Stars are massive objects that consist of gas and emit a certain amount of light. Are there stars that don’t have a glow or cannot be considered?
What is the stellar spectrum?
If you carefully look at the night sky, you can see that the stars differ in color and degree of illumination. The color of the star allows you to know the temperature of its photosphere - radiation. In turn, the spectrum depends on this. It provides a lot of valuable information about the star - its size, luminosity, temperature, etc.
In 1910, scientists Henry Russell and Einar Herzshprung (independently of each other) proposed a special diagram that is used to classify stars. It displays the relationship between the basic properties of stars, such as luminosity, spectrum, absolute value and temperature.
The spectra of stars that have common features form spectral classes. Latin letters are used for their designation (from O to M). Thus, there are several colors and intermediate shades that stars and their corresponding classes can have:
- blue (O);
- blue and white (B);
- white (A);
- yellow white (F);
- yellow (G);
- orange (K);
- red (M).
In this case, there is a difference between the true and visible color of the star. It is worth noting that this revised version of the spectral classification is considered the main one - Harvard. There are other less common versions.
Stars of red color (class M) are characterized by the lowest temperature, and blue (O) - by the highest. Each spectral class is divided into several subclasses, which are numbered 0-9. For example, in the class M there are subclasses: M0 - M1 - M2 - M3 - M4 - M5 - M6 - M7 - M8 - M9. This is the classification of stars.
Interesting fact: The closest star to Earth is the Sun. It belongs to the spectral class G (namely, G2).
What are the types of stars?
Depending on the stage of evolutionary development, stars are divided into 3 main categories:
- normal
- dwarfs;
- giants.
There are also variable stars, such as Wolf-Rayet, T Taurus, new, supernovae, hypernovae and others. According to the diagram, more stars are located on the main sequence. That's what it is normal stars. Their common property is the transformation of hydrogen into helium with a concomitant energy burst.
Dwarfs are small stars. They have their own classification depending on the spectral class, stage of evolution, size, and other parameters. Dwarfs are yellow, orange, blue, brown, white, black, red and sub-brown.
White dwarfs emit a strong glow, since they heat up to 100,000 K. Red ones differ in temperature up to 3500 K (they shine 10,000 times fainter than the Sun). However brown dwarfs and the subsequent sub-brown, black ones, do not emit light in the visible range.
The fact is that for a visible stream of light, the surface temperature of the star should reach at least 600-700 K (400 ℃). Brown dwarfs cool throughout their existence. Therefore, their temperature ranges from 300 to 3000 K.
The sub-brown dwarfs are even colder, and the black ones are the coldest. Thus, their radiation energy is not enough to create visible light. For stars of this category, separate spectral classes are provided - L, T and Y.
Interesting fact: The coldest brown dwarf with a temperature of about 300 K is WISE 1828 + 2650.
Giant stars most often called red. This is because most of their radiation falls on the red and infrared spectrum. Such stars usually have a large radius and relatively low temperature. Due to the size they glow very brightly.
All objects emit a certain amount of light. But for the formation of a visible light flux, the body temperature must reach at least 400 ℃ or 600 K. In the stellar classification there are dwarfs - brown, sub-brown and black, whose temperature is too low for sufficient luminosity. The surface temperature of brown dwarfs varies throughout the entire cycle of their existence and amounts to 300 - 3000 K.
