Introduction to the Universe

-Outline the general structure of the solar system-Distinguish between a stellar cluster and a constellation-Define the light year-Compare the relative distances between stars within a galaxy and between galaxies in terms of orders of magnitude.-Describe the apparent motion of the stars/constellations over a period of a year, and explain these observations in terms of the rotation and revolution of the Earth.

Solar System

SunCompared with the billions of other stars in the universe, the sun is unremarkable. But for Earth and the other planets that revolve around it, the sun is a powerful center of attention. It holds the solar system together; pours life-giving light, heat, and energy on Earth.The sun is a big star, about 1.4 million kilometers wide. If the sun were a hollow ball, more than a million Earths could stuff inside it. But the sun isn't hollow. It's filled with scorching hot gases that account for more than 99.8 percent of the total mass in the solar system. How hot? The temperature is about 5,500 degrees Celsius on the surface and more than 15.5 million Celsius at the core.Deep in the sun's core, nuclear fusion reactions convert hydrogen to helium, which generates energy. Like many energy sources, the sun is not forever. It is already about 4.5 billion years old and has used up nearly half of the hydrogen in its core. The sun will continue to burn through the hydrogen for another five billion years or so, and then helium will become its primary fuel. The sun will expand to about a hundred times its current size, swallowing Earth and other planets. It will burn as a red giant for another billion years and then collapse into a white dwarf about the size of planet Earth.

Planets

Ancient astronomers observed points of light that appeared to move among the stars. They called these objects "planets," meaning wanderers, and named them after Roman deities—Jupiter, king of the gods; Mars, the god of war; Mercury, messenger of the gods; Venus, the goddes of love and beauty, and Saturn, father of Jupiter and god of agriculture.

Since the invention of the telescope, three more planets have been discovered in our solar system: Uranus (1781), Neptune (1846), and, now downgraded to a dwarf planet, Pluto (1930).

PlanetsThe elliptical orbits of the planets have the Sun at one of the two foci of the ellipse. The orbit of the Earth is almost circular, that of Mercury is the most elliptical.

The planes of the orbits of the planets differ only slightly from the plane of the Earth’s orbit, with the exception of Mercury, which has an inclination of 7°.

All planets revolve around the Sun in the same direction

Planets

The Solar System The planets listed in order of increasing size

The relative size, mass and acceleration due to gravity at the surface of the planets compared with Earth.

MercuryMercury's elliptical orbit takes the small planet as close as 47 million kilometers and as far as 70 million kilometers from the Sun. If one could stand on the scorching surface of Mercury when it is at its closest point to the sun, the sun would appear almost three times as large as it does when viewed from Earth.Temperatures on Mercury's surface can reach 430 degrees Celsius. Because the planet has no atmosphere to retain that heat, nighttime temperatures on the surface can drop to -170 degrees Celsius.Because Mercury is so close to the sun, it is hard to directly observe from Earth except during twilight. Mercury speeds around the sun every 88 days, traveling through space at nearly 50 kilometers per second faster than any other planet. The length of one Mercury day is equal to 58.646 Earth days.Rather than an atmosphere, Mercury possesses a thin exosphere made up of atoms blasted off its surface by solar wind and striking micrometeoroids. Because of the planet's extreme surface temperature, the atoms quickly escape into space. With the thin exosphere, there has been no wind erosion of the surface and meteorites do not burn up due to friction as they do in other planetary atmospheres. Mercury's surface resembles that of Earth's moon, scarred by many impact craters resulting from collisions with meteoroids and comets.

