3. Stars

Question 1

What is meant by spectral class?

Answer 1

• letter label given to a star

- relates to its colour and temp

Question 2

Recite the letters of spectral class, from hottest to coldest

Answer 2

Only

Bats

And

Flies

Go

Kicking

Mice

Question 3

What colour is the spectral class O and B?

Answer 3

Blue

Question 4

What colour is the spectral class A and F?

Answer 4

Blue white

White

Question 5

What colour is the spectral class G?

Answer 5

Yellow white

Question 6

What colour is the spectral class K?

Answer 6

Orange

Question 7

What colour is the spectral class M?

Answer 7

Red

Question 8

Describe the evolution of stars of similar mass to the sun through the following stages:

• nebula
• star (main sequence)

Answer 8

• NEBULA
  1. Nebula collapses (dust + gas fall toward centre of mass due to gravitational attraction)
  2. GPE -> KE + THERMAL ENERGY of the gas (thus temp increases)
  3. Protostar forms at centre (planets form in disk surrounding it)
  4. temp in protostar continues to increase until hot enough for nuclear fusion to occur, now main sequence star
• MAIN SEQUENCE (stable)
  1. When size is not changing= stable
  2. Gravitational forces (trying to make star collapse) and thermal pressures (trying to make star expand) are in equilibrium

Question 9

Describe the evolution of stars of similar mass to the sun through the following stages:

• red giant
• white dwarf

Answer 9

• RED GIANT
  1. Runs out of hydrogen -> OuTER SHELL gravitationally collapses, (as gravitational force bigger than thermal pressures) increasing the temp
  2. Eventually hot enough for helium fusion to occur - increases temp further -> star expands to become a red giant (red because surface area so large, cooler than before) fusion of heavier elements becomes possible as temp increases
• WHITE DWARF
  1. Star expands rapidly due to increase in temp (caused by fusion of heavier elements) gravity not strong enough to keep star together -> outer layers of star are ejected (left with core of the star)
  2. Star core compresses until it becomes very small hot white dwarf
  3. Eventually becomes black dwarf when it radiates all of its energy

Question 10

Do high or low mass stars have a larger lifetime? Why?

Answer 10

Low mass as fusion occurs more slowly (not as hot)

Question 11

Describe the evolution of stars with a mass larger than the sun?

Red SUper giant and supernova

Answer 11

• RED SUPER GIANT
  1. After stable period, larger mass stars expand into red supergiants
  2. Fusion can occur up to creation of iron.
• SUPERNOVA
  3. Runs out of nuclear fuel -> rapid collapse of star (temperature decreases, gravitational forces overcome outward pressure)
  4. Followed by an explosion (enough GPE transferred to fuse elements heavier than iron)
  5. Material ejected by supernova spreads out into space in a nebula (where more stars are born - these with heavier elements)

Question 12

Scientists believe that the universe began in the Big Bang. It predicts that most of the universe would be hydrogen with small amounts of heavier elements. Explain where the other elements were created?

Answer 12

1. Elements < iron but > hydrogen occurred in centre of stars by fusion
2. Energy released in supernova enough to create elements > iron

Question 13

Describe the evolution of stars with a mass larger than the sun. Neutron star and black holes

Answer 13

NEUTRON STARS

• one remnant of supernova = neutron star = small and dense

Black holes

• if remnant is greater than 2.5 solar masses a black hole remains
• black hole so dense that light cannot escape its gravitational field

Question 14

Describe large mass main sequence stars

Answer 14

1. Hot
2. White + blue
3. Fusion occurs quickly
4. Relatively short lifetime

Question 15

Low mass main sequence stars

Answer 15

1. Cool
2. Red
3. Fusion occurs slowly
4. Large lifetime