Astronomy Facts

Astronomy [ə-'strŏ-nə-'mē]

The study of everything in space beyond the Earth's atmosphere.

The World of Space

Astronomy is the study of everything in the universe beyond Earth's atmosphere. This includes things we can see with our own eyes, such as the sun, the moon, the planets, and stars. It also includes things that we can only see with special telescopes, such as exoplanets and galaxies. Astronomy even includes mysterious things humans can’t see at all, even with our most powerful instruments.

starry sky

Looking up at the sky is fascinating for everyone, whether you are a backyard stargazer or a space scientist. We are learning more and more every day, as new discoveries reveal the depth and wonder of the universe. Let’s stretch our imaginations as we explore the world of space.

The Oldest Science in the World

Galileo's telescopes
Galileo's telescopes

As long as there have been people on the earth, they have looked up at the sky. That’s why astronomy is sometimes called “the oldest science in the world.” Long ago people thought that the sun, and all the other planets, revolved around Earth. They saw patterns in the stars (constellations) and “wanderers” among the stars (planets) and recorded their observations, but they were limited by what their eyes alone could see.

In 1608, a scientist named Galileo used a special tool called a telescope to observe the stars and planets more closely. He proved that the Earth and the other planets revolved around the sun. Visit this astronomy timeline to see where Galileo’s discovery fits in with other important events in astronomy, and learn more about how telescopes work.

Today, small telescopes are used by amateur astronomers to search the skies, while huge, powerful telescopes are used by professional astronomers in observatories. Space-based telescopes such as the Hubble Space Telescope orbit the Earth and are able to capture images of very distant, dim objects. The science of astronomy will continue to expand our knowledge of the universe as long as people continue to look to the skies.

Hubble Telescope in orbit above the Earth
Hubble Telescope image courtesy of NASA

The Universe

How big is it?

When you look up at the night sky you are looking at the universe. Everything is inside the universe, even us, and it is impossible to imagine how big the universe is.

Trifid nebula
Trifid nebula image courtesy of NASA

Special astronomers called cosmologists are trying to measure it. They analyze the light from a star with an instrument called a spectroscope. By studying the light spectrum, they can tell whether an object is moving away from Earth or toward Earth. Based on the information from this instrument, scientists have learned that the universe is still growing outward in all directions.

The universe is huge, too huge to measure fully. Learn more about why the universe is so difficult to measure.

How old is it?

Scientists believe that about 13.8 billion years ago, a powerful explosion called the Big Bang set the universe into motion which continues today. In the billions of years since the Big Bang, gases from the explosion expanded and cooled, and galaxies, stars, and planets formed. The matter that spread out from the Big Bang developed into everything in the Universe. Scientists know that the universe is continuing to expand, but they are not yet sure if the movement will stop, change direction, or keep going forever.

diagram of the Big Bang
diagram of the Big Bang image courtesy of NASA

Everything in the universe is in constant motion. The Earth itself spins on its own axis and revolves around the Sun. The Sun moves around the center of the galaxy, and the galaxies are moving too. Gravity, the force of attraction between objects, holds it all together. Learn more at Science Trek’s Gravity website.

How do you measure the universe?

Since distances are so huge in the universe astronomers need a way to measure distance on a big scale. They use a unit of length called a light year, which is the distance light can travel in one year. Light travels very fast, so in a year it can go 6,000,000,000,000 (6 trillion) miles, or 9,500,000,000,000 kilometers. For example, the closest star to our Sun, Proxima Centauri, is 4.3 light years away. That’s a lot easier than writing 25,000,000,000,000 miles!

astronomical measurements
astronomical measurements

BIG Numbers

As we've already seen, galactic sizes and distances are huge. Really big numbers have names we're not used to hearing, and they have so many places that scientists have adopted a shorthand way to express them. For example, one trillion can be written as 1012 (10 to the 12th power), where the raised 12 represents the number of zeros after the 1. Here are some names of numbers in ascending order (small to large):

  • hundred (100) (102)
  • thousand (1,000) (103)
  • million (1,000,000) (106)
  • billion (1,000,000,000) (109)
  • trillon (1,000,000,000,000) (1012)
  • quadrillion (1015)
  • quintillion (1018)
  • sextillion (1021)
  • septillion (1024)
  • octillion (1027)
  • nonillion (1030)
  • decillion (1033)
  • undecillion (1036)
  • duodecillion (1039)
  • tredecillion (1042)
  • quatuordecillion (1045)
  • quindecillion (1048)
Image of galaxies
Hubble image of galaxies courtesy of NASA

Currently, the farthest detectable galaxy from the earth is 13.4 billion light-years away or 134 nonillion (134 x 1030) kilometers – that’s 134 followed by 30 zeros!

