Are all stars the same?


Stars are not boring objects with the same physical features. They come in a wide variety of sizes and colors. To fully understand why, you need to know the physics behind these. Let me try to put this in a simple way:

Astronomers can detect colors of stars by taking their spectra (very similar to splitting of light using a prism, which was done by Newton). The spectrum of a star resembles that of a black body (though there are some differences), which is characterized by a single parameter, the temperature. Hence, one can tell the temperature of a star by looking at the shape of the spectrum of a star.

We see the temperatures of stars go all the way from around 3000 K to 50,000 K. Blue stars are hotter and red stars are cooler. So the next time you see stars, keep in mind that the reason why stars like Betelguese, Arcturus and Antares look red is because they are "cool" (cool here means about 4000 K), and stars like Vega are white because they are much hotter. Based on the temperature, stars are classified into categories as O, B, A, F, G, K and M. O stars are the hottest and M stars are the coolest. For example Vega is an A star, while our Sun is a G star. So the Sun is much cooler than Vega.

The brightness of a star depends on its luminosity and its distance from us. Thus, a star may be faint because it is intrinsically faint, or because it may be very far away. So to know which star is more luminous than the other, we need to know the distance to the star. There are techniques to determine the distance to a star like parallax, Cepheid variables, etc. But once the distance is known, we can determine the actual luminosity of the star. The luminosity of a star may be less than the Sun by a factor of 10,000, or may be as high as more than a million times that of the Sun.

Once we know the luminosity of the star, we can determine the radius of the star. We see that many stars have very large radii with respect to that of the sun; such stars are called Giant or Supergiant stars (an example is Betelguese which has about 500 times the radius of the Sun).

Depending of the temperature of the star, its surface features may vary. Cool stars have molecules like Titanium oxide on the surface, while hot stars have ionized atoms. So you can see that stars come in a HUGE variety which can boggle our minds.