Space 1.4

STAR GROUPS

Constellations are the groupings of stars we see as patterns in the night sky. Officially, there are 88 constellations recognized by the International Astronomical Union. As well, there are many unofficially recognized star groupings. These are asterisms. One of the most famous asterisms visible from the northern hemisphere is the Big Dipper, which is part of the constellation Ursa Major. The ancient Greeks saw the stars that make up Ursa Major as a bear. The early Black Foot nation of North America also saw a bear. Ancient Europeans saw a variety of different patterns including a chariot, a wagon, and a plough.

GALAXIES

A galaxy is a grouping of millions or billions of stars, gas, and dust. It is held together by gravity.

The galaxy we live in is a spiral galaxy called the Milky Way. It is shaped like a flattened pinwheel, with arms spiralling out from the centre.

Viewed from the side, a spiral galaxy looks a little like a compact disc with a marble in the middle sticking out evenly on either side. Our galaxy is believed to contain from 100 billion to 200 billion stars. There are two other main types of galaxies: elliptical and irregular. Astronomers have estimated there may be a billion billion galaxies in the universe.

The "protoplanet hypothesis" is a model for explaining the birth of solar systems. The process can be described in three steps:

1. A cloud of gas and dust in space begins swirling.

2. Most of the material (more than 90%) accumulates in the centre, forming the Sun.

3. The remaining material accumulates in smaller clumps circling the centre. These form the planets.

THE SUN

At the centre of our solar neighbourhood sits the Sun. For thousands of years, we learned all we knew about the Sun from looking at it, and that wasn't easy to do. After telescopes were invented it wasn't long before filters were designed to allow observers to gaze directly at the Sun.

Satellites have offered an even closer look. The Sun is almost 110 times wider than Earth. If the Sun were a hollow ball, almost a million Earths would be required to fill it.

The temperature at the surface of the Sun, which is constantly bubbling and boiling, is about 5500°C, while the core is close to 15 000 000°C. The Sun releases charged particles that flow out in every direction. This solar wind passes Earth at an average speed of 400 km/s.

Earth is protected from the solar wind by its magnetic field.

The planets that make up our solar system are as different as the people that make up a family. Every planet has its own unique features and characteristics. The solar system can be divided into two distinct planetary groups: the inner planets, also called terrestrial, or Earth-like, planets; and the outer, or Jovian (in reference to Jupiter), planets. The terrestrial planets tend to be smaller, rockier in composition, and closer to the Sun than the Jovian planets. The Jovian planets are large and gaseous and are located great distances from the Sun.

Technology has enabled us to learn a lot about our nearest neighbours in space. All the planets except Pluto have been visited by orbiting space probes. Mars and Venus have had robots land on their surface.

Asteroids

Between the orbits of Mars and Jupiter lies a narrow belt of small, rocky or metallic bodies travelling in space. These are called asteroids. They can range in size from a few metres to several hundred kilometres across. The largest asteroid, called Ceres, is over 1000 km wide. Scientists aren't certain where the asteroids came from.

Comets

Comets, often described as "dirty snowballs," are objects made up of dust and ice that travel through space. Their long tails and bright glow only appear when they get close to the Sun. When that happens, the Sun heats the materials on the comet and gases are released. These gases then get pushed away from the comet by the solar wind. The tails of some comets can be millions of kilometres long.

Comets spend most of their time slowly orbiting in the outer reaches of the solar system. Only when an event, such as the close passing of another body, occurs might a comet's path be pushed toward the inner solar system. Then a comet can end up in a regular orbit around the Sun.

Comets that orbit the Sun will make a predictable appearance because their paths are large ellipses. One of these is Halley's comet, which is visible from Earth every 76 years. The last time it was seen was in 1986.

Meteoroids, Meteors, and Meteorites

Small pieces of rocks flying through space with no particular path are called meteoroids. Meteoroids can be as small as a grain of sand or as large as a car. Practically invisible to most telescopes, we are usually only aware of meteoroids as they hurtle through our atmosphere. When one gets pulled into the atmosphere by Earth's gravity, the heat of atmospheric friction causes it to give off light and it is known as a meteor. These are the so-called "shooting stars" that can often be seen streaking across the night sky. If a meteor lasts long enough to hit Earth's surface, it is called a meteorite. Some meteor showers are predictable, such as the Leonids.

As you have learned in previous sections, the paths of the major bodies in the solar system are ellipses. Because astronomers understand the nature and geometry of elliptical orbits, as well as of celestial motion, they now understand the paths of planets and their moons. This means that a variety of events can be accurately predicted, including both solar and lunar eclipses. Recall from earlier studies that a solar eclipse occurs when the Moon, passing between the Sun and Earth, casts a shadow on Earth. A lunar eclipse occurs when Earth passes between the Sun and the Moon, casting its shadow over the Moon.

Knowing so much about orbital paths and speeds, astronomers can predict eclipses well into the future. Some enthusiasts make it a hobby to plan trips wherever they can around the world to witness eclipses.

Understanding of orbits has also led to the discovery of many different comets. The paths of comets are elliptical, too, but larger and longer than planetary paths. Through careful observation and the use of some basic mathematics, astronomers are able to calculate the paths of some known comets and predict when they will next be close enough to Earth to be seen. Halley's comet is one example.

Tracking and discovering comets is a job shared by sky-watchers at all levels, from astronomers who work for the National Aeronautics and Space Administration (NASA) to backyard enthusiasts. NASA also has a system for tracking meteors