Solar System

The solar system is the “neighborhood” in which our planet, Earth, resides. The Roman word for the sun was “sol,” so “solar system” refers to our sun’s system, including all the natural objects that orbit around it. Our solar system is about 4.6 billion years old. It includes one central star (the sun), eight planets, five dwarf planets, more than 200 moons, thousands of comets, over a million asteroids, and other space objects that travel in paths around the sun. Learn more at NASA’s Solar System site.

diagram of the solar system
diagram of the solar system

The sun is the biggest object in our solar system. About one million Earths could fit inside the sun, yet it is only a medium-sized star. The sun is the reason why there is life on earth. It provides the light and heat energy that living things need to survive. Like all stars, our sun has a life cycle and will use up its energy and die in about 5 billion years.

There are so many interesting things to learn about the solar system that you'll want to explore these other Science Trek topics:


When you look at stars at night, they appear as tiny points of light. But stars are huge, burning balls of gas, like our sun. Scientists know that you can see about 3,000 stars with your naked eye. That may sound like a lot, but the stars we can see are just a small part of the stars in our galaxy and beyond. With powerful telescopes, scientists can see billions and billions of other stars. The prefix “astro” means “star” in the Greek language, so that is the source of our words “astronomy,” “astronaut” and “asteroid.”

Hubble image of distant stars courtesy of NASA

Stars vary in their size, color, and brightness. A star can be red, orange, yellow, white, or blue. Its color depends on its surface temperature, which is determined by its age and mass. Stars go through many stages in their lifetimes. First, they are born in clouds of dust and gas called nebulae. Then, stars can shine for up to 10 billion years! When a star is nearing the end of its life, it may become a red giant. Over time, a star the size of our sun may eject its outer layers into space and leave behind a white dwarf. A larger star may end its life in a huge explosion called a supernova, leaving behind a neutron star or a black hole. Black holes have gravity so dense that nothing, not even light, can escape.

Crab nebula
Crab nebula image courtesy of NASA
orange sun

Follow the links below for more informative facts about stars: 


Galaxies are collections of billions of stars, gas, and dust held together by gravity. All stars belong to a galaxy. Our sun and all the stars you see when you look at the night sky belong to the Milky Way galaxy. Astronomers believe there may be 100 billion stars in the Milky Way galaxy, many of which may have their own solar systems of orbiting planets we call exoplanets.

spiral galaxy
Hubble image of a spiral galaxy courtesy of NASA

In the 1920’s, an astronomer named Edwin Hubble observed distant galaxies through his telescope and realized that the Milky Way galaxy isn’t the only galaxy in the sky. Today, scientists estimate there may be 50 billion galaxies in the universe. Our Sun is just one star out of billions of stars in our galaxy, and the Milky Way galaxy is just one out of billions of galaxies. Galaxies are classified by their shape: spiral, elliptical or irregular. The Milky Way is a spiral type.

Galaxy with quasar
Galaxy with quasar courtesy of NASA/ESA/Hubble

Scientists believe that quasars, located in faraway galaxies, are the most distant known objects in the universe. They can be a trillion times brighter than the Sun! Because they are so bright, they drown out the light from all the other stars in the same galaxy. It is believed that quasars derive their energy from massive black holes in the center of galaxies. Because light from faraway quasars takes so long to reach the earth’s atmosphere, the images that scientists receive show what the quasar was like billions of years ago. Astronomers who study them can learn about earlier stages of the universe.

Learn more at the Science Trek sites Galaxies and Exoplanets.

