Space+Science

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Most astronomers believe the Universe began in a Big Bang about 14 billion years ago. At that time, the entire Universe was inside a bubble that was thousands of times smaller than a pinhead. It was hotter and denser than anything we can imagine.

Then it suddenly exploded. The Universe that we know was born. Time, space and matter all began with the Big Bang. In a fraction of a second, the Universe grew from smaller than a single atom to bigger than a galaxy. And it kept on growing at a fantastic rate. It is still expanding today.

As the Universe expanded and cooled, energy changed into particles of matter and antimatter. These two opposite types of particles largely destroyed each other. But some matter survived. More stable particles called protons and neutrons started to form when the Universe was one second old.

Over the next three minutes, the temperature dropped below 1 billion degrees Celsius. It was now cool enough for the protons and neutrons to come together, forming hydrogen and helium nuclei.

After 300 000 years, the Universe had cooled to about 3000 degrees. Atomic nuclei could finally capture electrons to form atoms. The Universe filled with clouds of hydrogen and helium gas.

= = = = = Big Idea: The sun is one of billions of stars in one of billions of galaxies in the universe. =

//Students will be able to compare various models of the solar system.// Our understanding of the solar system has changed over many centuries. Scientists such as Ptolemy, Copernicus and Galileo added their ideas to our changing understanding. We observe the universe using a variety of equipment, from telescopes to orbiters and landers. =Models of the solar system =  The planets and the Sun orbit the Earth in Ptolemy's model. Our understanding of the universe has changed over time. Different civilizations have created different models to explain what the universe is and how the universe began. The Greek astronomer Ptolemy (c90-168AD) used measurements of the sky to create his **geocentric** model. This had the earth at the center and all the planets and the sun orbiting around it. The geocentric model lasted a long time. It wasn’t until the mid 18th century that Nicolaus Copernicus (1473-1543) came up with a different model. His heliocentric theory put the sun at the center if the universe. It was based on observations with the telescope – work pioneered by the Italian astronomer Galileo Galilei (1564-1642).

Essential Question: What makes up the universe?
//Students will learn what makes up the structure of the universe//

Stars and Galaxies When you look at the night sky you can see many beautiful stars. If you are out in the country or camping in the mountains or the desert away from the city lights, you may see thousands of them. You may even be able to see part of the Milky Way. In a town or city, you can't see nearly as many stars because the city lights create a glow in the sky masking many of them.

There are several different kinds of stars in the sky. Some are very big. A couple of stars have been found that are 100 to 200 times larger than the sun. Some very old stars are smaller than the Earth. Scientists study stars and place them in groups based on how they are alike and how they are different.

Structure and Size of Universe

Essential Question: What are some properties of stars?
// Students will be able to describe stars and their physical properties. //

Stars A star is a huge sphere of very hot, glowing gas. Stars produce their own light and energy by a process called nuclear fusion. Fusion happens when lighter elements are forced to become heavier elements. When this happens, a tremendous amount of energy is created causing the star to heat up and shine. Stars come in a variety of sizes and colors. Our Sun is an average sized yellowish star. Stars which are smaller than our Sun are reddish and larger stars are blue.

Stars are made of very hot gas. This gas is mostly hydrogen and helium, which are the two lightest elements. Stars shine by burning hydrogen into helium in their cores, and later in their lives create heavier elements. Most stars have small amounts of heavier elements like carbon, nitrogen, oxygen and iron, which were created by stars that existed before them. After a star runs out of fuel, it ejects much of its material back into space. New stars are formed from this material. So the material in stars is recycled.

Brightness and Luminosity A glance at the night sky above Earth shows that some stars are much brighter than others. However, the brightness of a star depends on its composition and how far it is from the planet.

Astronomers define star brightness in terms of apparent magnitude (how bright the star appears from Earth) and absolute magnitude (how bright the star appears at a standard distance of 32.6 light years, or 10 parsecs). Astronomers also measure luminosity — the amount of energy (light) that a star emits from its surface.

The ancient astronomers believed the stars were attached to a gigantic crystal sphere surrounding Earth. In that scenario, all stars were located at the same distance from Earth, and so, to the ancients, the brightness or dimness of stars depended only on the stars themselves.

In our cosmology, the stars we see with the eye alone on a dark night are located at very different distances from us, from several light-years to over 1,000 light-years. Telescopes show the light of stars millions or billions of light-years away.

Today, when we talk about a star’s brightness, we might mean one of two things: its intrinsic brightness or its apparent brightness. When astronomers speak of the luminosity of a star, they’re speaking of a star’s intrinsic brightness, how bright it really is. A star’s apparent magnitude – its brightness as it appears from Earth – is something different and depends on how far away we are from that star.

