Although astronomers could determine the diameter of some stars in the past, stars still remain only points in any telescope. Surface features cannot be observed directly. However, you can infer a lot even from the sight in binoculars. Just like our Sun, all the stars are luminous balls of gas that draw their energy from nuclear fusion, which takes place in the core of the star at several million degrees. The light that we see from the stars does not derive directly from the hot center, but from the so-called photosphere, which is sometimes also simply referred to as the solar surface.
The gases of the photosphere glow, and their color depends on the temperature of the star. An approximately 10,800° F (6,000° C) hot star like our Sun glows yellow, cooler stars (5,400 to 7,200° F or 3,000-4,000° C) emit red light. At a surface temperature of about 12,600° F (7,000° C) stars shine in a pure white, and blue stars have temperatures of 19,800 – 72,000° F (11,000-40,000° C). So, from the color you can infer the temperature of a star, and from the temperature you can draw initial conclusions on its mass – for precise statements you still need to know its distance or its absolute magnitude, which depends upon its distance and its apparent brightness.
For a more detailed analysis you have to split the light into its spectral colors. Massive stars consume their hydrogen fuel very quickly, so they are very hot and can be burned out after a few hundred million years. Cooler stars like our Sun shine around eleven billion years, and low mass, red dwarf stars last on their hydrogen storage for much longer. In the course of its life, the spectrum and hence the color of a star changes.
The color can be perceived only at very bright stars, because the eye perceives low luminosities only in black and white. It is helpful to observe the stars slightly out of focus, because then the light is distributed on a larger part of the retina, which facilitates the perception of color. Incidentally, there are no green stars – stars do not shine only in one color, they only have a maximum of brightness at a certain color. The other colors are weaker, but also provided. Since the eye mixes a picture out of three colors for color vision, we do not perceive stars as green, even if their maximum brightness is in the green part of the spectrum.
Nevertheless, some observers see green stars – however, these are always members of binaries, so that the eye is deceived by the color contrast and sees green where there is none. The perception of color also varies from observer to observer, so there are different descriptions for many stars. Especially bright stars that are close to the horizon appear colored too, because their light is refracted through the Earth‘s atmosphere and split into its spectral colors – to see the real colors, you must wait until the star is high above the horizon.
The apparent brightness of a star is no indication of how bright it is in reality. Even from ten light-years away our Sun would be barely recognizable. Most of the stars that we see in the night sky are very distant and very luminous giant stars whose diameter exceeds that of the Sun many times. Betelgeuse (α Orionis) at 30 light-years distance, for example, has a diameter which is likely to correspond approximately to the Earth‘s orbit around the Sun. Conversely, many stars in our vicinity are so faint that they can be discovered only in large telescopes. In 2003 Teegardens Star was discovered at a distance of only seven to ten light-years – with 15.4m a very inconspicuous object, indeed.