How many stars in a constellation, and why don’t they change?

We explore the curious questions that science can answer

The Southern Cross has more stars than most other constellations (Getty/iStock)

How many stars are there in a constellation?

The number varies a lot. They are counted by measuring the number of stars that can be seen in an area of the sky. The Southern Cross has lots of stars – 19.6 per 100 square degrees of sky. Most others have between five and six per 100 degrees.

An easier way is to count the number of bright stars. In the northern hemisphere, Centaurus has 18 bright stars. Canis Minor has only two.

Why does the atmosphere on any particular planet not ‘leak’ into space?

For two main reasons: gravity and energy (at the molecular level). Some molecules do leak into space, while others don’t have enough energy, as they are too heavy.

Atmospheres may be replenished by biological activity and also by the substances that are released from volcanoes.

How big is our galaxy?

Our galaxy is about 50 kilo parsecs (kpc) across. A parsec is 3.26 light years and a light year is the distance light travels in one year.

So that makes the galaxy 1,500,000,000,000,000,000 kilometres across.

If the furthest reaches of the universe are expanding at close to the speed of light then their masses will increase. Could this increase in mass, and associated increase in size, explain why the universe is expanding?

It is true (according to Einstein’s theory of special relativity) that objects moving close to the speed of light appear to increase in mass, but they will also appear to shrink in size. This is called the Lorentz contraction. The expansion of the universe is, therefore, not due to special relativity.

How many people have walked on the moon?

Between 1969 and 1972, a total of 12 American astronauts walked on the moon, before the Apollo programme was cancelled prematurely by the US space agency Nasa. The 12 travelled in six separate Apollo missions and collectively spent approximately 80 hours outside the lunar lander, during which time they collected rock samples, took photographs and set up experiments.

What's the difference between an asteroid and a comet?

They can be difficult to distinguish. Essentially an asteroid is an irregular, rocky body that is smaller and less massive than a planet. About a million are thought to exist in the asteroid belt between Mars and Jupiter, and many more are scattered through the solar system.

By contrast, comets trace elliptical, but regular, orbits that may take them far beyond Pluto and then close to the sun. They have icy material on their surfaces or within, and emit gases when heated by the sun.

Is anyone allowed to name a star?

Officially no. Except for the few dozen bright stars named by the ancients, stars are always designated by some alphanumeric system involving their placement in the sky, such as their ordering by position in a zone of declination, by brightness (or by variability of brightness) in a constellation, or simply by the digits of their coordinates in some system: in some instances the stars are simply given numbers in a catalogue by some astronomer or observatory who collected them for some specific purpose – such as their relatively rapid motions with respect to other stars or because of particular features in their spectra. There are organisations that will try to sell you a star, but they have no official right to do so.

Why do the sun and moon appear bigger when they are near the horizon?

This is a famous optical illusion that has no simple answer. If you take a ruler and measure the size of the sun – or, more easily, the moon – at the horizon, and then again when it’s higher, you’ll see that there is no difference. There has been much research into this topic. Many think that because we can see buildings and trees that we know to be not that far away, our brain tries to imagine the sun or moon at that distance too, and this makes them appear bigger. However, experiments in which people were asked to judge the size of the moon with and without reference objects such as buildings suggest this is not the case.

Why don’t the constellations change, if the stars are moving so fast?

Although the stars are indeed moving very fast (at a speed of many kilometres per second) they are so far away (typically trillions of kilometres) that they have to travel for thousands of years before we can see a difference in their positions. In one lifetime we won’t notice a difference in the constellations, but if you could come back in 50,000 years’ time, the sky would look very different.