Unit 3

Question 1

Define the term ‘constellation’

Explain how a constellation is different to an asterism

Answer 1

A constellation is an area of the sky that contains a specific, named pattern of stars

An asterism is a smaller group of stars that form a recognisable pattern. They usually contain only a few stars and make up part of a constellation.

Question 2

Describe the asterism called ‘the winter triangle’

Answer 2

This is a triangle made up of 3 very bright stars in different constellations - Betelgeuse in Orion, Cirrius in Cannis Major and Procyon in Cannis Minor

Question 3

Explain how the greek letter naming scheme for stars work

Answer 3

The greek letter describes the brightness of the star, the earlier the letter in the Greek alphabet the brighter the star in the constellation

The order is alpha, beta, gamma, delta, epsilon

The name of the constellation is then given a 3 letter abbreviation, this comes after the greek letter.

Example gamma ori is the third brightest star in the orion cluster.

Question 4

State 3 constellations that are visible in the UK all year

Answer 4

Cassiopia, Ursa Major and Ursa Minor

Question 5

Explain why different stars are visible at different times of year

Answer 5

During the winter the night sky looks in the opposite direction out into space than in the summer sky, this means that different constellations can be seen at different times of year

Question 6

What is a pointer?

Why are pointers important?

Answer 6

This is a part of an asterism where stars that are lined up form a line that, when extended, will continue on to other stars in the night sky

Pointers are important as it helps astronomers to navigate the night sky by helping them to know which star they are looking at.

Question 7

What are the pointers of the orion constellation

Answer 7

If you continue on the line of roions belt to the left you will eventually hit the star Cirius. If you continue the belt to the right you will hit Aldebaran, and further on is the open cluster called The Pleiades

Question 8

What are the pointers for the Plough (part of ursa major)?

Answer 8

Following the handle further along will lead to Arcturus. Taking the non-handle side and tracing it upwards will lead to Polaris (the north star)

Question 9

What are the pointers for Pegasus?

Answer 9

Connect up the stars to the right of the square and trace it downwards to find the star Formalhaut. A cuved line diagonally left upwards between stars in the square will lead to the Andromeda galaxy

Question 10

What is the celestial sphere?

Why is it useful?

Answer 10

This is a sphere around the Earth that we imagine all of the stars in the night sky to be painted on to

It is useful becuase it contains a coordinate system (declinaiton and right ascension) that can be used to locate stars in the night sky

Question 11

What is declination?

What is it measured in and what is the range?

Answer 11

Declination is the coordinate system in the North-south direction. It is a projection of lattitude onto the celestial sphere.

It is measured in degrees and positive values are north of the equator and negative values are south

The range of declination is + 90^o to -90^o

Question 12

What is right ascension?

What is it measured in and what is its range?

Answer 12

Right ascension is a co-ordinate system in the East-West direction that allows you to locate celesteal bodies in the celestial sphere.

It is measured in arc hours, arc minutes and arc seconds.

There are 24 hours in a circle, each hour contains 60 minutes and each minute contains 60 seconds.

It is measured from the Vernal equinox (the point at which the sun crosses from the south to the north hemisphere).

Question 13

Describe how the location of the North Star (polaris) can be used to find your lattitude on the Earth

Answer 13

The angle of elevation of polaris is equal to the latitude of the observer (e.g. if you see polaris 20^o above the horizon then you have a latitude of 20^o)

Question 14

Define the term circumpolar

Explain how you can calculate if a star is circumpolar or not

Answer 14

A circumpolar star is a star that can be seen in the night sky all year round

A star is circumpolar if the declination of the star is greater than 90^o minus the observers lattitude

When written as an equation:

declination > 90 - lattitude

Question 15

Describe how to use circumpolar stars to calculate the period of rotation of the Earth

Answer 15

1. Set up a camera with a long exposure time pointed at the North Star (Polaris)
2. Open the shutter on the camera for a prolonged period of time (e.g. 2 hours)
3. Measure the arc made by multiple stars (the number of degrees around Polaris it has moved) and take a mean average of them
4. Use this equation to calculate the period of rotation of the Earth

Question 16

Why do stars appear to move across the sky?

