Black Holes and Dead Stars Essay Example

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The Remains of Stellar Evolution

The Remains of Stellar Evolution

Black holes, neutron stars, white dwarfs, supernova remnants and planetary nebulae are the remnants of the earliest days of stellar evolution.

Black Holes and Dead Stars

Image 1 : Stellar Evolution, redOrbit.com, 2014

Black holes

Black holes are stars which are compressed as far as this is possible. According to NASA 1this occurs because of immense gravity. The more material that is gathered together then the greater the amount and power of gravity. If all nuclear combustion in a star greater than about 1.4 solar masses should come to a stop, and the star was then permitted to cool down, it would gradually solidify. This solidified material would not have the strength to carry its own weight. It would therefore collapse as the electrons it contains are pressed into the protons to form neutrons. The material of this neutron star is then stronger than before, but the star’s diameter would now be only about 20 km across. If more material were then added, eventually the point would be reached where the gravity is so great, and produces such power, that not even the stronger neutrons could support it. At this point the former star, now dead, collapses to form a black hole.

Black holes by definition do not emit light. They are therefore very hard to see in the darkness of space2. According to Moskowitz black holes are basically invisible3. Despite this astronomers are developing technology which can image the immediate areas surrounding of the black holes and in the near future it is possible that researchers may produce the first-ever picture of this environment. They may even be able to see the «shadow» of a black hole, something which is at present only known in theory. At present astronomers know black holes are present spot black holes because they can detect the high levels of energy radiated by the swirling matter which falls into them. Before such matter actually the point of no return, known in the case of black holes as the event horizon, any radiation emited by this matter, whatever it is, can still escape, and so can be detected. In a few years however , however, astronomers hope to be able to detect spot black holes by observing the warps produced in space-time, which occur as a result of their enormous gravity.

Neutron Stars

Neutron stars are extremely compact objects, the most dense objects discovered so far by astronomers. They are created in the hearts of massive stars when supernova explosions occur4. Typically they only have a diameter of about five miles to ten miles, but their mass is greater than that of the sun. Newman quotes astronomer Frank Shu as having said that:-

A sugar cube of neutron star stuff on earth would weigh as much as all humanity5.

Such a cube would weigh about 100 million tons. If it were possible to weigh a whole mountain on Earth it would weigh about as much.

When the core at the centre of such of a star collapses inwards , every proton is crushed and together with its corresponding electron, every electron-proton pair becomes a neutron. These neutrons are stronger and so can often prevent total collapse and so the material remains as a neutron star. In a similar way to white dwarfs, their less massive equivalents, the greater the weight of a neutron star the smaller it becomes, as it becomes more and more compact.

Neutron stars can sometimes be observed as an extremely small hot star within the remains of a supernova. They are however more likely to be observed when they are a pulsar or as part of an X-ray binary star system , that is when two such stars rotate around a common mass. A neutron star in such binary system gains mass from another star. There is a rapid transfer of gas between the two and this produces a very hot, about 1 million degrees K, accretion disk. X rays are produced and these can be detected using an x ray telescope. A neutron star is visible if powerful machines, such as the very sensitive Hubble telescope, are used. This is a Cassegrain reflector telescope which orbits the earth6. It is observable because the star produces so much heat that it emits a great deal of light per unit area. Even so, because of its very small size, even then it only shows up faintly as a point of light.

White Dwarf

According to Mattson 7 a white dwarf star is what all stars such as the Sun evolve into as their nuclear fuel becomes gradually exhausted. Towards the end of its nuclear power burning stage, this sort of star loses the majority of its exterior material. The result is a , planetary nebula consisting of only the still hot core of the star. This core is then a very hot white dwarf, its temperature being 100,000 Kelvin, which is why it appears to be white rather than red. This remaining core is so hot that it takes such a white dwarf star more than a billion years to cool down , and even this very long period of time can be extended if it taking matter from a neighbouring star. Many nearby, in space terms young white dwarfs have been observed as producers of soft, that is lower-energy, X-rays. The observation of these stars and their composition has been done using extreme ultraviolet and soft X-rays observations. A white dwarf is typically has half the mass of the Sun, yet is probably only a little larger than Earth.

Supernova Remnants

A Supernova is an extremely bright light which bursts suddenly into view in the night sky8. It is an exploding star at the very end of its life. Although very brief, such explosions can be brighter than whole galaxies, and it is possible that they emit more energy than the sun will in its entire existence, past, present and future. Such explosions are the main source of heavy elements within the universe. In a galaxy such as the Milk Way such an explosion takes place about twice in a century, which means that somewhere in the vast universe such an explosion takes place every second.

