Outline - March 4, 2010

• How does the sun shine? (pgs. 495-497, 499-503)

• Lifetimes of stars: gas guzzlers vs. econoboxes (pgs. 533-534)

• Where are the oldest known stars? (pgs. 536-538)

• Interstellar Medium (pgs. 544-547)

• How are stars made? (pgs. 549-551)

Proton-Proton Chain(all stars with M < 8 Msun)

Net result: 4 protons are fused, producing 1 helium nucleus

So where does the energy come from????

The mass of 4 protons is less than the mass of 1 helium nucleus.

The mass that is lost is converted into energy (in the form of light).

The sun (and all stars that are not white dwarfs or “neutron stars”) are very slowly losing mass in order to power themselves.

Note: only a tiny amount of mass is actually lost. By the end of its lifetime the sun will have lost about about 10% of its total mass to energy generation.

In principle, how long could the sun last by “burning” hydrogen at its present rate?

Mass of 4 protons = 6.690x10-27 kg

Mass of 1 helium nucleus = 6.643x10-27 kg

Mass lost (mlost) = 0.047x10-27 kg

Energy gained = mlost c2 = (0.047x10-27)(3.0x108)2 = 4.23x10-12 J

Energy produced by the sun every second = 3.8x1026 J

Sun must run this fusion reaction 8.9x1037 times every second or it would collapse under gravity!!!!

In other words, the sun must fuse 6.0x1011 kg of hydrogen every single second. That’s a lot of hydrogen, but the sun has a lot of mass…

In principle, how long could the sun last by “burning” hydrogen at its present rate?

The sun must fuse 6.0x1011 kg of hydrogen every single second.

The sun’s mass is 1.99x1030 kg, and at a current age of 4.5x109 years, we know that 70% of that mass is in hydrogen, or 1.39x1030 kg of hydrogen remains.

If the sun converted ALL of its remaining hydrogen into helium (at today’s rate of “nuclear burning”), how much longer could the sun live?

Remaining lifetime in seconds = remaining H mass / rate of H fusion

Remaining lifetime in seconds = 1.39x1030 / 6.0x1011 = 2.32x1018 seconds

Remaining lifetime in years = 73.4 billion years!!

In principle, how long could the sun last by “burning” hydrogen at its present rate?

So, if the sun could turn ALL of its hydrogen into helium at its present rate, you would think the sun would live a total of (4.5 + 73.4) = 77.9 billion years.

But, sadly, the sun’s lifetime is limited to only about 10 billion years because it can’t actually convert all of its hydrogen into helium.

HUGE structural changes will happen to the star long before it can “burn up” all of its hydrogen.

What determines a star’s Main Sequence lifetime?

It’s all about MASS.

The more massive is a star, the hotter and denser is the star in its core.

The hotter and denser it is in a star’s core, the FASTER the conversion of hydrogen to helium happens.

High-mass (> 8 Msun) stars are “gas guzzlers”

Low-mass (< 2 Msun) are “economy cars”

Main Sequence is a MASS Sequence

The highest mass stars live only a few million years. They have a lot of fuel and they’re burning it really fast.

The lowest mass stars live for 100’s of billions of years. They have very little fuel, but they’re burning it extremely efficiently.

Estimating the Age of the Universe(What are stars “good for”?)

It stands to reason that you are younger than your mother.

It therefore stands to reason that the objects within the universe cannot be older that the universe itself.

The ages of the oldest stars puts a limit on the minimum age of the universe!!

The Oldest Stars in the Milky WayGlobular Star Clusters

Spherical groupings of 10,000 to 1 million stars (about 158 known in our Galaxy). All of the stars formed at roughly the same time. Globular clusters have lots of RED stars, but no BLUE stars (because they died long ago and were not “replenished”).

Globular Cluster H-R Diagram

Globular Cluster M55

Globular clusters have short, stubby main sequences that “turn off” to the red giant region. The “turn off” point tells you the approximate age.

Oldest Stars in the Milky Way

Globular cluster M4 is one of the oldest known star clusters (about 13 billion years old), and contains many white dwarfs (the dead cores of low-mass stars

that used up all their fuel).