Temperature and Size

Two astronomers, Ejnar Hertzsprung from Denmark and Henry Norris Russell from the Unites States, both discovered that the brightness of a star depends on the surface temperature of the star. They each made this discovery on their own separately. Together, they came up with this diagram that explains the brightness, temperature and classes of stars.

The scale on the left shows how bright a star is. The letters across the bottom represent the spectral class of stars, or color of stars. O – Blue

B – Blue/White

A – White

F - White/Yellow

G – Yellow

K – Orange

M - Red The temperature of the stars measured across the bottom of the scale are measured in Kelvin. Zero Kelvin equals -273 degrees Celsius, -459 degrees Fahrenheit. As you can see, there are only a few categories of stars. Most stars in our universe are main sequence stars, including our sun Sol. Notice how the biggest stars are the brightest but not the hottest. The white dwarf stars are near the end of their life and losing much of their brightness but they are very hot. Have you ever noticed that stars shine in an array of different colors in a dark country sky? If not, try looking at stars with binoculars sometime. Color is a telltale sign of surface temperature. The hottest stars radiate blue or blue-white, whereas the coolest stars exhibit distinctly ruddy hues. Our yellow-colored sun indicates a moderate surface temperature in between the two extremes. Spica serves as prime example of a hot blue-white star, Altair : moderately-hot white star, Capella : middle-of-the-road yellow star, Arcturus : lukewarm orange star and Betelgeuse : cool red supergiant.

Essential Question: How do stars change over time?
Life Cycle of Stars //Students will learn that stars form in nebulae - giant gas clouds - through the force of gravity and the process of nuclear fusion. A low mass star eventually slows down, its outer atmosphere expands greatly and the star becomes a giant as it fuses helium and carbon. A high mass star eventually begins to fuse together atoms to create larger elements. This fusion triggers a supernova, which may leave behind a neutron star or a black hole.//

Nebula Imagine an enormous cloud of gas and dust many light-years across. Gravity, as it always does, tries to pull the materials together. A few grains of dust collect a few more, then a few more, then more still. Eventually, enough gas and dust has been collected into a giant ball that, at the center of the ball, the temperature (from all the gas and dust bumping into each other under the great pressure of the surrounding material) reaches 15 million degrees or so. A wondrous event occurs.... nuclear fusion begins and the ball of gas and dust starts to glow. A protostar, or new star, has begun its life in our Universe.

So what is this magical thing called "nuclear fusion" and why does it start happening inside the ball of gas and dust? It happens like this..... As the contraction of the gas and dust progresses and the temperature reaches 15 million degrees or so, the pressure at the center of the ball becomes enormous. The electrons are stripped off of their parent atoms, creating a plasma. The contraction continues and the nuclei in the plasma start moving faster and faster. Eventually, they approach each other so fast that they overcome the electrical repulsion that exists between their protons. The nuclei crash into each other so hard that they stick together, or //fuse //. In doing so, they give off a great deal of energy. This energy from fusion pours out from the core, setting up an outward pressure in the gas around it that balances the inward pull of gravity. When the released energy reaches the outer layers of the ball of gas and dust, it moves off into space in the form of electromagnetic radiation. The ball, now a star, begins to shine.

New stars come in a variety of sizes and colors. They range from blue to red, from less than half the size of our Sun to over 20 times the Sun’s size. It all depends on how much gas and dust is collected during the star’s formation. The color of the star depends on the surface temperature of the star. And its temperature depends, again, on how much gas and dust were accumulated during formation. The more mass a star starts out with, the brighter and hotter it will be. For a star, everything depends on its mass.

Throughout their lives, stars fight the inward pull of the force of gravity. It is only the outward pressure created by the nuclear reactions pushing away from the star's core that keeps the star "intact". But these nuclear reactions require fuel, in particular hydrogen. Eventually the supply of hydrogen runs out and the star begins its demise.

After millions to billions of years, depending on their initial masses, stars run out of their main fuel - hydrogen. Once the ready supply of hydrogen in the core is gone, nuclear processes occurring there cease. Without the outward pressure generated from these reactions to counteract the force of gravity, the outer layers of the star begin to collapse inward toward the core. Just as during formation, when the material contracts, the temperature and pressure increase. This newly generated heat temporarily counteracts the force of gravity, and the outer layers of the star are now pushed outward. The star expands to larger than it ever was during its lifetime -- a few to about a hundred times bigger. The star has become a red giant.

What happens next in the life of a star depends on its initial mass. Whether it was a "massive" star (some 5 or more times the mass of our Sun) or whether it was a "low or medium mass" star (about 0.4 to 3.4 times the mass of our Sun), the next steps after the red giant phase are very, very different.

