Astronomy: Lives of Stars

In this third installment of the Astronomy Series we�ll cover stars from their births to their deaths. Before we delve into that though, I�d like to explain the Hertzsprung-Russel (HR) Diagram.

The HR Diagram


From the HR Diagram nearly everything about a star can be understood. The diagram is broken into four main sections. There is the Main Sequence, where most stars reside, which curves from the upper left to the lower right. There are two giant sections; one for giants and one for red giants. Finally there is the white dwarf section, which is in the lower left. Blue giant stars, the upper left of the main sequence line, are very hot and very bright. Red dwarf stars, the lower right of the main sequence line, are very dim ad very cool. White dwarfs, the lower left of the diagram, are very hot but also very dim. Red giants and supergiants, the upper right of the diagram, are very cool but also very bright. The Sun lies in a G spectral class, with a luminosity of 1.

Birth of a Star
A star�s birth begins in a giant molecular cloud, which is the interstellar medium that contains concentrations of dust and gas. These clouds, called nebulae, are revealed by telescopes, infrared imaging, and the usage of radio telescopes.

Nebula is Greek for the word �cloud.� There are two types of nebula, diffuse and emission. Stars within and behind the nebula excite the molecular particles, which causes the nebula to glow from the inside. Inside some nebulae are �Bole Globules,� or dark concentrations within the nebulae that give rise to protostars.

A protostar is an early stellar form collapsing under gravity. An example of this is a T-Tauri star, which is a later protostar stage. In this stage the star has begun to evolve towards the main sequence. For a blue giant star this process takes about 10,000 years; for star about three times as massive as the Sun this takes 100,000 years; for a Sun-like star 30 million years; for a red dwarf 100 million years.

Zero Age Main Sequence (ZAMS)
The process of fusing hydrogen is called chemosynthesis. The hydrogen is fused into helium. Heavier elements are created when stars begin to die or in supernova blasts.

Once a star is on the Main Sequence it will burn its hydrogen at different rates. For Sun-like stars it will do so for about 10 billion years; for red dwarfs 200 billion years; for three Solar-mass stars 500-million years; for blue giant years 50 million years. After these times the stars hit their �turn-off� points, when they leave the main sequence as the fusion of hydrogen in the core has ended.

As stars leave the main sequence they become variable stars, or a star that varies in brightness. This is because as the star attempts to fuse heavier elements it has to expand and contract.

Stellar Death
The mass of a star is what determines its fate; the more massive the star, the more spectacular its death. New, heavier chemical elements are created in the deaths of massive stars. As a heavier star dies, the blast throws the heavier elements into interstellar space and eventually forms new stars and solar systems.

As a star dies it begins to fuse heavier and heavier elements in this order:

Elements higher than Fe are created in a Supernova blast.

Low Mass Stars
A low mass star is to be defined as a star from .5 Solar masses to 1.4 Solar masses.

When one of these stars begins to die the leave the main sequence to form a red giant star. This happens as the star begins to fuse helium into carbon. Though the star�s density drastically decreases, the pressures of the expanding star cause the core temperature to swell to over 100 million Kelvin. The star�s size can reach to the orbit of Mars, about 314 million miles in diameter.

Eventually the inner pressures of the star become to great and the star ejects about half of its mass into space, forming a planetary nebula. This phenomena is thought to last about 50,000 years.

What�s left behind from when the planetary nebula is formed is a white dwarf star. It is a small, dense stellar corpse supported by degenerate matter. These are about the size of the Earth.

Medium Mass Stars
A medium mass star is to be defined as a star from 1.4 to 3.0 Solar masses.

Like a low mass star, when one of these stars begins to die the leave the main sequence to form red super giants. These stars are typically variables due to their highly unstable interiors.

These stars though while expanding and contracting eventually undergo a sudden massive collapse. This collapse is too much to allow the star to expand again and a huge detonation called a supernova is created. During this explosion the star will outshine any other in the sky. Also, due to the extreme temperatures and pressures the heavier elements are created as well as a neutron star. The neutron star forms as electrons and protons are fused together.

The neutron star contains all the mass of the Sun compressed to the size of a city, or about 10 miles in diameter. They are extremely dense, and have a very rapid rotation due to the conservation of angular momentum. The neutron star sweeps out immense amounts of radiation from its poles, and when these beams hit Earth the star is called a pulsar.

High Mass Stars
A high mass star is to be defined as a star with a mass greater than 3 Solar masses.

Like a medium mass star, these stars end their lives in a supernova, though they do not form neutron stars. Instead they form blackholes, which is a massive, compact object whose gravity is so great that not even light can escape. They can only be observed by their influence on surrounding matter.