What do the stars tell us?

Astronomy, Astrology, andCosmology - a very quick tour of

our universe

Power and Magic in the Sky

You can keep precise track of theseasons by observing the apparentmotion of the Sun and the rising times ofbright stars. In early agrarian societies,such knowledge was critical for survival.

Power and Magic in the Sky

• Sunrise and sunset positions on thehorizon mark the seasons.

Power and Magic in the Sky

• The first time a bright star (such asSirius) can be seen rising ahead of theSun is another seasonal marker

Seven Wanderers

• Among the fixed stars, sevenwanderers (planets) were known inancient times: The Sun, the Moon,Mercury, Venus, Mars, Jupiter andSaturn.

The Origin of Astrology

• The Sun’s apparent motion carries vitalinformation 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 anyindividual.

Now we know a lot moreabout the planets

(Including what is a planet*)

and we know that they cannotinfluence individual events andpeople on Earth.

* and there are eight, not counting Sun and Moon

How many planets? or, whyis Pluto no longer a Planet?

• 1930: Pluto was thought to be perturbingNeptune. It isn’t.

• 1978: Pluto has a moon; this means Pluto iseven smaller.

• 2005: There is a body out there that is biggerthan Pluto (now called Eris).

• 2006: Either Pluto is a planet, and so is Eris,and so is Ceres, and perhaps so, also, isPluto’s moon Charon and a number of othernot-yet-discovered bodies …

OrPluto 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 andFuture

Today, the stars provide us withinformation on the more distant past andfuture of Earth, the Sun, the solar systemand our universe.Nearly all the light collected bytelescopes comes from stars.

Stars tell us how old galaxies are.

• Starlight comes from nuclear reactionsand/or gravity.

• We can determine the lifetime of a starfrom its fuel supply and how fast it isusing it.

• Stars with a lot of fuel spend it muchfaster, so they have short “lifetimes”.

• Stars like the Sun have fuel for 10billion years of stable consumption.

The ages of galaxies and starclusters

A young cluster has bright blue stars

Hot, blue ……….cool, red

Luminosity(Powerbeingradiated)

An old cluster has redgiants but no brightblue stars

The age of the universe as a whole:When everything was tightly packed together

z

NOW THEN

If the galaxies always moved awayat the same speed, then bymeasuring distance now and speednow we can deduce when they left:

Age = distance / speed

Measurements gave distance / speedbetween 10 and 15 billion years -10 Gyr is younger than the oldest stars!!!

A big problem in 1990

If the galaxies always moved awayat the same speed, then bymeasuring distance now and speednow we can deduce when they left:

Age = distance / speedWe expected that galaxies wereslowing down (from gravity) whichwould make this problem worse.

A big problem in 1990

Then came a surprise

The hard thing to measure is distance.

You need a good “standard candle” that isalso very bright (so you can see it far away).

Supernovae are very bright, but they arenot all alike.

However, one type, SN Ia, appear to follow arule relating how bright they are (as standardcandles) to how fast they fade.

Above:Different declinerates, differentpeak brightnessBelow:corrected to asingle standardcandle

SN IaThese eject material that has no hydrogenand therefore the star that exploded mustbe hydrogen-free; the most commonhydrogen-free stars are white dwarf starsmade of He, of C and O, or of Mg and Ne.

A white dwarf has a mass about like thatof the Sun crammed into a space the sizeof Earth - a volume a million times smaller.

Inside a white dwarf - SN IaElectrons are so close together that they runinto a limit: no more than two electrons perbox in six dimensions. The six dimensionsare position (3D) and speed (another 3D).When the low speed boxes are full, addedelectrons have to be moving at high speed.When the mass is ~ 1.4 solar masses, someof the electrons are moving at close to thespeed of light. These start nuclear reactionsthat detonate the supernova explosion.

SN IaTo reach 1.4 solar masses, the whitedwarf must be accreting mass.To explode with no hydrogen spectrum, itmust be accreting hydrogen-free material.The most likely source of hydrogen-freematerial is another white dwarf.Thus SN Ia are all explosions ofhydrogen-free white dwarf stars that growto 1.4 solar masses - this makes theexplosions nearly identical.

Then came a surpriseUsing these supernovae, distances weremeasured very carefully, by two groupsworking 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 bothgroups got the same answer.