The Fate of Sun-Sized Stars: Black Dwarfs Once a medium size star (such as our Sun) has reached the red giant phase, its outer layers continue to expand, the core contracts inward, and helium atoms in the core fuse together to form carbon. This fusion releases energy and the star gets a temporary reprieve. However, in a Sun-sized star, this process might only take a few minutes! The atomic structure of carbon is too strong to be further compressed by the mass of the surrounding material. The core is stabilized and the end is near.

The star will now begin to shed its outer layers as a diffuse cloud called a planetary nebula. Eventually, only about 20% of the star’s initial mass remains and the star spends the rest of its days cooling and shrinking until it is only a few thousand miles in diameter. It has become a white dwarf. White dwarfs are stable because the inward pull of gravity is balanced by the electrons in the core of the star repulsing each other. With no fuel left to burn, the hot star radiates its remaining heat into the coldness of space for many billions of years. In the end, it will just sit in space as a cold dark mass sometimes referred to as a black dwarf.

The Fate of Massive Stars: Supernovae! and Then... Fate has something very different, and very dramatic, in store for stars which are some 5 or more times as massive as our Sun. After the outer layers of the star have swollen into a red supergiant (i.e., a very big red giant), the core begins to yield to gravity and starts to shrink. As it shrinks, it grows hotter and denser, and a new series of nuclear reactions begin to occur, temporarily halting the collapse of the core. However, when the core becomes essentially just iron, it has nothing left to fuse (because of iron's nuclear structure, it does not permit its atoms to fuse into heavier elements) and fusion ceases. In less than a second, the star begins the final phase of its gravitational collapse. The core temperature rises to over 100 billion degrees as the iron atoms are crushed together. The repulsive force between the nuclei overcomes the force of gravity, and the core recoils out from the heart of the star in an explosive shock wave. As the shock encounters material in the star's outer layers, the material is heated, fusing to form new elements and radioactive isotopes. In one of the most spectacular events in the Universe, the shock propels the material away from the star in a tremendous explosion called a supernova. The material spews off into interstellar space -perhaps to collide with other cosmic debris and form new stars, perhaps to form planets and moons, perhaps to act as the seeds for an infinite variety of living things.

So what, if anything, remains of the core of the original star? Unlike in smaller stars, where the core becomes essentially all carbon and stable, the intense pressure inside the supergiant causes the electrons to be forced inside of (or combined with) the protons, forming neutrons. In fact, the whole core of the star becomes nothing but a dense ball of neutrons. It is possible that this core will remain intact after the supernova, and be called a neutron star. However, if the original star was very massive (say 15 or more times the mass of our Sun), even the neutrons will not be able to survive the core collapse and a black hole will form!

A. White/Black Dwarfs A star like our Sun will become a white dwarf when it has exhausted its nuclear fuel. Near the end of its nuclear burning stage, such a star expels most of its outer material (creating a planetary nebula) until only the hot (T > 100,000 K) core remains, which then settles down to become a young white dwarf. A typical white dwarf is half as massive as the Sun, yet only slightly bigger than the Earth. This makes white dwarfs one of the densest forms of matter, surpassed only by neutron stars.

White dwarfs have no way to keep themselves hot (unless they accrete matter from other closeby stars); therefore, they cool down over the course of many billions of years. Eventually, such stars cool completely and become black dwarfs. Black dwarfs do not radiate at all.

Many nearby, young white dwarfs have been detected as sources of soft X-rays (i.e. lower-energy X-rays); soft X-ray and extreme ultraviolet observations enable astronomers to study the composition and structure of the thin atmospheres of these stars.

B. Neutron Stars Neutron stars are typically about ten miles in diameter, have about 1.4 times the mass of our Sun, and spin very rapidly (one revolution takes mere seconds!). Neutron stars are fascinating because they are the densest objects known. Due to its small size and high density, a neutron star possesses a surface gravitational field about 300,000 times that of Earth.

Neutron stars also have very intense magnetic fields - about 1,000,000,000,000 times stronger than Earth's. Neutron stars may "pulse" due to electrons accelerated near the magnetic poles, which are not aligned with the rotation axis of the star. These electrons travel outward from the neutron star, until they reach the point at which they would be forced to travel faster than the speed of light in order to still co-rotate with the star. At this radius, the electrons must stop, and they release some of their kinetic energy in the form of X-rays and gamma rays. External viewers see these pulses of radiation whenever the magnetic pole is visible. The pulses come at the same rate as the rotation of the neutron star, and thus, appear periodic. Neutron stars which emit such pulses are called pulsars.

