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Solar Units 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”.
Temperature and Pressure • Gas Temperature: a measure of how fast atoms are moving in random directions • Gas Pressure: the “force” these atoms put on their surroundings via their collisions Temperature and Pressure are related through the Equation of State: where one goes, the others go! Temperature Pressure Temperature Pressure
The Beginning of Star Formation The interstellar medium is the gas and dust floating in space between the stars. This material is created by the death of stars, but it 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 Formation Where 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 Formation 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 gas pressure
Gravity pulls the star inward 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. Gas pressure resists gravity
Gravity pulls the star inward 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? radiation Gas pressure resists gravity
Central Temperature 100,000 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 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 A star’s 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.
( very rarely) Stopping the Collapse: Hydrogen Fusion In order to keep up the gas pressure (and prevent collapse), the center of the Sun must continually replenish the energy that is lost. This is done by nuclear fusion (of hydrogen). The energy produced maintains hydrostatic equilibrium. quickly The sequence of fusions is called the proton-proton chain. Net Result: 4 H 1 He + energy quickly
Why does fusion produce energy? • The net result of the proton-proton chain is to turn 4 hydrogen atoms into 1 helium atom. But there is a mass defect – the 4 hydrogen atoms have 0.7% more mass than the 1 helium atom (plus the other junk). Where did the missing mass go? E = m c 2Energy!!! • If the Sun had more mass, it would have more gravity, and its center would be under greater pressure. The greater the pressure, the greater the temperature, and the more violent the nuclear collisions. More fusion would occur, and more energy would be produced. • Fusion only occurs in the core, where the temperature and density are greatest. The rest of the star just sits there.
Brown Dwarfs: Stars without Fusion 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. If the mass is above 0.1 M, it’s a star Sun 1 M If the mass is below 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 stars. Each fragment is called a protostar.
Reddening and Scattering 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.
Emission Lines from Nebulae In addition to scattered light from the newborn stars, these nebulae produce emission line radiation, just like an aurora.
Characteristics of the Solar System Any theory for the formation of planets must explain: • The flatness of the Solar System (planets orbit in same plane) • All of the planets orbit in the same direction • 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 larger and larger bodies. • Over time, the planetesimals grow as more molecules condense out of the nebula
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 cold enough that ices could form, explaining why the outer planets have lower densities.
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.
The Age of the Solar System We 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 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.
