Stars appear to the naked eye as spiky, twinkling lights or scintillation, especially at night. The stars near the horizon also seem to flash and change color. The twinkling and flashing effects are not due to the stars themselves but to the Earth's atmosphere. Turbulent air currents cause the star's light to dance around. The spikiness of star images is due to optical effects in the observer's eyes. In reality, stars are spheres of gas similar to our own Sun. Stars are held together by gravity.

Astronomers classify stars in two ways. The first classification uses the star’s spectrum, the color(s) of light that the star emits. A star’s spectrum is caused by the temperature in the outer layers of the star. If the star is hot, it looks blue. If it is cool, it looks red. The chart below shows the basic classification of stars by spectral type. The temperatures for stars are measured in a unit called, Kelvin. The temperature of a star in degrees Centigrade is equal to its temperature in degrees Kelvin plus 273. In other words, real hot!

Try to guess what temperature and spectral type our Sun is from the chart below.

The Sun