Topic 7 - Astronomy

weight

The force acting on an object due to gravitational attraction

What does the gravitational field strength of a planet depend upon?

The mass - the greater the mass of the planet then the greater its gravitational field strength

Grav field strength on earth

10 N/kg

Where does gravitational force always act?

towards the centre of the larger body

On different planets, what factors of an object are constant and change?

Weight changes between planets

The mass of an object stays the same between planets

Weight equation

weight(N) = mass(kg) x grav field strength(N/kg)

satellite

an object which orbits a second more massive object

the two types of satellites

Natural

Artificial

natural satellites

The moons that orbit planets.

artificial satellites and eg

man made and can orbit any object in space i.e. ISS orbits Earth

asteroids

a small rocky object which orbits the Sun

Where is the asteroid belt?

lies between Mars and Jupiter

planets in solar system

Rocky planets:

Mercury

Venus

Earth

Mars

Gas planets:

Jupiter

Saturn

Uranus

Neptune

speed of light

3.0 × 10^8 m/s

geocentric model

earth was at the centre of the solar system

Other planets orbited the earth.

Heliocentric Model

A representation of the relationship between the Sun and planets in which the planets revolve around the Sun. Copernicus proposed the model.

Evidence for geocentric model in the old days

When observing the Sun, Moon, stars or planets with the naked eye, they appear to be moving across the sky and not just orbiting the Sun

Instead, they seem to appear to be going around the Earth

From simple observation, objects appear to move in the same direction and in a predictable pattern of movement which is the same each day

retrograde

When viewed from earth, the planet appears to be moving backward in its orbit

Evidence for heliocentric model

Moons of other planets are clearly seen to be orbiting that planet rather than earth

Telescopic observations show that planets don't move in an orbital path around earth

Retrograde motion of planets which can only be explained by heliocentric model

What does grav force of the sun cause planets to do?

Maintain circular orbit

Similarities in the orbits of different planets around the Sun

Their orbits are all slightly elliptical (stretched circles) with the Sun at one focus (approximately the centre of the orbit)

They all orbit in the same plane

They all travel the same direction around the Sun

Differences in the orbits of different planets around the Sun

They orbit at different distances from the Sun

They orbit at different speeds

They all take different amounts of time to orbit the Sun

Speeds and times of the orbits of planets closest to vs furthest from the sun

For closer planets:

less time , faster speeds

For further planets:

more time, slower speeds

speed in circular motion(don't memorise but helps to understand speed and time differences between orbits)

circumference/time

2πr/t

Describe comet orbit

Highly elliptical(oval)

speed of the comets change significantly as their distance from Sun changes

Not all comets orbit in same plane and some don't even orbit in the same direction

How does an artificial satellite get in circular orbit?

Needs to have a very specific speed to maintain circular orbit at a certain distance from object.

What happens if the speed of the satellite is too large?

Radius from satellite to the orbiting object

increases

Satellite spirals into space as grav. attraction isn't strong enough to keep it in orbit

What happens if the speed of the satellite is too low?

Radius from satellite to the orbiting object

decreases

Satellite moves towards the object it should be orbiting as grav attraction is too strong to maintain a constant orbital radius

How does an artificial satellite either in/decrease desired orbital radius?

The speed must change

A decreased speed results in a higher orbital radius

An increased speed results in a lower orbital radius

Why is the speed of an object in circular motion constant but always accelerating?

In a circular path, the direction in which the object is travelling will be constantly changing direction

A change in direction causes a change in velocity

Acceleration is the rate of change of velocity

Therefore, if the object is constantly changing direction then its velocity is constantly changing and so the object in orbit is accelerating

angle between direction of instantaneous velocity and resultant/centripetal force in circular orbit

90

Physical and energy changes in a comet as it approaches sun

loses gpe and gains ke

icy body of comet starts to melt and forms a tail pointing away from the sun

Changes to radius and speed in a non-circular orbit(comet)

As comet approaches the sun:

radius decreases

speed increases(stronger grav pull from sun)

As comet moves away from the sun:

radius increases

speed decreases(weaker grav pull from sun)

Nature of star equillibrium

Inward pull of outer layers due to gravity are equal and balanced with outward force from pressure of thermal expansion due to hot gases expanding

What is the result of thermal equillibrium?

Stars in the main sequence remain stable for millions of years

What happens if the temperature of star increases?

Outward pressure increases

causes star to expand

What happens if the temperature of star decreases?

Outward pressure decreases

Causes star to contract

What happens if thermal expansion and inward gravity are unbalanced?

Star no longer in thermal equillibrium

Stars expand/contract

5 steps of life cycle of solar mass stars(just steps)

1.nebula

2.protostar

3.main sequence star

4.red giant

5.white dwarf

nebula

The origin of all stars

a giant cloud of hydrogen gas and dust

nebula to protostar

The force of gravity within a nebula pulls particles closer together until it forms a hot ball of gas called a protostar.

As particles are pulled closer together protostar density increases

More frequent collisions between particles cause temp to increase

protostar to main sequence star

Once the protostar is hot enough, thermal expansion from fusion reactions occur in the core and the force of gravity keep the star in equillibrium.

