As the outer layers expand from the helium core, the star becomes a red giant
The helium at the centre of the star continues to increase until a helium core is formed. A Nuclear reaction then begins to spread outward. As the helium core grows heavier, the core's temperature also increases, and the outer layers begin to expand until the star becomes a massive red star known as a red giant.
In the case of a star that is about the size of our Sun, the gases of the outer layer are expelled, and then contract, so that the star becomes what it known as a white dwarf. However, if a star has a much greater mass than our Sun, the final stages of its giant star phase end in a supernova explosion. The giant star phase is about one tenth as long as the main sequence star phase.
Some stars collapse under their own weight, causing supernova explosions
When a star's mass is about three times that of our Sun, after the red giant phase has finished, it begins to collapse under its own weight, causing a supernova explosion that scatters it through space.
Its brightness at this point will be 100 billion times that of the Sun. When this happens, it looks as if a bright new star has appeared in the night sky. Supernova explosions of some exceptionally massive stars leave in their wake neutron stars, called pulsars, and black holes. Sometimes the scattered clouds of gas and particles become the stuff of new stars.
Our Star The Sun.
Scientists think that the core of the Sun is 15 million degree Celsius plasma, a soup of electrons and protons that are stripped from hydrogen atoms. This "soup," called plasma, makes up 90 percent of the Sun. Every second, thousands of protons in the Sun's core collides with other protons to produce helium nuclei in a nuclear fusion reaction that releases energy. Just outside the core, energy moves outward by a process called radiation.
Closer to the surface, the energy moves out by a process called convection - hot gases rise, cool, and sink back down again. As these masses of gas move, they push off of each other causing "Sun-quakes." These make the material in the Sun vibrate. These Sun-quakes help scientists determine the Sun's internal structure and the processes occurring at different locations underneath the Sun's surface.
NASA Photo of the Sun taken by Skylab in 1973. From the Astronomy Picture of the Day Archives,
http://antwrp.gsfc.nasa.gov/apod/ap960916.html
This drawing shows the major features of the Sun. The Sun actually consists of 90% hydrogen and a mixture of other gases.
In diameter, it is over 100 times bigger than the Earth.
What is a Star?
Stars are hot bodies of glowing gas that start their life in Nebulae. They vary in size, mass and temperature, diameters ranging from 450x smaller to over 1000x larger than that of the Sun. Masses range from a twentieth to over 50 solar masses and surface temperature can range from 3,000 degrees Celsius to over 50,000 degrees Celsius.
Its temperature determines the colour of a star, the hottest stars are blue and the coolest stars are red. The Sun has a surface temperature of 5,500 degrees Celsius, its colour appears yellow.
The energy produced by the star is by nuclear fusion in the stars core. The brightness is measured in magnitude, the brighter the star the lower the magnitude goes down.
There are two ways to measuring the brightness of a star, apparent magnitude is the brightness seen from Earth, and absolute magnitude, which is the brightness of a star seen from a standard distance of 10 parsecs (32.6 light years). Stars can be plotted on a graph using the Hertzsprung Russell Diagram (see picture on next page).
Hertzsrung Russell Diagram
It shows that the temperature coincides with the luminosity, the hotter the star the higher the luminosity the star has. You can also tell the size of each star from the graph as the higher the radius the higher the temperature and luminosity.
Star Formation:
Information and images for star formation from
http://zebu.uoregon.edu/~js/ast122/lectures/lec09.html
Stars form inside fairly dense concentrations of interstellar gas and dust known as molecular clouds. These regions are extremely cold (temperature about 10 to 20K, just above absolute zero).
At these temperatures, gases become molecular, the group together. Normally, molecules can’t form in space because photons of starlight will break them apart. But in the centre of dark clouds, molecules like CO and H2 (the most common molecules in interstellar gas clouds) can form. The deep cold also causes the gas to clump to high densities. When the density reaches a certain point, stars form.
Since the regions are dense, they are opaque to visible light and are known as Dark Nebula. Since they don't shine by optical light, we must use Infrared and radio telescopes to investigate them.
