Stars Chapter 30

30.1 The Sun The Sun is the largest object in the solar system in both size and mass The Sun is about 330,000 times as massive as Earth and 1048 times as massive as Jupiter The Sun contains 99% of all mass in the solar system Density of the Sun is similar to the densities of the gas giants Due to its size, the Sun controls all the motion in the solar system

Solar Structure Most of the mass of the Sun comes from hydrogen (70.4%) and helium (28%), which is very similar to the gas giants. Due to the similarities, it is suggested that they are representative of the interstellar cloud that formed our solar system.

The Sun’s Atmosphere The lowest layer of the Sun’s atmosphere is about 400km in thickness. This is called the Photosphere. This is the visible surface of the Sun. Above the photosphere is the chromosphere, which is about 2500km in thickness. The temperature is nearly 30,000 K at the top. The chromosphere is typically only visible during a solar eclipse, when the photosphere is blocked. The top layer of the Sun’s atmosphere is called the corona. This extends several million km from the chromosphere and has a temperature range of 1-2 million K. The density of the gas in the corona is quite low, which is why the corona is so dim.

Solar Wind Formed by the gas that flows outward from the corona at high speeds. Solar wind “bathes” each planet in the solar system with a flood of particles. At Earth, which is 1 AU from the Sun, solar wind flows at a speed of 400km/s. These charged particles are deflected by magnetic fields of Earth and trapped in rings called the Van Allen belts. The high-energy particles in the belts collide with gases in the atmosphere and cause a light to be given, in which we call the aurora.

Solar Activity Periodically, the Sun’s magnetic field disturbs the solar atmosphere and causes new features to appear. The most obvious of these is sun spots, which are dark spots on the surface of the photosphere. Sunspots are very bright, but they appear darker because they are cooler. Sunspots typically last a few months and occur in pairs with opposite magnetic polarities.

Solar Activity Cycle Scientists hypothesize that the solar activity cycle is 11.2 years in length, due to the number of sunspots changing regularly and on average. When the polarity of the Sun’s magnetic field is taken into account, the length doubles to 22.4 years. When the polarity of the magnetic fields reverse, the polarity of the sunspots also reverse. This causes the solar activity cycle to go from minimum spots to maximum spots. The magnetic field then switches polarity again and the solar activity cycle is repeated.

Other Solar Features Coronal Holes: located over sunspot groups, low density and act as “escape route” for solar wind Solar Flares: violent eruptions of particles and radiation from the surface of the Sun Prominence: an arc of gas that is ejected from the chromosphere, with an ability to reach up to 50,000 K

(Einstein’s Theory of Special Relativity… E = mc2) The Solar Interior Where does all of the energy that causes solar activity and light come from? Within the core of the Sun, where pressure and temperature are extremely high, fusion occurs. Fusion: combining of lightweight nuclei (hydrogen) into heavier nuclei Fission: splitting of heavy nuclei into smaller, lighter nuclei Fusion occurs in the core of the Sun. The mass lost through fusion of hydrogen to helium is converted to energy… which powers the Sun (Einstein’s Theory of Special Relativity… E = mc2)

Energy from the Sun If energy of the Sun is produced in the core, then how does it get to the surface before travelling to Earth? Radiative Zone: region above core where energy is transferred from particle to particle by radiation Convective Zone: region above the radiative zone where volumes of gas carries energy the rest of the way to the Earth’s surface through convection

Visible light arranged according to wavelengths Spectra Visible light arranged according to wavelengths

30.2 Measuring the Stars Groups of Stars: During ancient times, many civilizations looked at the brightest stars and named groups of them after animals, mythological characters, or everyday objects. These groups are called constellations. Today, we group stars by the 88 constellations named by ancient people. Some can be seen all year long Some can be seen in the northern hemisphere only (Big Dipper) Some can only be seen during specific times of the year (Orion ~ winter) As result, constellations are classified as summer, fall, winter, and spring constellations

Constellations

Zodiac Signs Did you know that…

Star Clusters Stars in constellations appear to be close to one another, but few are gravitationally bound to one another. They appear close because the human eyes cannot distinguish how far or near stars actually are. By measuring distances to stars and observing their interactions to one another, we can determine which ARE gravitationally bound… or star clusters.

Sirius shining brightly in the night sky Binaries When only two stars are bound together and orbit a common center of mass, they are called binary stars. More than half of all stars in the sky are either binary or part of a multi-star system. They are so close to one another, they appear to be one star to the human eye. Sirius shining brightly in the night sky

Stellar Positions and Distances Astronomers use two units of measure for long distances: Light year – the distance that light travels in one year (9.461 x 1012) Parsec – larger than a light year, equal to 3.26 light years (3.086 x 1013) It is important to astronomers to find exact distances to stars and they must use special tools due to parallax, which is a nearby shift in position of a nearby star as observed from Earth.

