Everything we know about the stars arrives as light

Word	What it means
Celestial sphere	The imaginary sphere of sky that the stars seem fixed to. It appears to turn once a day because Earth rotates.
Constellation	A named pattern of stars, and the region of sky around it.
Altitude / azimuth	Altitude: degrees above the horizon, from 0° to 90° straight overhead. Azimuth: compass direction in degrees clockwise from north — east 90°, south 180°, west 270°.
Geocentric / heliocentric	A model with Earth at the centre / with the Sun at the centre.
Astronomical unit (AU)	The average Earth–Sun distance, about 150 million km. Used within the solar system.
Light-year	The distance light travels in a year, about 9.5 trillion km. Used for stars and galaxies.
Star	A huge ball of hot gas releasing energy by nuclear fusion: joining hydrogen to make helium.
Spectroscope / spectrum	A spectroscope spreads light into its colours; that band of colours is a spectrum, and lines in it identify elements.
Red shift	Lines in a spectrum moved toward the red end: the source is moving away.
Optical / radio telescope	Optical telescopes collect visible light with lenses or mirrors; radio telescopes collect radio waves with dishes.
Interferometry	Linking telescopes spread far apart so they work like one much larger telescope.
Triangulation / parallax	Finding a distance from a baseline and the angle at each end. Parallax is a near object's apparent shift against a far background, seen from two places.
Geosynchronous orbit	About 36 000 km up, taking one day per orbit. Above the equator, a satellite in it stays over one spot.
Microgravity	The near-weightless condition in orbit. Gravity is still there; astronauts float because they and the station are falling around Earth together.

The sky as a calendar. Peoples everywhere used the Sun's rising point, the Moon's phases and the seasonal return of particular stars to time hunting, planting and ceremonies. First Nations on the plains carry this star knowledge in stories passed down orally; in ancient Egypt, Sirius appearing before dawn signalled the Nile's yearly flood.

Geocentric to heliocentric. For about 1400 years most astronomers in Europe and the Middle East used Ptolemy's Earth-centred model. In 1543 Copernicus put the Sun at the centre. The strongest early evidence came through a telescope: in 1610 Galileo saw moons circling Jupiter, so not everything orbits Earth, and saw Venus show a full set of phases, which is impossible if Venus circles Earth.

Worked: altitude and azimuth. A planet is at azimuth 135°, altitude 30°. Azimuth 135° is halfway between east (90°) and south (180°), so face south-east. Altitude 30° is a third of the way from the horizon (0°) to straight overhead (90°).

A check you can do tonight. Polaris, the North Star, sits due north at an altitude about equal to your latitude — in Edmonton, about 53.5°, a little more than halfway up. The celestial sphere seems to turn around it once a day, and from Alberta the Big Dipper, part of the constellation Ursa Major, circles it without ever setting.

Worked: distances. Light travels about 300 000 km every second, so sunlight crosses 150 million km in 150 000 000 ÷ 300 000 = 500 s — a little over 8 minutes. Proxima Centauri, the nearest star after the Sun, is about 4.2 light-years away: 4.2 × 9.5 trillion km ≈ 40 trillion km.

A star's life. Gravity pulls a nebula, a cloud of gas and dust, together until fusion starts. The star spends most of its life steadily fusing hydrogen, the main sequence; the Sun is about 4.6 billion years into roughly 10 billion. Then mass decides. A star like the Sun swells into a red giant and leaves a white dwarf, a hot core about the size of Earth. A star of roughly eight times the Sun's mass or more burns out far sooner, swells into a red supergiant and explodes as a supernova, leaving a neutron star or, from the most massive stars, a black hole, from which not even light escapes.

Spectra. Starlight spread out by a spectroscope is crossed by dark lines, and each element makes its own pattern, like a fingerprint; helium was identified in the Sun's spectrum in 1868, decades before anyone found it on Earth. A star's colour gives its temperature: blue-white is hottest, red coolest.

