---
title: "Stars and the Night Sky"
book: "Primary & Middle School Physics"
subject: physics
language: en
chapter: 33
exercises: 11
source: https://one-course.com/books/physics/1/en/chapter/33-stars-and-the-night-sky
---

# Chapter 33 — Stars and the Night Sky

Far from the city lamps, on a clear moonless night, the sky is crowded: thousands of sparks, faint and fierce, white and orange. Sailors steered by them, farmers planted by them, and every science began by wondering at them. What are they — and why do they wheel so tidily overhead while we sleep?

## 33.1 What a star is

**Example 33.1 (Suns beyond counting).**

Each [star](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) is a sun: a gigantic ball of glowing-hot stuff, pouring out its own [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) — many are mightier than our own Sun. They look like humble sparks for one reason only: distance. The Sun is our next-door [star](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star); the others are so far that even [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source), the champion runner that crosses a room in no time at all, spends *years* on the road from the nearest of them. The night sky is a crowd of suns, shrunk by distance to glitter.

![One streetlamp, seen near and seen far: brightness fades with distance. The stars are lamps at distances beyond all roads.](https://one-course.com/images/onecourse/chapters/physics-1/g5-stars-night-sky/fig-c5fe38c1257f.svg)

*One streetlamp, seen near and seen far: brightness fades with distance. The [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) are lamps at distances beyond all roads.*

![A truly dark night far from town: thousands of suns, and the soft band of the Milky Way crossing the sky.](https://one-course.com/images/onecourse/chapters/physics-1/g5-stars-night-sky/img-5029e823407d.jpg)

*A truly dark night far from town: thousands of suns, and the soft band of the [Milky Way](#ex-g5-stars-night-sky-milkyway) crossing the sky.*

**Example 33.2 (Star colors).**

Look carefully and the sparks are not all white: some burn orange-red, some yellow like our Sun, some blue-white. The color is a message about heat — and it runs opposite to taps and paint pots: the orange [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) are the *coolest* of the family, and the blue-white ones the hottest, hotter far than the Sun. A glowing coal and a white-hot blade of steel tell the same story here on Earth: hotter glows whiter.

**Remark 33.3 (Where the stars go by day).**

The [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) never leave. They shine above you at noon exactly as at midnight — but the daytime sky is flooded with sunlight scattered by the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air), a blue glare far brighter than any spark. A candle is invisible beside a bonfire. When the Sun sets and the glare drains away, the sky’s permanent crowd simply becomes visible again.

## 33.2 Maps made of stars

**Definition 33.4 (Constellation).**

A *constellation* is a named pattern of bright [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) — a hunter, a swan, a great bear — invented by sky-watchers long ago as a map and a memory aid. The [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) of a constellation are not really neighbors; they only line up as seen from Earth. But the patterns are steady: your grandparents learned the very same shapes.

**Method 33.5 (Finding the North Star).**

The sky offers a free [compass](https://one-course.com/books/physics/1/en/chapter/25-magnets-and-the-compass#def-g4-magnets-and-compass-compass):

1. find the Big Dipper — seven bright [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) , a saucepan with a long handle, easy to spot through the year;
2. take the two [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) at the front edge of the pan — the *pointers* ;
3. prolong the line from one to the other, onward across the sky, about five times their spacing;
4. the modest [star](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) you land on is the *North Star* — hang a plumb line from it down to the horizon, and there is north.

Check it against your [compass](https://one-course.com/books/physics/1/en/chapter/25-magnets-and-the-compass#def-g4-magnets-and-compass-compass): needle and [star](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) agree. Sailors bet their lives on that agreement for centuries.

![The night sky’s signpost: the Dipper’s two pointer stars aim straight at the North Star.](https://one-course.com/images/onecourse/chapters/physics-1/g5-stars-night-sky/fig-a91f36bbb8da.svg)

*The night sky’s signpost: the Dipper’s two pointer [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) aim straight at the [North Star](#met-g5-stars-night-sky-northstar).*

## 33.3 The wheeling sky

**Proposition 33.6 (The sky turns around the North Star).**

Through the night, the whole crowd of [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) wheels slowly across the sky in great circles — all around one nearly motionless point: the [North Star](#met-g5-stars-night-sky-northstar). Nothing up there truly moves so tidily; it is our own Earth spinning, as it spins the Sun across the daytime sky. The [North Star](#met-g5-stars-night-sky-northstar) barely stirs because it happens to stand almost exactly over the Earth’s spin axis — the one direction the spin leaves in peace.

![Point a camera at the North Star and let it stare all night: every star draws an arc around the still point. The sky’s wheel is the Earth’s spin, seen from inside.](https://one-course.com/images/onecourse/chapters/physics-1/g5-stars-night-sky/fig-247f8d0d14bf.svg)

*Point a camera at the [North Star](#met-g5-stars-night-sky-northstar) and let it stare all night: every [star](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) draws an arc around the still point. The sky’s wheel is the Earth’s spin, seen from inside.*

**Example 33.7 (Our city of stars).**

On the darkest nights, a faint milky band arches right across the sky: the *Milky Way*. Binoculars dissolve the milk into truth: countless [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star), too faint and crowded to separate by eye. It is our home city of [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) seen from inside — a flat, swirling island of more suns than there are grains of sand on a beach, with our Sun living in a quiet suburb. Every [star](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) you can see by eye is a neighbor from this one city.

