12.1 - Space

Question 1

Stars are at such enormous distances from the earth, what information can we get from them???

Answer 1

The only information we have about them is the electromagnetic radiation we receive from them.

Question 2

We know the electromagnetic radiation we receive from stars on earth, what can this be used for

Answer 2

From this limited information, we can measure various stellar properties. These allow us to classify stars into various groups which have quite enigmatic names, such as red giant, white dwarf and blue supergiant. They are much too far away for us to send probes to them, or even to send signals to them in the hope of detecting reflections.

However, the electromagnetic emissions from stars can tell us their temperature, chemical composition, speed of movement, approximate age, size and much more! 🤩

Question 3

Is there a better method to determine how bright a star is other than just observing it with the naked eye?

Answer 3

With the naked eye, we are only able to distinguish six different levels of how bright stars appear to us. This is insufficient for scientific use, as many stars of differing brightness would appear identical to our eyes.

Astronomers therefore use a more precise measure to classify the actual brightness of stars: their output power, which is known as luminosity.

Question 4

What can luminosity be defined as

Answer 4

We define luminosity as the rate at which energy of all types is radiated by an object in all directions. This depends upon both objects size and it’s temperature

Question 5

What does an object/stars luminosity depend on

Answer 5

The objects size and more importantly, it’s temperature

Question 6

Explain the black body radiation curve for different temperatures

Answer 6

Y axis = energy output
X axis = wavelength

The higher the temperature of the star, its energy out put peak will be further to the left and a higher peak overall.
The distribution is given by the Stefan-Boltzmann law. This tells us that the output power from a black body is proportional to its surface area and the fourth power of temperature.
Since L = sigma x A x T^4
ADD MORE STUFF TO THIS FROM BOOK

Question 7

What’s a perfect black body radiator

Answer 7

A perfect black body radiator will give off energy across the entire EM spectrum

Question 8

What is the Stefan-Boltzmann law

Answer 8

L = sigma x A x T^4

Where L = luminosity
A = surface area
T = temperature in kelvin (SI unit)

The Stefan-Boltzmann constant, sigma is 5.67 x 10^-8 W m^-2 K^-4

For a spherical object this equation would become

L = 4 x pi x r^2 x sigma x T^4
Working on the assumption that a star acts like a black body emitter, which is a very good approximation, this equation describes the luminosity of a star.

Question 9

What is important to remember about black body radiators

Answer 9

A black body radiator is a theoretical perfect emitter, which follows the Stefan-Boltzmann energy output curve for its temperature and also follows Wien’s law.

Remember black body radiation is a thermodynamics idea that can be applied in other areas of physics.

Question 10

What is the range of wavelengths emitted by a star also known as

Answer 10

It’s spectrum

Question 11

How can we determine the temperatures of stars

Answer 11

There are various methods, but we will focus on one that uses the wavelengths of light given off by a star

When we examine a stars spectrum, we find that some wavelengths are given off with more intensity than others.

Question 12

Tell me about Wien’s law

Answer 12

We saw from the Stefan-Boltzmann law that as the temperature of a black body increases, it emits more energy. At higher temperatures the black body radiation curve has a more pronounced peak, and the wavelength of the peak output gets shorter as the temperature rises. The relationship between the peak output wavelength and temperature is described by Wien’s law

Lamder(subscript max)x T = 2.898 x 10^-3 mK

The number 2.898 x 10^-3 m K is known as Wien’s constant

Question 13

Define red giant

Answer 13

A red giant is a large star, somewhat cooler than our sun, eg 3000 K

Question 14

Define a white dwarf

Answer 14

A white dwarf is a small hot star, perhaps 10000 K

Question 15

Define blue supergiant

Answer 15

A blue supergiant is a very large, very hot star, perhaps 25000 k

Question 16

Define luminosity

Answer 16

Luminosity is the rate at which energy of all types is radiated by an object In all directions

