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Flashcards in Stars & their evolution Deck (51):
1

How many miles is the Sun from Earth?

150 million miles or 1 Astronomical Unit.

2

What is a nebula?

Clouds of dust known as Nebula or 'cloud-forming regions'.

3

How can we view 'baby' stars?

As the baby star is surrounded by gas and dust, and relatively cool (red) we use infrared light.

4

Why is everything a disc?

Gravity pull things in but the centrifugal force counteracts gravity along the rotation axis causing the disc to form.

5

What causes a star to generate heat?

Heat originates from nuclear fusion. An accelerated charged particle (such as an electron) radiates energy as thermal radiation (light).

6

What is Stefan's Law?

Apparent luminosity is related to distance, temperature and area. (NB: the further away a object the smaller it looks).

7

What is Hertzsprung-Russell (H-R) Diagram?

This diagram shows us the evolution of a star i.e. stellar evolution - baby stars are born and appear in the lower-right of the H-R Diagram "Main Sequence".
The chart depicts the luminosity & colour of stars form a distinct pattern.

8

What does colour represented on the H-R diagram?

Colour represents temperature.

9

What does luminosity tell you on the H-R diagram?

Luminosity is telling you how big the star it is (width).

10

How is mass represented on the H-R diagram?

Mass determines the location of a star on the main sequence e.g. low mass stars are cool, red and dim.

11

What is hydrostatic equilibrium?

Hydrogen and helium pushes against gravity to keep the star's shape.

12

What is the most abundant element in the universe?

Hydrogen.

13

What is nuclear fusion?

The combination of two atoms to form another e.g. two hydrogen atoms to form helium. This is how stars are powered.

14

What is nuclear fission?

Take one large atom and pull it apart (e.g. uranium into plutonium) which results in left over neutrons that can cause Chernobyl-esque disasters.

15

Has nuclear fusion been created on Earth?

Yes. We have been able to recreate this on Earth (for about 1min) within the Joint European Torus (JET) nuclear fusion experiment in Oxford. However Hydrogen Fusion requires a lot of energy to force the two atoms together.

16

How is a star created?

1. Gas Cloud
2. The cloud collapses (usually by the shockwave of a nearby star collapsing)
3. Clumpy bits form as gravity pulls it together
4. Star is formed surrounded by dust and gas (the leftover gas and dust creates planets and forms a solar system)
5. The star eventually gets hot enough to create nuclear fusion.

17

What are the main layers of a star?

• Core
• Radioactive Zone (where light tries to escape - radiation is energy transfer by light)
• Convective Zone (where gas is moving in similar way to observing convection (energy movement) in a lava lamp!)

18

What is conduction?

Energy passed between particles.

19

How old is the Sun?

Approximately 5,000 million years old.

20

What is stellar evolution?

The fate of all stars is pre-determined by its mass - locked in place when star forms.

Each star is unique - low and high mass stars evolve differently but can be charted on the H-R diagram e.g.

• Small Star --> Red Giant --> Planetary Nebula --> White Dwarf

• Large Star --> Red Supergiant --> Supernova --> Neutron Star or Black Hole

21

What type of star will live the longest?

Low mass stars.

More mass = stronger gravity = higher temp & pressure = faster nuclear reaction rates = higher luminosity i.e. high mass stars burn out quicker than low mass stars.

22

What is a planetary nebula?

Nebula means cloud and when the core (what eventually becomes the white dwarf) is viewed through early telescope looks like a planet although a Planetary Nebula has nothing to do with planets or clouds!

23

What is a Degenerate Helium Core?

In a main sequence star as the hydrogen is being converted into helium, the helium accumulates like ash in a fireplace at the centre of the star. Once all the hydrogen has been used up at the centre of the star (in 5bn years for our sun), this causes a Degenerate Helium Core.

24

What is a Horizontal Branch Star?

A star burning helium to carbon at the core and then hydrogen to helium in outer layer.

25

What is an Asymptotic Giant Branch Star?

A star with helium depleted at the core so gravity wins over pressure and grows redder and hotter (again) and starts to blow off the outer cores to create the Planetary Nebulae.

26

Summarise a main sequence star's evolutionary process.

1. Hydrogen (H) is turned to Helium (He) until core is pure He
2. H turns into He in the outer layers only
3. Expands into sub-giant and the red giant
4. Core heats up until He fuses into Beryllium (Be) & Carbon (C)
5. This causes a Helium Flash
6. Core expands, cools, fusion slows and outer layers shrink
7. He now becomes C (Horizontal Branch Star)
8. Slowly heats & expands again (Asymptotic Giant Branch)
9. Outer layers gently blown away as Planetary Nebula
10. Core shrinks and fades as a White Dwarf Star

27

What is Electron Degenerate?