**Remark 33.8 (Keep looking up).**

Astronomy is the oldest science, and the only one whose laboratory opens free of charge every clear night. A blanket, a dark spot away from streetlights, patience while the eyes wake to the dark — that is the whole equipment list. The Dipper, the [North Star](#met-g5-stars-night-sky-northstar), a [planet](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-planet) or two and the [Milky Way](#ex-g5-stars-night-sky-milkyway) make a fine first expedition; the deeper laws of the [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star)’ [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) will find you well prepared, years from now, near the end of this course.

## 33.4 Exercises

**Exercise 33.1 ★.**

What is a [star](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star)? Why do [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) look like tiny sparks although many outshine our Sun?

**Solution of Exercise 33.1.**

A [star](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) is a sun: a gigantic ball of glowing-hot stuff making its own [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source). They look like sparks only because of their distance — even [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) spends years on the road from the nearest one.

**Exercise 33.2 ★.**

Are the [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) still in the sky at noon? Why can we not see them?

**Solution of Exercise 33.2.**

Yes — they shine at noon exactly as at midnight. The daytime sky is flooded with sunlight scattered by the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air), and no spark can show beside that glare, as no candle shows beside a bonfire.

**Exercise 33.3 ★.**

An orange [star](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) and a blue-white [star](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) shine side by side. Which is hotter? What everyday glow tells the same color story?

**Solution of Exercise 33.3.**

The blue-white one — [star](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) colors run from orange (coolest) through yellow to blue-white (hottest). A coal glowing orange versus steel heated white-hot tells the same story.

**Exercise 33.4 ★.**

What is a [constellation](#def-g5-stars-night-sky-constellation)? Are its [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) truly close to one another?

**Solution of Exercise 33.4.**

A named pattern of bright [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star), invented as a sky map and memory aid. Its [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) are not real neighbors — they only happen to line up as seen from Earth.

**Exercise 33.5 ★.**

Describe, step by step, how to find the [North Star](#met-g5-stars-night-sky-northstar) from the Big Dipper.

**Solution of Exercise 33.5.**

Find the Big Dipper; take the two pointer [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) at the front edge of its pan; prolong their line about five spacings across the sky; the [star](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) reached is the [North Star](#met-g5-stars-night-sky-northstar), and below it on the horizon lies north.

**Exercise 33.6 ★.**

What is special about the [North Star](#met-g5-stars-night-sky-northstar)’s place in the wheeling sky, and what everyday instrument does it replace?

**Solution of Exercise 33.6.**

It stands almost exactly over the Earth’s spin axis, so the wheeling of the sky leaves it nearly motionless — a fixed mark of north, replacing the [compass](https://one-course.com/books/physics/1/en/chapter/25-magnets-and-the-compass#def-g4-magnets-and-compass-compass).

**Exercise 33.7 ★.**

The [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) wheel across the night in tidy circles. What is really turning? What else does the same turning parade across the daytime sky?

**Solution of Exercise 33.7.**

The Earth is turning, carrying us around once a day. The same spin parades the Sun across the daytime sky — one turning, two parades.

**Exercise 33.8 ★.**

What does the milky band across the darkest skies turn out to be, seen through binoculars? What is it really?

**Solution of Exercise 33.8.**

Countless faint [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star), too crowded to separate by eye: the [Milky Way](#ex-g5-stars-night-sky-milkyway) — our own flat city of [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star), seen edgewise from inside.

**Exercise 33.9 ★★.**

Left alone all night, a camera aimed at the [North Star](#met-g5-stars-night-sky-northstar) records circles; aimed at the eastern horizon instead, it records rising slanted streaks. Explain both pictures with one spinning Earth.

**Solution of Exercise 33.9.**

The spinning Earth wheels the whole sky around the axis through the [North Star](#met-g5-stars-night-sky-northstar). Near that still point, [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) ride small circles — the camera records rings. Toward the east, the wheeling carries [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) up over the horizon along slanting paths — the camera records rising streaks. One spin, two views of it.

**Exercise 33.10 ★★.**

A shipwrecked sailor on a clear night has no [compass](https://one-course.com/books/physics/1/en/chapter/25-magnets-and-the-compass#def-g4-magnets-and-compass-compass), no phone — and no hurry. Explain how the sky hands her both a direction and, with patience, a clock. (Two chapter ideas, one wheeling sky.)

**Solution of Exercise 33.10.**

Direction: the Dipper’s pointers give her the [North Star](#met-g5-stars-night-sky-northstar), and with it north — no needle required. Clock: the Dipper itself wheels around the [North Star](#met-g5-stars-night-sky-northstar) like the hand of a great clock, one full turn per day; watching how far it has swung tells her how the night is passing.

**Exercise 33.11 ★★.**

The [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) of a far [star](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) spends years on the road to your eye. When you look at that [star](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) tonight, are you seeing it as it is *now*? What are you actually looking at — and why does this make every telescope a kind of time machine?

**Solution of Exercise 33.11.**

No: you see the [star](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) as it *was*, years ago, when tonight’s [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) left it — the messenger, not the sender. Every telescope therefore looks into the past: the farther the [star](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star), the older the news — which is precisely what makes distant [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) so precious to astronomers.