Star and Planet Formation

When referring to the properties of stars like mass and radius, astronomers normally use units of the Sun’s mass, radius, etc. instead of units like kilograms and kilometers. The symbol for the Sun is . For instance, if a star has a mass 10 times greater than the Sun’s mass, then its mass is 10 M , which is read as “10 solar masses”. If a star has radius that is 1/5 of the Sun’s radius, than that radius is written as 0.2 R , or “0.2 solar radii”. Solar Units

Temperature and Pressure Gas Temperature: a measure of how fast atoms are moving in random directions Gas Pressure: the force these atoms exert on their surroundings via collisions Temperature and Pressure are related: Temperature   Pressure  Temperature   Pressure 

The Beginning of Star Formation Our solar system is within a galaxy of 200 billion stars, called the Milky Way. In addition to these stars, there are huge clouds of gas and dust in the Milky Way. Some of this gas was created at the birth of the universe (the Big Bang), and the remaining gas and dust has been produced since then by dying stars.

The Beginning of Star Formation These clouds span a very large range of masses. The total mass of the gas and dust in a cloud can range anywhere from 1 M  to 100,000,000 M . The clouds of gas and dust in space cast a silhouette against the light from stars behind the clouds.

Clouds of gas and dust are very cold, just a few degrees above absolute zero. As a result, gas pressure within a cloud is also low. So there is little resistance to the inward pull of gravity, which causes the cloud to collapse and eventually become a star. gravity pressure gas The Beginning of Star Formation

As a newborn star contracts, it becomes more compact, so the pressure inside of it increases. This pressure should eventually become high enough to halt the star’s collapse. At this point, gravity and gas pressure would balance each other. Gravity pulls the star inward Gas pressure resists gravity

Like any blackbody, the interior of a star emits thermal radiation into space. As a star loses this energy, its interior pressure decreases. As a result, gas pressure can balance gravity and halt collapse for only a short time. This is analogous to a balloon steadily deflating because of a leak. In the case of a star, it is light that is leaking out. So why isn’t the Sun collapsing? Gravity pulls the star inward Gas pressure resists gravity radiation

Central Temperature 100,000 K As a star collapses, its interior pressure increases, and hence the temperature also increases. Gravity pulls the star inward Gas pressure resists gravity

Central Temperature 1,000,000 K Gravity pulls the star inward Gas pressure resists gravity As a star collapses, its interior pressure increases, and hence the temperature also increases.

Central Temperature 10,000,000 K Gravity pulls the star inward Gas pressure resists gravity As a star collapses, its interior pressure increases, and hence the temperature also increases.

Central Temperature 10,000,000 K As a star collapses, its center eventually becomes hot enough to ignite hydrogen fusion, which replenishes the energy that is lost through radiation. As a result, the pressure remains stable, and collapse is halted.

Hydrogen Fusion A hydrogen atom consists of 1 proton and 1 electron. Inside of a star, the protons and electrons are detached from each other, and move around freely by themselves. At lower temperatures, protons move slowly, so they repel each other before touching.

Hydrogen Fusion If the center of a star is hot enough (several million Kelvin), protons are able to collide fast enough to overcome their mutual repulsion and fuse together.

Hydrogen Fusion In a fusion reaction, 4 protons fuse to become 1 helium nucleus: During this reaction, a small amount of the matter in the original protons is transformed into energy. As a result, the mass of the 1 helium nucleus is slightly less than the mass of the original 4 protons. Einstein’s equation E=mc 2 tells us how much energy is produced from the transformed matter. The energy produced by fusion is in the form of photons of light. It is because of fusion that the Sun shines very brightly and produces a lot of radiation. = sunlight used to be matter

Light’s journey to the Sun’s surface Energy (light) is produced by hydrogen fusion at the center of the Sun. These photons of light do not leave the Sun immediately, and instead bounce from atom to atom until finally escaping into space. The journey of a photon from the center to the surface of the Sun takes 10 million years.

If the mass is ≥ 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 cool to ignite hydrogen fusion. These stars that are too small in mass for hydrogen fusion are called brown dwarfs. After their birth, they become steadily cooler, fainter, and smaller in diameter while maintaining a constant mass. Sun 1 M  Brown Dwarfs: Stars without Fusion If the mass is <0.1 M , it’s a brown dwarf

The Beginning of Star Formation Rather than collapsing to form just 1 star, most clouds fragment into many clumps, which then collapse to form many individual stars.

After stars are born in a cloud of gas and dust, their light reflects from the surrounding cloud, which significantly changes its appearance. A cloud that is illuminated by starlight is often called a nebula. before stars are born after stars are born Nebulae of Newborn Stars

Stars behind large amounts of dust appear red for the same reason that the Sun appears red during sunrise and sunset. Some parts of a nebula appear blue for the same reason that the sky on Earth is blue (blue light is scattered more than red light). Scattering of Light in Nebulae

Emission Lines in Nebulae In addition to scattered light from the newborn stars, these nebulae produce emission line radiation, like an aurora on Earth.

Clues to How Planets are Born  The planets orbit the Sun in roughly the same plane  The planets orbit in the same direction around the Sun  Planets close to the Sun are made of rocks and metals while the planets far from the Sun are made of mostly ices and gases

Formation of Planets A star like the Sun is born when a huge cloud of gas and dust collapses due to gravity. Most of the matter collapses to the center to form the star, but some of it is left behind in a disk that rotates around the star. Planets are born within this rotating disk, which is why our solar system’s planets orbit in roughly the same plane and orbit in the same direction.

Temperature vs. Distance from Sun Close to the newborn Sun, it was so hot that only rocks and metals could condense into solid bodies. Far from the Sun, it was much colder, so ices could form, and planets could hold onto light gases more easily, which is why the outer planets are primarily made of gases and ices.

Temperature vs. Distance from Sun

Disks of gas and dust have been directly detected around newborn stars. Planets will spend a few million years growing within these disks until the disks eventually dissipate.

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 is similar in composition to the Earth’s crust and mantle, but has as very small iron core.

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).