VenusVenus is covered by a thick, rapidly spinning atmosphere, creating a scorched world with temperatures hot enough to melt lead and a surface pressure 90 times that of Earth. Because of its proximity to Earth and the way its clouds reflect sunlight, Venus appears to be the brightest planet in the sky.Venus's atmosphere consists mainly of carbon dioxide, with clouds of sulfuric acid droplets. Only trace amounts of water have been detected in the atmosphere. The thick atmosphere traps the sun's heat, resulting in surface temperatures over 470 degrees Celsius. Probes that have landed on Venus have not survived more than a few hours before being destroyed by the incredibly high temperatures.The Venusian year is about 225 Earth days long, while the planet's rotation period is 243 Earth days. Venus rotates retrograde compared with Earth's prograde rotation. About 90 percent of the surface of Venus appears to be recently solidified basalt lava; it is thought that the planet was completely resurfaced by volcanic activity 300 million to 500 million years ago. Sulfur compounds, possibly attributable to volcanic activity, are abundant in Venus's clouds. The corrosive chemistry and dense, moving atmosphere cause significant surface weathering and erosion.

Earth

MarsMars is a small rocky body once thought to be very Earthlike. Like the other terrestrial planets—Mercury, Venus, and Earth—its surface has been changed by volcanism, impacts from other bodies, movements of its crust, and atmospheric effects such as dust storms. It has polar ice caps that grow and recede with the change of seasons; areas of layered soils near the Martian poles suggest that the planet's climate has changed more than once, perhaps caused by a regular change in the planet's orbit.At present, Mars is too cold and its atmosphere is too thin to allow liquid water to exist at the surface for long. There's water ice close to the surface and more water frozen in the polar ice caps, but the quantity of water required to carve Mars's great channels and flood plains is not evident on—or near—the surface today. Images from NASA's Mars Global Surveyor spacecraft suggest that underground reserves of water may break through the surface as springs.Mars has some remarkable geological characteristics, including the largest volcanic mountain in the solar system, Olympus Mons.

JupiterThe most massive planet in our solar system, with four planet-size moons and many smaller satellites, Jupiter forms a kind of miniature solar system. Jupiter resembles a star in composition. In fact, if it had been about eighty times more massive, it would have become a star rather than a planet.Jupiter's appearance is a tapestry of beautiful colors and atmospheric features. Most visible clouds are composed of ammonia. Water exists deep below and can sometimes be seen through clear spots in the clouds. The planet's "stripes" are dark belts and light zones created by strong east-west winds in Jupiter's upper atmosphere. Within these belts and zones are storm systems that have raged for years. The Great Red Spot, a giant spinning storm, has been observed for more than 300 years.The composition of Jupiter's atmosphere is similar to that of the sun—mostly hydrogen and helium. Deep in the atmosphere, the pressure and temperature increase, compressing the hydrogen gas into a liquid. At depths about a third of the way down, the hydrogen becomes metallic and electrically conducting. In this metallic layer, Jupiter's powerful magnetic field is generated by electrical currents driven by Jupiter's fast rotation. At the center, the immense pressure may support a solid core of ice-rock about the size of Earth.

SaturnLike Jupiter, Saturn is made mostly of hydrogen and helium. Its volume is 755 times greater than that of Earth. Winds in the upper atmosphere reach 1,600 feet (500 meters) per second in the equatorial region. (In contrast, the strongest hurricane-force winds on Earth top out at about 360 feet, or 110 meters, per second.) These superfast winds, combined with heat rising from within the planet's interior, cause the yellow and gold bands visible in the atmosphere.Saturn's ring system is the most extensive and complex in the solar system, extending hundreds of thousands of kilometers from the planet. In the early 1980s, NASA's two Voyager spacecraft revealed that Saturn's rings are made mostly of water ice. They also found "braided" rings, ringlets, and "spokes," dark features in the rings that circle the planet at different rates from that of the surrounding ring material. Material in the rings ranges in size from a few micrometers to several tens of meters.

UranusUranus has been revealed as a dynamic world with some of the brightest clouds in the outer solar system and 11 rings. The first planet found with the aid of a telescope, Uranus was discovered in 1781 by astronomer William Herschel.Uranus, with no solid surface, is one of the gas giant planets. Its atmosphere is composed primarily of hydrogen and helium, with a small amount of methane and traces of water and ammonia. Uranus gets its blue-green color from methane gas. Sunlight is reflected from Uranus's cloud tops, which lie beneath a layer of methane gas. As the reflected sunlight passes back through this layer, the methane gas absorbs the red portion of the light, allowing the blue portion to pass through and resulting in the blue-green color that we see.Uranus's rotation axis is nearly horizontal, as though the planet has been knocked on its side. This unusual orientation may be the result of a collision with a planet-size body early in Uranus's history, which apparently radically changed the planet's rotation.The temperature differences on the summer and winter sides do not differ greatly because the planet is so far from the sun. Near the cloud tops, the temperature of Uranus is -216 degrees Celsius.