Question 17

Define Stefan-Boltzmann law

Answer 17

The law is that the power output from a black body is proportional to its surface area and the fourth power of its temperature in kelvin

L = sigma x A x T^4

Question 18

Define Wien’s law

Answer 18

Wien’s law is that the relationship between the peak output wavelength and temperature for a black body radiator is given by the equation:

Wavelength(subscript max) x T = 2.898 x 10^-3 mK

Question 19

Tell me about star classes

Answer 19

Astronomers have classified stars into groups according to their temperature. This is a useful property to use since stars with similar temperatures tend to share many other features. The temperature determines the spectral output of the star, but it can also suggest chemical composition and age.

Question 20

What are the spectral classes and how can you remember them

Answer 20

From hottest to coldest 
O
B
A
F
G
K
M

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Question 21

Tell me about trends in spectral classes

Answer 21

The hotter stars tend to have more mass and are more luminous. The larger gravitational pressure at the centre of a massive star makes the nuclear fusion reactions within the star run very fast, producing a lot of energy and using the hydrogen fuel in the star at an incredible rate. In addition, more massive (as in higher mass) are also larger.

Therefore, with large size and high temperatures, the hotter stars are very luminous, giving off a great deal of energy. At the same distance away from us, these will then appear very bright in the night sky compared with a smaller, cooler star.

Question 22

Tell me about changes in colour between spectral classes

Answer 22

The overall impression of the O class spectrum is a bluish colour, whereas for the M class spectrum there is a pronounced red colour showing. These changes are a direct result of the variation in output curve we saw associated with Wien’s law.

The peak of the black body radiators wavelength output - will show the colour the star appears.

Wien’s law curve shows us what colour a star will appear.

Question 23

Why is the Hertzsprung-Russel diagram called a diagram and not a graph

Answer 23

If you were to plot a graph of luminosity against start temperature, you can confirm this trend. There is a general correlation.

However, the more data you add, the more complex the picture becomes. We must also remember that the temperature measurement assumes the star behaves as a black body, and the luminosity is similarly often not as accurate as we would like. In general, luminosity has to be determined from a calculation that includes the distance to the star, and measuring the distances to stars is by no means an exact science.
Such a lot can give us some very useful insights, but it’s not a graph In the true sense. It is known as a Hertzsprung-Russel diagram

Question 24

What is the Hertzsprung-Russel diagram like

Answer 24

Most stars we observe fall on a diagonal line across the Hertzsprung-Russel diagram, which is called the main sequence. These are stable stars which will exist in this state for the majority of their lifetime. Their correlation represents the connection between brightness and high temperature.

Note that the plot is ALWAYS drawn with hotter temperatures on the left hand side !

There are also other stages in a stars evolution, which appear in other places on the diagram, but these are much shorter than it’s stable period. Thus, there are far fewer stars in those parts of the diagram.

Question 25

What is the majority of ordinary matter in the universe

Answer 25

The majority of ordinary matter in the universe is hydrogen (roughly 75%) or helium (roughly 24%), and it is from these elements that stars are initially formed.

Question 26

Tell me about the general life cycle of a star

Answer 26

From an accretions collection of gases (hydrogen and helium), called a protostar, the life cycle of a star follows a number of stages, with the star ending its life as a white dwarf, neutron star or black hole.

As the star undergoes nuclear fusion, the binding energy differences of the nuclei before and after fusion mean that the process releases energy, often as electromagnetic radiation, to heat the star. The pressure from the vibration of its particles, and em faction trying to escape hold up the structure of the star against its collapse.

The initial mass of a star is a critical factor in determining how the evolution changes, and thus which life cycle a star will follow.

Question 27

What stops a star from collapsing

Answer 27

As a star undergoes nuclear fusion, the binding energy differences of the nuclei before and after fusion mean that the process releases energy, often as electromagnetic radiation, to heat the star.