Dark Matter and Dark Energy

Planets, stars, and galaxies are all things we can detect with our eyes or with specialized tools. But we cannot see most of the matter in the universe, no matter what instruments we use. Dark matter is an invisible material that doesn’t give off light like a galaxy or absorb light like a black hole, but scientists know it exists because they can observe its gravitational pull. Dark energy is a mysterious pressure working in opposition to gravity, pushing matter apart. Together, dark matter and dark energy make up 95% of the universe.

black hole
black hole image courtesy of NASA

Scientists don’t know very much about these mysterious things, but they are studying all the time to find out more. When it comes to the universe, there is always more to discover and more to learn.

Stargazing: Pictures in the Sky

A constellation is a group of stars that looks like an object, animal or person in the night sky. If you were to draw lines between the stars like a dot-to-dot puzzle, your imagination might let you see a picture. Ancient people imagined these starry figures and created stories about how these images, or constellations, came to be. Many of the constellations in the northern sky were created and named by the Greeks during the Greek Empire (about 50 BC to 480 AD). Over time, other cultures developed their own stories. Today, there are 88 officially recognized constellations.


Which constellations you can see at night depends on the time of year. Our view of the night sky changes as Earth orbits the sun. Your location on Earth also determines which constellations you see, and how high they appear to rise in the sky. The view from the Northern Hemisphere is different from what can be seen from the Southern Hemisphere.

The Big Dipper
The Big Dipper

You may already know familiar constellations such as The Big Dipper or Orion the Hunter. It can be fun to learn more about constellations that you can see in the night sky. All you really need to be a stargazer is a dark night and your own eyes. Binoculars and a star map can help too.

Stars such as those in well-known constellations can also be used to navigate. From ancient times, sailors out at sea have used stars to determine their location. Today, astronauts also train to use the stars as a backup to modern navigation systems.

Observatories and Telescopes

ALMA array
ALMA array image courtesy B. Tafreshi

Observatories are the homes of large, ground-based telescopes that gather light from distant objects and make the objects appear larger. They let scientists view the sky in more detail than the naked eye allows. There are observatories all over the world. Most optical observatories are built on mountain tops, far from city lights which interfere with viewing, where the air is calm and dry and the weather is clear. For example, the Mauna Loa Observatory in Hawaii is located high above 40% of Earth’s atmosphere, so atmospheric distortion is decreased and its telescopes’ images of stars and planets are much sharper. The percentage of clear nights at Mauna Loa is among the highest in the world, so it is an excellent location for an observatory.

Optical telescopes are one kind of telescope used to look into space at objects that reflect or give off light. But there is a whole spectrum of electromagnetic radiation beyond visible light. Objects in space emit or reflect other kinds of radiation, including ultraviolet, infrared, X-rays, and radio waves. Some ground-based telescopes, such as radio telescopes, can detect different wavelengths, but many kinds of rays are blocked by Earth’s atmosphere. When satellite-based telescopes were first launched into space, they gave access for the first time to the entire electromagnetic spectrum, including rays that had been blocked by the Earth’s atmosphere. The Hubble Space Telescope is equipped with infrared telescopes that can see objects that are smaller or further away, and its ultraviolet telescopes are able to see some of the hottest and brightest stars. Learn more about different kinds of telescopes and the electromagnetic spectrum.

Observatories Across the EM Spectrum
Observatories Across the EM Spectrum image courtesy of NASA

Ground-based telescopes are less expensive to build and easier to maintain, but space-based telescopes like Hubble have opened up much more of the universe to human eyes. Take a look at some of the amazing discoveries made possible by the Hubble Space Telescope.


Scientists who study space beyond Earth’s atmosphere are called astronomers. They may be observational astronomers who study and analyze information received from telescopes, or they may be theoretical astronomers who use mathematical models to describe and predict events and processes in space. In astronomy, there will be always be questions to answer and mysteries to solve.

telescope in the sunset

Citizen astronomers also play an important part; many discoveries have been made by amateurs who share their findings with scientists. Citizen astronomers also help to classify galaxies and search for exoplanets. People who love astronomy often hold community “star parties” where they set up telescopes for interested people to view stars, planets, or galaxies. Learning more about space helps all of us to better understand our place in the universe.

The universe is huge, and there is so much yet to discover. What will we find next?