Gwiazdy i ich ewolucja

Before you start, answer the questions.

• What is the star?

• What the galaxy is?

Stellar classification

The stellar classification is based on stars’ spectra. The stellar spectrum is characterised by three basic parameters:

• temperature

• gas pressure,

• chemical composition.

In the Morgan‑Keenan (MK) system, there are seven main spectral types of stars ordered of decreasing temperature: O, B, A, F, G, K, and M. Each type has subclasses from 0 to 9 (hottest to coolest star of certain type e.g. B4, G7). The colour of a star is determined by its surface temperature.

ROTT1cBzdGX5D1

Na rysunku przedstawiona jest klasyfikacja gwiazd według ich temperatury. Gwiazdy o mniejszej średnicy mają niższą temperaturę oraz są oznaczone ciemniejszym kolorem niż gwiazdy o większej średnicy. Od lewej do prawej strony widoczne są kule o stopniowo zwiększającej się średnicy, podpisane jako M, K, G, F, A, B oraz O. Kule od lewej do prawej mają barwę od żółtej do biało-niebieskiej.

An additional parameter used in the classification is luminosityluminosityluminosity of a star (The Yerkes Luminosity Classes).

The luminosityluminosityluminosity of stars is affected not only by their temperature, but depends also on the size of a star. The most luminous stars are these which are hot and large. For a group of stars with the same temperature, the luminosity class differentiates between their sizes (supergiants, giants, main‑sequence stars, and subdwarfs).

The Hertzsprung‑Russell diagram

The relationship between the average surface temperature of stars (spectral type) and their absolute luminosity is presented by the Hertzsprung‑Russell diagram (the H‑R diagram). The absolute luminosityluminosityluminosity of stars is telling how brigth they would appear if they were all the same distance, equal to 10 parsecs.

R158fv3Fdlj9M

Na rysunku przedstawiony jest wykres Hertzsprung‑Russell, czyli związek między średnią temperaturą powierzchni gwiazd (typem widmowym) a ich absolutną jasnością. Na osi pionowej brak skali, podpisana jest Luminosity, oś pozioma podzielona jest na równe odstępy, każdy oznaczony jest literą, O, B, A, F, G, K oraz M, podpisana Spectral Class. Na wykresie w postaci pasa przebiegającego w poprzek wykresu od lewego górnego rogu do dolnego prawego rogu zaznaczone za pomocą kropek są gwiazdy ciągu głównego podpisanego Main Sequence, o kolorze zmieniającym się płynnie od jasnoniebieskiego, białego, żółtego, pomarańczowego w ciemnoczerwony (symbolizującego czerwone karły podpisane Red Dwarfs). W prawym górnym rogu znajdują się kropki symbolizujące nadolbrzymy podpisane Supergiants, a pod nimi czerwone olbrzymy podpisane Red Giants. W dolnej środkowej części kropki symbolizujące białe karły podpisane White Dwarfs. Na środku strzałka do jednej z żółtych kropek z podpisem Sun.

Main sequence stars

Main sequencemain sequenceMain sequence stars are the central band of stars on the H‑R Diagram. They are usually young stars. Their energy comes from nuclear fusion (conversion of hydrogen into helium). About 90% stars belong to the Main Sequence Stars.

The Sun is a G2V type star – a yellow dwarf.

Dwarf stars

Dwarf stars are relatively small stars. Their size can be up to 20 times larger than the Sun’s size and luminosityluminosityluminosity up to 20000 times higher.

Yellow dwarfs are small stars of spectral type G, a weight between 0,7 and 1 times the solar mass, and a surface temperature of about 6000°C. They are bright yellow or almost white. Yellow dwarfs are about 10% of stars in the Milky Way.

Red dwarfs are small, cool, very faint, main sequencemain sequencemain sequence stars. Their surface temperature is under 4000 K. Red dwarfs are the most common type of star.

Giant and supergiant stars

To the giantgiantgiant and supergiant stars belong mainly the old large stars.

Red giants

• A red giant is an old star with a diameter about 100 times larger than it was at its beginning. Its surface temperature is under 6500 K.

Blue giant

• A blue giant is a huge, very hot, blue star.

Supergiant

• Supergiants are the largest known stars. Some of them are as big as our entire Solar System. They have extreme masses and hence relatively short lifetime of only 10 to 50 million years.

Dead stars

White dwarfs

• White dwarfs are small, very dense, hot stars. They are remnants of red giantgiantgiant stars. Their size is comparable with the Earth’s size, but these stars are much denser.

Neutron star

• A neutron starneutron starneutron star: is a very small, very dense star. It is composed mostly of neutrons and has a thin atmosphere consisting of hydrogen. It has a diameter of about 5‑15 km.

Pulsar

• A pulsarpulsarpulsar is a rapidly spinning neutron star. Its radiation can be observed only when the beam of emission is pointing toward the Earth.

Black holes

A black holeblack holeblack hole is a part of space where a great amount of matter is packed into a very small area. The gravitational field of black holes is so strong that nothing, no particles and no electromagnetic radiation can escape from inside it.

Evolution of stars

The radiation emitted by stars is a result of thermonuclear reactions taking place deep in their cores. In these reactions light elements are converted into heavier and enormous energy is released. There is enough pressure due to energy flow from the core to the outer parts of the star to keep it from collapsing under its weight. When nuclear reactions slow down due to the lack of elements a star starts to collapse. The dying star expands in the giantgiantgiant or supergiant phase. The star will eventually explode and become a planetary nebula or supernova. Finally it turns into a white dwarf stardwarf stardwarf star, neutron starneutron starneutron star or is getting a black holeblack holeblack hole. The final state of a star depends of its initial mass.

Remember

• The star classification is based on stars spectra.

• The relationship between the average surface temperature of stars and their absolute magnitude is presented by the Hertzsprung‑Russell diagram.

• All stars undergo an evolution.

Exercises

Glossary

Keywords

dwarf stardwarf stardwarf star

giantgiantgiant

luminosityluminosityluminosity

main sequencemain sequencemain sequence

wavelengthwavelengthwavelength