A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity.
This is the general definition of star that we observer in sky but this white looking twinkling dot are one of the most complex and important elements of Universe. It is predicted that...
in universe on an average of 100 billion stars per galaxy means that there are about 1,000,000,000,000,000,000,000 (that's 1 billion trillion) stars in the observable universe!
I know numbers are bit too big but so as universe. But when it comes to these absorbed amount of stars it is more than necessity for us to categories these in way that almost include all of the stars that we observer till now. And after observing night sky for hundreds of years astronomers came up with 2 major factor which is common in each and every star.
And these two element are
Spectra (the elements that they absorb) and temperature of the star.
And if we are talking about classification of stars, how can we forget the famous
Hertzsprung -Russell (H-R) Diagram.
Hertzsprung-Russel Diagram
The Hertzsprung -Russell (H-R) Diagram is a graph that shows stars color (spectral type or surface temperature) vs. its luminosity (intrinsic brightness).The diagram is one of the most easiest way to learn classification because on it, astronomers plot stars' color, temperature, luminosity, spectral type, and evolutionary stage.
But before we learn more about stars , let’s take quick look at luminosity
Luminosity
Is the total amount of energy that a star radiates each second (including all wavelengths of electromagnetic radiation).
So astronomers need to divide it into further category which is called The Yerkes Luminosity Classes.(which we see in second image ). In the Yerkes classification scheme, stars are assigned to groups according to the width of their spectral lines.
For a group of stars with the same temperature, the luminosity class differentiates between their sizes - super giants, giants, main-sequence stars, and sub dwarfs.
Now let’s learn about those main 3 types of star in Details.
Giant and Supergiant Stars
A giant star is a star with substantially larger radius and luminosity than a main-sequence star of the same surface temperature. They lie above the main sequence on the Hertzsprung–Russell diagram and correspond to luminosity classes II and II. And Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars spans from about 3,400 K to over 20,000 K
BLUE GIANT
- a blue giant is a hot star with a luminosity class of III (giant) or II (bright giant).
- They have temperatures from around 10,000 K upwards, zero age main sequence (ZAMS) masses greater than about twice the Sun (M☉), and absolute magnitudes around 0 or brighter. These stars are only 5–10 times the radius of the Sun (R☉), compared to red giants which are up to 100 R☉.
RED GIANT
- A red giant is a relatively old star whose diameter is about 100 times bigger than it was originally, and had become cooler (the surface temperature is under 6,500 K).
- Giant and Supergiant about 14,000 times brighter than the Sun,
- They are frequently orange in color. Examples of well-known stars in the RG phase are Aldebaran (Alpha Tauri) and Mira (Omicron Ceti)
SUPERGIANT
- A supergiant is the largest known type of star; some are almost as large as our entire solar system.
- The temperature range of supergiant stars spans from about 3,400 K to over 20,000 K.
- Betelgeuse (Red Super Gaint) and Rigel(blue super giant) are supergiant. These stars are rare.
- When supergiants die they supernova and become black holes.
Faint, Virtually Dead Stars
A degenerate dwarf stars, which is generally a stellar core remnant composed mostly of electron-degenerate matter.How such a state of matter could exist in these type od stars was beyond the understanding of physicists until a new theory of matter, quantum mechanics, was developed in the 1920s. This theory showed that matter can exist in so-called "degenerate" states of extremely high density. Even though the degenerate state of matter cannot be produced in a terrestrial laboratory, it is very common in the universe at large.
WHITE DWARF
- A white dwarf is a small, very dense, hot star that is made mostly of carbon.It is much hotter (25,000 K), but because of its small size its luminosity is low.
- These faint stars are what remains after a red giant star loses its outer layers. Their nuclear cores are depleted.They are about the size of the Earth (but tremendously heavier)! They will eventually lose their heat and become a cold, dark black dwarf.
- Our sun will someday turn into a white dwarf and then a black dwarf. The companion of Sirius is a white dwarf.
BROWN DWARF
- A brown dwarf is a "star" whose mass is too small to have nuclear fusion occur at its core (the temperature and pressure at its core are insufficient for fusion).
- A brown dwarf is not very luminous. It is usually regarded as having a mass between 1028 kg and 84 x 1028.
- Unlike the sun, they have no internal energy source and emit almost no visible light. Brown dwarfs are formed along with stars by the contraction of gases and dust in the interstellar medium,
NEUTRON STAR
- A neutron star is a very small, super-dense star which is composed mostly of tightly-packed neutrons
- It has a thin atmosphere of hydrogen. It has a diameter of about 5-10 miles (5-16 km) and a density of roughly 10 15 gm/cm3.
- A pulsar is a rapidly spinning neutron star that emits energy in pulses.
- Well known example of this type of star is Crab Pulsar. The Crab Pulsar is a relatively young neutron star. The star is the central star in the Crab Nebula, a remnant of the supernova SN 1054.
There is always an exception when we try to classify something and when it comes to astronomy there are two exception types which is turns out to be a very common types. I will upload different blog article next week for those 2 types. Stay Tune .....