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A1-Phys-23-1-A1 > 13: Our Place in the Universe > Flashcards

13: Our Place in the Universe Flashcards

1
Q

What is an astronomical unit?

A

Defined as the mean distance between Earth and Sun

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2
Q

Why is the size of the observable universe limited?

A

Light has a finite speed

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3
Q

What is apparent magnitude?

A

The brightness of stars measured on a logarithmic scale

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4
Q

Why are images of astronomical objects often displayed using a logarithmic scale of brightness?

A

So that the difference between the brightest and dimmest parts of the image is less intense

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5
Q

What is the Hertzsprung-Russell (H-R) diagram? What is it used for?

A

A plot of luminosity (power output) against temperature for stars. Both axes have a logarithmic scale - used to determine age of star clusters

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6
Q

What is it assumed about the age of stars in clusters?

A

Assumed to have formed at the same time - so same age

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7
Q

What is the main sequence of a star’s life?

A

The majority of their life - they fuse hydrogen into helium to produce energy

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8
Q

What does the position of a star on an H-R diagram main sequence depend on?

A

The mass`

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9
Q

What happens at the end of the main sequence? In respect to the H-R diagram

A

The stars move off the main sequence to the top-right of the H-R diagram

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10
Q

What is the relationship between mass and time spent in the main sequence?

A

More massive stars use their fuel more quickly and spend less time in this stage

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11
Q

Which starts (on the H-R diagram) leave the main sequence first?

A

More massive stars at the top-left of the main sequence leave the main sequence first

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12
Q

How do scientists calculate the age of a cluster based on the H-R diagram?

A

They identify the most massive star left on the main sequence. They assume that it’s about to leave the main sequence, and work out how old a star of that mass would be when it uses up all of its hydrogen fuel. This gives an estimate of the ages of the star, and therefore the age of the cluster

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13
Q

What are distances and velocities in the solar system measured with?

A

Radar

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14
Q

Describe how you can work out distance using radar

A

1) A short pulse of radio waves is sent from a radio telescope towards a distant object.
2) When the pulse hits the surface of the object, its reflected back to earth
3) The time the pulse takes to return is recorded (t)
4) Radio waves travel at c… so the distance = ct/2

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15
Q

What can space-time wordlines be used for?

A

To find the distance to an object from the time interval between sending and receiving a radar pulse

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16
Q

Describe how you would draw a radar pulse on a space-time diagram. How do you work out the distance to the object you are sending the pulse to?

A

Start the line showing the pulse at t=0. The gradient of the line should be 1 (as light travels 1 light-second in 1 second). The radar pulse changes direction half way between leaving and arriving back, so the line should too. The point at which it changes direction is the distance of the object that reflected it

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17
Q

How you use radar to find out the average velocity of an object, relative to earth?

A

1) You send 2 pulses separated by a certain time interval, to give 2 separate measurements of the object’s distance.
2) The difference between the distances shows how far the object has moved relative to Earth in the time interval
3) Speed = distance / time

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18
Q

What are the assumptions made when measuring an object’s speed or distance with radar?

A

1) The speed of the radio wave is the same on the way to the object and the way back to the telescope
2) The time taken for the radio waves to reach the object is the same as the time taken to return
3) Speed of radio wave must remain constant (speed of light)
4) Object’s speed must be much less than the speed of light to ignore any relativistic effects

19
Q

What type of EM wave is used for radar?

A

Radio

20
Q

What is a more accurate measurement of an object’s speed than radar?

A

Doppler shifts

21
Q

The motion of a wave’s source affects its []

A

Wavelength or frequency

22
Q

What is red shift?

A

When a light source is moving away from you and as a result the wavelength becomes longer and the frequencies become lower

23
Q

What does the amount of doppler shift depend on?

A

The velocity of the source

24
Q

What shift occurs when a light source moves towards us?

A

Blue shift

25
Q

Radiation is emitted from a very hot region of a star, called the [], in a [] spectrum

A

Photosphere
Continuous

26
Q

What produces dark absorption lines within the spectrum (of radiation emitted by a star)?

A

Atoms within the atmosphere of the star absorb certain wavelengths of the radiation - which result in the dark lines

27
Q

How can you work out what a stellar atmosphere is composed of?

A

Different atoms absorb different parts of the spectrum, resulting in a characteristic pattern for each atom. By looking at the absorption lines from a star, and comparing them with known spectra in the lab, the composition can be found

28
Q

When does time dilation happen?

A

When something moves close to the speed of light

29
Q

What is assumed when using radar or Doppler shift to calculate velocities/distances?

A

That the object, that you are looking at, is travelling much slower than the speed of light

30
Q

What is the effect called where time runs at different speeds for 2 objects moving relative to each other?

A

Relativistic time dilation

31
Q

What did Hubble discover about recessional velocity?

A

Recessional velocity is proportional to distance

32
Q

What tells you how fast a galaxy is moving away from earth?

A

The spectra from galaxies all show red shift. The amount of red shift gives the recessional velocity.

33
Q

What does the rate of expansion of the universe depend on in our calculations?

A

Hubble’s constant

34
Q

By showing that objects in the universe are moving away from each other, what did Hubble’s work provide strong evidence for?

A

That the universe is expanding

35
Q

What is cosmological red shift?

A

When space itself is expanding and the light waves are being stretched along with it

36
Q

Is the universe expanding uniformally?

A

Yes

37
Q

What is the rough age of the universe?

A

13 billion years old

38
Q

How can you work out the age of the universe from Hubble’s constant? Why is this only an estimate?

A

t = 1 / H₀
(time = distance/speed)
Because it is unlikely that the universe has always be expanding at the same rate

39
Q

What is the observable universe?

A

A sphere (with earth at its centre) with a radius equal to the maximum distance the light can travel during its age

40
Q

What is the hot big bang theory (HBB)?

A

The universe started off very hot and very dense and has been expanding ever since

41
Q

Why do we think that the universe started off in a very small point?

A

If the universe is expanding, then further back in time it must have been much smaller. If you trace time back, and assume that the expansion has always been happening, then the entire universe must once have been contained in a single point

42
Q

Explain why Cosmic Microwave Background Radiation is evidence for the HBB

A

1) The HBB model predicts that loads of EM radiation was produced in the very early universe
2) This radiation should still be observed today
3) Because the universe has expanded, the wavelengths of this Cosmic Background Radiation have been stretched, and are now in the microwave region
4) This radiation was accidentally discovered by Penzias and Wilson

43
Q

Describe CMBR

A

1) A continuous spectrum, corresponding to a temperature of 2.73K
2) Mostly isotropic (same in all direction) and homogeneous (same everywhere) - it’s about the same intensity everywhere you look

44
Q

What evidence is there for Earth (and our galaxy) moving through space towards some unknown mass (the Great Attractor)?

A

CMBR shows Doppler shift, indicating Earth’s motion through space. It turns out that our galaxy is rushing towards some unknown mass

A1-Phys-23-1-A1 (60 decks)

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