The pressure from the vibration of its particles, and the electromagnetic radiation trying to escape, hold up the structure of the star against gravitational collapse. It is this constant battle between the outward pressure and gravity that drives the evolution of a star throughout its lifetime.

Question 28

What does the initial mass of the star impact on

Answer 28

The initial mass of the star is a critical factor in determining which of the possible life cycles a star will follow.

The multiple possible life cycles for stars are usually grouped together into just two paths in which the outcomes are similar. These are the life cycle for low mass stars (such as our sun) and the life cycle for massive stars, which have at least four times the mass of our sun

Question 29

What are the two paths for Stellar evolution

Answer 29

The multiple possible life cycles for stars are usually grouped together into just two paths in which the outcomes are similar.

These are the life cycle for low mass stars (such as our Sun) and the life cycle for massive stars, which have at least four times the mass of our sun !

Question 30

Explain the life cycle of a low mass star

Answer 30

Once it has accreted about the mass of our sun, a low mass star will start to undergo nuclear fusion of hydrogen, converting this into helium. This is a stable stage of life in which radiation pressure and gravity are in equilibrium. The star will remain in this star for billions of years. Eventually, it will run low on hydrogen fuel, but will have produced so much energy that it will expand slightly. This expansion causes the temperature to fall and the star becomes a red giant. Once most of the hydrogen fuel is used, the stat will start fusing helium nuclei. This complex process can cause an explosion which throws some material from the star out into space, forming a planetary nebula.
As the fuel to produce energy to support the star runs out, the outward pressure from fusion drops and gravity takes hold, causing the star to contract to a much smaller size. This heats up the star significantly and it becomes a white dwarf. As time continues, the star will slowly run out of energy and die, passing through the red dwarf stage to become a black dwarf.

Note that the black dwarf stage is theoretical, as it takes a white dwarf longer than the current age of the universe to cool this much, so there has not been time for any to develop.

Question 31

Which stage of a low mass stars life cycle is theoretical

Answer 31

The black dwarf (end/death) stage is theoretical, as it takes a white dwarf longer than the current age of the universe to cool this much, so there has not yet been time for any to develop.

Question 32

Summarise the stages of a low mass star life cycle

Answer 32

Protostar, sunlike star, red giant, planetary nebula, white dwarf

Question 33

Explain the life cycle of a massive star

Answer 33

If a protostar has more than four times the mass of our sun, the star begins life as a blue supergiant. As with low mass stars, nuclear fusion begins and the star enters a stable stage of life in which heat pressure and gravity are in equilibrium.

However, the fusion processes happen at much higher temperatures than in lower mass stars. This means that it burns very quickly, and the conditions make it possible for further fusion of some of the larger nuclei it produces to occur. The fusion of helium can produce a variety of the larger elements, which have mass numbers which are multiples of 4 (helium has four nucleons), such as carbon, oxygen and silicon. There will then be stages of carbon and oxygen burning. A high mass star is likely to be on the main sequence for only up to a billion years.

When the material of such a star has been fused to the point where it is mostly iron, it can no longer undergo nuclear fusion and it stops producing energy. This happens even more abruptly than in low mass stars, and with the enormous gravitational forces produced by a large mass, it undergoes an incredible collapse. This sudden increase in density produces a sudden huge burst of energy, effectively bouncing the collapse back out. This explosion is called a (type II) supernova and is the most immense burst of energy ever witnessed. It is so bright, you can see the change in the night sky with the naked eye.

Within a supernova explosion there is so much more energy that nuclear reactions occur that produce the elements above iron in the periodic table. The natural occurrence of these elements is evidence that supernovae must have occurred in the past, as the binding energies of these heavy elements are such that they cannot be created in other natural processes in the universe.

It can then become a neutron star or black hole

Question 34

Summarise the stages of a massive stars life cycle

Answer 34

Protostar, blue supergiant, red supergiant, type II supernova, then black hole or neutron star

Question 35

What is evidence supernovas that supernovas have occurred in the past

Answer 35

Within a supernova explosion there is so much energy that nuclear reactions occur occur that produce the elements above iron in the periodic table.

