John Farndon - What Do We Know About Stars and Galaxies?

If you look into the sky on a clear night, youll see the Moon and thousands of twinklingpinpoints of light spread across the darkness. All these twinkling lights are stars.Stars twinkle because dust and gas surrounding Earth get in the way of your view,and also because the atmosphere itself fluctuates. You may see one or two pinpointsof light that dont twinkle. These are planets, our neighbor worlds that circle theSun along with the Earth. Although much smaller than stars, planets are so much closerto Earth that they form pinpoints of light too big to be blocked out by dust. Thisis why you wont see planets twinkling.

How many stars are there?

You can see just a few thousand stars with your eyes alone. With an amateur astronomicaltelescope you can see a few million. But the Universe is beyond enormous. All ofthe matter and space in existence make up the Universe. Every star in the Universeis part of a giant star city called a . Astronomers have found that there areabout 100 billion stars in each galaxy. They have also found about 100 billion galaxiesin the Universe. That means there must be at least 100 billion times 100 billionstars in the Universe!

There is another difference between stars and planets. If you watch the sky for longenough, it looks as if all the stars are moving slowly from east to west. In fact,it is not the stars moving but the Earth. If the Earth stopped spinning, youd seestars stay perfectly still, in a fixed pattern. It takes 24 hours for the Earth toturn around. So the star pattern returns to the same place every 24 hours.

Unlike the fixed stars far away, however, the planets actually move through the skyas they circle the Sun close by. Thats how in ancient times knew planetswere different from stars, even though both were just pinpoints of light. Today,we can actually see that planets are globes like the Moon.

The stars are much too far away to ever visit, and even the most powerful telescopesonly show them as pinpoints of light. However, telescopes have revealed the secretsof the stars in other ways.

In a photo that uses a long exposure, such as this one, the movement of the starsthrough the sky as the Earth turns means that the stars appear as long curving streaks.

One of the biggest challenges astronomers face is measuring distances to stars faraway across space. They can calculate how far a nearby star is by the method.As the Earth circles the Sun, the star shifts sideways slightly compared to starsfurther away, as the angle we see it from changes. The nearer the star is to Earth,the more it shifts.

This diagram shows a basic example of the parallax method. By measuring the anglesof a stars light from two points in Earths orbit, an astronomer can calculate howfar away the star is. The further away the star is, the smaller the parallax angle.

With distant stars, this side-shift is too small to measure. The secret is to comparebrightness. A stars relative is how bright it looks compared to others.But a dim star might be shining feebly that is, it has a low absolute magnitude.Or it may just be far away. We can tell which by its color. The whiter a star is,the hotter and brighter it glows; the redder it is, the cooler and dimmer it is.A dim white star is a star far away. A bright red star is a star nearby. Astronomershave made a graph showing the relationship between color and brightness for mostordinary or Main Sequence stars. Calculating distances using this graph is knownas Main Sequence fitting.