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Physics P7 > 7.3 - Mapping the Universe > Flashcards

7.3 - Mapping the Universe Flashcards

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1
Q

Briefly, what is parallax?

A

An apparent change in position of an object against a distant background.

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2
Q

In astronomy, what is a parallax angle?

A

HALF the angle moved against distant background stars over 6 MONTHS. The NEARER an object is to you, the GREATER the angle.

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3
Q

What are parallax angles measured in?

A

Arcseconds (“)

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4
Q

What is one Arcsecond equal to?

A

1/60 of a minute = 1/60 of a degree

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5
Q

What is a parsec (pc) ?

A

The distance to a star with a parallax angle of 1 arcsecond.

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6
Q

What is 1 parsec equal to in LIGHT YEARS?

A

3 ly

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7
Q

What is the formula for calculating a distance in parsecs?

A

Distance (pc) = 1 / parallax angle (ARCSECONDS)

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8
Q

Why is calculating Parallax not really effective for ALL stars?

A

It can only be used to identify nearby stars. As the more distant the star is, the less it appears to move so it is less noticeable to measure.

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9
Q

What is an Interstellar Distance? How many parsecs is it?

A

The distance between stars = A few parsecs

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10
Q

What is an Intergalactic Distance? How many parsecs is it?

A

The distance between galaxies = A few MEGAparsecs

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11
Q

How many Megaparsecs (Mpc) are there in parsecs?

A

1 000 000 (1 million) Parsecs.

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12
Q

What is “luminosity”?

A

The total power emitted by a star.

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13
Q

What does the luminosity of a star depend on?

A
  1. Its size (larger star = greater surface area to emit radiation)
  2. Its temperature (If temperature increases = radiation emitted increases)
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14
Q

What is “observed intensity / brightness” ?

A

How much energy emitted by a star REACHES THE EARTH.

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15
Q

What law does the spreading of radiation from a point source obey?

A

The INVERSE SQUARE law

Doubling distance = Spreads out 4x the area = 1/4
Tripling Distance = Spreads out 9x the area = 1/9

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16
Q

What is a ‘period’ of a star?

A

The time taken to complete one full cycle.

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17
Q

What is a Cepheid Variable star?

A

A star where its luminosity varies in a regular pattern, brightening and dimming again every few days.

18
Q

How do astronomers find the distance of a really distant star?

A
  1. Use parallax to calculate the distance to nearby Cepheid Variable stars.
  2. Use that information and how the bright the star appears to be to calculate the luminosity.
  3. Link this to the period of the star to calculate how the luminosity depends on the period.
  4. Estimate the distance to Cepheid Variable stars that are too far away for the Parallax method to be used.
  5. By studying the star to find its period you can work out its luminosity.
19
Q

If we know the luminosity of a star, what information can that give us?

A

We know how much light it gives out. By comparing this to how much light actually reaches Earth we can calculate how far away the star is.

20
Q

When looking at Cepheid Variable stars of the same period, the ________ the star, the ______ away it is.

A

The fainter the star the further away it is.

This is because they have the same period so they must have the same luminosity.

21
Q

What allowed an estimate of the size of a galaxy to be determined?

A

Cepheid Variable stars.

22
Q

What did Harlow Shapley use Cepheid Variable stars (in groups of stars called globular clusters) ?

A

To find their luminosity and the distance to them.

23
Q

What did Shapley assume when using Cepheid Variables?

A

That all globular clusters were the same luminosity and used this to measure the distance to many different clusters.

24
Q

The use of telescopes led to the discovery of fuzzy bright clouds of light. What are these scientifically called?

A

Nebulae

25
Q

What was the debate circulating around nebulae?

A

Some astronomers thought that nebulae were NEARBY CLOUDS of dust, possibly areas of planetary formation.
Other astronomers believed them to be distant galaxies so far away that they appeared fuzzy. (Island Universe Hypothesis)

26
Q

In the Curtis-Shapley debate, what did Heber Curtis propose?

A
  • Curtis argued for the Island Universe Hypothesis.
  • He said that spiral nebulae were distant galaxies, just like our own, or outside of our galaxy, which he thought was only 30K ly across.
27
Q

In the Curtis-Shapley debate, what did Harlow Shapley propose?

A
  • Shapley thought that our galaxy was much bigger: about 300K ly across and formed the ENTIRE universe.
  • He believed that spiral nebulae were nearby and just small clouds of gas.
28
Q

What evidence did Edwin Hubble provide to help end the debate?

A
  • He showed that spiral nebulae were other galaxies that were much further away than astronomers previously thought possible.
29
Q

Who won the Great Debate? Why?

A

Neither Curtis nor Shapley was 100% correct.

  • Curtis was correct in that spiral nebulae were other galaxies but his estimate of the size of the Milky Way was too small.
  • Shapley was wrong about the nebula but his estimate of the size of the Milky Way was pretty close.
30
Q

How did Edwin Hubble finally show that some nebulae were actually galaxies?

A

He discovered a Cepheid Variable star in the Andromeda nebula that had a period of 31 days (which meant that it should be very bright).
The star appeared to be very faint and when calculated it was 1 million light years away (well outside the Milky Way)

31
Q

What did Hubble make us realise?

A

That the Universe was far bigger than anyone had imagined. Andromeda is one of the closest galaxies to us.

32
Q

What is ‘redshift’?

A

When objects such as galaxies in the spectrum are shifted almost always towards the red end of the spectrum.

33
Q

What does this redshift indicate?

A

That almost all galaxies are moving away from us.

34
Q

What did Hubble find out about the speed-distance relationship when studying Cepheid variables?

A

The further away a galaxy is, the faster it is moving away from us.

35
Q

What did the discovery of galaxies moving away from us show?

A

Provided evidence for the Big Bang Theory.

36
Q

What graph from Hubble’s results formed a straight line?

A

A graph showing recessional velocity is proportional to the distance from Earth,

37
Q

What is the Constant of Proportionality known as?

A

Hubble Constant

38
Q

How would you calculate the Speed of Recession?

A

Speed of reccison (km/s) = Hubble constant (km/Mpc) x distance (km or Mpc)

39
Q

What value is the Hubble constant, generally?

A

~68-70 km/Mpc

40
Q

Because almost every galaxy is moving away from us and the further away it is the faster it is, what does this mean?

A

That space itself is expanding.

41
Q

What does the expansion of space suggest?

A

The Universe was much smaller in the past and so supports the Big Bang Theory.

Physics P7 (5 decks)

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