The natural occurrence of these elements is evidence that supernovae must have occurred in the past, as the binding energies of these heavy elements are such that they cannot be created in other natural processes in the universe.

Question 36

What happens to massive stars after the supernova stage

Answer 36

After a high mass star has exploded as a supernova, the entire star may be completely shattered. If there remains a central core of stellar material, this will be either a nurturing star (if the core was up to three solar masses) or a black hole (if the core retained more than three solar masses)

Neither of these is easy to detect, as they emit little or no light, and they are not plotted on the Hertzsprung-Russel diagram.

Question 37

What is a neutron star and how does it compare to a black hole

Answer 37

A neutron star consists almost entirely of neutrons, packed as densely together as the nucleons within the nucleus of an atom. They can hold three times the mass of the sun but are only about 10km in diameter.

Black holes are even smaller and hold even more matter than neutron stars. This means that their gravitational pull is immense, so strong that things travelling at the speed of light cannot escape

Question 38

What dictates if a star becomes a neutron star or black whole

Answer 38

When a high mass star explodes as a supernova, and their remains a central core of stellar material, this will be a neutron star or black whole

Neutron star if the core was up to three solar masses

A black whole if the core retained more than three solar masses

Question 39

What can we plot on to a Hertzsprung Russel diagram

Answer 39

You will see that a given star develops through its life as various different types of star. If we observe it at each of these points in its life, they would be plotted in different places on the H-R diagram. Thus, we can plot the life cycle of a star as a movement around the H-R diagram.

Question 40

Define the Hertzsprung Russel diagram

Answer 40

It’s a plot of stars, showing luminosity (or absolute magnitude( on the y-axis), and temperature (or spectral class) on the x-axis.

Question 41

Define the main sequence

Answer 41

It’s a rough diagonal line from the top left to bottom right of a Hertzsprung Russel diagram which marks stars that are in a generally stable stage of their existence

stable stage, when a small mass star is on the main sequence it is a yellow dwarf

Question 42

Define protostar

Answer 42

A protostar is dust and gas clumping together under the force of gravity, prior to the start of nuclear fusion in its core, which will go on to become a star.

Question 43

Define a neutron star

Answer 43

A neutron star is one of the possible conclusions to the life of a large mass star; small and very dense, composed of neutrons

Question 44

Define black whole

Answer 44

A black whole is one of the possible conclusions to the life of a large mass star; a region of space-time in which the gravity is so strong that it prevents anything from escaping, including EM radiation.

Question 45

Define planetary nebula

Answer 45

A planetary nebula is the remnants of an explosion at the end of the life cycle of a low mass star, material which may eventually agglomerate into new planets

Question 46

Define black dwarf

Answer 46

A black dwarf is the final stage of the life cycle of a small mass star, when nuclear fusion has ceased and it has cooled so that it no longer emits visible light.

Question 47

Define supernova

Answer 47

A supernova is the explosion of a large mass star at the end of its lifetime, when it becomes extremely unstable

Question 48

Why do astronomers use alternative distance units to things like metres

Answer 48

Astronomical distances are very large, using standard from notation helps write these numbers, but astronomers have defined a number of alternative distance units to cut down the magnitude of the numbers involved.

Question 49

What is a light year

Answer 49

A light year is the distance that light can travel in one year, which is about 10^16 m.

Question 50

What is AU

Answer 50

The astronomical unit (AU), is equal to the radius of the earths orbit around the sun

1AU = 1.5 x 10^11 m

Question 51

How can limitations of measuring distances to stars be overcome

Answer 51

Limitations can be overcome, or at least minimised by comparing the results from the different techniques on the same star, and refining the techniques to improve accuracy.

Some of the techniques later explained in this section are used on individual stars, and some are used on large groups of stars, such as galaxies or star clusters.