Whgich direction do they appear to move in the Northern hemisphere?

Answer 16

Stars appear to move across the sky becuase of the rotation of the earth. The motion of the observer relative to a fixed star is the same as the star moving while the observer stays still

The stars appear to move from left to right in the norther hemisphere

Question 17

Define the word culminate

Explain why stars appear brighter when they culminate

Answer 17

Stars culminate when they cross the observers meridian this means they are due south and at their highest point in the sky

They are at their brightest at this point becuase there is the smallest thickness of the astmosphere between the observer and the star (the light is shining straight downwards) meaning that the lowest amount of light is absobed by the atmosphere.

Question 18

Explain how to calculate the difference between the culmination times of two stars.

Example: if there are tow stars one with an RA of 6hours and 18 minutes and another with a RA of 6hours 23 min, and the first star culminates at 23.35 when will the second star culminate?

Answer 18

The the difference between the culmination times of two stars is equal to the difference in their right ascensions

For the example, the second star will culminate 5 min later, at 23.40 because the difference between their right ascensions is 5 min.

Question 19

Explain how to get the best results from a naked eye observing session of the night sky

Answer 19

1. Observe the sky on a clear night
2. Observe the sky during a new moon - during a full moon the light from the moon will be too bright, you eye will adapt to this and then not be able to see faint stars
3. Pick an area with low levels of light pollution
4. Dark adapted eye - star in a dark area for 30 min so that your eyesight is adapted to the dark. Record observations under the light of a red torch as this will have less of an effect on stopping your eye from being dark adapted.
5. Averted vision - The rods in your peripheral vision are better at detecting faint light compared to the central cones, so look at the stars out of the side of your eye
6. Do not strain - straining the eye to see causes the iris to contract and allows less light through the pupil, making it harder to see faint objects

Question 20

Explain what the Messier catalogue is and why it was made

Answer 20

The messier catalogue is a list of nebula that can be seen using the naked eye. A nebula is a fuzzy ‘cloud like’ object in the sky

The catalogue was made becuase Charles Messier was hunting for comets and kept on finding the same objects that were not comets. He made a catalogue of where they are so he could aviod them in future.

Question 21

Explain the difference between absolute magnitude and aparrent magnitude

Answer 21

Absolute magnitude is the brightness that a star would seem to be if you were viewing it from 10 parsecs away.

Aparrent magnitude is the magnitude of the star when viewed from earth

Question 22

Describe how the magnitude scale of brightness works

What is the faintest star that can be seen with the naked eye

Answer 22

The higher the number the dimmer the star. This means that a star of magnitude 2 is much brigher than a star of magnitude 7 for example.

The faintest stars that can be seen with the naked eye have an aparrent magnitude of 6.

Question 23

What does it mean if the aparrent magnitude is higher than the absolute magnitude for a star?

Answer 23

If the aparrent magnitude is higher then the star is dimmer when viewed from the Earth than it would be if viewed 10pc away. This means that the star must be further than 10pc from the Earth

Question 24

Describe how to comare the brightnesses of stars.

Example: how much brigher is a star of magnitude 3 than a star of magnitude 6.5?

Answer 24

To compare brightnesses you use this formula:

Multiplication factor for brightness = 2.5ⁿ

n = the difference between the absolute magnitudes of the stars

Example: the difference between the magnitudes is 3.5.

This means it is 2.5^3.5 = 24.7x brighter

Question 25

State 4 factors that affect the aparrent magnitude of a star

Answer 25

1. The actual brightness (power emitted) by the star
2. The distance of the star from the Earth
3. The amount of light absorbed by interstellar gas and dust in space
4. the amount of light absorbed by the atmosphere of the Earth

Question 26

How do you find the distance of a star from the earth if you know the absolute and aparrent magnitudes.

