The lifecycles of stars

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

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

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

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!

Energy source of stars proton – proton chain reaction

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)

Stellar Nurseries – the Orion nebula (1500ly)

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.

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.

Helium fuel runs out Once the star has exhausted its supply of helium, the outward pressure stops and the star collapses It collapses up to the point where ‘electron degeneracy’ provides an outward pressure to counter the inward pressure of gravity The outer layers of the star are puffed away in what is called a ‘planetary nebula’ What remains is a small hot star called a white dwarf

Electron degeneracy Electron degeneracy pressure is a particular manifestation of the more general phenomenon of quantum degeneracy pressure. The Pauli exclusion principle disallows two identical half-integer spin particles (electrons and all other fermions) from simultaneously occupying the same quantum state. The result is an emergent pressure against compression of matter into smaller volumes of space. From Wiki

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

Core of star

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 ‘Electron degeneracy pressure’ is overcome, with electrons being forced to combine with protons, forming neutrons 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

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 (‘neutron degeneracy pressure’ is overcome) If not, the star, which is spinning, can be detected as a pulsar

SN 1987A – before and after SN 1987A – before and after

M1 Supernova explosion remnant from 1054 AD

Crab nebula pulsar Imaged by Chandra X-ray telescope

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.

Pulsar

Black Holes Companion star X-rays Black hole Accretion disk