THE SUN AS A STAR
Susan Cartwright
University 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
100
10
Abundance (%)

1
0.1
0.01
H
He
O
C
Fe
Ne
N
Si
Mg
S
Rest
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

Joe Morris,
http://joemorris.mystarband.net/
THE SUN AS A STAR
HOW TYPICAL IS THE SUN?

Compare Sun with nearby stars
It is on the main sequence
 It is brighter and more massive
than average



It is not a binary


but fainter and less massive than
most bright naked-eye stars
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!