Thousands of stars twinkle in the sky – but take a good look: not all stars are the same. There are lighter ones and darker ones, red shining ones or more blue ones. Some stars are gigantic giants, others are small dwarf stars. Some are hot, others rather cool. There are ancient stars and very young. And stars go through an evolution – many are born as dwarfs, sometimes grow to giants, gradually burn up, or end in a spectacular explosion.
Yellow dwarfs are an important type of star for us: our sun is a yellow dwarf. Smaller stars of spectral type G, because they shine in yellow light. They are the average stars of the main sequence: cozy suns that gradually and quietly fuse the hydrogen inside to helium. They have about the size and mass of our sun – rather small compared to other stars. Their surfaces have moderate temperatures of about 5.000 degrees. For us this is only good – in the area of such a star it is easy to live on a planet. In addition, these suns have a long life expectancy: Our sun has been around for about four and a half billion years – and it has only used up about half of its hydrogen. But after that, helium will start to burn, and our star will inflate into a red giant. One of our neighboring stars, which you can see in the southern sky, is also a G2V star like the Sun: Alpha Centauri in the constellation Centaur. But much more common are stars like Proxima Centauri, the closest star to us: Red Dwarfs, which you can’t see, although they are the most common stars.
Spectral types – The colors of the stars
Alpha Centauri video on the subject
Dwarfs, giants, supernovae and quarks – what’s going on with stars? Prof has the answers. Harald Lesch.
You can easily tell some stars apart by their colors: Some tend to shine in white light, others red or blue. The Color of a star (the spectrum of its light) is dependent on its Surface temperature. Very hot stars shine blue, very cool stars red. The different Spectral classes of the stars will be with the letters O, B, A, F, G, K, M, L and T designated, with O stars being the hottest, blue stars with temperatures up to 30.000 degrees, while M, L, and T stars are very cool, glowing red with temperatures starting at 1.000 degrees Kelvin. For finer distinction, these classes are further subdivided into ten subclasses from 0 to 9, where 0 is again the hottest, 9 the coolest. So a G9 star is hotter than a K0 star. By the way, our Sun is a G2 star – a relatively hot one among the yellow shining stars.
Luminosity classes – The brightness of a star
In addition, the stars are distinguished according to eight Luminosity classes, depending on the absolute brightness of a star. For stars of the same spectral type – red stars, for example – can be of different brightness: The larger a star is, the more surface area it has. More surface but more luminous at the same temperature: A red giant is much brighter than a red dwarf. The luminosity classes are given in roman numerals. The luminosity class of the supergiants I is again divided into Ia and Ib subdivided. The main sequence stars all belong to class V, including our sun. White dwarfs have the lowest luminosity (VII). By the way: Spectral types and luminosity classes are not to be confused with the Star classes, the stars according to their apparent brightness into size classes. A first-class star is especially bright when seen from Earth, even if it does not belong to the highest luminosity class.
The Hertzsprung-Russell diagram
Luminosity and Spectral type are defined in the Hertzsprung-Russell diagram (HRD), which represents hundreds of stars in the immediate vicinity of our sun, each with its own point. This scheme is used to examine the developmental states of stars. At the top left of the diagram are the hottest, brightest stars (O stars), and at the bottom right are the coolest, faintest stars (M stars) . When this diagram was created, it showed that 90 percent of the stars are in the so-called Main branch of the HRD – a very limited line from upper left to lower right. Only one-tenth of the stars are not part of this Main series, but belongs to the giants, supergiants or white dwarfs. At the same time, the HRD traces the evolutionary path of individual stars – because some stars move across the diagram: They begin as main sequence stars, grow into red giants, and end as white dwarfs.