NeptuneNeptune was the first planet located through mathematical predictions rather than through regular observations of the sky. When Uranus didn't travel exactly as astronomers expected it to, a French mathematician, Urbain Joseph Le Verrier, proposed the position and mass of another as yet unknown planet that could cause the observed changes to Uranus's orbit. After being ignored by French astronomers, Le Verrier sent his predictions to Johann Gottfried Galle at the Berlin Observatory, who found Neptune on his first night of searching in 1846.Neptune's atmosphere extends to great depths, gradually merging into water and other "melted ices" over a heavier, approximately Earth-size solid core. Neptune's blue color is the result of methane in the atmosphere. Uranus's blue-green color is also the result of atmospheric methane, but Neptune is a more vivid, brighter blue, so there must be an unknown component that causes the more intense color that we see. The planet has six rings of varying thicknesses, confirmed by Voyager 2's observations in 1989. Neptune's rings are believed to be relatively young and relatively short-lived.

PlutoThe world was introduced to dwarf planets in 2006, when petite Pluto was stripped of its planet status and reclassified as a dwarf planet. What differentiates a dwarf planet from a planet? For the most part, they are identical, but there's one key difference: A dwarf planet hasn't "cleared the neighborhood" around its orbit, which means it has not become gravitationally dominant and it shares its orbital space with other bodies of a similar size. Because it has not cleared the neighborhood around its orbit, Pluto is considered a dwarf planet. It orbits in a disc-like zone beyond the orbit of Neptune called the Kuiper belt, a distant region populated with frozen bodies left over from the solar system's formation. The dwarf planet is 5.9 billion kilometers from the sun, and its average temperature hovers around -215 degrees Celsius.Pluto's surface is composed of a mixture of frozen nitrogen, methane, and carbon monoxide ices. The dwarf planet also has polar caps and regions of frozen methane and nitrogen.

All the planets except Mercury and Venus have moons orbiting them. The Earth’s moon has a mass of 7.35x1022 kg, a radius of 1.74x106 m, and an orbti radius around the Earth of 3.84x108 m, and an orbital period of 27.3 days.

AsteroidsAsteroids are essentially chunks of rock that measure in size from a few meters to several kilometers in diameter. (Small asteroids are called meteoroids.) The largest asteroid, Ceres, is about 950 kilometers wide. Like most asteroids, it lies in the asteroid belt between Mars and Jupiter. Scientists have found more than 90,000 asteroids in the belt. Many astronomers believe the belt is primordial material that never glommed into a planet because of Jupiter's gravitational pull. Other astronomers say the belt is a planet that was broken apart during a collision.

CometsComets are balls of rock and ice that grow tails as they approach the sun in the course of their highly elliptical orbits. As comets heat up, gas and dust are expelled and trail behind them. The sun illuminates this trail, causing it to glow. The glowing trails are visible in the night sky.While there are perhaps trillions of comets ringing the outer fringes of the solar system, bright comets appear in Earth's visible night sky about once per decade. Short-period comets such as Halley's were perturbed from the so-called Kuiper belt out beyond the orbit of Neptune and pass through the inner solar system once or twice in a human lifetime. Long-period comets come from the Oort Cloud, which rings the outer reaches of the solar system, and pass near the sun once every hundreds or thousands of years.

The motion of the starsThe observation of the motion of the stars is greatly complicated by the fact that the Earth is itself moving. In the course of a night, stars and constellations appear to move across the sky from east to west. The ancient astronomers noticed, however, that the relative positions of the stars and constellations remained unchanged. That gave rise to the notion of the celestial sphere, a huge sphere surrounding and rotating around the Earth on whose surface the stars and constellations were firmly embedded. We know now that the rotation of the stars and constellations is a consequence of the rotation of the earth about its axis. The star Polaris (North Star) is right in the celestial north pole and so appers not to move at all. The rest of the stars and constellations appear to rotate about it.