At this point the star is born and it becomes a main sequence star and it is in equillibrium

MSS to red giant

after several billion years the reactions in the star eventually die down as it runs out of fuel

Therefore inward force of gravity > outward force of diminishing pressure

The core shrinks and heats up

As core shrinks more reactions will cause the outer part of the star to expand making it a red giant

It is red as the outer surface starts to cool

red giant to white dwarf

Star eventually becomes unstable and ejects outer layer of dust and gas as planetary nebula

Core collapses completely and the star becomes a white dwarf

A white dwarf is cooling down and the energy it emits decreases

Life cycle of larger stars (just steps)

1.nebula

2.protostar

3.main sequence star

4.red supergiant

5.supernova

6.neutron star/black hole

MSS to red supergiant

after several billion years the reactions in the star eventually die down as it runs out of fuel

Therefore inward force of gravity > outward force of diminishing pressure

The core shrinks and heats up

As core shrinks more reactions will cause the outer part of the star to expand making it a red supergiant

It is red as the outer surface starts to cool

A red supergiant is MUCH larger than a red giant

red supergiant to supernova

When reactions inside the red supergiant finally finish the core of the star will suddenly collapse and rebound, causing a massive explosion; this is a supernova.

The outer remnants of the star will then be ejected into space during the supernova

supernova to neutron star/black hole

at the centre of the explosion a dense body called a neutron star forms

In the case of the largest stars the neutron star continues to collapse under gravity until it forms a black hole

A black hole is an extremely dense point in space that not even light can escape from

The Big Bang theory

Around 14 billion years ago, the Universe began from a very small region that was extremely hot and dense

Then there was a giant explosion, which is known as the Big Bang

This caused the universe to expand from a single point, cooling as it does so, to form the universe today.

Each point/galaxy expands away from the others

What can be said about galaxies as the Universe expands

Density decreases

This is due to the space within the galaxy expanding

Steady state theory

Main rival to big bang until recently

The idea that the universe has always existed more or less the same as it is seen today.

Expansion is the universe is explained by the idea that as galaxies move apart, new ones are formed in the gaps between them, implying that galaxy density is constant as Universe expands

IT IS NOT ACCEPTED AS A THEORY TODAY as it can't explain other evidence in favour of Big Bang

What is the Doppler effect

Usually an object emitting waves spreads the wavefront symmetrically

But if the wave source moves then the waves are either squashed together or stretched out

As a result there will be a change in frequency and wavelength observed

If the source of waves moves then waves in front of the soucre are squashed, their wavelength decreases and frequency increases whilst waves in front of the souce are spread out, their wavelength increases and frequency decreases

What is redshift?

If an object moves away from an observer the wavelength of light increases

This is known as rddshift as the light moves towards the red end of the spectrum

How does the Doppler effect prove redshift?

If an object moves away from the observer, then there will be an increase in wavelength and decrease in frequency which results in light being moved towards the red end of the spectrum(Doppler effect affects light waves and red has the highest wavelength)

blueshift

If an object moves towards an observer the wavelength of light decreases

This is known as blueshift as the light moves towards the blue end of the spectrum

What does redshift reveal about the expansion of the Universe

By observing light spectra from other galaxies we can see that light is redshifted in those galaxies

However, we also find that galaxies further away are redshifted more than others. This shows that further away galaxies expand faster away compared to nearby galaxies

What does the Big Bang theory suggest about the universe's origin and what are the results of said suggestion?

The early Universe was extremely hot and extremely dense

As a result of this, it must have emitted thermal radiation

What is cosmic microwave background radiation(CMBR)?

radiation in the microwave region of the EM spectrum that is though to have been the remains of the thermal energy from the Big Bang. It is in the microwave region due to redshift and the radiation comes from all directions

What characteristics about CMBR lead to being in favour of BB theory

very uniform and has the exact profile that is expected to be emitted from a hot body that cooled down a long time ago

What theory does CMBR disprove?

The SS theory, whilst redshift can be explained by Steady State, CMBR is a phenomenon that the theory cannot explain, hence it is not an accepted theory

Does CMBR have a constant temp?

Yes for the most part, although there can be very tiny temp fluctuations

Development of Universe Observation - the old age

Started out with ancient astronomers splitting the sky into constellations.

They sketched by hand their observations

Development of Universe Observation - the telescope

the invention of the astronomical telescope is the most important astronomy development

It allowed astronomers to develop more accurate models of the SS i.e. heliocentric, and also observe features of planets and moons for the first time

How can the atmosphere restrict observation quality

air currents distorting starlight path

reflection of light from atmospheric moisture causing interference

Certain wavelengths of EM waves are absorbed by atmosphere restricting observations from astronomical objects that produce said type of waves i.e. x ray, gamma ray,

Development of Universe Observation - the space telescope

To get around atmospheric restrictions, telescopes are often launched into space, the most famous being the Hubble Space Telescope

Benefits of space telescopes

They lead to the discovery of objects not detectable by visible light

More information and data can be collected

Different EM waves can give different types of information about astronomical objects

They can produce much more detailed and magnified images

They produce clearer images which are unaffected by Earth's atmosphere and light pollution

Downsides- of space telescopes

The telescopes are much harder to repair

They cannot be made too large since they need to fit into a rocket to be launched

They are much more expensive