L21. What Is a Star? Brightness, Colour, Distance

Unit 5 · The Lives of Stars

R-report

L21. What Is a Star? Brightness, Colour, Distance

Two stars look equally bright tonight. One is a modest star nearby; the other is a monster thousands of times more powerful, far across the galaxy. How do astronomers tell them apart?

A star is a ball of gas that fuses

A star is a huge sphere of hot gas — mostly hydrogen and helium — held together by its own gravity, with a core hot and dense enough for nuclear fusion. That last part is the definition. Gravity does the squeezing; fusion provides the energy. An object without enough mass to ignite fusion is not a star: it is a brown dwarf, sometimes called a failed star. The range of sizes is enormous. - Red dwarfs, the most common stars, can be as little as 8% of the Sun's mass. - The Sun is fairly average. - The largest known stars are hundreds of times wider than the Sun; placed where the Sun is, some would swallow the orbit of Jupiter. Despite this variety, all stars are doing the same fundamental job: converting light elements into heavier ones and releasing energy. The differences between them come mostly down to one property — mass.

Bright, or just close?

How bright a star looks from Earth is called its apparent brightness, and by itself it tells you very little. Brightness falls off rapidly with distance. Move twice as far away and a light appears four times fainter. So a dim nearby star and a brilliant distant one can look identical. What astronomers really want is luminosity — the true energy output of the star, independent of how far away it is. To get it, they need the distance. The most direct method is parallax. Observe a nearby star in January and again in July, when Earth is on the opposite side of its orbit. The star appears to shift very slightly against the far more distant background. The larger the shift, the closer the star. Hold a finger at arm's length and look at it with one eye, then the other: it jumps against the background. Parallax is the same trick, with Earth's orbit as the distance between the two "eyes". The shifts involved are minuscule, which is why no one measured a stellar parallax until 1838.

What colour tells you

The colour of a star is a direct clue to its surface temperature, and temperature connects to nearly everything else about it. - Blue: hottest, above about 20,000 °C. - White: around 10,000 °C. - Yellow: around 5,500 °C, like our Sun. - Orange and red: coolest, down to about 3,000 °C. When astronomers plot stars by temperature against luminosity, the result is not a random scatter. Most stars fall along a single diagonal band called the main sequence, running from hot and bright at one end to cool and dim at the other. This chart is called the Hertzsprung-Russell diagram, and it is one of the most useful tools in astronomy. Stars off the main band are doing something different: red giants are cool but very bright because they are enormous, and white dwarfs are hot but faint because they are tiny. The pattern is not decoration. It is the reason the next lessons can describe how stars are born, live, and die — the diagram is essentially a map of stellar life stages.

Putting the investigation together

A star is a self-gravitating ball of gas with fusion running in its core. How bright it looks depends on both its true luminosity and its distance, so measuring distance — usually by parallax for nearby stars — is what turns an appearance into a real measurement.

The Hertzsprung-Russell diagram is the payoff. Once you can place stars by temperature and true brightness, the tidy diagonal band that emerges shows that stars are not arbitrary. They follow patterns, and those patterns turn out to be stages in a life story.