1. STARS

2. For most of recorded history the Earth was thought to be the center of the universe and never moved. The constellations were named and stories were told about them as they appeared through the seasons. Without a telescope or other aid the stars seem to be just points of light that move across the sky at night. Some wandered about which we now know are planets. Let's look at some facts about stars.

3. Stars Versus Planets Stars and Planets  → A star is matter that emits huge amounts of energy.  → A planet is matter, generally spherical, that revolves around a star. Feature Planet Star Location In the solar system Far beyond the solar system Distance from Earth Fairly near Very far Real size Smaller than most stars Usually larger than planets Reason we see object Reflects light from Sun Emits its own light Surface temperature Usually cool or very cold Very hot What object is made of Usually rocks or gases Gases under high pressure and temperature Observable feature Has a steady light Appears to twinkle

4. Gas pillars in the Eagle Nebula where dust and gases swirl around to eventually form stars. The Hubble telescope photographed this picture.  A star is a spherical ball of gas. It is made up of mostly hydrogen (75% by mass) and Helium (23% by mass), with a trace amount of elements heavier than helium (total of 2% by mass).  The structure of a star is such that the pressure (and density and temperature) increases inward because of the weight of the overlying layers. Star Facts

5.  Our closest star, besides the Sun, is Proxima Centauri which is 4.2 light years away from the Sun.  Our Sun and solar system belong to the Milky Way galaxy The Milky Way is thought to be composed of about 200 billion stars - the Sun is one of those stars.  Astronomers have observed that some stars occur in clusters. An open cluster is a collection of 50 to 1 000 stars that are dispersed across the Milky Way and other galaxies. Globular clusters are collections of 10 000 to 1 000 000 stars. There are about 200 globular clusters in our galaxy. More Star Facts... The Pleiades, an open cluster of stars in the constellation of Taurus. Courtesy of NAS

6. Star Characteristics Many astronomers study stars to gain more knowledge about the possibility of life existing on other planets that are orbiting stars in the universe. Some of the main characteristics of stars that are studied include:  colour  size  mass  temperature  brightness or luminosity

7. Colour Temperature Range (degrees Celsius) blue25 000 - 50 000 bluish-white11 000 - 25 000 white7 500 - 11 000 yellowish-white6 000 - 7 500 yellow5 000 - 6 000 orange3 500 - 5 000 red2 000 - 3 500 Stellar Classes Stellar classes are based on properties of stars, including the temperature on their surfaces. Stars vary in colour depending on their approximate surface temperature.

8. Dwarf stars can be 1000 times smaller than the sun. Stars can be packed so close as to be seen as Star Clouds, grouped like a ball as in Globular Clusters or rather loosely assembled in Open Clusters. Patterns are called Constellations and parts of these like the Big Dipper in Ursa Major are called Asterisms. Stars come in different sizes. Supergiant stars range from 100 to 1000 times the diameter of the sun and giants range from 10 to 100 times the diameter of the sun. Groups and Types of Stars

9. More Types... Double Stars are two stars that look like one to the naked eye but separate in a telescope view. They may not be orbiting each other as in a true Binary Star but appear to line up that way. Variable Stars are stars that vary in brightness by various reasons. Novas and Supernovas are exploding stars that are blowing off their outer shells.

10. Larger Groups... GALAXIES We need to set the stars in context. All those you see at night are part of our local collection of stars, all part of our Galaxy. A galaxy is like an island in space made up of gas, dust and millions of stars. Our own galaxy, the Milky Way, includes about a trillion (a thousand billion) stars in a disk shape with our sun being at the edge. The spiral galaxy seen at right is like our own galaxy. We are about two-thirds of the way out on one of the arms of our galaxy. We are in a group of galaxies called the Local Cluster.

11. The picture at left shows the Andromeda Galaxy and two companion galaxies. It is over 2.4 million light- years away from us. Trillions of other galaxies flock the Universe, ours being one of the larger ones. Galaxies are classified using Edwin Hubble’s scheme describing spiral, barred spiral, elliptical, peculiar and irregular shapes.

12. Just like the Solar System......  Observations suggest that stars are born out of gas clouds (the solar nebula theory).  The contraction of a cloud causes it to spin faster. The centre increases in mass and becomes denser and hotter.  Eventually, it gets hot and dense enough to combine 2 hydrogen nuclei (which are protons) into a helium nucleus. This process is called nuclear fusion, and it releases a large amount of energy (like a hydrogen-bomb!).  This energy makes its way to the surface and a star is born! How Are Stars Born?

13. Since a star is mostly made of hydrogen, there is a lot of “fuel” to produce energy for a long time (for a star like the sun, this lasts about 10 billion years) however.....

14. All stars evolve. A star like the Sun will eventually use up the hydrogen fuel in the core and stops producing energy. In its final stages, the Sun will expand so much that its atmosphere will envelope the Earth. This expanded atmosphere will eventually escape the Sun into the space between the stars. What will be left of the Sun is the dense and hot core (the innermost part of the original Sun), called a White Dwarf. Rare, massive stars end their lives as SUPER NOVAS and become neutron stars or black holes!!! How Do Stars Die?