Accelerating ?!?!I didn’t believe it until I saw that it solvedseveral problems at once:

If the expansion is accelerating, then it wasslower, and the Universe is older -comfortably older than the oldest star.

The best mathematical theory of theoriginal “Big Bang”, inflation theory,predicted that space should be very flat,and with this accelerating expansion, it isvery flat.

Acceleration and the shape ofthe universe

The curvature of the universe is determinedby ! = !matter + ! darkmatter +! other Regular matter (stars, planets, dust, gas.. allwe can see) !matter = 0.04.Dark matter (matter we know is there onlybecause of its gravity) !darkmatter = 0.24

We had <30% of what was needed!With the measured acceleration, there is a“dark energy” !darkenergy = 0.72so that ! = 0.72+0.24+0.04 = 1.

It all addsup to the

rightanswer

matter: 0.28 = 0.04 normal and 0.24 dark

Sum = 1Flat Universe

Dark Energy0.72

Best fit age = 14.5±1 GyrIf flat: = 14.9± 1 Gyr

Figure fromPerlmutter et al.

OK, that’s the past. What ofthe future?

• When our Sun runs out of fuel at thecenter, the center shrinks and theoutside expands; the Sun will becomea red giant.

• It will convert about 55% of its massfrom H to He and then from He to Cand O.

• Conversion of 10% of the H to Hetakes 10 billion years; the next 45% toHe and then C and O takes < 2 billionyears.

• At the end, the Sun will fill the orbit ofthe Earth; the Sun’s diameter will bemore than 200 x larger than now.

For our Sun:

6000 5000 4000 3000Surface Temperature, Kelvins

10,000

1000

100

L/LSun

10

1

Pre-mainsequence

Red GiantBranch

HorizontalBranch

Asymptotic GiantBranch

shell flashingand mass loss

Now

From Boothroyd, Sackmann, and Kramer 1993, Ap. J. 418, 457

Theevolution of

the Sun

Late stages of the Sun

• The total power put out by the Sun willrise to several thousand times what itputs out today.

• Earth will be very hot and will probablyend up inside the Sun, where it maycontribute to the dust that helps theSun dump the unprocessed fuel (45%of its mass) back into space.

Life on Earth is in troublemuch sooner than that

• Calculations of Earth’s climate by J.Kasting and others tell us that in about1 billion years the atmosphere will heatup until the oceans boil and Earth willbecome inhospitable to all known lifeforms.

The Power of the Sun

-1

0

1

2

3

4

0 5 10 15

m10x70z02.mixing14.z01RGBlogL

log

L

time, Gyr

10,000

1000

100

10

1

0.1

Luminosity/ Sun’s Lnow

0 5 10 15 time, in billions of years

Now

Oceans boil

2000 3000 4000

1.5

1.0

Mars

Earth

Distancein AU

Luminosity / Sun Today

Marsgetsaway(barely)

Inside the Sun

If the Sun loses20% of its masslong before theMira stage, thenthe Earth mayget away.

Stellar radius

2

1.5

1

0.5

Mars

Earth

Venus

-16

-14

-12 -10

log(density) =

L/Lnow2000 3000 4000

It loses mass, revealing thedead core = new white dwarf

• During the roughly 10,000 yearsthat it takes to go from red giant tovery hot white dwarf, it may beseen as the central star of a“planetary nebula”.

The shape of this nebula may be the result ofwind-planet or wind-companion starinteractions.

Fire and Ice:

Some say the world will end in fire,Some say in ice.From what I've tasted of desireI hold with those who favor fire.But if it had to perish twice,I think I know enough of hateTo know that for destruction iceIs also greatAnd would suffice

Robert Frost

What do the stars tell us?The minimum age of the universe= about 12-14 billion yearsThe 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/energyThe fate of the Sun and the Earth: The Earthwill (probably) spiral into the Sun when theSun becomes large enough; the Sun will endas a 0.55MSun carbon + oxygen white dwarf.

We’ve come full circle …

• From using the apparent motions of celestialbodies to map the seasons and predict theimmediate future, to using observations andmodels of stars to tell us the ultimate fate ofthe Sun and the Earth.

• Taking this farther: A book by ISU AlumnusFred Adams, The Five Ages of the Universe.

The End

PlanetShock Mira