths cannot be produced in the laboratory. Explain this phenomenon (2)

wavelengths are shifted

universe is expanding

Doppler Shift

(b) Describe how Hubble’s law is used to determine the distance from the Earth to distant galaxies (2)

recessional speed of a galaxy is proportional to its distance from the Earth: v = Hd

v is obtained from spectral lines

Doppler Shift

(c) Explain why Hubble’s law is not used to measure distances to nearby stars or nearby galaxies (such as Andromeda). (2)

the expansion of the universe is very small on local scales;

it would be impossible to distinguish between random velocities and expansion;

5. This question is about galactic motion.

The K-line of light from singly ionised calcium has a wavelength of 393.3 nm when measured in a laboratory. The same line in the spectrum of galaxy NGC 4889 has a wavelength of 401.8 nm. The value of the Hubble constant may be assumed to be 70 km s–1 Mpc–1.

Deduce a value for the distance of NGC 4889 from Earth. (4)

6. This question is about the Hubble constant.

(a) The value of the Hubble constant H0 is accepted by some astronomers to be in the range 60 km s–1 Mpc–1 to 90 km s–1 Mpc–1.

(i) StateandexplainwhyitisdifficulttodetermineaprecisevalueofH0 (2)

v =H0 d the further galaxies are away the more difficult it is to accurately determine their distances

because of the difficulty of locating a Cepheid variable

/ difficulty of accurately measuring luminosities (of the galaxy / of the standard candle

(ii) State one reason why it would be desirable to have a precise value of H0. (1)

to determine an accurate value of the age of the universe

/ to determine an accurate value of the rate of expansion of the universe

/ to determine an accurate value to very distant galaxies;

7. This question is about expanding Universe.

(a) Quasars are the most distant objects whose redshifts have been measured. Quasar 3C273 has an emmision line at 475 nm. One corresponding to particular quantum jump in the hydrogen atom. The normal line is at 410.2 nm.

(i) Is this a red shift or a blue shift? Explain your answer.

larger λ at 475 nm

red shifted

(ii) What is the fractional shift z in wavelength? (2)

(iii) Calculate the velocity of Q3C273 with direction WRT Earth.

(iv) If the present scale of the Universe is taken to be 1 and the present age of the Universe

is 16 bilion years, what was the scale and age of the Universe when this light was emitted from the quasar?