Think about the night sky. What can you see? Stars might be one of the first things to come to mind. There are too many stars for scientists to count them all. There are probably billions and billions of stars in the universe. Do all stars look exactly the same? It might be hard to tell the differences among stars from here on Earth, but in fact they are all very different. No two stars are the same. We classify stars by their size, color, brightness, mass, and surface temperature. A star is an object in the sky made of hot gases so hot that they glow with light! Size Scientists often describe a star s size as a ratio to the size of the Sun. For instance, when scientists compare a star s radius, the Sun s radius is called 1 solar radius, so a star with a measurement of 0.75 solar radius is equal in size to three-fourths of the Sun. Stars can appear larger due to their distance from Earth. Although the Sun appears much larger than the stars we see at night, it is not the largest star. The farther away a star is from Earth, the smaller it will appear. The distance of a star from Earth is measured in light years. The Sun is the closest star to Earth, which makes it appear to be the largest. The picture below shows the size of our Sun compared to one of the largest stars, VY Canis Majoris, if you could put them side by side. You can see that our Sun is not nearly as large as it appears when we are looking at it from here on Earth. The Sun looks so much larger because it is so much closer to us than VY Canis Majoris. VY Canis Majoris Sun radius radius: the distance from the center of a sphere to its surface; half its diameter What is a light year? It is the distance light can travel, when moving in a straight line, in one year. It is equal to 9,500,000,000,000 km (or 9.5 x 10 12 km) Sun VY Canis Majoris Distance to Earth 149,600,000 km (0.1496 billion km) ~4,900 light years (46,550,000 billion km) Solar Radius 1 solar radius >1,800 solar radii 1
Color and Surface Temperature A star s color can tell scientists about its surface temperature. The hottest stars appear blue in color, and the coolest stars are red. Stars with mid-range temperatures are white, yellow, and orange. The Sun is a yellow star in the center of our solar system. It s called a yellow star because, even though it emits light in every single color of the rainbow, the color the Sun emits the most strongly is yellow. All stars emit one color of light the strongest, and this color is determined by the temperature of the star. Coolest stars Hottest stars The Sun s temperature is about 5,800 K. The Kelvin temperature scale is related to the Celsius scale: 1 Kelvin equals 273 C. Red stars typically have temperatures less than 3,500 K, while blue stars can reach temperatures of over 30,000 K! Most stars in the universe are red, while most stars we see in the night sky are either blue or blue-white. Brightness A star s absolute brightness, also called luminosity, is the total amount of energy (or light) emitted by a star. It is related to a star s mass and surface temperature. A greater mass relates to a greater brightness more energy and light given off by the star. A hotter temperature also means greater luminosity. This is different from a star s apparent brightness, which is affected by how far a star is from Earth. A star closer to Earth will look brighter to us than another star farther away from Earth with the same mass and surface temperature. Absolute brightness is also called absolute magnitude, while apparent brightness is called apparent magnitude. Sirius has the highest apparent brightness of any star in our night sky, but is in reality a dim star. It appears so bright because it is very close to the Earth when compared to the brightest stars. Mass Similar to a star s size, a star s mass is also usually reported as a ratio with the Sun s mass, called solar mass. The Sun is the most massive object in our solar system with a mass of 1.989 x 10 30 kg (over 300,000 times the mass of Earth). The star VY Canis Majoris is estimated to have a solar mass of 25 (so it is about 25 times larger than the Sun in terms of mass). Because of differing density, two stars of the same size will not necessarily have the same mass. luminosity: the amount of energy or light given off by a star 2
We use telescopes to see objects in space. You will construct your own telescope to observe the stars in the night sky. Here are the materials you will need: 2 lenses of different strengths Paper towel roll 1 sheet of black construction paper Tape Steps: 1. Roll your sheet of construction paper into a tube similar to the paper towel roll. Use tape to keep the paper in a rolled up position. 2. Take one of your lenses, the one weaker in strength, and carefully tape it to the end of the rolled up sheet of paper. This will be your eyepiece. The eyepiece is the side you will look into. 3. Take your stronger lens and carefully tape it to one of the ends of the paper towel roll. 4. Gently put the rolled up construction paper tube inside the other end of the paper towel roll. 5. You now have your own telescope to experiment with! Have you ever been outside in the country during the night? If so, what did the sky look like? Did it look different than the sky you would see in a city? In the country, stars in the sky appear brighter and clearer, and there appears to be more of them. In a city, light pollution makes it harder to see all of the stars in the sky. The night sky can look very different depending on where you are when you re looking at it. What do you know? Read the statements below and write true or false. If the statement is false, re-write the statement to make it true. 1. The Sun is the largest Star. 2. The coolest stars are red. 3. The brighter the star, the bigger it is. 4. 5. All stars are made out of gases and are the same size and color. Telescopes are instruments used by astronomers to help see objects in space. 3
Look at the table below, which shows information about four different stars. Use the table to answer the questions that follow. Star Color Temperature in Kelvin Distance in Light-Years Actual Brightness in Suns Apparent Brightness in Suns Sirius White 9900 8.6 25.4 310 Alpha Centauri Yellow 5800 4.3 1.5 103 Betelgeuse Red 3600 1,400 90000 51 Achernar Blue 15000 69 3150 33 1. Which star is the brightest? How do you know? 2. Which star is the hottest? 4
Stargazing Stargazing is a fun family activity to do in your own backyard, and the best part is that it s free! Before you begin stargazing with your child, you might find it beneficial to search the internet for some printable materials at home: Discuss the following questions with your child: What patterns do the stars form? Do all stars look the same? How are they different? How are stars different from planets? star map a map of the night sky star clock uses the stars to tell the time star wheel used to help you navigate the night sky Any of the stargazing materials listed above will help guide you and your child when observing Earth s night sky. Take time to learn together how to use each of these materials. With some practice, you and your child will quickly become expert stargazers! Do any stars in the night sky appear larger than our Sun? Do the stars change location? What causes stars to appear to move? 5