Basic Properties of Stars Basic properties include: Diameter Mass Brightness energy output (power) Surface temperature Composition

Magnitude of Stars Magnitude: how bright the star appears in the night sky Types of Magnitude: Apparent Magnitude – how bright the star appears to be, without taking distance into consideration Absolute Magnitude – how bright the star appears to be at a specific distance Luminosity: the energy output from the surface of a star per second

Spectra of Stars Stars have dark absorption lines in their spectra and are classified according to their patterns of absorption lines. Stars are classified on the spectrum in the following order: O, B, A, F, G, K, and M with O being the hottest and M being the coolest. All stars have nearly identical compositions, including the Sun, with nearly 73% of the mass being hydrogen, 25% helium, and the remaining 2% is other elements.

Wavelength Shifts Spectral lines are shifted in wavelength by motion between the source of light and the observer. If a star moves toward the observer, the lines are blueshifted (shorter wavelength) If a star moves away from the observer, the lines are redshifted (longer wavelengths) The higher the speed, the larger the shift. There is no shift for motion that is sideways, so we can only learn about the portion of shift toward or away from Earth.

H-R Diagrams The properties of mass, luminosity, temperature, and diameter are closely related and demonstrated on a graph called the Hertzsprung-Russell diagram (H-R Diagram) with absolute magnitude plotted on the vertical axis and temperature plotted on the horizontal axis. About 90% of all stars are main sequence stars, including the Sun. Main sequence stars range from hot and luminous to cool and dim. Red Giants, a star more than 100 times as large as the Sun in some cases, are cool but very luminous so must be very large White Dwarfs, about the size of Earth with a mass as large as the Sun’s, are dim, so must be small, but very hot

30.3 Stellar Evolution Mass and composition of a star determine nearly all of its other properties. The more massive a star is, the greater the gravity pressing inward, and the hotter and denser the star must be inside to balance the gravity. The temperature inside a star governs the rate of nuclear reactions, which determine the star’s energy output (luminosity). The star must balance itself, by it’s mass, so it does not expand or contract and become unstable.

Fusion Inside a star, conditions vary in the same way they do inside of the Sun. Density and temperature increase toward the center, where energy is generated by nuclear fusion. Stars on the main sequence all fuse hydrogen into helium, like the Sun. (Stars off the main sequence fuse different elements.) Once a star’s entire core has been converted to helium, helium can fuse to form carbon, then carbon fuses to form heavier elements, and so on. The energy given off by fusion provides the pressure needed to counteract gravity.

Stellar Evolution and Life Cycles A star changes as it ages because its internal composition changes as nuclear fusion reactions in the star’s core convert one element into another. As the core composition changes, density increases and temperature rises. The luminosity of the star also increases until the nuclear fuel eventually runs out. Formation: stars begin as a cloud of gas and dust called a nebula, which collapses in on itself due to gravity. As the nebula contracts and rotates it creates a disk shape with a hot condensed object in the center called a protostar. The temperature in the protostar becomes hot enough for fusion to occur, providing pressure to balance gravity. It is now a star on the main sequence. What happens next depends on the star’s mass.

A Protostar, formed from a disk of gas and dust

The Sun’s Life Cycle As the Sun fuses hydrogen into helium, density and temperature rise making it more luminous. After about 10 billion years for a star with the mass of the Sun, all of the hydrogen in the core is used up. Outer layers of the star begin to expand and cool and the star becomes a red giant and loses gas from its outer layers as they expand and cool. The helium core begins fusing into carbon, but the core will never get hot enough for carbon to fuse so all the fuel gets used up. The outer layers expand and are driven off in a shell of gas called planetary nebula. The remaining core is a white dwarf star.

Life Cycles of Massive Stars For stars more massive than the Sun, evolution is very different. A more massive star begins its life in the same way, but much higher on the main sequence, with hydrogen being converted to helium. More massive stars use their fuel quickly, due to their high luminosity. It can go through many more cycles of fusion, so becomes a red giant many times as it expands following the end of each reaction stage. It eventually drives off the outer layers becoming a white dwarf.

Supernovae Some stars do not lose enough mass to become white dwarfs and their cores become so massive that they collapse in on themselves. This creates a neutron star as electrons and protons merge to form neutrons and provide resistance to further collapsing. These are incredibly dense, about 100 trillion times more dense than water. A neutron star forms quickly, while the outer layers of the star are still falling quickly. The gas that is still collapsing inward rebounds off the surface of the star, creating a massive explosion that occurs when the outer layers are blown off. The massive explosion is referred to as a supernova.

Black Holes Some stars are so massive that the resistance of the neutrons cannot stop the collapsing. The core will continue to collapse, forever, compacting matter into a smaller and smaller volume. The small, but extremely dense, object that remains is called a black hole. The gravity of the black hole is so immense that nothing, not even light, can escape it.