Red shift. A familiar pattern of lines shifted toward red means the source is moving away; toward blue, coming closer. Almost every galaxy is red-shifted, and the farther away, the bigger the shift — the evidence that the universe is expanding.

On the test. Beside a lab spectrum of hydrogen, a star shows the same pattern of lines shifted toward red. So the star contains hydrogen and is moving away. The shift says nothing about how hot it is; that comes from its colour.

Telescopes. A wider lens or mirror shows fainter objects. Radio waves pass through dust and cloud, so radio dishes work day or night, and in 2019 dishes across the world, linked by interferometry as the Event Horizon Telescope, produced the first image of a black hole's shadow. Space telescopes escape the air, which blurs starlight, blocks much infrared and ultraviolet light, and stops X-rays entirely; the James Webb Space Telescope, launched in 2021, carries a Canadian-built guidance sensor and science instrument.

Worked: triangulation. Two students stand 50 m apart on a riverbank. From A, a tree on the far bank is at 90° to the baseline; from B, it is at 45°. The angles add to 180°, so the angle at the tree is also 45°, and a triangle with two equal angles has two equal sides: the tree is 50 m from A. A scale drawing at 1 cm : 10 m gives the same answer.

Parallax is triangulation for stars. Close each eye in turn and a raised thumb jumps against the background, more when it is closer. Astronomers photograph a nearby star six months apart, from opposite sides of Earth's orbit — a baseline of about 300 million km — and measure its shift. A smaller shift means a farther star, so it works only for the nearer ones.

Orbit	Height	Used for
Low Earth orbit	A few hundred to about 2000 km	The space station (about 400 km up, circling Earth in about 90 minutes)
GPS	About 20 000 km	Navigation: a phone times signals from at least four satellites
Geosynchronous	About 36 000 km	Weather and communication satellites that stay over one region

Canada in space. RADARSAT satellites, the first launched in 1995, image Earth by radar, which works through cloud and darkness — ideal for Arctic sea ice. Canadarm first flew on the U.S. space shuttle in 1981, and Canadarm2 has worked on the International Space Station since 2001; its robotics led to neuroArm, a surgical robot built at the University of Calgary. Marc Garneau was the first Canadian in space (1984), Roberta Bondar the first Canadian woman (1992), and Chris Hadfield the first Canadian to command the station (2013).

Hazard	The problem	The answer
Vacuum	No air, no pressure	Sealed, pressurized cabins and suits
Temperature	Very hot in sunlight, very cold in shade	Insulation, reflective surfaces, heaters
Radiation	No thick atmosphere to block it; it damages cells	Shielding, limits on time in space
Microgravity	Bones and muscles weaken	About two hours of exercise a day
Space debris	At orbital speed, a paint fleck can chip a window	Shielding, tracking, moving out of the way

Life support is recycling. On the space station, well over 90% of the water — from breath, sweat and urine — is recovered and purified. Oxygen is made by splitting water with electricity, and carbon dioxide is removed from the air.

Questions science cannot settle alone. Is a space program worth billions when there are problems on Earth? Who owns the Moon? The Outer Space Treaty of 1967 says no country can claim it, but it was written long before private rocket companies. Who cleans up debris, when one collision in 2009 turned two satellites into more than two thousand trackable pieces?

On the test: whose perspective? A company plans thousands of small internet satellites. “Remote northern communities would finally get fast internet” is social. “The satellites streak across telescope images” is scientific. “It will create jobs and pay for itself” is economic. “More satellites means more debris” is environmental. Science can say what will happen; it cannot, on its own, say whether the trade is worth it.

• Treating a light-year as a time. It is a distance.
• Saying the Sun will become a black hole or a supernova. It lacks the mass; it will end as a white dwarf.
• Reading red shift as “the star is red.” The lines moved, because the star is moving away.
• Thinking a bigger parallax shift means a farther star. Bigger shift, closer star.
• Saying there is no gravity in orbit. Astronauts are falling around Earth.
• Swapping altitude and azimuth, or measuring azimuth from anywhere but north.