Star formation begins when the denser parts of the cloud core collapse under their own weight/gravity. These cores typically have masses around 104 solar masses in the form of gas and dust. The cores are denser than the outer cloud, so they collapse first. As the cores collapse they fragment into clumps around 0.1 parsecs in size and 10 to 50 solar masses in mass. These clumps then form into protostar and the whole process takes about 10 millions years.
Proto star formation:
Once a clump has broken free from the other parts of the cloud core, it has its own unique gravity and identity and we call it a protostar. As the protostar forms, loose gas falls into its centre. The in falling gas releases kinetic energy in the form of heat and the temperature and pressure in the centre of the protostar goes up. As its temperature approaches thousands of degrees, it becomes an IR source.
During the initial collapse, the clump is transparent to radiation and the collapse proceeds fairly quickly. As the clump becomes denser, it becomes opaque. Escaping Infrared radiation is trapped, and the temperature and pressure in the centre begin to increase. At some point, the pressure stops the in fall of more gas into the core and the object becomes stable as a protostar.
The protostar, at first, only has about 1% of its final mass. But the covering of the star continues to grow as in falling material is accreted. After a few million years, thermonuclear fusion begins in its core, and then a strong stellar wind is formed which stops the in fall of new mass. The protostar is now considered a young star since its mass is fixed, and its future development is now set.
T-Tauri Stars:
Once a protostar has become a hydrogen-burning star, a strong stellar wind forms, usually along the axis of rotation. As a result, many young stars have a bipolar outflow, a flow of gas out the poles of the star. This is a feature, which is easily seen by radio telescopes. This early phase in the life of a star is called the T-Tauri phase.
One result of this collapse is that massive, opaque; circumstellar disks usually surround young T Tauri stars gradually accrete onto the stellar surface, and so radiate energy both from the disk (infrared wavelengths), and from the position where material falls onto the star at (optical and ultraviolet wavelengths).
Somehow a fraction of the material accreted onto the star is cast out vertical to the disk plane in a highly collimated stellar jet. The circumstellar disk eventually dissipates, probably when planets begin to form. Young stars also have dark spots on their surfaces, which are similar to sunspots but cover, a much larger part of the surface area of the star.
The T-Tauri phase is when a star has:
- Vigorous surface activity (flares, eruptions)
- Strong stellar winds
- Variable and irregular light curves
A star in the T-Tauri phase can lose up to 50% of its mass before settling down as a main sequence star, thus we call them pre-main sequence stars.
Their location on the HR diagram is shown below:
The arrows indicate how the T-Tauri stars will develop onto the main sequence. They begin their lives as slightly cool stars, then heat up and become bluer and slightly fainter, depending on their initial mass. Very massive young stars are born so rapidly that they just appear on the main sequence with such a short T-Tauri phase that they are never observed.
T-Tauri stars are always found implanted in the clouds of gas from which they were born.
One example is the Trapezium cluster of stars in the Orion Nebula.
The evolution of young stars is from a cluster of protostar deep in a molecular clouds core, to a cluster of T-Tauri stars whose hot surface and stellar winds heat the surrounding gas to form an HII region (HII, pronounced H-two, means ionised hydrogen). Later the cluster breaks out, the gas is blown away, and the stars evolve as shown on the next page.
Often in galaxies we find groups of young stars near other young stars. This occurrence is called supernova induced star formation. The very massive stars form first and explode into supernovas. This creates shock waves into the molecular cloud, causing nearby gas to squeeze and form more stars. This allows a type of stellar coherence (young stars are found near other young stars) to build up, and is responsible for the pinwheel patterns we see in galaxies.
Brown Dwarfs:
If a protostar forms with a mass less than 0.08 solar masses, its internal temperature never reaches a value high enough for thermonuclear fusion to begin. This failed star is called a brown dwarf, halfway between a planet (like Jupiter) and a star. A star shines because of the thermonuclear reactions in its core, which release enormous amounts of energy by fusing hydrogen into helium.
