Day and night would not be the same without stars! Our sun, which is so essential to life on Earth, is a star almost 900,000 miles across. That’s only an average-sized star! Nuclear fusion in the sun’s core produces light energy: hydrogen atoms, which have light atomic nuclei, join together to form a heavier helium nucleus under the sun’s extremely hot temperatures, releasing energy in the process. Although the sun is almost 93 million miles from Earth, its light travels at about 186,000 miles per second; so it only takes eight minutes through space for the sun’s light to reach Earth.

The sun does not just provide light, though. Our whole solar system revolves around it. The earth, other planets, and satellites—like the moon—are all part of this solar system. Earth turns daily on its axis (think of it as an imaginary line between the earth’s north and south poles), one half of the planet facing the sun at a time, and it orbits yearly around the sun. Each of the other planets also orbit the sun, at varying speeds. Although the solar system itself seems pretty vast in size, it’s only a tiny part of our Milky Way galaxy. A galaxy is a collection of stars, gas, and dust, perhaps 1,500 to 300,000 light-years across (a light-year is the distance light can travel in a year, almost 6 trillion miles). And there are countless galaxies in the universe.

Like the other stars, the sun is formed by gases: it is about 92% hydrogen and 8% helium, with some other elements. Scientists are able to identify the gases that different stars are made up of by doing a spectral analysis. They use a spectroscope to break up the light emitting from the star into the spectral colors, each a unique pattern of colors that occurs only from a certain element or combination of elements. (You can use a qualitative spectroscope to observe the spectral colors of light in your house.)

Not all stars have the same brightness, as you can see if you go out at night and look up at the sky. Some appear much brighter than others, don’t they? Scientists measure brightness in apparent magnitude and absolute magnitude. Apparent magnitude is how bright a star appears from Earth. Sometimes one star looks brighter than others because it is close to Earth. Absolute magnitude, on the other hand, measures the inherent brightness of a star—how bright it appears from a set distance. Thus, one star’s inherent brightness can be compared to that of another. About 2,000 years ago, the Greek Hipparchus classified stars by brightness, with the lowest numbers indicating the greatest magnitude in brightness. Now the brightest stars are assigned apparent magnitudes with negative values: the sun’s magnitude is -26.7, the moon’s is -12.7 when full, Sirius (the brightest star) has -1.4 and Betelguese has +0.4. With unaided eyes you should be able to see stars up to 5-6 magnitude, and with binoculars you should see up to eight or nine. A good telescope will allow you to view much dimmer stars, with magnitudes of around 13.

If you look at a star chart, you’ll see that stars are often shown in patterns, or constellations. Many of the constellations that are visible in the Northern Hemisphere of the earth cannot be seen in the Southern Hemisphere, and vice versa. Which constellations are visible also varies with season. This is because the earth is daily spinning on its axis inside the ‘celestial sphere,’ the stars and night sky objects surrounding it. The Southern Hemisphere of Earth sees mostly stars in the southern part of the celestial sphere. Then, since Earth is orbiting the sun yearly, the view changes with the seasons. The stars visible in the night sky seem to travel east to west as the earth itself travels around the sun.

The ‘Summer Triangle’ can be seen in the night sky through September. It’s actually formed by three stars from different constellations: Vega (from the constellation Lyra), Altair (from Aquila), and Deneb (Cygnus). Follow this link for a ‘map’ of the Summer Triangle. When using a star chart outside at night, cover your flashlight with red cellophane before shining it on the chart. Red-shaded light is less distracting to your eyes.

Science Lesson: The Planets

Can you name the eight main planets orbiting our sun? To keep them all straight, your children might find it helpful to make a planet chart. Write down each one’s size, distance from the sun, and other interesting facts about each one. Be sure to include pictures! (You can find this kind of information in an encyclopedia, the internet, or a good science dictionary. You can find pictures at the Exploring the Planets site.)

Mercury is only visible at twilight (in the west) and sunrise (in the east) because its orbit is so close to the sun. Just how close is that, though? From Mercury, the sun is ‘only’ 36 million miles away. Mercury’s temperatures change radically during the day: 800 ° to -200 °F between day and night. Although small, Mercury is the densest planet after Earth.

Venus, the second-closest planet to the sun, has the highest planetary temperature: 900 °F/475 °C. It also has the longest rotational period of any of the planets, the equivalent of 243 Earth days. You would need more than an umbrella if you got caught in a storm on Venus: it rains sulfuric acid there! Venus is the third brightest object in the sky, followed by Jupiter.

