Stars Thursday, February 28

Thu, Feb 28: Stars Tue, Mar 4: Planets Problem Set #7 due Thu, Mar 6: Past & Future Problem Set #7 returned 1:30 pm Final Exam Tue, Mar 11, 1:30 pm Final Exam

What is a star? Examples of stars: SunBetelgeuse

Pleiades: known traditionally as the “seven sisters” (6 bright stars, 1 fainter) With his telescope, Galileo saw many more stars.

Average density of ordinary matter in the universe = 0.04 ρ crit = 3.6 × kg/m 3 Density of Sun = 1400 kg/m 3 = 4 quadrillion quadrillion × average Stars are lumps of ordinary matter compressed to small size and high density.

What is a star? A luminous ball of gas nuclear fusion powered by nuclear fusion in its interior.

“Why do stars shine?” Stars are dense (Sun is 40% denser than liquid water). Stars are opaque (because they’re made of ionized gas). Stars are hot (surface temperatures > 2000 Kelvin).

Hot, dense, opaque objects emit light! stars Today, we call hot, dense, opaque objects that emit light “stars”. Flashback slide!

Question: Why do stars shine? Short answer: Stars shine because they are hot. Follow-up question: Why don’t stars cool down?

There’s a continuous fossil record of life on Earth for over 3 billion years. Sun’s luminosity can’t have been wildly variable – if it had, life would have scorched or frozen.

Sun must have an interior power source to replace the energy carried away by photons. What’s the power source? The Sun’s mostly hydrogen – what about burning hydrogen?

2 H + O → H 2 O + energy Burning 1 kg of hydrogen releases 1.4 × 10 8 joules of energy. Sun’s mass = 2 × kg. (1.4 × 10 8 joules/kg) × (2 × kg) = 2.8 × joules

The Sun throws away energy at a rate L sun = 3.9 × watts = 3.9 × joules/sec. Time to “burn up” the Sun = 2.8 × joules / 3.9 × joules/sec = 7.2 × seconds = 23,000 years

We need a power source that gives us more bang for the buck (more joules for the kilogram…) nuclear fusion The Sun’s mostly hydrogen – what about nuclear fusion, converting hydrogen into helium?

lot 4 H → He + a lot of energy Fusing 1 kg of hydrogen into helium releases 6.3 × joules of energy. That’s 4.5 million times what you’d get by burning the hydrogen. billions Sun’s hydrogen supply adequate for billions, not thousands, of years.

If nuclear fusion is such a great energy source, why don’t we all have “Mr. Fusion” units?

positron neutron neutrino proton photon Helium Fusion inside the Sun

The fusion chain starts with combining two protons. Protons are positively charged; overcoming their electrostatic repulsion requires high speeds. T > 10 million Kelvin.

only Fusion occurs only in the hot, dense central regions.

Energy is generated in the Sun’s hot core. Energy is radiated from the Sun’s surface, 700,000 km away. How does the energy get from the core to the surface?

Photons Photons are good at carrying energy from point A to point B. If If the Sun were transparent, photons could travel from its center to its surface in 2.3 seconds. A B

not The Sun is not transparent. Photons travel only an inch before being scattered in a random direction. It takes 200,000 years (on average) for light to stumble its way to the surface.

Galaxies form because ordinary matter can cool down (by emitting photons) and fall to the center of dark halos. Why do galaxies curdle into tiny stars, instead of remaining as homogenous gas clouds?

now Look at where stars are forming now. In the Whirlpool Galaxy, we see newly formed stars in dense, cold molecular clouds.

In regions where the gas is cooler and denser than elsewhere, hydrogen forms molecules (H 2 ). molecular clouds These cool, dense regions are thus called “molecular clouds”.

Consider a small, dense molecular cloud. Mass = 1 M sun Radius = 0.1 pc = 4,000,000 R sun Temperature = 10 Kelvin = T sun /580

Molecular clouds are usually stable; but if you hit them with a shock wave, they start to collapse gravitationally. shock waves Once the collapse is triggered, it “snowballs”.

Once gravity has reduced the radius of the cloud by a factor of 4,000,000, it’s the size of a star. black hole Why doesn’t the molecular cloud collapse all the way to a black hole?

Escape speed from molecular cloud ≈ 0.3 km/sec Escape speed from star ≈ 600 km/sec Escape speed from black hole = 300,000 km/sec

When the gas temperature is high enough (T ≈ 10 million Kelvin), nuclear fusion begins! As the gas of the molecular cloud is compressed, it becomes denser. As the gas is compressed, it also becomes hotter.

temperature pressure Nuclear fusion keeps the central temperature and pressure of the star at a constant level. hydrostatic equilibrium The star is static (not contracting or expanding) because it’s in hydrostatic equilibrium.

Hydrostatic equilibrium = a balance between gravity and pressure. Pressure increases as you dive deeper into the ocean: pressure increases as you dive deeper into the Sun.

Gas flows from regions of high pressure to regions of low pressure.

For a fish in the ocean, pressure creates a net upward force, gravity creates a downward force. hydrostatic equilibrium The fish is in hydrostatic equilibrium.

For gas in the Sun, pressure creates a net outward force, gravity creates a inward force. hydrostatic equilibrium The Sun is in hydrostatic equilibrium.

The Sun is like a fat guy on an inflatable chair. pressure gravity fusion energy

Hydrostatic equilibrium tends to be very stable. Put a six-pack on fat guy’s lap. Gravitational force increases. Gas in chair is compressed. Upward pressure force increases.

Hydrostatic equilibrium is common throughout the universe. Sun is in hydrostatic equilibrium. Oceans are. Earth’s atmosphere is. Earth’s interior is. Fat guys in inflatable chairs are.

Tuesday’s Lecture: Reading: none Planets Formation and Evolution of Planets