Star and Planet FormationStar and Planet Formation

The Beginning of Star FormationThe interstellar medium is the gas and dust floating in space between the stars. This material is created by the death of stars, but also provides the ingredients for making new stars.

Clouds in the interstellar medium can contain anywhere from 1 to 100,000,000 M of gas and dust. So a given cloud could produce 1 newborn star, or millions of them!

The Beginning of Star FormationWhere there is gas, there is also dust, which absorbs and scatters light. Dust in space can be seen in silhouette, as it blocks out the light from more distant stars.

The Beginning of Star FormationSince dust blocks the light, the temperatures within these clouds can be just a few degrees above absolute zero!

The Beginning of Star FormationSince the temperature is so low inside these clouds, gas pressure is almost non-existent. There is nothing to stop gravity from collapsing the cloud. The cloud will get smaller and increase in density.

The Beginning of Star FormationRather than collapsing to form just 1 star, most clouds fragment into many clumps, which then collapse to form stars. Each fragment is called a protostar.

After stars are born in an interstellar cloud, their light reflects from the surrounding cloud, which significantly changes its appearance.

before stars are born after stars are born

Reddening and Scattering

Stars behind large piles of dust will be reddened. Other parts will appear blue, due to the scattering by dust.

This is just like the daytime sky.

Reddening and Scattering

Emission Lines from NebulaeIn addition to scattered light from the newborn stars, these nebulae produce emission line radiation, just like an aurora.

Formation of a Disk

As a protostar collapses, conservation of angular momentum causes the material to spin rapidly. The centripetal force fights the collapse in the plane of rotation, but not at the poles. As a result, the material collapses into a disk.

Central Temperature

100,000 K

The protostar collapses and gets smaller and smaller (and, due to the increased central pressure), hotter and hotter.

Gravity pulls the star inward Gas pressure

resists gravity

Central Temperature1,000,000 K

The protostar collapses and gets smaller and smaller (and, due to the increased central pressure), hotter and hotter.

Gravity pulls the star inward Gas pressure

resists gravity

Central Temperature10,000,000 K

Core hot enough for Hydrogen fusion

The star’s center eventually becomes hot enough to ignite hydrogen fusion, which stops its collapse, and the star stably fuses hydrogen for a long time.

If the mass is above 0.1 M,

it’s a star

In order to fuse hydrogen, the center of a star must be hot enough. If a star’s mass is too low, its core will be too cold to ignite hydrogen fusion. These objects lack a source of energy and can’t shine like a normal star. They are called brown dwarfs. They grow cooler, fainter, and smaller forever, like a dying ember.

Sun 1 M

Brown Dwarfs: Stars without Fusion

If the mass is below 0.1 M, it’s

a brown dwarf

Characteristics of the Solar SystemAny theory for the formation of planets must explain:

The flatness of the Solar System All of the planets orbit in the same direction The separation of Terrestrial and Jovian planets The decrease in planet densities with distance from the Sun

Formation of Planets

The densest region of the disk (the center) becomes the Sun. Eventually, fusion in the Sun will occur.

Atoms orbiting in the disk bump together and form molecules, such as water. Droplets of these molecules stick together to form planetesimals.

Over time, the planetesimals grow as more molecules condense out of the nebula

Formation of Planets

Differential rotation (due to Kepler’s laws) will cause particles in similar orbits to eventually meet up. One will accrete onto the other, forming a bigger body.

The bigger the body, the greater its gravitational force, and the more attraction it has for other bodies. Further accretion will occur. Protoplanets form.

Formation of Planets

While protoplanets are forming, the Sun’s luminosity is growing, first due to gravitational contraction, then due to nuclear ignition.

Regions of the nebula close to the Sun will get hot; the outer regions will stay cool. In the hot regions, light elements will evaporate; only heavy elements will condense out of the nebula

Temperature vs. Distance from Sun

Inside the orbit of the Earth, only metals can condense out of the solar nebula. Rocky (silicates) can condense near Mars. In the outer solar system, water and ammonia ice can survive.

Inside the orbit of the Earth, only metals can condense out of the solar nebula. Rocky (silicates) can condense near Mars. In the outer solar system, water and ammonia ice can survive.

