10. Space

Question 1

Define luminosity

What symbol and units are used?

Answer 1

The rate of light energy released or power output of a star.

Symbol: L
Units: W or Js⁻¹

Question 2

Define Intesity

What symbol and units are used?

Answer 2

The power received from a star (its luminosity) per unit area.

Symbol: I
Units: Wm⁻²

Question 3

What about a star follows the inverse square law?

Answer 3

It’s intensity.

Showed by I=L/4πd².

(4πd² beacuse light from a star is assumed be emitted equally in all directions from a point.)

Question 4

What is parallax?

Answer 4

The apparent change of position of a nearer star in comparison to distant stars in the background

Question 5

Why does parallax happen?

Answer 5

Because the orbit of the Earth around the Sun.

Question 6

How is parallax measured

Answer 6

Angle of parallax (θ).

by measuring the angle to a star and seeing how this angle changes as the Earth changes position.

Question 7

What can you deduce from the angle of parallax about a star’s distance?

Answer 7

The greater the angle of parallax, the closer the star is to the Earth.

Question 8

What is meant by Astronomical Unit (AU)?

Answer 8

The average distance between the centre of the Earth and the centre of the Sun.

Question 9

What is meant by Parsec (pc)?

Answer 9

The distance at which the angle of parallax is 1 arcsecond (1/3600th of a degree).

Question 10

What is meant by Light Year (ly)?

Answer 10

The distance that an EM wave travels in a year in a vacuum.

Question 11

What is a standard candle?

Answer 11

An object of known luminosity.

Question 12

How are standard candles used to measure distances?

Answer 12

By measuring the intensity detected from the light source on Earth and using the inverse square law equation to calculate its distance away.

Question 13

What does a Hertzsprung-Russell (HR) diagram show?

Answer 13

The stellar luminosity of a star against its temperature

Question 14

What can you infer from a Hertzsprung-Russell (HR) diagram?

Answer 14

What spectral class that star belongs to.

Question 15

Where are Red Giants found on a Hertzsprung-Russell diagram?

Answer 15

Top Left

with red supergiants being at the very top

Question 16

Where are Blue Giants found on a Hertzsprung-Russell diagram?

Answer 16

Top left.

Question 17

Where are Main Sequence Stars found on a Hertzsprung-Russell diagram?

Answer 17

From top left to bottom right in a y=-x³ shape.

Question 18

Where are White dwarfs stars found on a Hertzsprung-Russell diagram?

Answer 18

Bottom left.

Question 19

What direction do the scales go on a Hertzsprung-Russell diagram?

Answer 19

X-axis: Temp Decreasing

Y-axis: Luminosity Increasing.

Both exponential.

Question 20

What main feature dictates the lifecycle of a star?

Answer 20

Its mass.

Question 21

How are protostars formed?

Answer 21

• Clouds of gas and dust (nebulae) have fragments of varying masses that clump together due to gravity.
• Irregular clumps rotate gravity/conservation of angular momentum spins them inwards.
• This forms a denser centre – a protostar.

Question 22

What happens to form a main-sequence star?

Answer 22

• High temps and pressures allow for fusion.
• The inward force of gravity and the outward force due to fusion are in equilibrium – the star is stable.
• Hydrogen nuclei are fused into helium.
• The greater the mass of the star, the shorter it’s main sequence period because it uses it fuses at a faster rate.

Question 23

What happens to form a Red Giant star?

Answer 23

• Once the hydrogen runs out, the temperature of the core increases.
• It begins fusing helium nuclei into heavier elements (E.g. Carbon, Oxygen and Beryllium).
• The outer layers of the star expand and cool.

Question 24

What happens to form a White Dwarf star?

Answer 24

• When a red giant has used up all its fuel, fusion stops and the core contracts as gravity is now greater than the outward force.
• The core becomes very dense.
• A white dwarf will eventually cool to a black dwarf.

Question 25

What happens to form a Red Supergiant star?

Answer 25

• A high-mass star runs out of hydrogen nuclei.

The same process for a red giant occurs but on a larger scale.

Question 26

What happens to form a Supernova?

Answer 26

• When all fuel runs out, fusion stops and the core collapses inwards very suddenly and becomes rigid (as the matter can no longer be forced any closer together).
• The outer layers of the star fall inwards and rebound off of the core, launching them out into space in a shockwave.
• As the shockwave passes through surrounding material, elements heavier than iron are fused and flung out into space.
• The remaining core depends on the mass of the star.

Question 27

What happens to form a Neutron Star?

Answer 27

• The core of a large star collapses, gravity is so strong that it forces protons and electrons together to form neutrons and neutrinos (which are flung out in the supernova).

Question 28

What happens to form a Black Hole?

Answer 28

• When the core of a giant star collapses, the neutrons are unable to withstand gravity forcing them together.
• The gravitational pull of a black hole is so strong that not even light can escape.

Question 29

What is the Doppler Effect?

Answer 29

The compression or spreading out of waves that are emitted or reflected by a moving source.

Question 30

What effect does the Doppler Effect have on line spectra?

Answer 30

Causes the line spectra of distant objects to be shifted either:

• Towards the blue end of the visible spectrum when they move towards the Earth (blue-shift)

OR

• Towards the red end of the spectrum when they move away from the Earth (red-shift).

Question 31

Why is redshift evidence fro the expanding universe?

Answer 31

• Distant objects are red-shifted.

- The more distant the object, the greater its red-shift.

Question 32

When using the redshift formula what is the exception for its use?

Answer 32

It can only be used when v is much smaller than c.

Question 33

What is Hubble’s law used for?

Answer 33

For objects at cosmological distances.

Question 34

In Hubble’s law, what are the units of distance?

Answer 34

Megaparsecs.

Question 35

How can we suggest the universe began from one point.

Answer 35

• The redshift of distant objects shows that they are moving away from us.
• This suggests that the universe is expanding.
• It would be reasonable to assume that the universe began from one point
• This point is a singularity that was infinitely small and infinitely hot.

Question 36

How can we estimate the age of the universe?

Answer 36

Using Hubble’s Law:

v = H₀d
v/d = H₀

1/H₀ = d/v = time

∴ t = 1/H₀

Question 37

If we can’t detect dark matter, how do we know it exists?

Answer 37

• By considering the centripetal force exerted on stars in the outer orbits of a galaxy we expect them to travel slower than stars closer to the galactic centre.
• As centripetal force is inversely proportional to the distance from the centre.
• But it has been observed that all stars in a galaxy tend to travel at the same speed regardless of distance from the centre of the galaxy.
• This suggests that the stars have a larger mass than they appear.
• This extra mass is believed to be caused by dark matter.