Stars and Stellar Distances
Ancient Greek Astronomers believed the stars were burning objects. Today, we know that stars are composed of very hot gases. Stars produce energy by nuclear fusion. If you observe the sky on a clear night, you can see as many as 2000 stars.
Some stars in the night sky are brighter than others. However, the brightest points of light in the night sky may not be stars. Some planets; Venus, Mars, Jupiter and Saturn may be brighter at times than most stars. A planet usually appear as a point of light with a steady glow. Stars usually twinkle.Twinkling is caused by the reflection of the start light as it passes through the earth’s atmosphere.
A star’s apparent brightness depends on its size type, and distance from Earth. A large hot star produces more light than a small cool star. Thus the large star will be brighter than the small star if both are at the same distance from earth.
An analogy is a light held near you then is being moved away, you will notice that the light will be decreasing in brightness with increase in distance from you. That is how stars work too, the more the distance from earth, the less bright it appears on the sky.
The actual brightness of a star can be used to detect the amount of light it produces. To compare actual brightness of stars, astronomers calculate what their brightness will be if the star were at the same distance. Using this method, brightness ca also be used to indicate a star’s distance from Earth.
Stellar distances can be measured by parallax. Parallax is the apparent change in position of an object caused by an actual change in position of the observer. Nearby stars will appear to change position relative to a background star as the Earth revolves around the sun. Using the distance between Earth’s first and second positions and the parallax, astronomers can compute the distance to the star.
The parallax method works only for stars that are relatively close to Earth. Parallax decreases as distance increases.When a star is very far from Earth, no parallax is observed. About 1000 stars are close enough to use parallax.
Astronomers work with very large numbers, the star closest to Earth is about 38,000,000,000,000 km (38 trillion km) away.Astronomers measure distance with a unit called the “light-year“. A light-year is the distance light travels in one year which is 9.5 trillion kilometers. Alpha Centuari, the nearest star is about 4 light-years away.
About 40 stars in the sky are within 16 light-years of Earth. The brightest star Sirius is 9 light-years away. Betelguese, one of the largest known stars, is 520 light-years away. Stars more than 5 billion light years away have been identified. This distance is impossible to grasp. Yet the universe extends at least this far in every direction.
Try it Yourself: Parallax Experiment
- Put a series of vertical lines 10cm apart on a large board hanged on the wall. Number them in order.
- Clamp a pencil to a ring stand and place it between you and the board.
- Stand 1m from the pencil. Use your left eye (closing your right eye) to observe the pencil’s position against the lines on the board.
- Without moving from your position, repeat the observation with your right eye (closing your left eye). The apparent change in the position of the pencil is the parallax. For example, if the pencil is aligned with line eight on the board when seen with your right eye and aligned with line four when seen with your left eye, the parallax is 4 (8 – 4 = 4).
- Repeat steps 3 and 4 at 2m, 3m and 4m from the pencil. Record the amount of parallax in each case. Then note the relationship between each amount of parallax and your distance from the pencil.