Temperature vs. Distance from Sun

Radiation Pressure and the Solar Wind

Two other processes are also important for driving light gases from the inner part of the solar system.

Radiation pressure: Photons act like particles and push whatever particles and dust they run into.

Solar wind: The Sun constantly ejects (a little) hydrogen and helium into space. This solar wind pushes whatever gas and dust it runs into.

Accretion Once the major bodies of the solar system were formed, most of the remaining debris was either ejected out of the solar system or accreted onto other bodies by gravitational encounters.

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Observations of Protostellar Disks

The solar nebula theory states that young stars should be surrounded by a disk consisting of molecular gas and dust. These are now being observed.

The Age of the Solar SystemWe can estimate the age of the Solar System by looking at radioactive isotopes. These are unstable forms of elements that produce energy by splitting apart (i.e., fission).

The radioactivity of an isotope is characterized by its half-life – the time it takes for half of the parent to decay into its daughter element. By measuring the ratio of the parent to daughter, one can estimate how long the material has been around.

Radioactive Elements

Isotope#

protons#

neutrons

Daughter Half-life (years)

Rubidium-87 37 50 Strontium-87 47,000,000,000

Uranium-238 92 146 Lead-206 4,510,000,000

Uranium-235 92 143 Lead-207 710,000,000

Potassium-40 19 21 Argon-40 1,280,000,000

Aluminum-26 13 13Magnesium-87

730,000

Carbon-14 6 8 Nitrogen-14 5,730

Each of these isotopes spontaneously decays into its daughter. In each case, the daughter weighs less than the parent – energy is produced.

Age of the Solar System

When rocks are molten, heavier elements (such as uranium) will separate out from other elements. (In liquids, dense things sink, light things rise.) Once the rocks solidify, radioactive decay will then take over.

• On Earth, the oldest rocks have ages of 3 billion years

• The oldest asteroids have ages of 4.5 billion years

• Rocks from the “plains” on the Moon have ages of about 3 billion years. The oldest Moon rocks have ages of 4.5 billion years.

The solar system is therefore 4.5 billion years old.

Four major theories have been proposed for formation of the Moon:

Fission: the Moon broke off of the Earth

Co-formation: Moon formed like the Earth, right next to the Earth

Capture: Moon formed elsewhere in the solar system and was later captured by Earth’s gravity

Large impact: Mars-size planet collided with the Earth and the Moon formed from the debris

Formation Scenarios for the Moon

The Moon has a similar composition as the Earth’s crust and mantle, but has a much smaller iron core. If the Moon formed by fission or co-formation, it should have a larger iron core like the Earth. If it formed through capture, it shouldn’t match the composition of the Earth’s crust and mantle.

Formation Scenarios for the Moon

The large impact theory is widely believed to be correct.

The iron core of the impacting planet could have merged with the Earth’s core, while the Moon formed from crust and mantle thrown into space. This explains why the Moon in similar in composition to the Earth’s crust and mantle, but has as very small iron core.

The impact theory also explains why rocks on the Moon contain no water or other volatiles (easily evaporated materials).

Effects of the Moon on the EarthThis impact that created the Moon may have caused the Earth’s spin axis to become tilted. So we might not have seasons if it wasn’t for this collision. The Moon has several effects on the Earth that are probably beneficial for life. Without the Moon:

• There would be no lunar tides in the ocean. Only the much smaller tides from the Sun would remain. Tides may have helped wash minerals into the ocean that were needed for the formation of life.

• The Earth’s spin would not have slowed down, and the day would be as short as when the Earth was born (6 hours). With such fast rotation, the atmosphere would have much stronger winds, producing stronger ocean waves. Early organisms would have taken more of pounding.

• The tilt of the Earth’s axis would have been unstable, and could have changed drastically from time to time, which would have produced huge climate changes and earthquakes.

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When the Moon formed, it was about 10 times closer to the Earth than it is now. At this distance, the tidal forces would have been 1000 times stronger, resulting in ocean tides that rushed miles inland and out to sea every day (which was only 6 hours long).

Could it happen again?