Question 52

Explain fully, how to use Trigonometric parallax to measure the distance to a star

Answer 52

Method used to measure distance to relatively close stars

As the earth moves around the sun, a relatively close star will appear to move across the background of more distant stars. This optical illusion is used to determine the distance of a star.

The star itself does not move significantly during the course of the observations. To determine the trigonometric parallax you measure the angle to a star, and observe how that changes as the position of the earth changes. We know that six months the earth will be exactly on the opposite side of its orbit, and therefore, will be 2 AU from its location today.

Using observations of the star to be measured against a background of much more distant stars, we can measure the angle between the star and the earth in these two different positions in space, six months apart.

As we know the size of the earths orbit, geometry allows calculations of the distance to the star.

The angle measured will be double the angle we want to use, as we have to make a right angled triangle to use SOH CAH TOA

We use tan (theta) = r/d and rearrange to get d

At small angles, as we are usually using in astronomy, tan theta = theta , the distance, d, will come out in the same units to measure r, so we have

d = r/theta

Question 53

What is a parsec

Answer 53

A parsec (pc) is a measure of distance. It is an abbreviation of the ‘parallax second’

It is the distance a star must be from the sun in order for the angle earth-star-sun to be 1 arcsecond

1 parsec = 3.27 light years , 3.09 x 10^16 m

1 arc second = 1/3600 of a degree

Question 54

How can we simplify the formula for using trigonometric parallax

Answer 54

The triangle geometry means that if we have the angle measurement in arcseconds, theta, the distance, d, to the star, measured in parsecs is easily obtainable from

d = 1/theta

Question 55

What is the accuracy of trigonometric parallax dependant on

Answer 55

The accuracy of the trigonometric parallax is dependant on the accuracy of the angle measurement. With atmospheric interference for earth based telescopes, this was for many years limited to stellar distances of about 100 light years. But now with an orbiting telescope, we can accurately measure the distances to stars as far away as 135000 light years.

Question 56

What do we units do we use for measuring the angle in trigonometric parallax

Answer 56

Each degree can be split into 60 minutes, and each minute of arc (an arc is part of a circle) can be split into 60 seconds

1 degree = 3600 seconds
1 arcsecond = 1/3600 of a degree

Question 57

What does the apparent brightness of a star tell us

Answer 57

How bright it looks to us, or how much energy we receive from it - depends upon how far away the star is. If we know the power output of the star, then how bright it appears to us on earth would give away its distance.

Question 58

Tell me about calculating the distance to a star by using the inverse square law/ standard candles

Answer 58

The general equation we had for intensity was I = P/A. The inverse square law means that the energy emitted by a star will spread out in all directions over the surface of an ever increasing sphere. As the surface area of the sphere is pi x r^2, this gives us an equation for the radiant energy intensity at a certain distance, d, from a star:

I = L/ 4 x pi x d^2

Where luminosity, L, is in Watts

Question 59

What are and how can we use standard candles

Answer 59

Some stars, including some variable stars and supernovae, have properties which mean their luminosity can be determined quite separately from other measurements. These are known as standard candles. If we have a figure for luminosity, and measure the energy intensity (brightness) of a star reaching the earth, we can then calculate how far away it is by comparing it with a standard candle with the same luminosity.

Question 60

Tell me about using variable stars to measure the distances to stars

Answer 60

Henrietta Leavitt monitored and found that some stars, their brightness changes, varying in a repeating cycle. The time period of this oscillation in brightness was constant and, importantly in direct proportion to the luminosity of each star.

It was possible to calculate the intrinsic luminosity of these stars, As they were close enough to use trigonometric parallax to find their distance. A longer time period for oscillation meant an intrinsically brighter star. Astronomers then took this relationship and used it to determine the luminosity of variable stars at much greater distances.

RR Lyrae variable stars allow us to measure distances to about 760,000 parsecs. However, we may look at Cepheid variable stars. There are two types, the more luminous, type I cepheids give us the greatest distance measurements using the standard candle technique, out to about 40 million parsecs.

