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Astronomers define the observable universe as objects that can be observed and detected. Planets, moons and asteroids are all objects in our solar system that we can observe. The first tool used to observe space was a telescope used by Galileo Galilei in the early 1600s. Spectroscopes tell us what elements are present in space objects. They also tell temperature and speed of these objects.
In 2006, astronomers defined the characteristics of a planet:
- A planet is a large object that orbits a star.
- A planet must have enough mass in gravity to have a round shape.
- A planet's gravity must be strong enough to pull all the smaller objects around it into it.
The sun is the star the planets in our solar system orbit. The process of nuclear fusion within stars causes them to give off a tremendous amount of radiant (strong light) energy and thermal (heat) energy.
Stars are not living, yet they are considered to pass through a life cycle. Stars are "born" in a nebula, a cloud of dust and gas. Gravity pulls the dust and gas together. Stars "die" when they run out of their hydrogen supply, the main source for nuclear fusion. A large star eventually explodes in an event called a supernova. A smaller star sheds its outer material to become a white dwarf. Groups of stars make up galaxies. Galaxies are formed by spinning clouds of gas, and individual stars are born within galaxies. Stars have a variety of colors, temperatures and sizes. Their luminosity, or actual brightness, may also be different. Scientists use the Hertzsprung-Russell diagram (HR diagram) to help classify and compare stars. The vertical (y-axis) axis shows luminosity (brightness) of a star and the horizontal (x-axis) axis shows the temperature of the star. The hottest stars are on the left and coldest stars are on the right. The sun is much smaller and less luminous than the giants and supergiants. The sun is cooler but more luminous than the dimmest.
Asteroids are huge masses of rock. They orbit the sun in the asteroid belt between Jupiter and Mars. Smaller pieces of asteroids are called meteorites. Meteoroids are space rocks made mostly of iron. Meteorites that enter the atmosphere are called meteors and burn due to friction, causing them to light up. Meteors are often called shooting stars. Meteorites make it through the atmosphere and land on Earth. Comets are made of dust and ice. Comets are named after the person who discovered them. Comets orbit the Sun. As they approach the Sun, the heat from the Sun causes a part of the comet to melt. This causes a tail which streams behind them away from the Sun.
The subjects science, technology, engineering, and mathematics use the acronym STEM. Career examples include astronauts, atmospheric scientists, computer technicians, and aerospace engineers. NASA is the agency of the United States government that studies space. The internet is a great resource to learn more about these and other careers.
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