1. The lifecycles of stars

2. What can we learn from observing stars?
Intensity: affected by intrinsic luminosity/distance
A bright distant star can look the same as a dim star which is closer
Colour/spectra: affected temperature/relative velocity
Hotter stars appear bluer, cooler stars appear redder
Stars which are moving away have their spectra shifted to longer wavelengths – ‘Red Shift’

3. Hertzsprung-Russell diagram

4. Lifecycles
A star will plot at different locations on the Hertzsprung-Russell diagram as it passes through its lifecycle
The lifecycle of a star is determined by its mass
More massive stars have shorter, brighter lives than less massive stars

5. Small mass star: 0.8 to 8 x mass of Sun
Large mass star: greater than 8 x mass of Sun

6. Star Formation
Stars form in cool (10 K) clouds of gas and dust in interstellar space, giant molecular clouds (GMCs) – seen as dark nebulae
Denser regions of giant molecular clouds contract due to gravity – dense cores
Eventually pressures and temperatures become high enough to start nuclear fusion of hydrogen – a star is born!

7. Nuclear fusion reminder
The nuclei of light elements are fused together to make heavier elements
In this process, some mass is converted to energy
This process will give out energy all the way up the periodic table until iron
Elements heavier than iron won’t give out energy

8. Giant Molecular Cloud (GMC)
Horsehead nebula (B33) in Orion
Giant Molecular Cloud seen as dark nebula
Appears in silhouette in front of emission nebula (flame nebula, NGC 2024)

9. Stellar Nurseries – the Orion nebula (1500ly)

10. Nebula
A cloud of dust and gas attracted together by gravity

11. Birth of the Sun
Nuclear fusion reactions take place at centre – hydrogen nuclei are fused together to make helium, releasing enormous amounts of energy

12. Planetary formation
Inner planets made from more dense material (Rocky), outer planets are mainly gas (Gas Giants)

13. Small Mass star evolution
Hydrogen burning phase – ‘Main Sequence’. Balance between gravitational forces acting inwards and radiation pressure acting outwards.
Stars spend most of their lives in this phase (Sun 10 billion years, smaller stars much longer)
When hydrogen runs out in the core outward radiation pressure decreases and the core is squeezed to higher temperatures and pressures causing helium to start fusing.

14. Red Giant phase
The outer layers expand dramatically and cool (become redder). The star becomes a Red Giant.
The luminosity of the star increases considerably because the size of the Red Giant star is so large.
On the Hertzsprung-Russell diagram, the star moves away from the main sequence towards the Red Giant star zone.

17. Larger Mass star evolution
Stars that are >8 x mass of Sun will have much shorter lives, and burn much hotter
A star 25 x mass of Sun gets through its life 1000 times faster
After a time fusing hydrogen (‘main sequence’), these larger stars start fusing heavier and heavier elements in shells (like an onion) with the heaviest elements at the core
The star becomes a Red Giant or Red Supergiant

18. Core of star

19. A Supernova
Once iron has formed, no more energy can be released by nuclear fusion
The enormous gravitational pressure compresses the core to a million million kilograms per cubic metre and raises its temperature to 10,000 million degrees Celsius
The core collapses in 1/10 second from 12,000 km in diameter to 20 km – forming a neutron star
The outer layer collapse in and rebound off the core releasing enormous amounts of energy in a Type II supernova explosion

20. Heavier elements (above iron) are formed in the supernova explosion
For a few weeks the supernova is brighter than a whole galaxy
For very large stars it is possible for the neutron core to collapse to become a black hole
If not, the star, which is spinning, can be detected as a pulsar

21. SN 1987A – before and after
SN 1987A – before and after

22. M1
Supernova
explosion
remnant
from 1054 AD

23. Crab nebula pulsar
Imaged by Chandra X-ray telescope

24. Black Holes and Pulsars
Black Holes are caused when the concentration of mass is so large that the light itself can’t escape the pull of gravity. They can’t be observed directly but by their gravitational effect on other bodies. Material being sucked into a black hole gets accelerated to such high speeds that X-rays are emitted.
Pulsars are rotating neutron stars that emit short bursts of radiation at very regular intervals. The radiation pulses are caused by the rotating magnetic field.

25. Pulsar

26. Black Holes
Companion star
X-rays
Black hole
Accretion disk