LIFE OF A STAR

By Andy Hu, Mitchell Dawes, Jeremy Zhang and Eamon Roche

NebulaeOur galaxy is filled with these irregular shaped clouds called “nebulae” which form stars. There are nebulas with animal shaped things like the horsehead nebula and the eagle nebula nebulas are cold as -263 degrees Celsius, which scientists think is 10 degrees from absolute zero! These nebulas are cold and thin but they have the perfect ingredients for giving birth to a star. The death of a light-weight star can form a nebula. The perfect example is the Helix nebula , which now is being illuminated by the dying star inside it.

Name of the nebula

How far away Diameter

Eagle nebula 7000 light-years 315 light-years

NGC 1999 1500 light-years 1 light-year

Horsehead nebula

1300 light-years 4 light-years

Helix nebula 680 light-years 5 light-years

How a star is bornA star’s life begins when clumps of gas and dust come together from it’s own gravity. Gravity pushes these clumps together to make more pressure. Small amounts of gas and dust fizzle out. If the clump is big enough, the centre of the forming star boosts over 10 million degrees Celsius. A nuclear reaction begins as hydrogen atoms fuse together to make helium. The heat makes the star shine and that star’s life begins in the vast universe.

Cycle of a Star

Life of a starThere are different types of stars like blue ones which don’t live long and middle-aged stars like our Sun or big dying stars called supergiants. For example: Betelgeuse and Antares, which is 700 times bigger than our sun and there could be a star in the constellation of Auriga that is 3 billion km across(4000 times bigger than the Sun!) . While a star is burning, first, it burns the hydrogen for about 10 billion years , than burns the helium and any other flamable things like dust.

A Star

DEATH OF A STAR The pressure at the centre of a star becomes so enormous in the red giant stage

that silicon and carbon fuse together to make iron. Once iron is formed, the star fails to give off energy and drastically and catastrophically collapses into an explosion. An example of this is Eta Carinae. In 1841, Eta Carinae suffered a violent outburst after suddenly collapsing , and blew 2 giant clouds of dust that have been expanding since. The two clouds just became twice as bright as before and it will explode as a supernova in the next few thousand years. After collapsing as a supergiant, it will either become a neutron star ( a star that is dying and is sending out intense radio signals), a white dwarf or a black hole.

The death

Eta carinae

A supernova is an explosion that is more energetic than a nova. Supernovae are extremely bright and cause a burst of radiation that often briefly outshines a galaxy, before fading from view. During this short time a supernova can radiate as much energy as the Sun is expected to emit over its entire life span.

The Big Bang produced hydrogen, helium, and traces of lithium, while all heavier elements are made in stars and supernovae. Supernovae tend to enrich the surrounding space with elements other than hydrogen and helium.

Each stellar generation has a slightly different composition, going from an almost pure mixture of hydrogen and helium to a more metal-rich composition. The different abundances of elements in the material that forms a star affect the star's life, and may affect the possibility of having planets orbiting it.

Black holesBlack holes are probably the most puzzling objects in astronomy. A black hole’s gravity is so intense that it bends light! Black holes are infinitely small and have infinite space. There are supermassive black holes which lies at the centre of most galaxies and there are small ones which suck in nearby stars and just gets sucked in by a supermassive one. Black holes have a ring of shredded up nebulas and stars called accretion.

RED GIANTSA red giant is a luminous giant star in a late phase of evolution, with between half and 10 times the mass of our sun. They have a large radius (up to hundreds of times larger than our sun) and their surface temperature is 5,000° Kelvin and lower. They are yellow-orange to red in colour.

Red giants have burnt up all the hydrogen fuel in their cores. When this happens, nuclear reactions stop, the core begins to get smaller, and a shell outside the core heats up and starts fusing hydrogen to helium. The higher temperatures make enough energy to increase the star's brightness by a factor of 1,000–10,000. The outer layers then expand, beginning the red giant phase.

The Sun is predicted to become a red giant. As its radius expands it will become large enough to swallow the inner planets, up to Earth. But don’t panic, because this won’t happen for about 4.5 billion years.

WHITE DWARFS

Composition of a white dwarf

A white dwarf is a small star composed mostly of electron-degenerate matter.

 

If a red giant has not enough mass to generate the core temperatures required to fuse carbon, a mass of carbon and oxygen will build up at its center. After shedding its outer layers to form a planetary nebula, it will leave behind its core, which forms the white dwarf.The visible radiation emitted by white dwarfs varies over a wide range, from an O-type to a M-type. White dwarf surface temperatures can be over 150,000 K to under 4,000 K. A white dwarf is very hot when it is formed but it has no source of energy so it will gradually radiate away and cool down. Over a very long time, a white dwarf will cool to temperatures at which it won’t be visible, and become a black dwarf. But since no white dwarf can be older than the age of the Universe, no black dwarfs exist yet.

NEUTRON STARS If the collapsing core at the centre of a supernova contains between 1.4 and 3

solar masses, the collapse continues until electrons and protons combine to form neutrons, producing a neutron star. Because it contains so much mass packed into a small volume, the gravity at the surface of a neutron star is very big. If a neutron star forms in a multiple star system it can collect gas by stripping it off nearby companions. Neutron stars have powerful magnetic fields which accelerate particles around its poles producing powerful beams of radiation. These beams sweep around as the star rotates. If the beam is oriented so that it points toward the Earth, we observe it as pulses of radiation that happen whenever the pole of the neutron star sweeps past the line of sight. In this case, the neutron star is known as a pulsar.

HERTSPRUNG-RUSSELL DIAGRAM

Hertzprung-Russell diagrams plot each star on a graph measuring the star's absolute magnitude or brightness against its temperature and colour.

Stars tend to fall only into certain regions on the diagram.

The most predominant is the diagonal, going from the upper-left (hot and bright) to the lower-right (cooler and less bright), called the main sequence.

White dwarfs are found in the lower-left, and the subgiants, giants and supergiants are above the main sequence.

The Sun is found on the main sequence at luminosity 1 (absolute magnitude 4.8).