THE SUN AS A STARSusan CartwrightUniversity of Sheffield

WHAT DO WE KNOW ABOUT THE SUN? Its size and its distance from us Its luminosity (energy generated per second) Its (surface) chemical composition and

temperature How it generates energy

Its (approximate) age from radioactive dating of solar system material

Its history and future from our understanding of stars in general

THE SUN’S COMPOSITION AND TEMPERATURE The spectrum of the Sun tells us both its

composition and its temperature

HOW THE SUN GENERATES ENERGY Fusion of hydrogen to helium How does this work?

He atom weighs slightly less than 4 H atoms (0.7%)

E = mc2

How do we know? Only mechanism

that lasts long enough!

We detect neutrinos

THE SUN AS A STAR

Joe Morris,http://joemorris.mystarband.net/

HOW TYPICAL IS THE SUN? Compare Sun with nearby

stars It is on the main sequence It is brighter and more

massive than average but fainter and less massive than

most bright naked-eye stars It is not a binary

this is not rare, but probably more than half of all stars are binaries

It has planets this is probably very common

THE SUN’S LIFE AND DEATH: YOUTH The Sun was born from a giant cloud of cool

gas this contracted under gravity as it contracted it heated up eventually the core reached 10 million degrees

and hydrogen fusion began

Most of following images taken from http://outreach.atnf.csiro.au/education/senior/astrophysics

THE SUN’S LIFE AND DEATH: YOUTH

THE SUN’S LIFE AND DEATH: PRIME The Sun’s main sequence

lifetime is 10 billion years Less massive stars live

longer More massive stars live

less long The Sun gets slightly

brighter as it evolves on the main sequence “faint young Sun

problem” Why didn’t early Earth

freeze solid??

THE SUN’S LIFE AND DEATH: OLD AGE When the Sun has used up its core hydrogen,

it will become a Red Giant Red giants still use hydrogen as fuel, but outside

the (pure helium) core The helium core will get larger and hotter

Eventually it will reach 100 million degrees and helium fusion will begin

this is much less efficient and will not last as long

THE SUN’S LIFE AND DEATH: OLD AGE

THE SUN’S LIFE AND DEATH: DEATH As helium fusion continues, the Sun will shed its

outer envelope the hot inner core will be revealed as a White Dwarf about 0.6 times the Sun’s current mass, but only

the size of Neptune

THE SUN’S LIFE AND DEATH: DEATH

THE LIVES OF OTHER STARS Stars up to about 8 times the Sun’s mass live

and die like the Sun only more massive ones do it faster

Stars more than 8 times the Sun’s mass can fuse heavier elements they are responsible for

making most of the Periodic Table!

But they die young in a spectacular explosion called a Supernova

THE LIFE AND DEATH OF STARS

Simulations from http://rainman.astro.uiuc.edu/ddr/stellar/beginner.html

PLANETARY SYSTEMS

OTHER STARS WITH PLANETS 276 planets detected to date!

CONCLUSIONS The Sun is a star

a little brighter than most a bit more massive than most not a binary

Studying the Sun can tell us about stars it is far closer than any other star and can be

studied in much more detail Studying other stars can tell us about the Sun

range of ages, masses and chemical composition available for study

planets are common – range of very different planetary systems can be studied

GLIESE 581

Probably like Venus

Possibly habitable!