Mars has a day length just slightly longer than Earth’s, but its average surface temperature is a chilly -80 °F. Do this experiment to demonstrate one of theories about why Mars has such a red surface. Put a layer of sand in the bottom of a ceramic baking dish. Cut some steel wool into 2 cm (1 inch) pieces and mix them with the sand, then cover the mixture with water. (You could also sprinkle iron filings on top of the sand instead of using steel wool.) You might need to add more water each day as some evaporates. Every day, check on the experiment and record how the surface is changing. How long did it take before the sand turned red like the surface of Mars? Scientists believe Mars’ color occurs because of iron oxide (rust) in the soil.

Jupiter, named after the king of the Roman gods, has a diameter more than 11 times the size of Earth’s. One day on Jupiter is only 10 hours, though, since the planet rotates very quickly. One of Jupiter’s moons, Io, has the most violent volcanic eruptions known in our solar system. You can actually see Io and three of Jupiter’s other moons (Europa, Ganymede, and Callisto) with just binoculars. Galileo was the first to see these moons, using a telescope in the 1600s.

Saturn is one of the ‘gas planets,’ along with Jupiter, Uranus, and Neptune. Like Jupiter, Saturn probably has a rocky core and outer envelope composed of liquid metallic hydrogen. However, Saturn is notable as the planet with the lowest density: it’s less dense than water. Although all of the gas planets have rings around them, Saturn’s are the brightest and most famous. In the mid-1800s, Scottish physicist James Clerk Maxwell correctly hypothesized that Saturn’s rings must be made up of solid particles (such as ice) or else the rings could not maintain their stability.

Uranus and Neptune are mostly rock and ice, with around 15% hydrogen. Uranus’s satellites (moons or huge rocks orbiting it) are named after characters from Shakespeare’s plays and from an Alexander Pope poem. Neptune has a really, really long sidereal period, or year: 165 Earth years would equal just one year on Neptune! Since Neptune wasn’t discovered until 1846, the planet has yet to make a full orbit around the sun while we’ve been aware of it.

Pluto was classified as one of the ‘nine planets’ for most of the 21st century, until a ruling by the International Astronomical Union in 2006 declared it a dwarf planet instead. Debate continues even among scientists whether this is an accurate definition, since it orbits a star (our sun) but is not a star or a moon itself, just like the other planets. However, Pluto has a somewhat wacky orbit and sometimes nips in closer to the sun and leaves Neptune behind as the farthest planet. Pluto has colder temperatures than the eight bigger planets: -400 °F/-250 °C. In fact, it’s so cold there that it ‘snows’ methane gas crystals.

Noteworthy Scientists: William & Caroline Herschel (1738-1822, 1750-1848)

William Herschel was born in Hanover, Germany in 1738, followed twelve years later by his sister, Caroline. Both were destined to become famous astronomers, although they originally pursued careers in music. Their father was a musician for the military. He saw to it that not only his four sons were educated, but Caroline as well, in spite of her mother’s wishes that she and her sister learn exclusively ‘homemaking’ skills. At the age of 10, Caroline came down with typhoid, which stunted her growth. She never grew taller than 4’3′ and was frail throughout her life, but she lived to be 98 years old!

William moved to England to compose and teach music, and in 1872 Caroline joined him. She trained to be a singer, even getting a solo role in Handel’s Messiah. However, she and her brother both gave up their musical careers after William discovered the planet Uranus in 1781. King George III gave William a royal appointment and paid him a pension; he also gave a pension to Caroline for her work as William’s assistant. During this time, William would study the night sky through his telescope and Caroline would record the observations as well as do the mathematical calculations that were involved.

Caroline is famous for her observations of comets, five of which had never been discovered before. She was also the first woman scientist to be elected to the Royal Society, though her position was honorary rather than active. In addition to discovering a planet, William made important observations about the sun’s motion in space and the approximate size and shape of the Milky Way galaxy. He used a process of ‘star gauging,’ taking sample counts of the stars visible in his telescope’s field of view. Although he underestimated the size of the galaxy, he was correct in supposing that the Milky Way is disc-shaped. Both he and Caroline also did extensive work cataloging stars (making lists of known stars).

The Herschel astronomy tradition was passed on to William’s son, John, who discovered more than 500 nebulae.