The Universe Chapter 20

Stars Stars: huge sphere of very hot gas that emits light and other radiation Formed from clouds of dust and gas, or nebulas, and go through different stages as they age Stars are located at various distances from the Earth We measure this distance in light-years (ly) Distance light travels in one year - 9.5 × 1012 km UNIT OF DISTANCE, it would take us 10 million years to “drive” 1 light year

Life Cycle of Stars Like all things, stars have a natural progression from birth, through development, and then death About 90% of the stars in our galaxy, including the sun, are around midlife They are converting hydrogen into helium in their interiors (with nuclear fusion!).

Life Cycle of Stars Nebula: thin cloud of gas and dust Gravity causes nebula to collapse inward and begin spinning – creating a protostar Hydrogen atoms begin fusing into Helium The onset of this fusion marks the birth of a star

Life Cycle of Stars Hydrostatic Equilibrium: The fusion reactions in the core of the sun produce energy and outward pressure, this balances the inward pressure from gravity This creates the round shape of the sun

Life Cycle of Stars A star that is stable and midlife, like our sun, is called a Main Sequence Star When a star’s supply of hydrogen runs out, it begins to fuse heavier elements, all the way up to iron The outer layers of dust and gas expand, and the star swells to a Red Giant – a large reddish star in its late life cycle

Life Cycle of Stars The outer layers eject themselves as a planetary nebula In the sun’s case, these layers will engulf Mercury, Venus, and possibly Earth and Mars The remaining core will shrink to an Earth-size ball – called a White Dwarf

Life Cycle of Stars Stars larger than the sun will become supernovas Supernova: a stellar explosion The collapse of the core rebounds with a shock wave that violently blows the star’s outer layers away from the core. A supernova can become a black hole or a neutron star

Life Cycle of Stars Neutron stars are only a few dozen kilometers in diameter, but they are very dense. Just a teaspoon of matter from a neutron star would weigh more than 100 million tons on Earth.

Life Cycle of Stars If there is enough mass, a black hole will form after the supernova A black hole consists of matter so massive and compressed that nothing can escape its gravitational pull, not even light. The only way to detect one is by observing the radiation of light and X rays from the objects that revolve rapidly around them

Energy of a Star Stars have various layers that differ in number and depth Energy moves slowly through the layers by a combination of radiation and convection.

Energy of a Star Convection: hot gas moves away from the star’s core Radiation: energy is transferred to individual atoms; the atoms absorb the energy and transfer it to other atoms in random directions; atoms near the surface give off the energy into space as electromagnetic radiation

Studying Stars Ancient Greeks classified stars by their color and brightness Telescopes allowed astronomers to study stars in more detail for the first time

Traits of Stars Brightness of a star depends on the star’s temperature, size, and distance from the Earth. Stars produce energy in different wavelengths of electromagnetic radiation, such as high energy X rays and low energy radio waves

Traits of Stars A star’s color is related to its temperature Hotter Objects: colors that are more intense, shorter wavelengths, toward the blue end Cooler Objects: have longer wavelengths, closer to red The wavelength at which a hot object emits the most light will determine the color we see when looking at it

Traits of Stars Spectral lines reveal the composition of stars When you pass light through a spectrograph, it makes a unique pattern The pattern is determined by what types of elements make up the gas emitted from the light Each element has its own unique “fingerprint” when passed through this fancy prism and comparing these will give the make up of a star

H-R Diagram Diagram that shows how stars evolve This diagram doesn’t show where stars are literally, only their progression as they age Y-axis: luminosity, or the brightness of stars. X-axis: surface temperature of the stars, with hotter temperatures on the left side

Galaxies Galaxy: a collection of millions to billions of stars Grouped in clusters The Milky Way and the Adromeda galaxy are two of the largest, with a cluster of more than 30 galaxies Superclusters contain thousands of galaxies They are the largest known structures in the universe

Galaxies Gravity holds galaxies together, and the solar system revolves around the center of the galaxy because of this gravity It takes our solar system about 226 million years to complete one orbit of our galaxy

Galaxies Can be divided into three main types: Spiral, Elliptical, and Irregular Each has many stars, but differs in structure Interstellar Matter: the medium needed to create new stars, mostly gas and dust

Galaxies Spiral: Spiral arms made of gas, dust, and stars, has a lot of interstellar matter Milky Way Elliptical: Little gas or dust, no spiral arms, spherical or egg shaped, often reddish in color Irregular: Lack regular shapes and well defined structures, some have little interstellar matter while others have a lot

Galaxies Over Time When a scientist observes a galaxy that is 1 billion years away, they are observing light that left the galaxy 1 billion years ago Scientists don’t know what the galaxy looks like now, but can study similar closer galaxies to piece together the evolution of galaxies The gas, dust and stars that make up galaxies is in constant motion, as they consume their gas/dust they can no longer make stars The gas/dust from nearby galaxies can collide and set off rapid burst of new star formation

The Universe Universe: consists of all space, matter and energy that exists- now, in the past or in the future We see the universe now as it was in the past The farther an object is, the older the light that we get from that object is The sun is 8 light minutes away, that means we are seeing what the sun looked like 8 minutes ago

The Universe Most of the universe is empty space Space is a vacuum with no air and no air pressure

The Big Bang Scientists use telescopes to study the ancient light emitted by stars Scientists have theorized that the universe formed during a cataclysmic event known as the big bang The Big Bang Theory: States that the world began with a giant explosion 13 billion to 15 billion years ago (universe is believed to be about 13.7 billion years ago)

The Big Bang According to this theory, before the big bang there was: Nothing No time No space But out of this big nothing came the vast system of space, time, matter, and energy that now makes up the universe

The Big Bang According to the Big Bang Theory: Immediately after, the universe was extremely hot and made up of pure energy There was a period of rapid expansion that caused the energy to cool, and allowed electrons, neutrons and protons to form Hydrogen nuclei started to form but it was still too hot to form stable atoms About 380,000 years after the big bang is when electrons could combine with atomic nuclei to form atoms The first stars were born about 400 million years after the big bang

The Big Bang There are several theories being tested, and as new information is found, we might revise what we believe The Big Bang is the most supported by current evidence cosmic background radiation and observation of the movement of distant galaxies

Edwin Hubble 1929: Announced the universe is expanding based on observations of spectral lines in the light from other galaxies He found these lines were almost always shifted toward the red end of the spectrum

Edwin Hubble The red shift can be explained by the Doppler Effect Light waves from an object moving away would be stretched out The faster the object moves, the longer the wavelength

Cosmic Background Radiation 1965: Arno Penzias and Robert Wilson were making adjustments to a radio antenna they built There was a steady but dim signal they kept intercepting They realized it was cosmic background radiation The detected microwaves are remnants of radiation produced by the Big Bang

Future of the Universe The future of the universe is uncertain The universe is still expanding but it will not do this forever The combined gravity of all the mass in the universe is also pulling the universe inward

Future of the Universe Possible outcomes: 1. the universe will keep expanding forever 2. the expansion of the universe will gradually slow down and the universe will approach a limit in size 3. The universe will stop expanding and start to fall back on itself

Future of the Universe If there isn’t enough mass – gravity will not be strong enough to stop the expansion Just right amount of mass – the expansion will slow down but not end completely Too much mass – gravity will overcome the expansion and the universe will start to contract (the big crunch), becoming very hot and small, at this point the universe could end, or they cycle would start again