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sns > Astrophysics topic test 3 > Flashcards

Astrophysics topic test 3 Flashcards

1
Q

Universe

A

a large collection of billions of galaxies

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2
Q

Galaxy

A

a large collection of billions of stars

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3
Q

Our Sun is just one of many billions of stars that form the

A

Milky Way Galaxy

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4
Q

Our solar system is in which galaxy

A

the Milky Way Galaxy

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5
Q

Force which keeps stars together in a galaxy

A

Gravity

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6
Q

moons orbit ?

in a

A

planets

fairly circular

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7
Q

planets orbit?

in a

A

the Sun

fairly circular

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8
Q

artificial satellites orbit

in a

A

earth

fairly circular orbit

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9
Q

Comets + Asteroids orbit ?

in a

A

sun

highly eliptical

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10
Q

A comet’s highly eliptical orbit means it is fastest at which point

A

when it is closest to the sun

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11
Q

What happens to gravitational potential energy as comets approach the sun

A

It loses gravitational potential energy and gains kinetic energy, causing it to speed up to its fastest point nearest the sun (least GPE, hence most KE)

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12
Q

How does gravitational force cause an orbit ? (4 steps)

A

1) circlular motion means changing direction /acceleration

2) due to centripetal force of gravity w/ object already moving causing acceleration towards object of orbit.
instantaneous velocity keeps circle

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13
Q

In what direction should force arrows of gravity be drawn

A

form centre of mass of object experiencing force TO centre of mass of object exerting force
(BALL TO WHIRLY STICK!)

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14
Q

Gravitational field strength depends on

A

1) Mass

2) Distance

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15
Q

More mass = (gravity)

A

larger gravitational field strength

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16
Q

More distance = (gravity)

A

smaller gravitational field strength

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17
Q

Why does an object weigh less on the moon than on earth

A

The moon is less massive, hence gravitational field strength is weaker meaning less pull of object towards centre of mass and less force thus generated (weight)

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18
Q

If an object in orbit speeds up, what happens to the radius of its orbit

why?

A

It decreases / gets smaller / gets closer to / star

gravitational field strength decreases with distance,

a closer object will have stronger gravitational attraction, higher centripetal force, and due to object already moving , higher acceleration to centre of mass, which requires a higher instantaneous velocity to level it out and keep it in a circle.

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19
Q

What is the value of G (gravitational field strength) on earth

A

9.8 N/Kg / M/s^2

10 N/kg

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20
Q

Orbital speed can be calculated with S= D/T

How do you measure Distance and Time

A

Distance = circumference of orbit = 2pi R (R= distnace form centre of planet to object that’s orbitting it)

Time = time period

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21
Q

Orbital speed equation

A

Orbital speed = 2 x Pi x orbital radius / time period

V= 2 x pi x r / T

22
Q

Why can’t we use orbital speed equation for comets

A

it has an eliptical orbit

23
Q

Comets eliptical orbit starts at one side of the X and comes towards the Y

A 
X = solar system 
Y = sun
24
Q

Define the function of the two competing forces inside a star

A 
Gravity - pulls layers of star inwards
External pressure (thermal expansion) - pushes layers away from centre
25
Q

As temp increases, pressure

A

increases and thermal expansion occurs, expanding the star

26
Q

As temp decreases, perhaps due to nuclear fusion stopping, outward pressure

A

decreases, causing star to contract and collapse

27
Q

How does a star equal in mass to the sun form?

A

1) All stars form from a cloud of dust and hydrogen called a nebula
2) Gravity pulls dust and gas together to form a PROTOSTAR
3) As protostar gets denser as particles collide, temperature increases. At a point, the hydrogen nuclei can now undergo nuclear fusion to form helium nuclei. This gives out lots of energy, keeping the core of the star hot. MAIN SEQUENCE STAR is born
4) Now a long stable period during which OUTWARD PRESSURE caused by THERMAL EXPANSION (from nuclear fusion) balances GRAVITY pulling layers of the star inwards. This lasts several billion years. The heavier the star the shorter its time on main sequence because it must consume lots of fuel to retain luminosity.
5) Eventually hydrogen gas in the core runs out and thermal expansion no longer occurs because no nuclear fusion, therefore gravity is greater than outward pressure and the star is compressed untill it is dense and hot enough that nuclear reactions start again and thermal expansion occurs and outward pressure makes the outer layers of the star expand and well up to form a RED GIANT. It becomes red when the surface cools
6) Once this second stage of reactions has finished, the star becomes unstable and ejects its outer layer of dust and gas to from a PLANETARY NEBULA , leaving behind the hot, dense, solid core - a WHITE DWARF

28
Q

How does a star greater in mass to the sun form?

