When we look at the night sky, we can identify some shapes: they are called constellations, and they are formed by stars that seem to be near. Ancient civilizations – and, more recently, astronomers – saw in these configurations different figures, often connected to mythology or deity. They gave a name to them, invented stories about them. They were – and still are – very useful to identify the position of planets and astronomical objects in the sky, and to find one’s way during the night, when there is no Sun to be oriented.
In fact, stars in a constellation are not really near to each other; they only appear like that because in the night sky is very difficult to appreciate distances, and it seems to us like a flat dome moving according to Earth rotation.
Today we see the same constellations our ancestors saw in ancient times, as if stars are fixed in the sky. This is simply an illusion: stars have proper motions, and change their mutual position in time. These movements happen in a very long time, because stars are very far from us, and this is the reason why we cannot appreciate them until we were able to develop adequate instruments for measuring them.
The first scientist to measure a proper motion was Edmund Halley – the same that discovered the Halley comet, named after him – in 1719. He noticed that three very bright stars, namely Sirius, Arcturus and Aldebaran, were away from the positions measured by both Ptolemy, a Roman astronomer (c. 100-170 AD), and Tycho Brahe (XVI sec).
The Satellites Era
Now we have many instruments that can measure the slightest movements of stars. One of the most important was Hipparcos, a satellite launched by ESA (European Space Agency) in 1989 and that operated until 1993. The satellite has been named after the great Greek astronomer Hipparchus (190-120 BC), perhaps the greatest astronomer of antiquity. Hipparchus measured with great precision the position of the stars, and, comparing them with oldest measurements, he discovered the precession of the equinoxes, i.e. the apparent change in position of the stars due to the change of orientation of earth rotational axis. The satellite main goal was to measure with great accuracy the position of as much stars as possible; measuring these positions multiple time in different epochs, it was able to determine their proper motion too. Hipparcos produced a catalogue of 2.5 million stars.
In 2013, ESA launched a second satellite, Gaia, in order to extend the Hipparcos catalogue and perform more accurate measurements. It produced a catalogue of 1.8 billion stars, enhancing in meantime the measurement performed by Hipparcos. Data analysis is still ongoing, so more and richer catalogues are expected to be released.
In this actitivty, you will be using Stellarium (https://stellarium.org/), a free open source planetarium software that can be used with your computer. Stellarium can be used to simulate a realistic sky as seen from any place on Earth and at any time, just like what you would see with the naked eye, binoculars or a telescope.
Stellarium is based on Hipparcos data, that is more than enough to evaluate stars constellations motion; these stars are in fact very bright, in general too bright for Gaia, that has been designed to observe fainter and less studied stars.
The Ursa Major Constellation
In this activity, we propose to work on Ursa Major constellation, one of the most famous and visible in the Northern Hemisphere. Here we provide you some historical and astronomical information about it and a power point presentation that you can use in classroom to introduce it.
Its name means “great bear” in Latin, and this asterism has been associated with this animal in many ancient cultures. This could be explained with an ancient tradition correlating the asterism with the bear that dates, according to come studies, 13000 years ago.
Image: The Ursa Major constellation Credits: Starry Night Software / A.Fazekas Link
According to Greco-Roman tradition, it was believed that Zeus, father of all gods, lusted after a nymph (a minor Greek deity related to nature) called Callisto (“the most beautiful” in Greek). His wife Hera, jealous, decided to transform the nymph in a bear. One day, Callisto and Zeus’ son, Arcas, met her mother in the woods and, not recognizing her, tried to kill her. Zeus avoided the tragedy whisking both of them in the sky, Arcas in the form of Boötes constellation, and Callisto as Ursa Major.
Jewish people too identified the constellation with a bear, and cited it in the Bible along with a few others; this is true for Native American people too, among them Lakota and Wampanoang. For Iroquois, the asterism represents a hunt scene, while for Wasco-Wishram it shows five wolves and two bears left in the sky by Coyote, a mythological animal.
Other civilizations saw in the asterism a wagon or a dipper; the first is the case of Norse tradition, in which the Ursa Major is known as the Woden’s wagon, one of the Odin names, the latter of China and Japan, in which the constellation is known as the Big or North Dipper. In the Sami traditions, part of the constellation represents a bow.
Image: Stars in the Ursa Major constellation. Credits: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg) - Link
From an astronomical point of view, Ursa Major is one of the biggest constellations, and at medium and northern latitudes, it appears circumpolar, meaning that it never sets behind the horizon. Two stars, Dubhe (form the Arabic word “dubb”, meaning “bear”) and Merak (from the Arabic word al-maraqq, meaning “the loins” of the bear) could be used to identify Polaris the star that is now marking the north pole by simply connecting the two with a line, and multiplying their distance for five. Another star, Mizar, is a double star, because it seems to be very close to another one, Alcor; probably they are not gravitationally bounded, but the first one is a quadruple system (meaning it counts four stars that revolve around each other) and the second one a binary system (meaning it is formed by two stars gravitationally bonded); anyway, the two systems move together.
Image: How to find the Polaris using Ursa major. Credits: Link
Image: Alcor e Mizar Credits: sebastien lebrigand from crépy en valois, FRANCE, CC BY-SA 2.0, Wikimedia Commons
For each constellation, the IAU (International Astronomical Union) defines a region in the sky; all objects in this area are considered associated to the constellation. In this sense, there are many interesting object in Ursa Major Region:
- 47 Ursa Major is a star with three exoplanets, meaning planets that orbit around a star that is not the Sun;
- M81 and M82 are two interacting galaxies; a galaxy is collection of stars gravitationally bounded. M81 has a spiral form, while M82 is irregular. They are connected by filamentary gas structures.
Image: 47 Ursa Major Credits: Link
Image: M81 and M82 location Credits: NASA, ESA, Z. Levay (STScI) and A. Fujii Link
Image: M81 and M82 Credits: NASA Link
Image: M81 Credits: Image Credits: Subaru Telescope (NAOJ), Hubble Space Telescope; Processing & Copyright: Roberto Colombari & Robert Gendler Link
Image: M82 Credits: NASA, ESA, STScI, AURA, Hubble Heritage Project (STScI, AURA)Link