Image: Deep sky image of the constellation Orion. Credit: Mouser/Wikimedia
What is a constellation?
Constellations are imaginary patterns that farmers, poets, navigators, or astronomers invented to easily identify stars. Stars are big balls of burning gases that emit light and heat. During the day, the light coming from our closest star, the Sun, is too strong to see the other stars. During the night, when it is very dark, we can see from 1000 to 1500 stars. It would be very hard to tell which star is which by looking just at one! Constellations, with their familiar shapes, help localise stars and allow us to find our way through the night sky. There are 88 constellations, adopted by the International Astronomical Union, across the sky between the northern and southern hemispheres so some constellations are only visible in one hemisphere. Stars belonging to a constellation are not on the same plane and can be at various distances from the Earth. Changing our position in the galaxy would change the relative positions of a group of stars, so we would see different constellations.
See how the night sky would appear from a different part of the galaxy: http://nautil.us/issue/19/illusions/a-quick-spin-around-the-big-dipper https://www.youtube.com/watch?v=lD-5ZOipE48
Origin of the Constellations
Ever since people first wandered the Earth, great significance has been given to the celestial objects seen in the sky. Throughout human history and across many different cultures, names and mythical stories have been attributed to the star patterns in the night sky to more easily remember and recognize them, giving birth to what we know as constellations.
Constellations were used by people for many practical reasons. For example, in agriculture, constellations could help determine when the seasons were coming before the birth of calendars. Constellations also helped navigators and explorers to find their way across the planet. These activities fastened the discovery of new constellations. Nowadays, astronomers still use the names of constellations to explain where a celestial object can be found in the sky.
Text adapted from the constellations page - International Astronomical Union, http://www.iau.org/public/themes/constellations/
Introduction to the constellation of Orion
Orion is a constellation that has the shape of a man and is made of 7 main stars. It has been observed and drawn since prehistoric times. The name ‘Orion’ derives from Greek mythology. Orion was a gigantic, supernaturally strong hunter of ancient times.
How do astronomers calculate the distance to a star?
Astronomers developed two methods to determine the distance of a star. The first one, known as parallax, uses triangulation. Astronomers observe the position of a star on one day (position 1 in the drawing below) and again six months later (position 2 in the drawing below). They can see the angle formed with the star and the Sun. Knowing that the diameter of the Earth’s orbit around the Sun is about 300 million kilometres, astronomers use trigonometry to calculate the distance. This technique works for stars within about 400 light years of Earth.
Another method is related to the brightness of a star. Astronomers observe and determine the real colour spectrum of the star. They compare the real brightness of the star to the brightness seen on Earth. Using these two values, astronomers can deduce the distance of the star from Earth.