A

1 - 4 same
5) when hydrogen in core runs out, and star is compressed due to gravity vs outward pressure / thermal expansion imbalance, the new nuclear reaction form a RED SUPERGIANT

6) From here, big stars glow brightly as they continue to make heavier element s when undergoing more fusion. Expand and contract several times due to thermal expansion vs gravity imbalances. Eventually explode in a SUPERNOVA
7) Exploding supernova throws outer layers of dust and gas into space creating a NEBULA, leaving very dense core called NEUTRON STAR. If neutron star is massive enough, it collapses due to gravity and becomes a BLACK HOLE.

29
Q

What colours are used to classify stars

A

Blue, white, yellow, orange , red

30
Q

What is hottest / coldest

A

hottest = blue / coldest = red

31
Q

Why does hot = blue, red =cold

A

The hotter the star, the shorter the wavelength of light it will emit

32
Q

Two things a star’s brightness depends on

A

1) Temp
2) Size
bigger, hotter = brighter

33
Q

Why is classifying stars brightness difficult

A

because star’s brightness also depend on distance as well as temp, so close to earth could seem brighter.

34
Q

Define absolute magnitude

A

a measure of how bright a given star would appear to be if it was a fixed distance from Earth (~3.1 x10^17m).

35
Q

Advantage of absolute magnitude

A

allows us to compare the brightness of stars without worrying about they distance

36
Q

Confusingly, the lower the absolute magnitude (negative values), the …. the star

A

brighter

37
Q

Scale to classify stars other than visual colour, with mnemonic Oh Be A Fine Girl Kiss Me

A

OBAFGKM (O = blue, M = red)

38
Q

Doppler effect is when a source of waves is moving relative to observer, and thus undergo an apparant change in… and … when observed compared to when emmited

A

frequency, wavelength

39
Q

Red shift def

A

When light source is moving away from you, more distance = more energy needed, hence peaks aren’t as big as they are when emitted and wavelength increases, so frequency decreases and light appears to be shifted toward the lower frequency end of the visible part of the EM spectrum - red-shifted.

40
Q

Light from distance stars is …

A

red-shifted

41
Q

The red-shift of stars mean that stars must be

A

moving away from the earth

42
Q

How is light frequency from galaxies detected

A

Different elements are exposed to the light and because they absorb different frequencies or wavelengths of light, a pattern of dark lines is produced with a dark line at each frequency absorbed

This is compared to the light given out from hydrogen + helium gas on earth

43
Q

Amount that light from a galaxy is redshifted =?

A

change in wavelength / reference wavelength = velocity of a galaxy / speed of light

44
Q

What is the “reference wavelength”

A

The wavelength of the light when it was emmited, before it was red-shifted

45
Q

The Hertzsprung -Russell diagram is a graph of X against Y for many, many stars

A

Absolute magnitude against temperature

46
Q

Where are red giants and supergiants found ?

Describe their size and temp and brightness

A

Top right, cool but very large, very bright

47
Q

White dwarfs are found where? Describe them

A

Bottom left. They are very hot but small, so are dim

48
Q

Main sequence stars found where? Describe

A

diagonally from top left to bottom right . since all roughly same size, a brighter star = higher temp

49
Q

Two pieces of evidence for the Big Bang

A

Red-shift, Microwave radiation (CMB)

50
Q

How does red-shift prove the universe is expanding ?

How does it prove the Big Bang?

A

More distant galaxies have greater red-shifts than nearer ones, hence they are moving faster and the universe is expanding

Something must have set these galaxies into motion - a big explosion - the Big Bang

51
Q

4 point thought process of Big Bang theory

A

1) Initially all matter occupied a single point
2) This tiny space was very dense and very hot
3) This single point then “exploded” - the Big Bang .
4) Space started expanding, and the expansion is still going on

52
Q

What is CMB in relation to Big Bang

A

CMB radiation is the leftover energy of this initial explosion

sns (4 decks)

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