Example calculation: How far is cirius from the earth - Aparrent magnitude = -1.44, Absolute magnitude = 1.45

Answer 26

1. Calculate the difference between the absolute and aparrent magnitudes (n) (Aparrent - absolute)
2. Perform the calculation 2.5ⁿ to get the difference in brightness (how much more dim it is)
3. Square root this number to get the difference in distances
4. Multiply this number by 10 to find out how many parsecs away it is

Example:

1. difference = -2.89
2. x dimmer = 2.5^-2.89 = 0.071
3. difference in distances = √0.071 = 0.267
4. distance in parsecs = 0.267 x 10 = 2.67 Pc away

Question 27

Explain how you use the distance modulus formula to find the Absolute magnitude of a star if you know the aparrent magnitude and the distance away.

Example = What is the absolute magnitude of a star with an aparrent magnitude of 5.5 at a distance of 50 pc away

Answer 27

The distance modulus formula states that:

M = m + 5 - 5logd

d = distance in parsecs

M = absolute magnitude

m = aparrent magnitude

Example: M = m + 5 -5logd

= 5.5 + 5 - 5log50

= 2.0

Question 28

What is a binary star?

Explain what you would see if observing a binary star through a telescope?

Answer 28

A binary star is two stars that are orbiting their common centre of gravity (orbiting each other)

When looking at a binary star it will look like a bright point of light which will periodically become fainter as one of the stars moves behind the other, reducing the total light intensity reaching the earth.

the dip is not the same each time, as it is brighter when the primary (larger star) is behind the seconday star, compared to when they are the other way round.

Question 29

Describe how the light intensity of a Cephied variable star changes over time

How is the brightness of the star related to the period of light change?

Answer 29

Cephied vairble stars get periodically brighter and dimmer over time in a regular pattern

The period of the star is the time it takes for the star to go from brightest to brightest. the longer the period of the star, the greater the absolute magnitude of the star. This can be found by looking at a chart of period against absolute magnitude

Question 30

How can Cephied variables be used as a standard candle to work out stellar distances.

Answer 30

The period of a Cephied variable star can be used to find the absolute magnitude of the star. The average aparrent magnitude of the star can also be found by looking at the centre of the pattern on the graph.

Once both of the magnitudes are known then the distance of the star can be calculated by comparing the two magnitudes.

For example, this star has an average aparrent magnitude of 4.

Question 31

Define what the word parallax means

Describe how to use parallax to find the distance to a star

Answer 31

Parallax is when a nearby object appears to move in relation to a far away object when your viewpoint changes

To use parallax to find stars you observe the position of a nearby star inrelation to one in the background. 6 months later when the earth is on the other side of the sun you repeat the process.

Half of the angluar shift in the position is called the parallax angle. The distance to the star can then be found from this formula

d = 1/p

d = distance in parsecs (pc)

p = parallax angle (arc seconds)

In this formula 1 degree contains 60 minutes of arc and 1 minute contains 60 seconds of arc.

Question 32

Describe how stars are formed in a nebula

Answer 32

1. Nebula are large clouds of hydrogen and helium gas, some areas of the nebula are more dense than others.
2. The stronger gravitaitonal force of the more dense areas pulls the surrounding gas in the cloud towards it
3. As the gas falls towards the denser area energy is transferred from the gravitational potential energy store to the kinetic energy store. The gas particles get faster and therefore hotter. This is now an object called a protostar
4. As the protostar collapses in on itself it gets hotter and hotter. When it reaches 15 million kelvin it is hot enough for the hydrogen neucli to form into helium.
5. When fusion ignites the outer layers of the protostar are ejected into orbit around the star, this material will eventually coalesce into planets.

Question 33

What is a main sequence star?

what is the relationship between the mass of the star and how long it remains on the main sequence.

Describe the forces acting in a main sequence star

Answer 33

A main sequence star is a stable star. Stars will remain on the main sequence for the majority of their lifetime. The lower the mass of the star, the more slowly it uses up its fuel so the longer it remains on the main sequence.

Inside a main sequence star, the gravitational force trying to make the star collapse inwards are balanced by the radiation pressure from the fusing hydrogen neuclii which is exerting a force outwards.