Question 61

How can we determine the luminosity of star using its temperature-luminosity relationship

Answer 61

One of the simples methods of determining the luminosity of a star is simply to look at its spectrum. The peak wavelength gives the temperature from Wien’s law, and the width of spectral lines can determine whether or not it is a main sequence star. If it is, and you find its place on the main sequence of the H-R diagram, you can read the luminosity from the y-axis. However, this is one of the least reliable standard candle methodologies.

Question 62

Define a light year

Answer 62

A light year is the distance that light can travel in one year, which is about 10^16 m

Question 63

Define an astronomical unit (AU)

Answer 63

An astronomical unit is the radius of the earths orbit around the sun, 1AU= 1.5 x 10^11 m

Question 64

Define trigonometric parallax

Answer 64

Trigonometric parallax is a method for measuring the distance to relatively close stars. It works by comparing their measured angle in the sky at 6 monthly intervals and using these measurements to calculate the distance using trigonometry.

Question 65

Define parallax angle

Answer 65

A parallax angle is the difference in angular observation of a given star, for use in the trigonometric parallax method of measuring the distance to a star.

Question 66

Define a parsec

Answer 66

A parsec (pc) is the distance a star must be from the sun in order for the parallax angle earth-star-sun to be 1 arcsecond.
1 pc = 3.09 x 10^16 m

Question 67

Define standard candles

Answer 67

Standard candles are stars with properties which mean their luminosity can be determined from measurements other than brightness.

Question 68

How did we discover red shift

Answer 68

When astronomers first began to look at the spectra of stars in other galaxies during the 1920’s, they noticed that the spectra looked very similar to the spectra from stars in our own galaxy but that all the features present were shifted by the same relative amount towards the red end of the spectrum, this phenomenon became known as the red shift.

Question 69

What is red shift/ the shift due to

Answer 69

This shift is due to the relative motion of other galaxies with respect to ours, in an effect called the Doppler effect: an observer receiving waves emitted from a moving body observed that the wavelength of the waves has been altered to a new wavelength.

Question 70

Explain the Doppler effect

Answer 70

Doppler effect: an observer receiving waves emitted from a moving body observed that the wavelength of the waves has been altered to a new wavelength.

Eg. When a car is stationary, there is the same frequency in all directions

When a car is moving, behind the car, where it’s moving from, the wavefronts are stretched out, since the car is travelling in the opposite direction to the waves.

Ahead of the car, where it’s moving towards you, the wavefronts are compressed, since the car is travelling in the same direction as the waves.

The Doppler effect causes a change in frequency and wavelength if there is relative motion between the wave source and observer.

Question 71

What does the red shift/ Doppler effect look like for a receding star/galaxy

Answer 71

A receding star or galaxy emits light that appears to have a longer wavelength than expected. We also experience this effect when, for example, we hear a car coming towards us and driving past at a steady rate. As it approaches, the note of its engine rises to a maximum pitch, and then falls as the car travels away. You could imagine the waves getting squashed closer together (shorter wavelength) as the car drives towards you, and then stretched further apart (longer wavelength) as it drives away.

Question 72

How can we calculate the speed of a galaxy from earth

Answer 72

Comparison of light from distant galaxies with light produced in (stationary) earth-based experiments can allow calculation of the galaxys speed away from earth.

Question 73

Tell me a formula for calculating the amount of red shift a galaxy exhibits

Answer 73

The amount of red shift a galaxy exhibits, z, allows us to calculate how fast it is moving. This can be done using measurements of either wavelength or frequency changes.

z = triangle lander/ wavelength =(squiggly equals sign) triangle frequency/ frequency =(squiggly equal sign) v/c

Question 74

What did Hubble discover/ what is Hubbles laws

Answer 74

Astronomers quickly realised that redshift implied that galaxies surrounding us were travelling away from us. In 1929 Edwin Hubble published his finding that the value of a galaxy’s red shift is proportional to its distance from us - the further away a galaxy is, the faster it is moving.

