What do the stars tell us?
Astronomy, Astrology, and
Cosmology - a very quick tour of
our universe
Power and Magic in the Sky
You can keep precise track of the
seasons by observing the apparent
motion of the Sun and the rising times of
bright stars. In early agrarian societies,
such knowledge was critical for survival.
Power and Magic in the Sky
• Sunrise and sunset positions on the
horizon mark the seasons.
Power and Magic in the Sky
• The first time a bright star (such as
Sirius) can be seen rising ahead of the
Sun is another seasonal marker
Seven Wanderers
• Among the fixed stars, seven
wanderers (planets) were known in
ancient times: The Sun, the Moon,
Mercury, Venus, Mars, Jupiter and
Saturn.
The Origin of Astrology
• The Sun’s apparent motion carries vital
information about the calendar.
• The Moon’s variations are also useful.
• Maybe the other wandering bodies (planets)
are trying to tell us something, too?
• From: Events affecting all, to predictions
concerning royalty, to predictions for any
individual.
Now we know a lot more
about the planets
(Including what is a planet*)
and we know that they cannot
influence individual events and
people on Earth.
* and there are eight, not counting Sun and Moon
How many planets? or, why
is Pluto no longer a Planet?
• 1930: Pluto was thought to be perturbing
Neptune. It isn’t.
• 1978: Pluto has a moon; this means Pluto is
even smaller.
• 2005: There is a body out there that is bigger
than Pluto (now called Eris).
• 2006: Either Pluto is a planet, and so is Eris,
and so is Ceres, and perhaps so, also, is
Pluto’s moon Charon and a number of other
not-yet-discovered bodies …
Or
Pluto and Eris are dwarf planets and there are five
kinds of objects in the solar system:
• Terrestrial (Earth-like) planets Mercury, Venus,
Earth and Mars - rock & metal
• Jovian (Jupiter-like) planets Jupiter, Saturn,
Uranus and Neptune - mostly gas
• Dwarf planets: Big enough to be round (shaped
by gravity) and not orbiting other planets.
• Moons: Some are bigger than Mercury.
• Small solar system bodies: Not big enough to be
round.
The Stars - Key to Past and
Future
Today, the stars provide us with
information on the more distant past and
future of Earth, the Sun, the solar system
and our universe.
Nearly all the light collected by
telescopes comes from stars.
Stars tell us how old galaxies are.
• Starlight comes from nuclear reactions
and/or gravity.
• We can determine the lifetime of a star
from its fuel supply and how fast it is
using it.
• Stars with a lot of fuel spend it much
faster, so they have short “lifetimes”.
• Stars like the Sun have fuel for 10
billion years of stable consumption.
The ages of galaxies and star
clusters
A young cluster has bright blue stars
Luminosity
(Power
being
radiated)
Hot, blue ……….cool, red
An old cluster has red
giants but no bright
blue stars
The age of the universe as a whole:
When everything was tightly packed together
z
NOW
THEN
A big problem in 1990
If the galaxies always moved away
at the same speed, then by
measuring distance now and speed
now we can deduce when they left:
Age = distance / speed
Measurements gave distance / speed
between 10 and 15 billion years 10 Gyr is younger than the oldest stars!!!
A big problem in 1990
If the galaxies always moved away
at the same speed, then by
measuring distance now and speed
now we can deduce when they left:
Age = distance / speed
We expected that galaxies were
slowing down (from gravity) which
would make this problem worse.
Then came a surprise
The hard thing to measure is distance.
You need a good “standard candle” that is
also very bright (so you can see it far away).
Supernovae are very bright, but they are
not all alike.
However, one type, SN Ia, appear to follow a
rule relating how bright they are (as standard
candles) to how fast they fade.
Above:
Different decline
rates, different
peak brightness
Below:
corrected to a
single standard
candle
SN Ia
These eject material that has no hydrogen
and therefore the star that exploded must
be hydrogen-free; the most common
hydrogen-free stars are white dwarf stars
made of He, of C and O, or of Mg and Ne.
A white dwarf has a mass about like that
of the Sun crammed into a space the size
of Earth - a volume a million times smaller.
Inside a white dwarf - SN Ia
Electrons are so close together that they run
into a limit: no more than two electrons per
box in six dimensions. The six dimensions
are position (3D) and speed (another 3D).
When the low speed boxes are full, added
electrons have to be moving at high speed.
When the mass is ~ 1.4 solar masses, some
of the electrons are moving at close to the
speed of light. These start nuclear reactions
that detonate the supernova explosion.
SN Ia
To reach 1.4 solar masses, the white
dwarf must be accreting mass.
To explode with no hydrogen spectrum, it
must be accreting hydrogen-free material.
The most likely source of hydrogen-free
material is another white dwarf.