Question 34

Describe the life cycle of a small main sequence star, such as our sun

For each part of the life cycle state the elements created in the star

Answer 34

1. A protostar forms within a nebula
2. Fusion ignites in the protostar and it becomes a main sequence star, it remains here for most of its lifetime

Hydrogen is fused to form helium here

3. The core of the star runs out of hydrogen to fuse and gravitational forces now overwhelm the thermodynamic pressure forces and the star collapses inwards. As it collapses it gets hotter and hotter until a layer of the star surrounding the core starts to undergo fusion. The outward pressure force increases and the star swells to become a red giant.

Hydrogen is fused to form helium

4. the Red giant runs out of fuel and collapes in on itself again. When it has collapsed enough to be hot enough to create fusion, helium is fused into larger elements in one great and brief event that ejects the outer layers of the star to form a planetary nebula

Helium is fused to form heavier elements here

5. The inner part of the star collapses to create a dense hot white dwarf star. This is very dense and no fusion occurs. it gives of light as it cools to become a red dwarf then a brown dwarf

Question 35

What is the life cycle of a star much larger than our sun

Include the elements that are created by the star at each stage

Answer 35

1. A protostar forms within a nebula
2. Fusion ignites in the protostar and it becomes a main sequence star, it remains here for most of its lifetime

Helium is fused to form hydrogen here

3. The core of the star runs out of hydrogen to fuse and gravitational forces now overwhelm the thermodynamic pressure forces and the star collapses inwards. As it collapses it gets hotter and hotter until a layer of the star surrounding the core starts to undergo fusion. The outward pressure force increases and the star swells to become a red supergiant.

This star is hot enough to neuclei together to form all of the elements up to iron

4. The Red supergiant runs out of fuel and collapes in on itself again. When it has collapsed enough to be hot enough to create fusion, the outer core undergoes a massive wave of fusing creating a massive explosion. This is a supernova.

All of the known elements are formed during this stage

5. The inner part of the star then collapses again. If the mass of the core is less than 1.4x the entire mass of our sun it collapses to become an incredibly dense neutron star. If the mass of the core is greater than 1.4x the mass of our sun then the core collapses into a singularity - a black hole

Question 36

Explain why neutron stars appear to give off pulses of radio waves (pulsars)

Answer 36

Neutron stars spin at an incredibly fast rate. As they spin they give off radio waves in beams from the magnetic north and south poles, which are in a different place to the pole around which it spins

This acts like a lighthouse beam, when the beam is pointing directly at us we experience a bright radio beam, then as it passes the radio wave intensity drops off to nothing again.

Question 37

Explain why black holes cannot be seen directly

How do we detect black holes?

Answer 37

Black holes have a strong enough gravitational force that even light cannot escape its gravitational pull. This means that light cannot leave the black hole and get to the Earth, meaning that we cannot see it

As material falls into a black hole it gains energy and gets hotter. As it heats off it gives off light and infrared waves which reach the earth and can be detected

Question 38

What is a stellar spectrum?

What information can they tell us about stars?

Answer 38

This is the spectrun of all of the light given off by a star.

The absorbtion lines correspond to wavelngths absorbed by particular elements in the atmosphere of the star, It can therefore tell us what the star is made of

Different temperature stars give of different colours of light in different amounts. By looking at the proportions of light given off we can tell how hot the star is.

Question 39

What is the Hertzsprung-Russell diagram?

Explain why this diagram is useful

Answer 39

The Hertzsrpung-russel diagram is a chart that plots each star on a scale where its temperature is compared to its brightness.

It can be used to show which stars are on the main sequency and which ones are red giants, red supergiants and white dwarfs. This classification can then be used to predict what will happen to the star in the future and to infer some of its characteristics from similar stars.

Question 40

How can the Hertzsprung-russell diagram be used to determine the potential lifespan of a main sequence star

Answer 40

The further you go in the diagonal up left direction of the main sequence stars, the shorter the lifespan of the star. The further you go in the down right direction of the main sequence stars, the longer the lifespan