The star's core outward pressure reduces, gravity wins and the core becomes very dense and the electrons desperately repels each other.

28

What is the difference between Giant and Super-Giant Stars?

Giant stars will turn off halfway through the main sequence (H-R diagram) whereas Supergiant stars will go through the whole sequence getting hotter and hotter (and more blue) before turning off.

29

What causes a star to pulsate?

As a star expands, it cools until gravity is stronger than the thermal expansion and then energy trapped inside the star heats the gas causing it to expand again. This cycle continues.

These are found on the instability strip on the H-R diagram.

30

What are examples of pulsating stars?

Cepheid and RR Lyrae.

31

What are examples of red dwarfs?

Proxima Centuri and Gilese.

32

What are examples of supergiant stars?

Polaris, Betelgeuse and Antares.

33

What is the CNO Cycle?

In supergiant stars, after fusing hydrogen and helium, the star will then go on to fuse Carbon, then Sodium, Neon, Magnesium. The heavier atoms will fall to the centre. Because the star is so hot at its core it can fuse heavier elements. As the star increases in age you will have layers of these different elements fusing together to form iron in the centre layering out to Hydrogen fusing to form Helium.

34

What are solar winds?

Our Sun releases mass through solar winds which are seen as the "Northern Lights" at the north or south poles on Earth.

35

What happens to giant stars at the end of their lives?

At the end of their lives they explode in a supernova.

36

Why are supernova important?

Supernova are essential for life as they synthesize elements heavier than iron (neutron capture) as well as those within the star itself and these are found throughout the galaxy.

37

What causes a neutron star?

Stars with more than 3x mass the Sun will cause a neutron star.

They are a billion times denser than white dwarfs and 10x15 times denser than water.

38

What is the Large Magellanic Cloud?

Our closest galaxy.

39

What happens to supermassive giant stars at the end of their lives?

More massive stars collapse as gamma-ray bursts into a black hole.

40

What are gamma-ray bursts?

Gamma-ray bursts indicate a black hole has been formed.

41

What is a pulsar?

Rapidly rotating neutron star which pulsates radiowaves.

Due to angular motion as it collapses the rotation speeds up and is measured through xHz i.e. per second

42

What are long gamma-ray bursts?

Caused by collapsing massive stars (about a minute).

43

What are short gamma-ray bursts?

Caused by two neutron stars merging (in less than a second) which collapse and form a black hole.

NB: about half stars in sky are binary stars.

One researcher believes this is the main source for the creation of gold.

44

What is the SWIFT telescope?

The SWIFT telescope (NASA) contains optical (UVOT = ultra violet optical telescope), x-ray (XRT) and gamma-ray telescopes.

We see several black holes forming every week (as SWIFT detects GRB every ~3 days) but Gamma-ray bursts only last a few minutes.

45

What are Cataclysmic Variable (CV) stars?

Believed to be binary star systems where gas is flowing between the stars.

For example: one which is similar to our sun and one that is a white dwarf which is giving off UV light but the accretion disc is what makes it brighter through the 'avalanche of material' i.e. gas streaming from the companion star to the very fast spinning white dwarf star.

46

Who first discovered CV stars?

First discovered by J.R. Hind on 15/12/1855. Star was called U Gem (U = varies in brightness, Gem = in constellation of Gemini).

47

When does a CV star not have an accretion disc?

If the white dwarf has a strong magnetic field there will not be an accretion disc as the gas will be channeled to the north and south poles.

48

What is an X-ray Binary?

Close binary system where a neutron star (or rarely a black hole) accretes matter from what is usually a main sequence star.

X-ray binaries are some of the most luminous X-ray sources in the sky. X-rays are produced as material from the companion star is drawn to the compact object either through Roche-lobe overflow into an accretion disk (low-mass X-ray binaries) or through direct impact of a stellar wind onto the compact object (high-mass X-ray binaries).

49

What is a red dwarf?

A small and relatively cool star on the main sequence, either late K or M spectral type.

Red dwarfs range in mass from a low of 0.075 solar masses (the upper limit for a brown dwarf) to about 50% of the Sun and have a surface temperature of less than 4,000 K.

50

What is the most common type of star?

Red dwarfs are by far the most common type of star in the Milky Way, at least in the neighborhood of the Sun, but due to their low luminosity, individual red dwarfs cannot easily be observed.

51

What has a hotter core and cooler surface?

Red Giants.