Skip to main content

What is an accretion disk?

An accretion disk is an astronomical term that refers to the rapidly spiraling  matter that is in the process of falling into an astronomical object. In principle, any  star could have  an accretion disk, but in practice, accretion disks are often associated with highly collapsed stars such as black holes or  neutron stars. 


The matter that serves as the base of the accretion disk can be obtained when a star passes through a region where the  interstellar matter is thicker than normal. Normally, however, a star gets an accretion disk from a companion star.  When two stars  orbit each other, there is an invisible figure eight around the two stars, called the Roche lobes.  The Roche lobes represent all the points in space where the gravitational potential from each star is equal.  Therefore any matter on the Roche lobes could just as easily fall into either star. If one star in a binary system becomes larger than the Roche lobes, matter will fall from it onto the other star, forming an accretion disk.

The matter falling into a collapsing star hole tends to form a disk because a spherical  mass of gas that is spinning will tend to flatten out.  The faster it is spinning, the flatter it gets. So, if the falling material is orbiting the central mass, the spinning flattens the matter into an accretion disk. 

Black holes are objects that have  collapsed to the point that nothing, not even  light, can escape the incredible  force of their gravity. Because no light can escape, however, there is no way to directly observe it. However, if the  black hole has an accretion disk, we can observe  the black hole indirectly by observing the accretion disk, which will emit x rays.  Without accretion disks there would be little hope of astronomers ever observing black hole.

 Accretion disks can also occur with a  white dwarf in a binary system.  A white dwarf is a collapsed star that is the final stage in the evolution of stars similar to the  Sun. White dwarfs contain as much mass as the Sun, compressed to about the size of  Earth. Normally the nuclear reactions in a white dwarf have  run out of fuel, but the  hydrogen from the accretion disk falling onto a white dwarf fuels additional nuclear reactions.  White dwarfs have  some unusual properties that do not allow them to expand slowly to release the  heat pressure generated by these nuclear reactions.  This heat pressure therefore builds up until the surface of the whited dwarf explodes.  This type of explosion is called a  nova (not to be confused with a  supernova), and typically releases as much  energy in the form of protons in less than a year as the Sun does in 100,000 years.

Comments

Popular posts from this blog

How Energy conversion from coal is done?

Single generator sets of over 600 MW are now used in the UK, though there are many smaller generators in use. A 600 MW generator can supplythe average needs of over 1 million UK households. Three or four such generators are typically installed in a single large coal-fired station which isoften sited close to a coal mine, away from the city dwellers who consume the electricity. Such generators are usually driven by a compound arrangement of highpressure, intermediate-pressure and low-pressure turbines, increasing in size as the pressure decreases. Modern turbines rotate in a speed range from 1500 to 3500 r.p.m., usually 3000 r.p.m. for the UK’s 50 Hz system. For large coal-fired plant the steam pressure could be 25 megapascals (MPa) with steam temperatures of 500–600 °C to improve the thermodynamic efficiency. In nuclear reactors, which operate under less demanding conditions, the steam is superheated to about 5 MPa and 300 °C. Modern water tube boilers are complex and have ...

All about Electronics & Instrumentation Engineering)(EI)

  Vision : To be globally recognized as a seat of learning and innovation in Electronics for application to Communication, Transportation, Defence, Industry, Health Care, Entertainment, and many other consumer products.  Mission : 1). To produce quality engineers in the field of Electronics and related domains.  2) To conduct research and develop products in the fields of Chip design, Communication Systems, Electronics Instrumentation, Signal Processing and other related areas with strong emphasis on critical state of the art applications.  3) To grow in the area of Design and Manufacturing of chips, boards and systems.     Program Educational Objectives :  I.  To provide students a successful career in industry that meets the needs of national and multinational companies.  II.  To develop the ability among students to synthesize data and technical concepts for application to electronic product design and to solve real...

5 TOP FACTS LIGHTHOUSES

1  The builder of the Lighthouse of Alexandria, Sostratus – disobeying orders from the pharaoh Ptolemy – engraved his name and a dedication to the sea gods on the tower base. Fame 2  The technical term for the study of lighthouses is ‘pharology’, a word derived from Pharos, the island upon which the great Lighthouse of Alexandria once stood. Academia 3  George Meade built many notable lighthouses in the US during the classical lighthouse period. He is remembered in history as the winning general in the Battle of Gettysburg. War 4  The tallest lighthouse in the world is the Yokohama Marine Tower in Yokohama, Japan. The structure fl ashes alternately green and red every 20 seconds. Tallest 5  Originally lighthouses were lit merely with open fi res, only later progressing through candles, lanterns and electric lights. Lanterns tended to use whale oil as fuel.