This meant instead of being static, the universe was expanding!

Question 75

Simply what is hubbles law

Answer 75

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

We can plot a graph of velocity of recession (y axis) against distance (m), it should be a straight line ! Directly proportional line.

As virtually all galaxies show red shifts in their spectra, Hubble concluded that all the galaxies must be moving apart from each other and the universe is expanding.

Question 76

What is the equation for Hubbles law

Answer 76

v = H(subscript 0) x d

Where v = the recessional velocity

H(subscript 0) = the constant of proportionality, the Hubble constant

d = distance to object

Question 77

Tell me about the Hubble constant

Answer 77

The Hubble constant, can be found from the gradient of a graph plotted with recessional velocity (y) and distance (x)

This has had many values over the years, which demonstrates the immense difficulties involved in accurately determining astronomical distances.

It is hoped that data from the Gaia survey will improve the value more

The current value is considered accurate to within 1% and is 70.9 kms^-1Mpc^-1

Astronomers can now also use Hubbles law to determine distances to newly observed objects.

Question 78

How can we calculate the age of the universe

Answer 78

As all distant objects show a red shift, they are all moving away from us. This implies that the universe as a whole is expanding. Everything must have started from one thing, this would be the time of the Big Bang, when everything first exploded outwards from that single point. If we can find the Hubble constant, it well tell us how quickly the universe is expanding. From this we can work out when it all started.

For an object to travel a distance, d(subscript 0) from the beginning for time, a speed of v(subscript 0), the time taken, T(subscript 0), can be calculated from the basic equation for speed

Speed = distance/time

v(subscript 0) = d(subscript 0) / T(subscript 0)
Means

T(subscript 0) = d(subscript 0) / v(subscript 0)

If we consider the gradient of the Hubble graph
H(subscript 0) = v(subscript 0)/ d(subscript 0)

T(subscript 0) = 1/H/subscript 0)

Time is the age of the universe, this comes out as about 13.8 billion years.

Note that in this you need to take note of units as Hubble constant is quoted as kms^-1Mpc^-1 so time will be in seconds.

Question 79

Define red shift

Answer 79

Red shift is the shift in wavelength of a stars spectrum, caused by increasing separation between the star and earth

Question 80

Define the Doppler effect

Answer 80

The Doppler effect occurs when an observer receiving waves emitted from a moving body observes that the wavelength of the waves has been altered, to a new wavelength, as a consequence of the relative motion

Question 81

Define Hubble’s law

Answer 81

Hubbles law is that the recession velocity of a galaxy is directly proportional to the distance to it

Question 82

Define Hubbles constant

Answer 82

Hubbles constant is the constant of proportionality in Hubbles law

Question 83

Define the Big Bang

Answer 83

The Big Bang is the theory that the universe expanded outwards, from a single point to the currently observed situation

Question 84

How has the Hubble constant changed to give us different ages of the universe

Answer 84

There has been huge variations in values for the Hubble constant depending on the observational methods used to find it. This variation can give us even larger variation in possible ages for our universe, over the years, giving answers ranging from 10 billion to 20 billion years.

The age of our universe is not a fixed answer, but keeps changing as experimental methods improve.

Question 85

Will the universe stop expanding?

Answer 85

The universe is expanding - will this change in the future? The answer critically depends on the mass of the universe and more specifically the density of matter.

Gravity is the force that could slow the expansion down to a stop, and possibly even then start to cause the universe to contract back inwards.

Question 86

What’s the Big Crunch theory

Answer 86

The force of gravity between particles decreases with the square of distance. Thus, if the matter in the universe is only sparse, then the gravitational forces between particles, stars and galaxies will be generally weak. The universe will continue expanding forever. However, if the matter in the universe has more than a critical density then the gravitational forces will succeed in causing the Big Crunch.