Interstellar Medium (ISM)

• Material between the stars

• Most of space is a better vacuum than can be made in a laboratory!

• About 1/5 as much mass in the ISM as in stars in our Galaxy

• Some regions of space contain clouds gas (some clouds are hot: > 10,000 K, some clouds are very cold: 10K-30 K)

• Chemical composition of ISM: 70% H, 28% He, 2% other elements (by mass)

Why should you care about the ISM?

• Stars had to come from somewhere (the Big Bang didn’t make stars)

• When stars die, their guts have to go somewhere

• If those “somewheres” weren’t the same place, we wouldn’t be here! (a topic for after Spring Break)

Association Between Cold Clouds and Stars

“Heir ist wahrhaftig ein Loch im Himmel”

Wm. Herschel

Image taken in optical / visible light

Cold clouds are transparent in the infrared and radio

Milky Way: Optical

Cold clouds obscure our view at visible wavelengths, but infrared and radio light penetrates the clouds.

Milky Way: Infrared

Milky Way: Radio

Cold (Molecular) Clouds in the Milky Way

The Boston University-Five College Radio Astronomy Observatory

Galactic Ring Survey

Molecular gas clouds, as revealed by radio light emitted by the molecule CO (carbon monoxide).

The full moon would appear to be this big on

the image above.

Molecules are fragile - they are easily broken apart by high energy light or strong collisions (both of which happen in high-temperature environments)

Cold clouds contain many types of molecules

Molecules in the cold clouds range from simple molecules like carbon monoxide (CO) to more complex molecules like alcohol (CH3CH2OH)

HCO+

N2H+HNC

HCNHCO+ HCN

HNC N2H+

Some cold clouds have intriguing shapes

Some cold clouds are long and snaky

The “Nessie” NebulaSize > ~100 pc x 0.5 pc

Molecular CloudsStellar Nurseries

• Very, very cold (10K to 30K)

• Typical density is 300 molecules per cubic centimeter (vastly less than the density of air at sea level, but vastly more than the density of the ISM on average in our Galaxy)

• Gas is primarily H2 molecules, but you can’t detect them directly! (Note: Helium does not form molecules because it is chemically inert.)

• Most common “tracer” molecule is CO (carbon monoxide)

• About 1% of the mass in molecular clouds is in “dust”

“Dust” in the ISM

• Not dust bunnies, more like the microscopic particles in smoke

• Size of dust grains is smaller than bacteria (typical size is 1 micron = 10-6m)

• Dust grains made mostly of some combination of carbon, silicon, oxygen, and iron

• Dust blocks wavelengths of light that are smaller than the size of the grains ( < 10-6 m)

• Dust easily blocks UV and visible light, but IR and radio light can (usually) pass right through

Horsehead Nebula (in Orion), optical image

Cloud Structure: Gravitational Equilibrium

A stable cloud has a balance of two forces:

INWARD: Gravity

OUTWARD: Pressure

No net force => No motion

The “Jeans Mass”

Sir James Jeans showed that gravity becomes stronger than pressure when the mass gets large enough.

If clouds (or fragments of clouds) acquire enough mass, the cloud will collapse.

Problem: Although gravity is the most pervasive, long-range force in nature, it is also the weakest force in nature.

How can gravity get the upper hand and win the pressure-gravity tug of war?

What do we mean by “pressure” in a cloud?

• Why does a balloon maintain its shape?

• What happens to a balloon if you blow it up at room temperature, then put it in the freezer for a couple of hours?

• This is what is known as “thermal” pressure (the common pressure for gasses)

• Easiest place for gravity to “win” over pressure is in a cloud of gas that is very cold (= low pressure)

Collapsing Clouds

This cold, dark cloud is collapsing and forming cores that will eventually become stars

This is a cloud where gravity has won the tug-of-war!

Most Stars are Born Inside Clusters

Pleiades Star Cluster

Most molecular clouds contain MUCH more mass than would make a single star

Most molecular clouds are very LUMPY (not smooth)

Likely scenario is that many lumps (which are more dense than the average) contract to form stars at about the same time

Single star formation is possible but probably very rare (because you need an unusually dense, yet low-mass cloud)