Stars are massive, glowing balls of hot gases, mostly hydrogen and helium. They are formed from collapsing clouds of gas and dust, often called nebulae or molecular clouds. Some stars are alone in the sky, others have companions (binary stars) and some are part of large clusters containing thousands to millions of stars. Not all stars are the same. Stars come in all sizes, brightnesses, temperatures and colors.
Some stars are extremely hot, while others are cool. You can tell by the color of light that the stars give off. If you look at the coals in a charcoal grill, you know that the red glowing coals are cooler than the white hot ones. The same is true for stars. A blue or white star is hotter than a yellow star, which is hotter than a red star. So, if you look at the strongest color or wavelength of light emitted by the star, then you can calculate its temperature (temperature in degrees Kelvin = 3 x 10^6/ wavelength in nanometers). A star’s spectrum can also tell you the chemical elements that are in that star because different elements (for example, hydrogen, helium, carbon, calcium) absorb light at different wavelengths.
When we look at the night sky, we can see that some stars are brighter than others. Two factors determine the brightness of a star: Luminosity and distance. Luminosity is how much energy it puts out in a given time. While, distance is how far it is from us.
We can measure a star’s brightness (the amount of light it puts out) by using a photometer or charge-coupled device (CCD) on the end of a telescope. If we know the star’s brightness and the distance to the star, we can calculate the star’s luminosity:
[luminosity = brightness x 12.57 x (distance)²].
In 1924, the astronomer A. S. Eddington showed that the luminosity and mass of a star were related. The larger a star (i.e., more massive) is, the more luminous it is (luminosity = mass³).
Some stars are moving away from us and some are moving toward us. The movement of stars affects the wavelengths of light that we receive from them, much like doppler effect. By measuring the star’s spectrum and comparing it to the spectrum of a standard lamp, we can measure the amount of the Doppler shift. The amount of the Doppler shift tells us how fast the star is moving relative to us. In addition, the direction of the Doppler shift can tell us the direction of the star’s movement. If the spectrum of a star is shifted to the blue end, then the star is moving toward us; if the spectrum is shifted to the red end, then the star is moving away from us. Likewise if a star is spinning on its axis, the Doppler shift of its spectrum can be used to measure its rate of rotation.
In around 1910, Danish astronomer Ejnar Hertzsprung and American astronomer Henry Norris Russell independently graphed the luminosity vs. temperatures for thousands of stars and found a surprising relationship as shown below.
This diagram called a Hertsprung-Russell or H-R diagram revealed that most of the stars lie along a smooth diagonal curve called the main sequence with hot, luminous stars in the upper left and cool, dim stars in the lower right. Off of the main sequence, there are cool, bright stars in the upper right and hot, dim stars in the lower left.
If we apply the relationship between luminosity and radius to the H-R diagram, we find that the radius of the stars increases as you proceed bottom left diagonally to top right.
If you apply the relationship between mass and luminosity to the H-R diagram, you find that stars along the main sequence vary from the highest (approximately 30 solar masses) at the top left to the lowest (approximately 0.1 solar mass) at the bottom right. As you can see from the H-R diagram, our sun is an average star.
The table below summarizes the types of stars in the universe according to luminosity:
White dwarfs stars are not classified because their stellar spectra are different from most other stars. The H-R diagram is also useful for understanding the evolution of stars from birth to death.
So, how stars are formed?
Usually, some type of gravity disturbance happens to the cloud such as the passage of a nearby star or the shock wave from an exploding supernova. The disturbance causes clumps to form inside the cloud. The clumps collapse inward drawing gas inward by gravity. The collapsing clump compresses and heats up. The collapsing clump begins to rotate and flatten out into a disc. The disc continues to rotate faster, draw more gas and dust inward, and heat up. After about a million years or so, a small, hot (1500 degrees Kelvin), dense core forms in the disc’s center called a protostar. As gas and dust continue to fall inward in the disc, they give up energy to the protostar, which heats up more. When the temperature of the protostar reaches about 7 million degrees Kelvin, hydrogen begins to fuse to make helium and release energy. Material continues to fall into the young star for millions of years because the collapse due to gravity is greater than the outward pressure exerted by nuclear fusion. Therefore, the protostar’s internal temperature increases. If sufficient mass (0.1 solar mass or greater) collapses into the protostar and the temperature gets hot enough for sustained fusion, then the protostar has a massive release of gas in the form of a jet called a bipolar flow. If the mass is not sufficient, the star will not form, but instead become a brown dwarf. The bipolar flow clears away gas and dust from the young star. Some of this gas and dust may later collect to form planets.
The young star is now stable in that the outward pressure from hydrogen fusion balances the inward pull of gravity. The star enters the main sequence; where it lies on the main sequence depends upon its mass.
Now that the star is stable, it has the same parts as our sun: core, radiative zone, and convective zone. However, the interior may vary with respect to the location of the layers. Stars like the Sun and those less massive than the sun have the layers in the order described above. Stars that are several times more massive than the sun have convective layers deep in their cores and radiative outer layers. In contrast, stars that are intermediate between the sun and the most massive stars may only have a radiative layer.