As the Earth rotates around the Sun, the night sky appears to be changing. At different times of the year the night hemisphere points along different directions in the sky and hence the view of stars and constellations is different as well. Since the Earth completes one revolution in one year, it follows that the change in the direction from one night to the next is 360°/365 = 0.986°, or about one degree. This is too small to be detected by the unaided eye but in the course of a few weeks the changes can easily be detected.

The motion of the stars

Stellar clusterSome of the stars in the universe are part of multiple star systems known as stellar clusters. Most appear to be part of a binary system where two stars orbit a common center of gravity. A few are even part of a triple star system. But some stars are also part of a larger group. They can be found together in associations known as stellar clusters. Stellar clusters are groupings of stars held together by a common gravitational bond. They vary greatly in size and shape as well as the number of stars. They also vary in age from just thousands of years to billions of years old. Gravity is the force that binds these cosmic swarms together. Astronomers have divided stellar clusters into two main types according to their shape and number of stars. They can all be classified as either open clusters or globular clusters.

Stellar clusterOpen clusters usually contain somewhere between a dozen and a thousand stars. They are held together by mutual gravitational attraction and have a common center of mass. Open star clusters are composed of hot, relatively young stars. It is estimated that there are about 20,000 open stellar clusters in our galaxy. The reason open clusters are so young is because they don't last very long. Gravitational interactions between the stars and other objects will cause these clusters to eventually disperse over time. Open clusters are formed when several stars are formed at the same time from the same cloud of dust and gas. Our own Sun is part of an open cluster than includes other nearby stars such as Alpha Centauri and Barnard's star. All of these stars are believed to have formed from the same primordial nebula around 5 billion years ago.

Globular clusters are much older and usually contain between ten thousand to a million stars, which are gravitationally bound in a tight concentration. The stars are usually packed into a spherical arrangement with the highest density of stars occurs in the center of the cluster. Globular clusters are found just outside our galaxy. They orbit around the galaxy's central halo or bulge like a swarm of bees. Their concentration increases closet to the galaxy's center. They orbit the galaxy in highly elliptical orbits, which take them far outside the Milky Way. There are about 200 known globular clusters surrounding our galaxy. They have also been observed around many other galaxies as well. Most globular clusters are believed to be between 14 and 16 million years old. They are believed to have formed from the same primordial matter that initially formed the galaxies.

ConstellationA constellation is a collection of stars that form a recognisable group as viewed from Earth. The ancient Greeks named many of the constellations as we know them now. Constellations are useful “landmarks” for finding one’s way around the night sky.

Gallaxies

Galaxies are large systems of stars and interstellar matter, typically containing several million to some trillion stars, of masses between several million and several trillion times that of our Sun, of an extension of a few thousands to several 100,000s light years, typically separated by millions of light years distance. They come in a variety of shapes: Spiral, elliptical and irregular. We live in a giant spiral galaxy, the Milky Way Galaxy of 100,000 light years diameter and a mass of roughly a trillion solar masses; our Sun is one of several 100 billions of stars of the Milky Way. The nearest dwarf galaxies, satellites of the Milky Way, are only a few 100,000 light years distant (and closer in case of some dwarfs which are currently merged with the Milky Way), while the nearest giant neighbor, the Andromeda Galaxy (M31), also a spiral, is about 2-3 million light years distant.

Light yearWe define the light year as the distance traveled by the light in one year. Thus1 ly = 3 x 108 x 365 x 24 x 60 x 60 m = 9.46 x 1015 m.

The average distance between stars in a galaxy is about 3.3 ly.

The distance to the nearest star (Proxima Centauri) is abourt 4.3 ly.

The average distance between galaxies varies from 105 ly to 106 ly.

The diameter of the Universe is at leat 93 billion ly.