The Life of a StarThe Life of a Star

The View From EarthThe View From Earth If you have ever spent If you have ever spent

time looking up at the sky time looking up at the sky on a clear night, you have on a clear night, you have probably noticed that probably noticed that some stars look as though some stars look as though they are grouped together they are grouped together into a distinct pattern. into a distinct pattern.

Perhaps the best-known Perhaps the best-known star pattern in the star pattern in the northern hemisphere is northern hemisphere is the Big Dipper the Big Dipper

Different cultures around Different cultures around the world refer to this the world refer to this collection of seven stars collection of seven stars by other names, such as by other names, such as the Plough, the Ladle, and the Plough, the Ladle, and the Great Cart.the Great Cart.

ConstellationsConstellations

The Big Dipper is part of a The Big Dipper is part of a larger star pattern known as larger star pattern known as Ursa Major, which is Latin for Ursa Major, which is Latin for Great Bear. Ursa Major is an Great Bear. Ursa Major is an example of a constellation. example of a constellation.

A A constellation constellation is a group of is a group of stars that, from Earth, stars that, from Earth, resembles a recognizable resembles a recognizable form. form.

Astronomers have officially Astronomers have officially listed a total of 88 listed a total of 88 constellations. constellations.

Examples, along with Ursa Examples, along with Ursa Major, include Cassiopeia, Major, include Cassiopeia, Orion, Pegasus, Sagittarius, Orion, Pegasus, Sagittarius, and Ursa Minor.and Ursa Minor.

Ursa MajorUrsa Major

CassiopeiaCassiopeia

OrionOrion

PegasusPegasus

SagittariusSagittarius

Ursa MinorUrsa Minor

Is the Big Dipper a Constellation?Is the Big Dipper a Constellation?

Smaller recognizable Smaller recognizable star patterns within a star patterns within a larger constellation larger constellation are known as are known as asterismsasterisms. .

The Big Dipper is an The Big Dipper is an asterism. asterism.

Other asterisms Other asterisms include: Orion’s belt include: Orion’s belt and the great squareand the great square

A Star is BornA Star is Born

All stars form inside a All stars form inside a collapsing nebula, a collapsing nebula, a cloud of dust and cloud of dust and gases. gases.

A nebula’s collapse A nebula’s collapse can be triggered by a can be triggered by a disturbance such as disturbance such as the gravitational the gravitational attraction of a nearby attraction of a nearby star or the shockwave star or the shockwave from an exploding from an exploding star.star. Crab

Nebula

Other NebulaeOther Nebulae

Eagle Nebula

Mycn18 Nebula

Other NebulaeOther Nebulae

Bug Nebula Horsehead Nebula

The Birth of the StarThe Birth of the Star Inside a collapsing nebula, the region with the

greatest amount of matter will start to draw material towards it through gravity. This is where the star will form.

Material falling inward to the core has excess energy. This energy causes the central ball of material to begin to spin.

Extremely high pressures build up inside the ball, which in turn causes the tightly packed atoms to heat up.

As the temperature climbs, the core begins to glow. This is a protostar, a star in its first stage of formation.

Star EvolutionStar Evolution How a star evolves in its

lifetime depends on the mass it had when it originally formed.

Astronomers describe stars in three general mass categories: low, medium, and high.

A low mass star, for example, advances through different phases than a high mass star does.

Stellar Evolution VideoStellar Evolution Video

http://videos.howstuffworks.com/nashttp://videos.howstuffworks.com/nasa/13499-chandra-stellar-evolution-via/13499-chandra-stellar-evolution-video.htmdeo.htm

Low Mass StarsLow Mass Stars Low mass stars use their nuclear fuel much

more slowly than more massive stars do. Low mass stars burn so slowly that they can

last for 100 billion years — more than eight times the current life span of the universe.

With less gravity and lower pressures than other stars, the nuclear reactions in the core of low mass stars happen at a relatively slow rate. The stars therefore exist a long time, shining weakly as small red stars called red dwarfs.

Medium Mass StarsMedium Mass Stars Medium mass stars burn their fuel faster than low

mass stars do, using their hydrogen up in about 10 billion years.

The Sun is a medium mass dwarf star. At the end of this long, stable period, the hydrogen

fuel in a medium mass star begins to run out and the star slowly collapses under its own gravity.

This process of collapsing raises the temperature and pressure again inside the star.

This is enough to start the fusion of helium, which has been accumulating in the core.

The star reignites. As the core heats up this time, the star expands rapidly into a red giant.

Eventually, even the helium fuel burns out and the star collapses again and slowly burns out.

High Mass StarsHigh Mass Stars High mass stars are those that are more than 10 times the

mass of the Sun. In a high mass star, as gravity pulls matter into the centre of

the star and squeezes the core, the nuclear reactions accelerate.

As a result, a high mass star is hotter, brighter, and bluer than other stars.

High mass stars always come to a violent end. After using up its hydrogen fuel, typically in less than about 7 billion years, such a star collapses just like a low or medium mass star does.

The heating and compression cause helium to begin to fuse. During this process, tremendously high temperatures result, causing the star to expand into a supergiant.

When the helium fuel runs out, the core again collapses into itself. The star continues to go through many cycles of collapse and expansion, as new elements, including iron, are formed in its core.

SupernovaSupernova So fast and intense is the

collapse of a star that the core of the star heats up to many hundreds of millions of degrees and explodes.

An exploding star is called a supernova.

The explosion releases enough energy to cause various elements to fuse in various combinations.

In this way, all the elements of the periodic table have been formed.

The blast sends these heavy elements far out into space. Some of the debris and elements from the old star create new nebulae out of which new star-and-planet systems may begin to form.

Neutron StarsNeutron Stars If the star was between 10

and 40 times the mass of the Sun, it will become a neutron star.

A supernova explosion is directed not only outward, but also inward.

This force causes the atoms in the star’s core to compress and collapse. When an atom collapses, it forms neutrons, particles that are at the centre of most atoms already.

When the star’s core becomes little more than a ball of neutrons only about 15 km across, it is called a neutron star.

Neutron stars are made of the densest material known.

Black HolesBlack Holes If the star was more

than 40 times the mass of the Sun, it will become a black hole.

After exploding as a supernova, the star’s core is under so much gravitational force that nothing can stop its collapse, not even the formation of neutrons.

In this case, the effect of gravity is so great that space, time, light, and other matter all start to fall into a single point.

The The HertzspruHertzsprung-Russell ng-Russell DiagramDiagram

The Hertzsprung-Russell The Hertzsprung-Russell DiagramDiagram

The stars are arranged as follows:

by colour – Red stars are plotted on the right, and blue stars are plotted on the left. Other stars, such as the yellow Sun, are plotted in between.

by luminosity – The brightest stars are plotted at the top, and the dimmest stars are plotted at the bottom. A star with a luminosity of 100 is 100 times brighter than the Sun.

by surface temperature – The hottest stars are plotted on the left, and the coolest stars are plotted on the right.

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