Thus SN Ia are all explosions of
hydrogen-free white dwarf stars that grow
to 1.4 solar masses - this makes the
explosions nearly identical.
Then came a surprise
Using these supernovae, distances were
measured very carefully, by two groups
working independently to check each other.
They found that the expansion is speeding up
- accelerating - instead of slowing down.
We were all sure they were wrong …
..but no one could find an error, and both
groups got the same answer.
Accelerating ?!?!
I didn’t believe it until I saw that it solved
several problems at once:
If the expansion is accelerating, then it was
slower, and the Universe is older comfortably older than the oldest star.
The best mathematical theory of the
original “Big Bang”, inflation theory,
predicted that space should be very flat,
and with this accelerating expansion, it is
very flat.
Acceleration and the shape of
the universe
The curvature of the universe is determined
by ! = !matter + ! darkmatter +! other
Regular matter (stars, planets, dust, gas.. all
we can see) !matter = 0.04.
Dark matter (matter we know is there only
because of its gravity) !darkmatter = 0.24
We had <30% of what was needed!
With the measured acceleration, there is a
“dark energy” !darkenergy = 0.72
so that ! = 0.72+0.24+0.04 = 1.
It all adds
up to the
right
answer
Dark Energy
0.72
Best fit age = 14.5±1 Gyr
If flat: = 14.9± 1 Gyr
Figure from
Perlmutter et al.
Sum = 1
Flat Universe
matter: 0.28 = 0.04 normal and 0.24 dark
OK, that’s the past. What of
the future?
• When our Sun runs out of fuel at the
center, the center shrinks and the
outside expands; the Sun will become
a red giant.
• It will convert about 55% of its mass
from H to He and then from He to C
and O.
For our Sun:
• Conversion of 10% of the H to He
takes 10 billion years; the next 45% to
He and then C and O takes < 2 billion
years.
• At the end, the Sun will fill the orbit of
the Earth; the Sun’s diameter will be
more than 200 x larger than now.
10,000
The
evolution of
the Sun
shell flashing
and mass loss
1000
100
Asymptotic Giant
Branch
Horizontal
Branch
Red Giant
Branch
L/LSun
10
Pre-main
sequence
From Boothroyd, Sackmann,
and Kramer 1993, Ap. J. 418, 457
Now
1
6000
5000
4000
3000
Surface Temperature, Kelvins
Late stages of the Sun
• The total power put out by the Sun will
rise to several thousand times what it
puts out today.
• Earth will be very hot and will probably
end up inside the Sun, where it may
contribute to the dust that helps the
Sun dump the unprocessed fuel (45%
of its mass) back into space.
Life on Earth is in trouble
much sooner than that
• Calculations of Earth’s climate by J.
Kasting and others tell us that in about
1 billion years the atmosphere will heat
up until the oceans boil and Earth will
become inhospitable to all known life
forms.
The Power of the Sun
logL
m10x70z02.mixing14.z01RGB
10,000
4
10003
Oceans boil
logL
Luminosity 1002
/ Sun’s L
now
101
Now
10
-1
0.1
0
0
5
10
5
10
time, in billions of years
time, Gyr
15
15
Mars
gets
away
(barely)
Mars
1.5
Distance
in AU
1.0
Earth
Inside the Sun
2000
3000
Luminosity / Sun Today
4000
2
If the Sun loses
20% of its mass
long before the
Mira stage, then
the Earth may
get away.
Mars
log(density) = -16
1.5
-14
Earth
-12
1
-10
Venus
Stellar radius
0.5
2000
3000 L/Lnow 4000
It loses mass, revealing the
dead core = new white dwarf
• During the roughly 10,000 years
that it takes to go from red giant to
very hot white dwarf, it may be
seen as the central star of a
“planetary nebula”.
The shape of this nebula may be the result of
wind-planet or wind-companion star
interactions.
Fire and Ice:
Some say the world will end in fire,
Some say in ice.
From what I've tasted of desire
I hold with those who favor fire.
But if it had to perish twice,
I think I know enough of hate
To know that for destruction ice
Is also great
And would suffice
Robert Frost
What do the stars tell us?
The minimum age of the universe
= about 12-14 billion years
The amount of “bright matter” in the universe about 4% of the total matter/energy.
The amount of “dark matter” in the universe about 24% of the total matter/energy
The fate of the Sun and the Earth: The Earth
will (probably) spiral into the Sun when the
Sun becomes large enough; the Sun will end
as a 0.55MSun carbon + oxygen white dwarf.
We’ve come full circle …
• From using the apparent motions of celestial
bodies to map the seasons and predict the
immediate future, to using observations and
models of stars to tell us the ultimate fate of
the Sun and the Earth.
• Taking this farther: A book by ISU Alumnus
Fred Adams, The Five Ages of the Universe.
Shock
Planet
The End
Mira