Almost all the energy which remains in a Super Nova Remnant (SNe) is set free as energetic neutrinos. The remaining energy, about 1% of the whole, is transformed into kinetic energy9. This speeds up the remaining stellar material to speeds exceeding the speed of sound. A shock wave moves out from the core of the explosion. The high velocity stellar material flies outwards into the interstellar medium (ISM). It compresses and heats the ambient gas and so it enriches the ISM with the remnants of exploding star. The material expands, and may also , collect other material on its travels across the interstellar medium, and so creates a supernova remnant (SNR).

According to Springer10, although some SNR’s can be seen by amateurs using very ordinary telescopes, the majority require radio telescopes in order to be observed effectively.

Planetary Nebulae A planetary nebulae gets this name because, if viewed through a small telescope, perhaps by an amateur, it looks like in the first instance a planet, although there is very little about them which is solid.
Such nebulae form at a very late stage of the life of a sun-like star and are basically shells of gas11which have the mere appearance of being solid. These stars have used nuclear fusion to change hydrogen into helium for billions of years. Then they evolve as their hydrogen stores are gradually used up and the core gets smaller. Eventually the crises which take place climax in the star increasing a hundred-fold in size to transform itself into a red giant.

They can be observed using a 114 mm telescope. The Nightsky info web site12also suggests the use of a prism, which would allow the planetary nebula’s spectrum to be seen. Another method would be to use an OIII filter. The area is observed with the filter in place and then again, after this has been removed. When the filter is in place the light form the nebula can be seen. This technique helps to differentiate it from a star or planet.

Conclusion

These are only brief notes on each of these phenomenon, but it is clear from the research undertaken that there is still much to be discovered. Also, in many instances there is more than one way in which these parts of the universe can be observed, both by actual observation, but also by the effects they cause.

References

Image 1 : Stellar evolution , redOrbit.com, 2014 retrieved from http://www.redorbit.com/education/reference_library/space_1/universe/2574776/stellar_evolution/

Goudarzi, S., (2007) The tricky task of detecting black holes, Space.com, retrieved from http://www.space.com/3457-tricky-task-detecting-black-holes.html

Harrington, J., Anderson,J., and Edmonds, P., (2012) Chandra: Exploring the Invisible Universe, Nasa
retrieved from
http://www.nasa.gov/mission_pages/chandra/multimedia/nebula_abell30.html

HEASARC (2011) Introduction to Supernova Remnants, NASA, retrieved from http://heasarc.nasa.gov/docs/objects/snrs/snrstext.html

Hubble
Site, (2000) The Telescope: Hubble Essentials, retrieved from http://hubblesite.org/the_telescope/hubble_essentials/

Mattson, B., (2010) White Dwarf Stars, Nasa’s Imagine the universe.
retrieved from http://imagine.gsfc.nasa.gov/docs/science/know_l2/dwarfs.html

Moskowitz,C., (2013) How to see a black hole, Fox News,
retrieved from http://www.space.com/3457-tricky-task-detecting-black-holes.html

Newman,P., (2010) Neutron Stars, Nasa’s Imagine the Universe,
retrieved from http://imagine.gsfc.nasa.gov/docs/science/know_l1/neutron_stars.html

NightSky info(undated) Planetary Nebulae, retrieved from http://www.nightskyinfo.com/planetary_nebulae/

Palmer,D. and Safi-Harb,S. (2005) Ask an astrophysicist: How can a star be compressed to form a black hole, Nasa, retrieved from
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/990210b.html

Springer,( Undated) Chapter 13, Observing Supernova Remnants, retrieved from http://link.springer.com/chapter/10.1007%2F978-0-387-46269-1_13#page-2

Thompson, A., (2009) What is a Supernova, Space.com retrieved from http://www.space.com/6638-supernova.html

1Palmer,D. and Safi-Harb,S. (2005) Ask an astrophysicist: How can a star be compressed to form a black hole.Nasa

2 Goudarzi, S., (2007) The tricky task of detecting black holes, Space.com

3 Moskowitz,C., (2013) How to see a black hole, Fox News

4 Newman,P., (2010) Neutron Stars , Nasa’s Imagine the Universe

5 Newman,P., (2010) Neutron Stars , Nasa’s Imagine the Universe

6 Hubble Site,( 2000) ,The Telescope: Hubble Essentials

7 Mattson, B., (2010) White Dwarf Stars, Nasa’s Imagine the universe.

8 Thompson, A., (2009) What is a Supernova, Space.com

9 HEASARC (2011) Introduction to Supernova Remnants, NASA

10 Springer, Chapter 13, Observing Supernova Remnants

11 NightSky info,(undated) Planetary Nebulae,

12 NightSky info,(undated) Planetary Nebulae