For the fusion reactions to occur, though, the temperature in the star's core must reach at least three million Kelvin’s and because core temperature rises with gravitational pressure, the star must have a minimum mass: about 75 times the mass of the planet Jupiter, or about 8 percent of the mass of our sun. A brown dwarf just misses that mark-it is heavier than a gas-giant planet but not quite massive enough to be a star.
For decades, brown dwarfs were the "missing link" of celestial bodies: thought to exist but never observed. In 1963 University of Virginia astronomer Shiv Kumar theorized that the same process of gravitational contraction that creates stars from vast clouds of gas and dust would also frequently produce smaller objects. These hypothesized bodies were called black stars or infrared stars before the name "brown dwarf" was suggested in 1975. The name is a bit misleading; a brown dwarf actually appears red, not brown.
In the mid-1980s astronomers began an intensive search for brown dwarfs, but their early efforts were unsuccessful. It was not until 1995 that they found the first indisputable evidence of their existence. That discovery opened the floodgates; since then, researchers have detected dozens of the objects. Now observers and theorists are tackling a host of intriguing questions: How many brown dwarfs are there? What is their range of masses? Is there a continuum of objects all the way down to the mass of Jupiter? And did they all originate in the same way?
The halt of the collapse of a brown dwarf during its formation occurs because the core becomes degenerate before the start of fusion. With the onset of degeneracy, the pressure can’t increase to the point of ignition of fusion.
Brown dwarfs still emit energy, mostly in the IR, due to the potential energy of collapse converted into kinetic energy. There is enough energy from the collapse to cause the brown dwarf to shine for over 15 million years (called the Kelvin-Helmholtz time). Brown dwarfs are important to astronomy since they may be the most common type of star out there and solve the missing mass problem (see cosmology course next term). Brown dwarfs eventual fade and cool to become black dwarfs.
Relative sizes and effective surface temperatures of two recently discovered brown dwarfs -- Teide 1 and Gliese 229B -- compared to a yellow dwarf star (our sun), a red dwarf (Gliese 229A) and the planet Jupiter, reveal the transitional qualities of these objects. Brown dwarfs lack sufficient mass (about 80 Jupiter’s) required to ignite the fusion of hydrogen in their cores, and thus never become true stars. The smallest true stars (red dwarfs) may have cool atmospheric temperatures (less than 4,000 degrees Kelvin) making it difficult for astronomers to distinguish them from brown dwarfs. Giant planets (such as Jupiter) may be much less massive than brown dwarfs, but are about the same diameter, and may contain many of the same molecules in their atmospheres. The challenge for astronomers searching for brown dwarfs is to distinguish between these objects at interstellar distances.
Eagle Nebula Lagoon Nebula, Orion Nebula (Hubble Space Telescope)
Neither planets nor stars, brown dwarfs share properties with both kinds of objects: They are formed in molecular clouds much as stars are, but their atmospheres are reminiscent of the giant gaseous planets. Astronomers are beginning to characterize variations among brown dwarfs with the aim of determining their meaning among the Galaxy's constituents. In this image a young brown dwarf is eclipsed by one of its orbiting planets as seen from the surface of the planet's moon.
How Do We Know What Stars Are Made From?
What is incredible is that just by looking at starlight astronomers can discover what a star is made from. The light that we see is just one kind of radiation known as 'visible light'.
Different stars give out varying amounts of these signals, known as their 'spectrum'. When measured, the spectrum appears as a series of bright and dark lines positioned at specific frequencies.
This is the blueprint of a star and provides a wealth of information about what is happening inside. As elements are heated inside the star, they absorb and emit energy, creating a 'blip' in the star's spectrum. So the position and strength of these lines reveal what elements are inside the star. Stars are classified into 'spectral types' according to the shape of this spectrum.
Picture Of Stars That I found
This image shows the Orion Nebula or . This image is the outflow (coloured red) and protostar.
A Planetary Nebula (Above, ). A Red Super giant, above).
The images above were from the