Black Holes Black holes are objects so dense that not even light can escape their gravity and, since nothing can travel faster than light, nothing can escape from inside a black hole. Nevertheless, there is now a great deal of observational evidence for the existence of two types of black holes: those with masses of a typical star (4-15 times the mass of our Sun), and those with masses of a typical galaxy. This evidence comes not from seeing the black holes directly, but by observing the behavior of stars and other material near them!

Galaxy-mass black holes are found in Active Galactic Nuclei (AGN). They are thought to have the mass of about 10 to 100 billion Suns! The mass of one of these supermassive black holes has recently been measured using radio astronomy. X-ray observations of iron in the accretion disks may actually be showing the effects of massive black holes as well.

Essential Question: What are the properties of the sun?
//Students will be able to describe the structure and rotation of the sun, energy production and transport in the sun, and solar activity on the sun.//

Sun Structure

The Romans called the sun Sol, which in English means sun. In ancient Greece, the sun was called Helios. Our Sun is not unique in the universe. It is a common middle-sized yellow star which scientists have named Sol, after the ancient Roman name. This is why our system of planets is called the Solar System. There are trillions of other stars in the universe just like it. Many of these stars have their own systems of planets, moons, asteroids, and comets.

The Sun was born in a vast cloud of gas and dust around 5 billion years ago. Indeed, these vast nebulae are the birth places of all stars. Over a period of many millions of years, this gas and dust began to fall into a common center under the force of its own gravity.

At the center, an ever growing body of mass was forming. As the matter fell inward, it generated a tremendous amount of heat and pressure. As it grew, the baby Sun became hotter and hotter. Eventually, when it reached a temperature of around 1 million degrees, its core ignited, causing it to begin nuclear fusion.

When this happened, the Sun began producing its own light, heat, and energy.

What is Thermonuclear Fusion? Thermonuclear fusion is the process in which a star produce its light, heat, and energy. This happens at the core of the star. The core is super-heated to millions of degrees. This heat travels towards the surface and radiates out into the universe. Through this thermonuclear process, stars "burn" a fuel known as hydrogen. The result is that they create another type of fuel known as helium. However, stars do not burn in the same way that a fire does, because stars are not on fire.

Energy Transfer

Heat rises, while cooler gas falls. Have you ever noticed that your basement is always much cooler than upstairs. The same laws of physics apply within stars. Because heat rises while cooler gases fall, the gas within a star is constantly rising and falling. This creates massive streams of circular motion within the star. This is called convection.

As the gases near the core of the Sun are heated, they begin to rise towards the surface. As they do so, they cool somewhat. Eventually they become cool enough that they begin to sink back down towards the core. It can take an atom millions of years to complete one complete cycle around a convection stream. As a result of this process, the temperature on the surface of the Sun is around 10,000 degrees Fahrenheit, which is much cooler than its super-heated core.

Solar Activity Sun Spots We don't often think of the Sun as having cooler areas on its surface. The Sun is far too hot for an astronaut to ever visit, but there are areas which are slightly cooler than others. These areas are known as sun spots. Sun spots are still very hot. However, because they are slightly cooler than the rest of the surface of the Sun, they appear slightly darker in color. The gravitational forces in Sun spots are also stronger than the other hotter areas. Of course, you cannot look directly at the Sun to see these spots because you would damage your eyes. Astronomers have to use special telescopes with filters and other instruments to be able to see the cooler spots on the surface of the Sun.

Sun spots come and go on a regular basis. At times, there are very few, if any sun spots. At other times there are far more. They generally increase in intensity and then decrease over a period of 11 years. This 11 year cycle is known as the Saros Cycle. To learn more, click here.

Solar Flares During periods of high solar activity, the Sun commonly releases massive amounts of gas and plasma into its atmosphere. These ejections are known as solar flares. Some solar flares can be truly massive, and contain impressive power. On occasion, these more powerful flares can even cause satellites orbiting the Earth to malfunction. They can also interact with Earth's magnetic field to create impressive and beautiful light shows known as the Northern and Southern lights. In the northern hemisphere, these lights are commonly known as the Aurora Borealis.

Solar Winds As the Sun burns hydrogen at its core, it releases vast amounts of atomic particles, or pieces of atoms, into outer space. These atomic particles along with the Sun's radiation create a sort of wind, known as the solar wind.

This wind blows particles outward in all directions from the Sun. Even as you read this, there are atomic particles which are traveling from the Sun towards you. Often, particles pass right through your body without you ever realizing it.

Eventually this wind reaches out beyond the Solar System and begins to mix with the winds from other stars. The bubble around the Sun where the solar winds are still strong enough to blow outward is known as the heliosphere (note the Greek name Helios). The area of space where the winds are too weak to continue pushing outward and instead begin to mix with the winds of other stars is known as the interstellar medium.

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