Question 87

What are the three things our universe could do

Answer 87

Our universe could expand for ever (open universe)

Our universe could keep slowing down but never reaching a standstill (flat)

Or our universe could slow to a standstill and start collapsing (closed universe)

Question 88

What is dark matter? How did it come about?

Answer 88

To determine the fate of the universe, scientists have been trying to work out its density. However, they have hit upon some big problems. They found that stars in the galaxies tend to rotate at similar speeds, whatever their distance from the galactic centre. This is wholly unlike the solar system, in which the further planets from the sun orbit much more slowly than close in.

Galaxies emit a certain amount of light. (We can measure this by measuring their apparent brightness and accounting for the inverse square law decrease in this caused by the distance to the galaxy) from the actual luminosity of a galaxy, we can estimate the mass of all its stars. Galaxies rotate and this means that all the stars they contain must be experiencing a centripetal force towards the centre of the galaxy’s rotation. when astronomers measure the rotational speed of the stars in the galaxies, they find that the mass suggested by the luminosity calculations is not nearly enough to create the centripetal force needed to keep the galaxy spinning. In fact, the mass of the stars is generally only about 10% of that needed.

This suggests that galaxies must contain a lot of mass that does not emit light. This has been dubbed dark matter.

Question 89

What is dark matter

Answer 89

Astronomers have not yet discovered what the dark matter could be. Most stars will have a planetary system, but the mass of all these planets is not nearly enough to hold a galaxy together. Similarly, black holes or interstellar gases are good candidates for dark matter, but these (and other similar suggestions) are also not observed in sufficient quantities. We can account for some dark matter with straightforward things such as these, but scientists are convinced that the majority of dark matter is another as yet unknown form.

Question 90

What is gravitational lensing

Answer 90

Einstein’s theory of gravity explains that large masses will deform space-time - the fabric of the universe - so that it accelerates things. This is quite different from Newton’s explanation of gravity as a force.

The curving of space time predicts that the direction of travel of light will be affected. Very large masses, such as galaxies or black holes, will cause a bending of light rather like when it passes through a lens. This effect has been widely observed in photographs in deep space.

Using gravitational lensing, astronomers have further verified the existence of dark matter, to the extent that they have made incredibly detailed maps of its locations in space.

Question 91

Why do we think dark energy exists and what is it?

Answer 91

Until the end of the twentieth century, it was not known which of the three possible fates of the universe would happen.

The Hubble space telescope has observed not only that the expansion of the universe is not slowing, as we would expect from the gravitational attractions of all matter in it, but it is actually accelerating!

The universe is getting larger at a faster and faster rate. What could cause this cosmic acceleration is not yet known, but it has been called dark energy.

All we know about dark energy is how much it is affecting the expansion of the universe, and thus what proportion of the universe it is. It turns out that dark energy is approximately two thirds of everything that exists - and we have no idea what it is or how to observe it.

Question 92

What are the proportions of matter in the universe

Answer 92

Ordinary matter (matter we can see and feel) is only 4.9% of matter!!

Dark matter = 26.8%

Dark energy = 68.3%

Question 93

What do scientists believe about dark matter and energy

Answer 93

Most physicists believe that the as yet unidentified dark matter is the solution to the unobserved mass, dark energy is much more of a mystery.

Some scientists have suggested that by tweaking the mathematics in the theory of gravity, the unexplained observations could be accounted for.

However, Einstein’s gravity theory works so well for many observations that we are not ready to alter it until a better version is developed.

Question 94

Define the Big Crunch

Answer 94

The Big Crunch is a possible future for the universe, in which it contracts inwards to a single point

Question 95

Define dark matter

Answer 95

Dark matter is a material in the universe that explains some anomalous behaviours of celestial objects, as yet its nature has not been identified.

Question 96

Define dark energy

Answer 96

Dark energy is a source of a force that is causing the expansion of the universe to accelerate, as yet its nature has not been identified