Clearly, stars have been an observed feature of the night sky ever since the dawn of human civilisation. Of course, with the advances in modern technology and the deeper understanding of the physical process within a star, i.e. fusion, cosmologists have been able to piece together a comprehensive model of the types of stars that must exist throughout the universe, as a whole. This, in-turn, has provided the means by which cosmologists have been able to speculate about larger scaled structures within the universe. The following diagram is known as the Hertzsprung-Russell diagram and provides an indication of the 'spectrum' of star types, which range in luminosity, temperature and size.
The vertical scale of the diagram above shows the relative luminosity or brightness with respect to our own sun and its absolute surface temperature in degrees Kelvin. Along the top is an indication of the spectral class of the stars, which is then described in a little more detail in the table below.
|O||Blue||+25,000 K||60||15||1,400,000||Singly ionized helium lines (H I) either in emission or absorption. Strong UV continuum.|
|18||7||20,000||Neutral helium lines (H II) in absorption.|
|3.2||2.5||80||Hydrogen (H) lines strongest for A0 stars, decreasing for other A's.|
|1.7||1.3||6||Ca II absorption. Metallic lines
|1.1||1.1||1.2||Absorption lines of neutral metallic atoms and ions, e.g. once-ionized calcium.|
|0.8||0.9||0.4||Metallic lines, some blue continuum.|
|M||Red||<3,500 K||0.3||0.4||0.04||(very faint) Some molecular bands of titanium oxide.|
There are many distinct types of stars, which result from the original mass accreted by the star on formation within a stellar cloud. This is linked to the rate of fusion and the subsequent temperatures and pressures generated as a function of the original mass. For example, a star of 1 solar mass equates to the size of our sun and is typically expected to have a lifetime of some 10-12 billion years. Normally, the mass of a star will range between 0.1 to 100 solar masses, but a mass of less than 0.1 will not have enough gravitational pressure to trigger fusion and are called brown dwarves. In contrast, massive stars burnt so fierce that their lifetime is a fraction of a normal star. They can also end their lives in a spectacular explosion called a supernova in which the heavy elements are formed. Without these heavy elements, life as we know it could not exist.
Dwarf stars are relatively small stars, but can range up to 20 times larger than our sun and up to 20,000 times brighter. Our sun is a dwarf star.
- Brown Dwarf
Brown dwarfs are sub-stellar objects, of between 5 to 90 Jupiter masses, which do not fuse hydrogen into helium and heavier chemical elements in their cores, as per stars on the main sequence, but have fully convective surfaces and interiors, with no chemical differentiation by depth. There is some question as to whether brown dwarfs are required to have experienced fusion at some point in their history; in any event, brown dwarfs heavier than 13 Jupiter masses (MJ) do fuse deuterium. As a consequence, a brown dwarf is not very luminous.
- Red Dwarf
Red dwarfs comprise the vast majority of stars. They have a mass of less than one-third that of the Sun and a surface temperature of less than 3,500 K. They emit little light, as little as 1/10,000th that of the sun. Due to the slow burn rate of hydrogen, red dwarfs have an enormous lifespan, e.g. billions to trillions of years. As such, there has not been sufficient time since the Big Bang for red dwarfs to evolve off the main sequence. Red dwarfs never initiate helium fusion and so cannot become red giants. The stars slowly contract and heat up until all the hydrogen is consumed. Red dwarfs are the most common type of star. The nearest star Proxima Centauri is a red dwarf. The fact that red dwarfs remain on the main sequence while older stars have moved off the main sequence allows one to date star clusters by finding the mass at which the stars turn off the main sequence. In addition, the fact that no red dwarfs have evolved off the main sequence has been observed as evidence that the universe has a finite age. However, there is another fact related to the lack of red dwarf stars with no metals. The Big Bang model predicts the first generation of stars should have only hydrogen, helium and lithium, and given their lifespan, 1st generation red dwarfs should still be observable today, but are not. Either there is a problem with the big bang theory or red dwarves cannot form without heavier metals. This position requires that the 1st generation of stars were extremely high mass, which died quickly and produced the metals necessary for low mass stars to form subsequently.
- Yellow Dwarf
In astronomy, a yellow dwarf is a small star, ~1-1.4 solar masses, which is in its main sequence, i.e. in the process of converting hydrogen to helium in its core by means of nuclear fusion. Our Sun is an example of a yellow dwarf. A yellow dwarf's lifespan is about 10 billion years, when its supply of hydrogen runs out, the star expands to many times its previous size and becomes a red giant. The star Betelgeuse is an example of a red giant. Eventually the red giant sheds its outer layers of gas, which become a planetary nebula, while the core collapses into a small, dense white dwarf.
- White Dwarf
Almost all small and medium-size stars will end up as white dwarfs, after all the hydrogen they contain is fused into helium. Near the end of its nuclear burning stage, such a star goes through a red giant phase and then expels most of its outer material until only the hot (T>100,000K) core remains, which then settles down to become a young white dwarf which shines by virtue of its residual heat. A typical white dwarf has half the mass of the Sun, yet is only slightly bigger than Earth. This makes white dwarfs one of the densest forms of matter (109 kg/m3), surpassed only by neutron stars and black holes. The higher the mass of the white dwarf, the smaller the size and so there is an upper limit to the mass of a white dwarf, which is called the `Chandrasekhar limit` of about 1.4 solar masses. When this limit is exceeded, the pressure exerted by electrons is no longer able to balance the force of gravity, and the star continues to contract, eventually forming a neutron star.
Typically, giant stars are rather old, large stars:
- Red Giant:
A red giant is a relatively old star whose diameter is about 100 times bigger than its original size. Its surface temperature has cool below 6,500K and as a result are frequently orange/red in colour. Betelgeuse is a red giant that is about 20 times as massive as the Sun and about 14,000 times brighter. It is approximately 600 light-years from Earth.
- Blue Giant:
A blue giant is a huge, very hot, blue star. It is a post-main sequence star that burns helium.
- Super Giant :
A super-giant is the largest known type of star; some are almost as large as our entire solar system. Betelgeuse and Rigel are super-giants. These stars are rare and when they die they can become supernova and may subsequently become black holes.
- Neutron Star:
A neutron star is a very small, super-dense star, which is assumed to be 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 1015 gm/cm3.
A pulsar is a rapidly spinning neutron star that emits energy in pulses.
- Black Hole:
A black hole was a star massive enough to eventually collapse under its own gravity to a point where the escape velocity exceeds the speed of light. The boundary at which the escape velocity exceeds the speed of light is called the event horizon; behind which a much smaller theoretical core exist. The term theoretical is used because the laws of physics behind the event horizon are still essentially hypothetical.